precipitate-based ion-selective...
TRANSCRIPT
PRECIPITATE-BASED ION-SELECTIVE ELECTRODES
E. PUNGOR and K. TOni
Inst itute for General and Analytical Chemistry, Technical University,Budapest, Hungary
ABSTRACT
A critical and wide-ranging revicw of the whole field of precipitate-basedion-selective electrodes is presented. The current theories of the mechanismof the electrode response are discussed and related to the experimental data.The most important analytical and physicochemical applications of the elec-trode are summarized, and, in particular, some examples of the use of elec-
trodes to study complex formation are treated in detail.
INTRODUCTIONWhen Kolthoff79 first proposed fused silver chloride and silver bromide
discs for the detection of chloride and bromide in 1937, and later, in 1961,when we122 pointed out the possibility of highly selective direct measure-ment of iodide ion, using silver iodide precipitate incorporated in paraffinwax, it could not have been foreseen how great a development had beeninitiated. In contrast to the lack of success in the fifties in the search forion-selective electrodes, in the sixties many such electrodes, based onprecipitates, were developed (Table 1) and many studies of their mostimportant parameters were carried out.
Parallel with the discovery of other types of electrodes not based onprecipitates. great efforts have been put into clarifying the mechanism of theelectrode response of the precipitate-based type, and in finding new lines ofdevelopment for novel ion-selective electrodes. Unfortunately, in thisrespect we are still very much in the early stages and so it is understandablethat there has been practically no increase in the range of selective electrodes(with respect to the ionic species) available since the middle of the sixties.
In this paper the conclusions and data are drawn partly from the literatureand partly from our findings for the precipitate-based electrodes. Opinionsgiven in the literature are contradictory in some cases and we propose toadd our own comments on some of them later. Furthermore, on the basisof up-to-date results we will offer our opinion about further developments.
We use the nomenclature 'precipitate-based electrodes' instead of solid-state electrodes because there are other types of solid-state ion-selectiveelectrodes such as ion-exchange electrodes. In all solvents the properties ofthe precipitate-based electrodes depend on the solubility of the activecomponent of the electrode in the appropriate solvent, whereas the propertiesof other types of solid-state electrode will depend on other parameters.
105
Tab
le I.
Pre
cipi
tate
-bas
ed
ion-
sele
ctiv
e el
ectr
odes
Sens
itive
to
ion
Solid
stat
e A
ctiv
e m
ater
ial
Mat
rix
Sele
ctiv
ity (
Com
men
ts)
Wor
kers
R
efer
ence
s
Ca2
+
Het
erog
eneo
us
Ca-
oxal
ate a
nd
Para
ffin
+ n
on-i
onic
Fa
irly
acc
epta
ble'
T
ande
loo
and
Kri
ps
167,
168
othe
r Ca-
salts
de
terg
ent
on g
auze
(1
957)
Ba2
' B
a504
SO
j H
eter
ogen
eous
B
aCrO
4 Pa
raff
in w
ithou
t gau
ze
Not
per
fect
ly s
elec
tive
Fisc
her
and
Bab
cock
46
to
eith
er a
nion
s or
(195
8)
catio
ns.
(Slo
w to
atta
in
Fisc
her
and
Yat
es (
1960
) 47
equi
libri
um)
K
Het
erog
eneo
us
K-t
etra
phen
ylbo
rate
Poly
styr
ene
+ ga
uze
Not
sel
ectiv
e T
ande
loo a
nd K
ripp
s (1
959)
16
9
Ca-
+
Het
erog
eneo
us
Ca-
stea
rate
Pa
raff
in a
s ab
ove
Stro
nger
res
pons
e to
Tan
delo
o an
d va
n de
r 17
0
Ca2
+ th
an o
xala
te. N
o V
oort
(196
0)
resp
onse
to K
+
Z
Ca2
+
Het
erog
eneo
us
Ca-
oxal
ate
Para
ffin
+ de
terg
ent
Non
-spe
cifi
c, p
orou
s C
loos
and
Frip
iat (
1960
) 28
('Exh
ibit
mem
ory'
) H
eter
ogen
eous
A
g!
Para
ffin
K
CI
does
not
inte
rfer
e Pu
ngor
and
Hol
lós-
12
2 >
. (±
SmV
rep
rodu
cibi
lity)
R
okos
inyi
(196
1)
Mg2
', Sr
2 M
ultil
ayer
C
a-st
eara
te
Gre
gor,
Gla
tz a
nd
55
Mg-
stea
rate
Sc
honh
orn (
1963
) r
F
Het
erog
eneo
us
Agi
Si
licon
e ru
bber
10
M
KC
I do
es n
ot
Pung
or, T
óth
and
Hav
as
130
, in
terf
ere
(196
6)
SO
Het
erog
eneo
us
BaS
O4
Silic
one
rubb
er
Phos
phat
e in
terf
eres
Pu
ngor
, Tót
h an
d H
avas
11
9, 1
20,
(196
4)
121
Ag'
,X
AgX
PO
H
eter
ogen
eous
M
n P0
4 Si
licon
e rub
ber
Hig
h se
lect
ivity
(th
e Pu
ngor
, Hav
as a
nd T
óth
126,
131
Al3
t A
l-ox
ine
elec
trod
es fo
r P0,
(1
965)
N
i2
Nic
kel-
DM
G
Al3
', N
i2',
Bi3
' are
B
i3 , P
0 —
B
iPO
4 no
t sel
ectiv
e)
Co2
' H
eter
ogen
eous
C
o3(P
04)2
C
ollo
dion
, pa
raff
in
Lev
elle
d by
KC
I, L
i250
4 M
oraz
zani
, Pel
letie
r and
10
5
(por
ous m
embr
ane)
B
affl
er (1
965)
Hom
ogen
eous
R
are
eart
h —
(S
ingl
e cr
ysta
l)
Fran
t and
Ros
s (19
66)
49
fluo
ride
s H
t OH
- T
i02,
Fe2
03, 5
n02
Poly
ethy
lene
N
a Z
r02
Pol
ypro
pyle
ne
Al3
+
Het
erog
eneo
us
A12
03
Para
ffin
(T
itrat
ions
only
stu
died
) G
eyer
and
Syri
ng (1
966)
53
SiFt
, K
K2S
iF6
Aga
r or p
aper
B
oth
ions
A
g4Fe
(CN
)6
Aga
r B
oth
ions
Pb
WO
4 Pa
raff
in
Ca2
H
eter
ogen
eous
C
a-ox
alat
e Pa
raff
in+
non-
ioni
c N
ot c
ompl
etel
y Sh
atka
y (19
67)
12,
155
dete
rgen
t + ga
uze
perm
sele
ctiv
e; n
ot
spec
ific
. (G
auze
nece
ssar
y fo
r con
duct
ion.
No
en
diff
eren
ce b
etw
een
pure
pa
raff
in a
nd p
araf
fin +
C
a-ox
alat
e)
F-
Hom
ogen
eous
L
aF3
(For
mic
roch
emic
al
Dur
st an
d T
aylo
r (19
67)
40, 4
1 an
alys
is)
SO
BaS
O4
3 .
3t
.. .
. P
04
Het
erog
eneo
us
BiP
O4
Silic
one
rubb
er
Rec
hnitz
, L
in an
d Z
amoc
hnic
k 138
en
(196
7)
a A
g, S
2 H
omog
eneo
us
Ag2
S L
ight
and
Sch
war
tz (
1968
) 92
5
BaS
O4.
R
espo
nd to
tota
l fre
e Z
N
i-D
MG
ca
tions
I
Div
alen
t ion
s H
eter
ogen
eous
C
o3(P
04)2
Pa
raff
in
(Con
ditio
ning
very
. B
ucha
nan a
nd S
eago
23
r
Ni3
(P04
)2
Sili
cone
rubb
er
impo
rtan
t cry
stal
line
(196
8)
CuH
PO4
form
; hyd
ratio
n and
M
nHPO
4 an
ion
not i
mpo
rtan
t)
J M
nCO
3 H
eter
ogen
eous
C
aF2
Sili
cone
rubb
er
Sel
ectiv
e res
pons
e to F
T
hF4
Not
rep
rodu
cibl
e M
cDon
ald a
nd T
óth
103
LaF
3 Se
lect
ive
resp
onse
to
F (1
968)
A
gt s2
H
eter
ogen
eous
A
g25
Sili
cone
rubb
er
Una
ffect
ed by
C1,
Pu
ngor
, Sch
mid
t an
d T
óth
125
BC
, F
(196
8)
a C
r A
gC1
BC
H
omog
eneo
us
AgB
r Sm
, Pr,
Eu,
Ho
dope
d V
esel
y an
d Ji
ndra
(196
8)
177
I—
AgF
si
ngle
cry
stal
L
aF3
NO
; H
eter
ogen
eous
N
i(N
03)2
Ph
OH
+ H
CH
O+
NH
3 R
espo
nsiv
e to
uni
vale
nt
Dob
beis
tein
and
Die
hl
36
anio
ns a
nd H
; (1
969)
un
resp
onsi
ve to
m
ultiv
alen
t ani
ons a
nd
catio
ns
Tab
le 1
(con
tinue
d)
Sens
itive
to
ion
Solid
sta
te
Act
ive
mat
eria
l M
atri
x Se
lect
ivity
(Com
men
ts)
Wor
kers
R
efer
ence
s
F Cu2
+
Cu2
+
s2-
Cu2
+
—
2+
o °°
Cl0
NH
PO -
Het
erog
eneo
us
Inor
gani
c Si
licon
e ru
bber
G
uilb
ault
and
Bri
gnac
58
phos
phat
e sal
ts
(196
9)
1, B
C. C
1 H
eter
ogen
eous
A
gI
AgB
r, A
gCl
Poly
ethy
lene
M
asci
ni a
nd L
iber
ti (1
969)
10
0
Cd2
+
Hom
ogen
eous
C
dS +
Ag2
S B
rand
, Mili
tello
and
Rec
hnitz
(19
69)
14
Hom
ogen
eous
E
u'1'
dop
ed s
ingl
e cr
ysta
l W
illia
ms
(196
9)
180
Het
erog
eneo
us
Cu2
S Si
licon
e ru
bber
H
irat
a an
d D
ate
63
r1
Hom
ogen
eous
Het
erog
eneo
us
Cu2
S
Ag2
S Po
lyet
hyle
ne
Cer
amic
mem
bran
e (1
970)
H
irat
a, H
igas
hiya
ma a
nd
Dat
e (19
70)
Mas
cini
and
Lib
erti
(197
0)
65
101
a
Hom
ogen
eous
A
g2S +
MeS
(P
aten
t)
Fran
t and
Ros
s (1
970)
51
Hom
ogen
eous
Pb
2, B
i3t Y
3 Sc
3 or
L
anth
anid
e F
Non
-pol
ariz
ed
ion-
se
lect
ive
elec
trod
e (S
emic
ondu
ctor
m
embr
ane a
nd a
Pb
and'
or B
i met
al
cont
act)
(pa
tent
)
Farr
en a
nd S
taun
ton
(Per
kin—
Elm
er) (
1970
) 45
' Z H
eter
ogen
eous
A
liquo
t 336
E
poxy
tre
ated
gla
ss fr
it T
ated
a, F
ritz
and
Itan
i (1
970)
16
5
Het
erog
eneo
us
K-s
alt b
iolo
gica
l m
ater
ial
Litt
le in
terf
eren
ce f
rom
ot
her c
omm
on ca
tions
(B
eckm
an M
odel
396
22)
Kru
ll, M
ask
and
Cos
grov
e (1
970)
83
Het
erog
eneo
us
NH
4-sa
lt bi
olog
ical
m
ater
ial
Litt
le in
terf
eren
ce f
rom
ot
her c
omm
on ca
tions
(B
eckm
an M
odel
396
26)
Cos
grov
e, M
ask
and
Kru
ll (1
970)
32
F H
omog
eneo
us
LaF
3 Sm
, Pr,
Eu,
Ho,
Nd
dope
d L
aF3
sing
le
crys
tal
Ves
ely
(197
1)
175
Pb2
+
Hom
ogen
eous
Pb
S C
eram
ic m
embr
ane
Hir
ata
and
Hig
ashi
yam
a 67
(1
971
Pb2t
H
eter
ogen
eous
Pb
S Si
licon
e ru
bber
H
irat
a an
d D
ate (
1971
) 64
Cu2
t H
eter
ogen
eous
C
uS +
Ag2
S
Poly
ethy
lene
M
asci
ni a
nd L
iber
ti 10
2
(197
1)
S2
Ag2
S Pp
t. on
the
surf
ace
of
Hal
ides
A
g-ha
lides
a g
raph
ite ro
d R
uzic
ka an
d L
amm
14
5, 1
46
Hg2
+
HgS
(S
elec
trod
e—th
e uni
vers
al (
1971
)
Agt
H
omog
eneo
us
Ag2
S io
n-se
lect
ive
solid
-sta
te
Cu2
+
CuS
el
ectr
ode)
Pb
2t
PbS
Cd2
CdS
Cu2H
Ag
Hom
ogen
eous
T
CN
Q s
alts
Sh
arp
and
Joha
nsso
n 15
4
NH
: (1
971)
6
e H
omog
eneo
us
CuS
G
ordi
evsk
ii et
al.
(197
1)
163
v205
C
N
Hom
ogen
eous
A
g ((
3-gl
ucos
idas
e in
Rec
hnitz
and
Lle
nado
13
9
acry
lam
ide
gel)
(1
971)
Cu2
H
omog
eneo
us
Cu2
S G
ordi
evsk
ii et
a!.
(197
1)
164
Pb2
+
Hom
ogen
eous
Pb
S C
eram
ic m
embr
ane
Hir
ata
and
Hig
ashi
yam
a 66
Ag2
S w
ith s
inte
red m
etal
(1
971)
Cu2
5 su
lphi
des
CN
H
omog
eneo
us
AgI
+ A
g2S
Flee
t and
Sto
rp (
1971
) 48
Cl
Het
erog
eneo
us
AgC
l M
ethy
l m
etha
cryl
ate
(Pat
ent)
N
eff,
Sam
buce
tti a
nd C
arlo
s 10
7
(197
1)
c C
st
Het
erog
eneo
us
Cs 1
2-m
olyb
do-
Silic
one
rubb
er
Coe
tzee
and
Bas
son (
1971
) 29
phos
phat
e C
t
Cu2
t H
omog
eneo
us
Cu1
, Se
(S
ingl
e cr
ysta
l)
Ves
ely
(197
1)
176
Hg2
t Cu2 +
Cd2
t Pb2t
Hom
ogen
eous
A
g2S
+ M
eS
On
the
surf
ace
Anf
ält a
nd J
agne
r (19
71)
4
Zn2
t of
a s
ilver
wir
e
Ni2
t C
o2t
Tab
le I
con
tinue
d)
Sens
itive
to
ion
Solid
stat
e A
ctiv
e m
ater
ial
Mat
rix
Sele
ctiv
ity (C
omm
ents
) W
orke
rs
Ref
eren
ces
Cu2
A
g H
omog
eneo
us
Sele
ctiv
ity,
sens
itivi
ty,
Ada
met
zova
and
Gre
gr
1
S2
infl
uenc
e of
pH. r
espo
nse
(197
1)(C
rytu
r m
odel
) C
N
time
are
disc
usse
d Z
C
a2
Hom
ogen
eous
C
aF2
+ La
F3
Mg2
, Sr2
, Pb2
, Ba2
, Fa
rren
(197
1)
44
Na,
K io
ns in
terf
ere
(Per
kin—
Elm
er)
Ca2
+
Het
erog
eneo
us
Ca-
dide
cylp
hos-
5 °.
PV
C i
n N
a, M
g, N
i, B
a, S
r,
Cat
tral
l and
Fre
iser
27
phat
e in
dio
ctyl
- cy
cloh
exan
one
Pb, Z
n in
terf
ere.
(1
971)
0'
ph
osph
onat
e C
oate
d w
ire
elec
trod
e
Cu2
H
eter
ogen
eous
C
u2 _
S Si
licon
e ru
bber
V
ery
high
sel
ectiv
ity
Pung
or, T
óth
and
Pick
13
2
for t
he c
oppe
r (1
972)
PRECIPITATE-BASED ION-SELECTIVE ELECTRODES
The precipitate electrodes can be prepared in three ways: from singlecrystals; from pressed crystals, and from crystals embedded in a suitablematerial. The first two kinds of electrodes form the group of homogeneouselectrodes, while the third type gives the heterogeneous electrodes. However,no basic difference exists between these two types of electrodes (except forsome effects, which have not yet been explained). There are of course,differences between the mechanical properties of the electrodes, their life-time and so forth, but from the electrochemical point of view they shouldbe treated together.
With regard to the construction of the electrodes, the electrode membranes,either homogeneous or heterogeneous, are sealed onto the end of glass orplastic tubes, or screwed on with the help of an inert material which doesnot swell or cause a short circuit between the two sides of the membranelayer.
THEORETiCAL SECTIONMechanism of the electrode response
The correlations and ideas derived from glass and the so-called ion-exchange electrodes greatly helped in the theoretical description of theprecipitate-based electrodes. From these guidelines the electrode behaviourmay be interpreted in two ways. The first is based on ion transport throughthe membrane, derived for ion-exchange electrodes. The second is relatedto the theory given for the mechanism of the glass electrode, in which it isproposed that different ions take part in the ion-transport phenomenonthan in the surface equilibria.
At the beginning of our research we tried to interpret the membranephenomena by ion-transport122. However, later on, experimental resultswere obtained which were contradictory. No electrochemical difference wasfound experimentally between electrodes made as a sandwich (silver orplatinum plates between two membrane layers assuring electrical contactonly through the metal) and normal electrodes' 18
Our opinion about the surface ion-exchange reaction was strengthenedby radioactive measurements of the exchange rate of the iodide ions on thesilver iodide precipitate-based electrodes' 18 The rate of the exchangereaction was fast and was dependent on the surface charge of the precipitateembedded in the electrode membrane. Of course the charge transportthrough the crystal and its mechanism are also very important in theinterpretation of the behaviour of the electrodes. In this context we wouldlike to mention the paper of Durst and Ross42, describing the application ofan LaF3 electrode doped with Eu for the coulometric generation of F,with an efficiency of 99.2 per cent. Sher and co-workers' showed that theLaF3 electrode has a conductivity of iO ohm' cm' which can beexplained by the following reaction:
LaF3 + vacancy —* LaF + F
Silver halides and silver sulphide are also ionic conductors (silver suiphideis partly an electric conductor). By analogy with LaF3, Ross'42 has stated
111
E. PUNGOR AND K. TOTH
that the theory of solid-state electrodes is simple because no ions other thanthose composing the crystal go through the membrane, and the potential isa result of the ion transport through the membrane. This statement showsthat he has not taken into consideration the ion-exchange theory describedin the literature24' 1 18, Brand and Rechnitz'5, using impedance measure-ments, have provided further evidence for the surface-exchange theory.However, their detailed discussion seems to be scarcely justified by thesmall amount of experimental data. In their work they classify four types ofelectrodes, according to electrical analogies to the response mechanism.The first group, to which the LaF3 electrode belongs, have, in the analogy,two frequency-dependent impedances in series, one of which is accountedfor by the La(OH)3 film on the surface of the electrode, while the other isdue to the LaF3 itself. Brand and Rechnitz assume that there is no differencebetween the fluoride transport in the two layers. They put the silver chlorideelectrode into the second group, where there is no surface film on the silverchloride. According to them, in both groups the halide is transportedthrough the membrane and ion-exchange equilibria are established on thesurfaces. In the third group, consisting of Br, 1, S2, Pb24 and Cd2electrodes and in the fourth group, the copper electrode, ion transportthrough the membrane has not been proved, and in an electric field chargeaccumulates on the surface. The authors are of the opinion that measurementof the electrode impedance is a good way of evaluating the rate of the ion-exchange reaction on the surface.
A new contribution has recently been made by Hirata and co-workers6466'67 to the study of the electrical conductivity of the membranephase. They found that electrically conducting sintered PbS has no responseto Pb2 In spite of this the electrode functions if the sintered phase containsa large amount of Ag2S in addition to PbS. In this case ion transport throughthe membrane and the exchange reaction on the surfaces are affected bydifferent ions. It is interesting to mention that they found a Pb2 responseif the PbS was incorporated into silicone rubber. This phenomenon so far,however, cannot be explained.
The importance of surface ion-exchange reactions in the response mecha-nism has been underlined by our experiments carried out studying theadsorption of various anions on the surfaces of Ag! precipitates' 17,124,149From these experiments, it was concluded that halide and pseudo-halideions, after losing their coordinated water, were adsorbed on the surface ofthe silver iodide, while other ions such as PO and SO are adsorbedwithout loss of the coordinated water. Furthermore, the investigationshowed that the anions forming slightly soluble silver salts are exchangedin a very fast reaction by other anions which form less soluble salts withsilver.
Since the development of the precipitate-based ion-selective electrodes ithas been stated unambiguously that electric conductivity is a necessarycondition for the electrode response in addition to the surface-exchangeequilibria. The conductivity can be assured by crystal defects or doping ofsingle crystals as suggested by Frant and Ross49 who used Eu for dopingLaF3. The last method needs further investigation as emphasized by thework of Vesely' who found that there is no unambiguous correlation in
112
PRECIPITATEBASED ION-SELECTIVE ELECTRODES
the case of the LaF3 electrode between the doping and the electrochemicalbehaviour. His finding is in accordance with the opinion of Lingane94 whobelieves that doping is unnecessary.
From our research98 we have concluded that there is a correlation betweenthe number of crystal defects and the standard potential of the electrode.Table 2 shows the difference between the standard potentials of electrodesof the second kind and the corresponding ion-selective electrodes, the activecomponent of which has been exposed to light for different time intervals.
Table 2. Effect of exposure time on the differences between the potentials of membrane electrodesand electrodes of the second kind
Membrane electrode Exposure time (mm) EsE (mV)
AgCI 51020
807260
AgBr 5102030
15713913173
AgI 102030
177151104
As the slopes remained constant the calibration curves of the electrodeswere shifted parallel to each other. In a simple case when the membraneprecipitate consists of univalent ions, there is a correlation between theshift of the standard potential and the number of crystal defects. Thiscorrelation is as follows:
AE=Jln(ax) (ic)lwhere K is the solubility product of the precipitate, K is the equilibriumconstant for the formation of the defect in the crystal, and (ax). is the activityof the component X in the crystal and varies between 1 and K.
The electrode function, however, is not influenced by the standardpotential. This was proved not only by us but also by Loshkarev andYudina95, who found this for the sulphide electrodes.
The migration of the crystal defects can be described by the Nernst—Plank equation. Buck24 has applied this equation for the derivation of anexpression for the lower detection limit of ion-selective electrodes andreached basically the same conclusion which we reached using the theory ofDonnan potentials established on the surfaces131.
E — 21ln [CAg+(_L) + [Cg+(_L) + 4K9(y±Y2]1 YAg+(_L)—F [ CAg+(L) + [Cg+(L) + 4K59(y)2] i YAg+U)
where (L) and (—L) denote the two sides of the membrane and is thesolubility product.
113
E. PUNGOR AND K. TOTH
For the calculation of the lower detection limit Covington33 gave asimpler equation, which also takes into account the solubility of silver halidesin halide solutions.
Naturally this lower detection limit varies from electrode to electrode andwith the medium used for measurements. Furthermore, the limit shifts tolower concentrations with higher ionic strength. Shifts of several decadescan be achieved by alteration of some parameters—as will be discussedlater on.
In solutions containing more than one potential-determining ion, themembrane potential of the precipitate-based electrodes can be described onthe basis of the ion-exchange equilibrium by the following equation'18,which is analogous with the Nikolsky equation
E = E0 + Tlna1 EKIkF k a1
where KIk is the selectivity constant of the electrode. (We have derived anequation for the calculation of Kk, which is given later.) Accordingly theselectivity constant may be determined unequivocally from the solubilityproducts of the precipitates built into the membrane and also those formedduring the exchange reaction.
Buck24 has modified this concept and stated that it is valid only for suchimmiscible precipitates as silver chloride and silver iodide, and he hasproposed a more complex expression used for mixed crystals such as mixedLa(OH)3 and LaF3, for which the selectivity constant is:
UOH Y0H1'OH.F X
UF Yr
where u is the mobility of the ion in the crystal, and y is the activity co-efficient in the crystal.
For the practical determination of the selectivity constant the methodused by Eisenman43 for glass electrodes was first employed. This calculationis based on the potential difference measured with the electrode in solutionscontaining only the reversible ion, and then in solutions containing onlythe interfering ion, both solutions containing the same counter-ions withactivity 0.1 M. The equation for the calculation is
AE = !JlnK
This method has a fundamental error. It ignores the fact that the concentra-tion of the potential-determining ion, when in a pure solution of the inter-fering ion, is determined solely by the dissolution of the precipitate. So themeasuring conditions do not relate to the required conditions and are notwell defined: these defects in the measuring conditions are admirablyexemolified by the poor experimental results'36.
114
Tab
le 3
. Se
lect
ivity
con
stan
ts o
f hal
ide i
on-s
elec
tive
elec
trod
es f
or so
me a
nion
s.
K de
note
s val
ues
mea
sure
d by
dir
ect
met
hod;
K8
deno
tes v
alue
s m
easu
red b
y in
dire
ct m
etho
d
(M
Chl
orid
e-se
lect
ive
elec
trod
e B
rom
ide-
sele
ctiv
e el
ectr
ode
Iodi
de-s
elec
tive
elec
trod
e
Ion
KIk
cal
c.
K, m
eas.
K
8 m
eas.
K
k ca
lc.
K m
eas.
K
* m
eas.
K
1k c
aic.
K
mea
s.
K*
mea
s.
C1
1 1
2.0
x iO
1.
8 x
iO
6.0
x iO
9.
6 x
1O
3.7
x B
r 1
1 2.
0 x
101
2.1
x 10
1.
8 x
SCN
1.
5 x
10°
0.2
x 10
° 3.
0 x
t04
2.4
x to
0H
1.
0 x
10-8
0.
9 x
10_8
C
rO
5.2
x 1O
4.
5 x
1O
2.5
x iO
1.
! x iO
5.
0 x
10"
6.6
x 10
_li
CO
6.
3 x
iO
4.6
x i0
3.
1 x
10
1.0
x 10
PO
1.
3 x
1O
0.5
x i0
6.
3 x
iO
3.1
x i0
1.
2 x
10- 10
0.
2 x
10_b
A
sO
3.3
x i0
" 2.
0 x i0
1.
6 x
10-6
1.
2 x
t0_6
3.
2 x o
10
2.6
x o
10
Fe(C
N)r
2A
x 1
06
3.5
10
I—
1 1
C
-l
H
Ct
rn C 2 r rn
C
H r rn
C
H
C
rn
Ct
E. PUNGOR AND K. TOTFI
Therefore we have worked out methods'28, on a theoretical basis, for thedetermination of the selectivity constant. In this calculation the activities atthe coprecipitation point have been used. The expression for the selectivityconstant is as follows:
s 1/a ( 'tb/a— —- j7 -
jk kJSand the activities at the coprecipitation point are substituted in the calcula-tions. In the formula, S and SJk are the solubility products of precipitates,('j)a (Ii)b. and (I) (Ik)m, while (a,)5 and (ak). are the activities of theprincipalion and the interfering ion at the coprecipitation point. The determinationof the selectivity constants can be carried out either by direct or indirectmethods. Some results obtained by such methods are collected in Tables 3,4 and 5.
Table 4. The selectivity constants of the sulphide-selective electrode for some anions
pK5Interfering anion (X) Measured Calculated*
7.8 7.85-8.60Br' 11.6 11.51—12.09(TV 14.2 14.10—14,500H 16.2 I.00—1(.73SCN 12.1 11.86—12.13
SO 21.2 22.00
PO 16.1 14.75—18.45
CN 3.20 3.30 5.96
S2O 10.70 10.47-11,42
* The values were calculated using the highest and lowest solubility constant data of Ag2S found in the literature.
In these tables the calculated and measured selectivity constants arecompared and they are in good agreement. Furthermore, it can also be seenthat the selectivity of some electrodes to many ions is so good that thereare practically no interferences to the electrode response.
Naturally, the electrode potential is a result of the difference in the electro-chemical potentials of solvated and desolvated ions, which are bonded in
Table 5. The selectivity constants of the sulphide-selective electrode for some cations
PKAg. YInterfering cation (Y) Measured Calculated*
TI 24.2 24.48—29.25Cu2 13.7 10.91-13.30Pb2 21.8 20.93—21.80Cd2 22.4 20.46—22.30Ni2 23.3 21.70-28.90
Zn2 27.9 22.27-26.80Fe2 31.5 26.82-31.20Mn2 36.2 33.24—38.80La3 40.3 44.17
* The values were catculated iiiig he highest an d lowest soluhility constant data of Ag2S found in the literature.
116
PRECIPITATE-BASED ION-SELECTIVE ELECTRODES
the crystal lattice. In this respect the research of Morf and Simon'°6 isvery valuable; they proposed a rather simple method for the calculation ofthe hydration enthalpy of the cations. A similar method for anions wouldbe welcome for the theoretical interpretation of anion-selective electrodes.
Many authors have investigated the effect of complexes on the precipitate-based electrodes. The studies have been carried out in two directions. Thefirst was to clarify the nature of the electrode response in solutions containingcomplexes of the appropriate ion. Secondly, it was fascinating to study thebehaviour of the electrodes in solutions of ions which dissolve the precipitateas a complex.
A further complication in the measurement of ions which form complexesis that they may react with the solvent, for example basic anions, such ascyanide, will form acids. Using sulphide and cyanide anion-selective elec-trodes it was clearly demonstrated that they measure only freeanions'25' 148, 173
pH7 8 9 10 11 12 13
I I I I I II /,I /
pkd?d2.65 1
toW2
Ar,,, ,A' /Ar o,Ar/ .3
Ar,C' 'Ar,ot U)Ar' " a0/',Ar,
Ar,0,,0/,'1/Ar',, -6
/,/.7//
Figure 1. Relation between pH and the sulphide concentration measured with a sulphide-selective electrode in Na2S solutions
For sulphide the results, shown in Figure 1, give good agreement betweenthe theoretical and experimental values. Wemeasured, at 25°C at an ionicstrength of 0.1 M, a value of pK2 = 12.65 compared to the literature valuepK2 = 12.92. Hseu and Rechnitz69 using the Orion sulphide electrode
117
E. PUNGOR AND K. TOTH
measured the actual suiphide concentration at various pHs. On the basisof their experimental results, they calculated a value for pK2 which agreedwith one other determination (pK, 14.92) made more than forty yearsago20' 21 78, Since the difference between the pK value of Hseu and Rechnitzand that found by us is quite large, and because we have repeated suchmeasurements and calculations with different systems with unexceptionableresults, we concluded that their results must remain open to doubt.
In the case of cyanide we determined the stability constant and found avalue of 9.20 which is in agreement with the literature value'73.
Various authors investigated the association and dissociation problemsof fluoride. Shrinivasan and Rechnitz'57 and later on, Vanderborgh'74published results for the stability constant of hydrogen fluoride using afluoride ion electrode, which agree well with literature data publishedpreviously. For example, Broene and de Vries'9 using a glass electrodefound pK = 3.173 at 25°C and Vanderborgh using an Orion fluorideelectrode found a pK value of 3.189 at 25°C.
If any ion present in the solution is a complex ing agent, then it may reactwith the membrane. A good example of this is cyanide ion and silver halidemembrane electrodes. Due to the formation of complex silver cyanide, thecyanide ion liberates halide on the surface of the electrode, which diffusesin the opposite direction to the cyanide ion reaching the surface from thebulk of the solution. From this the actual cyanide concentration at thesurface may be assumed to be virtually zero, except at very high cyanideconcentrations. This was the basis for our deduction of the dependence ofthe potential on the cyanide activity.
E = E0 + 1n (a + KaN)
where a,( is the activity of halide liberated and present at the surface of theelectrode this will equal aCN in the absence of the appropriate halide inthe solution. K is the dissolution constant, for which we173 have foundexperimental values of between I and 0.1. The electrode behaviour, indicatedby the equation, is illustrated in Figure 2.
Concerning the details of the mechanism, it is doubtful whether the reaction
AgX + 2CN Ag(CN) + Xalone determines the electrode response. Fleet and Storp48, who investigatedthe iodide-based cyanide electrode, propose that the simple precipitation-exchange reaction also plays a role:
AgX + CN AgCN + X
Silver halides can be completely dissolved in a cyanide solution when thecyanide present is equivalent to that of the halide. However, if complexformation needs a large excess of complexing agent, the electrode alsoresponds to that kind of complexing ion. This is the situation when an iodideelectrode is used for the determination of In this case the K valuelies between 102 and iO. The same is also true when measuring cyanidewith a sulphide electrode.
118
PRECIPITATE-BASED ION-SELECTIVE ELECTRODES
E(mV)
1001
0 1 2-log aCN-
Figure 2. Calibration curve for the determinationpoint.
of cyanide; —,calculated curve ;Q, measured
Whatever basic halide electrode is used for the determination of cyanidethe electrode responds with the same efficiency to the cyanide as to theanion composing the electrode. The selectivity constant for other ionsremains the same as it would be in the presence of the anion correspondingto the appropriate electrode. This was one of the fundamental experimentson the basis of which we deduced the potential equation mentioned pre-viously. This is demonstrated in Table 6, where an iodide-based electrodewas used and selectivity constants for various ions in the presence of bothcyanide and iodide ions in the solution were determined. It is worth notingthat the complexing agent destroys the electrode, and so the lifetime ofelectrodes is limited.
If the anion investigated forms complexes with metal ions, then the elec-trode potential allows us to observe the complex formation and to getinformation about the stability data. Szabót62 has been able to demonstrate,by using an iodide-selective electrode, the formation of the I complex.This was also shown by Liberti and Mascini88. There are now many resultsof complex formation studies in the literature obtained from work withion-selective electrodes. It is generally found that complexes of low stability
Table 6. Potentiometric selectivity of iodide membrane electrodes
IonSelectivity constants in
CNthe presence of
L
CL 10_5_106 10_bBr io—io' iO'1 I ——
CN — 1
NH 10_s_b_b 10°SO bo_5._lo_6 io
119
I
3 4
E. PUNGOR AND K. TOTH
produce no interference with the electrode response. For example Rech-nitzt36' 137 has shown that metal ions forming complexes of low stabilitywith halides do not influence the calibration curve of halides. In the presenceof stable complexes the free ligand activity or that of complex-formingcations can be measured. In our laboratory173 we have investigated cyanidecomplexes and found that one can distinguish, in the case of silver the twocomplexes, Ag(CN) and Ag(CN); for copper the Cu(CN) ion, andfor mercury and nickel the Hg(CN) and Ni(CN)ions. No cyanidecomplexes of Cd and Zn could be observed. Fleet and Storp48 explainedthis finding by saying that the hydroxide complexes were formed preferentiallyto the cyanide complexes.
We should also like to mention a very detailed study of the copper(ti)complexes of ethane- 1-hydroxy- 1, 1-diphosphoric acid, which was made witha copper electrode by Wada and Fernando' 78 They compared their resultswith those obtained by spectrophotometry and by potentiometry using aglass electrode. This study underlined the advantages of the copper electrodefor such purposes.
Kinetic studies of the formation of FeF2 + and A1F2 + complexes havebeen performed by Shrinivasan and Rechnitz'5 .Theywere able to distinguishthe kinetic steps of A1F2 + complex formation in the presence of aqueousand hydroxo-aluminium complexes.
A further example of a study of metal—fluoride complex formation isprovided by Bond and Hefter'3 who calculated the formation constants oflead fluoride complexes using a fluoride electrode.
Very important results concerning the lower detection limit of electrodeshave been obtained by the investigation of complexes. Our investigationswith cyanide lend weight to the fact that the lower detection limit shiftsfurther down in the presence of complexes. At the same time Mesmer'°4 hasdemonstrated similar phenomena with beryllium complexes, with which thelowest detectable fluoride concentration in 1 M sodium chloride solutionwas7 x 108M.
Aziz and Lyle5 successfully investigated fluoro-complexes of the metalsgadolinium, europium, scandium, iron, magnesium, and calcium: the fivalues are all lower than 1O I mol -' and this was found to be the limit ofthe method. Their investigation therefore excluded a study of the thoriumcomplexes. In spite of this, Baumann'° reports that she investigated thezirconium, thorium and lanthanum fluoro-complexes in solutions of ionicstrength of 0.5 to 2 M. Among the various complexes, she was able to dis-tinguish the ZrF3 + and ZrF + complexes for zirconium, the ThF3 + andThF + complexes for thorium and the LaF2 + complex in the case oflanthanum.
One problem encountered in this work is that lanthanum fluoride influ-ences the fluoride measurement by the dissolution of its surface, whichreleases fluoride ions into the solution. In this respect the polycrystallineelectrodes have a further disadvantage by having a higher dissolution ratethan the single crystals.
Baumann also reports that the Nernstian response of the fluoride electrodeextends down to 5 x iO M in the presence of thorium and zirconium,8 x iO M in the presence of lanthanum and 3 x iO M in the presence
120
PRECIPITATE-BASED ION-SELECTIVE ELECTRODES
of perchioric acid. At the same time, however, in pure fluoride solutions theresponse is linear down to only 8 x 10 M.
Buffers
Unfortunately, among the ion-selective electrodes, we only have buffersof high capacity for pH electrodes. Because of this lack of suitable buffersfor other ions, the calibration of electrodes responsive to these other ionsis difficult. Moreover, junction potentials are a further problem which hasbeen discussed in a paper by Bates and Alfenaar7. However, following theneed for further scales of ionic activity, some practical methods have beendevised for their construction. For concentrations greater than i0 M thecalibration can be carried out directly with solutions of the appropriate ion,the ionic strength being maintained by electrolytes for whose componentsthe selectivity constants are lower than
Another calibration method which is available uses the complexes orprecipitates which are in equilibrium with the appropriate ion. It should bementioned that such systems generally have low-buffer capacities. To thisgroup belong those buffers suggested by Havas and his co-workers61; theyhave used saturated solutions of silver halides for calibration points. Thedisadvantage of this method is the great susceptibility to change of thesolubility of the silver halides by any one of a number of parameters.
There are many papers published in the literature on the application of thefluoride electrode. Because the fluoride ion takes part in an acid-baseequilibrium and is also a good complexing agent for various metal ions, it isnecessary to ensure reproducible conditions for the fluoride measurements.Frant and Ross5° suggested the TISAB buffer (Total Ionic Strength AdjustingBuffer). This has a pH of 5.0 and an ionic strength of 1.75 M: it consists of1.0 M sodium chloride, 0.25 M acetic acid, 0.75 M sodium acetate and 0.001 Msodium citrate. In this medium fluoride determination can be carried outdown to 106 M fluoride ion activity without interference.
The behaviour of the electrodes in non-aqueous solvents
A systematic study of the behaviour of the precipitate-based electrodeshas been carried out by Kazarjan and Pungor74' The silver halide elec-trodes were used in alcohols, ketones and other solvents such as dimethyl-formamide. In this investigation aqueous and aqueous/non-aqueous solventmixtures in different ratios were used. The solubility product of the silverhalides and the selectivity constants of the electrodes were measured in thedifferent solvent mixtures. The calculated and measured selectivity constantsare compared in Tables 7 and 8.
It may be seen from the tables that the theoretical values agree with themeasured values and thus one may conclude that the ion-exchange conceptis also valid in these systems.
In these experiments we found that it was necessary to first conditionthe electrode in the appropriate solvent mixtures as the membrane is hetero-geneous; during the soaking we observed a swelling effect.
As expected the theory for the lower detection limit of the electrodesderived for aqueous solvents is also valid for other media. So, for example,
121
E. PUNGOR AND K. TOTH
Table 7. Selectivity constants of an iodide-selective electrode to bromide in mixtures of distilledwater and an alcohol
Ratio of Mole pKSolvent solvent fraction
(vv 0)) of solvent
CHOH 10:90 0.04740:60 0.22860:40 0.40090:10 0.802100: 0 1.00
10:90 3.20 3.62 3.7040:60 16.20 3.50 3.5560:40 29.80 3.34 3.3090:10 67.70 3,00 2.76100: 0 100.00
10:90 2.60 3.70 3.7420:80 5.60 3.58 3.5430:70 920 3.58 3.4640:60 13.70 3.58 3.42
in 90 per cent ethanol, the lower detection limit of the iodide electrode liesat about iO M, this decrease being produced by a difference of approxi-mately 1.5 units between the solubility product exponent in this mediumand in water.
For measurements with the fluoride electrode, Lingane94 reported thatin alcoholic solvents the lower detection limit of the fluoride electrodeshifts downward by nearly one decade.
Another study in non-aqueous solvents, using the lead electrode, con-sisting of lead sulphide plus silver suiphide, has been carried out by Rechnitzand Kenny134. They used this electrode with success in methanol, dioxane,acetonitrile and dimethylsuiphoxide.
TahleS. Selectivity constants of an iodide-selective elec1rde to bromide in non-aqueous solvents
Ratio of pKDMFtoH2O
Experimental (v/v %) Theor. Experimental
3.75 10:90 3.74 3.753.60 40:60 3.26 3.20
60:40 2.64 2.60
Theor. Caic. ApK
3.64 3.78 —0.143.48 3.60 -0.123.34 3.38 —0.043.02 3.02 —0.002.93
C,HsOH
n-C3H -OH
iso-CH OH
—0.08—0.05—0.04+0.24
10:9020:8030:7040:60
—0.04+ 0.04+ 0.12+0.16
2.60 3.745.60 3.649.20 3.56
13.70 3.50
3.78 —0.043.60 + 0.043.49 + 0.073.16 +0.34
In mixtures of distilled water andIn mixtures of distilled water and acetone
dimethylformamide
Ratio of acetone pKto 1-120 .
(vv Theor.
10:90 3.8220:80 3.68
30:70 3.62 3.56
40:60 3.56 3.55
122
PRECIPITATE-BASED ION-SELECTIVE ELECTRODES
Transient phenomena of the electrode
The transient phenomena of the precipitate electrodes are very importantfrom both theoretical and practical points of view, but primarily becauseof the information afforded concerning continuous measurement tech-niques129. Despite this importance, there has been little research in this field.This may be explained partly by the intrinsic measurement difficulties andpartly by the complexity of the interpretation of the results.
The immersion and injection techniques for producing transient pheno-mena are equally useful. In our review paper summarizing the state of theart, we concluded that, for the halide electrodes, the literature data werenot always useful because the results were sometimes influenced by the res-ponse lag of the measuring systems. Our results172 and those of Rechnitz1have indicated that the response time is, for halide electrodes, less than onesecond.
Our most important conclusions, reached with our heterogeneous elec-trodes, are as follows:
(i) The response time can be described by one exponential equation.(ii) The response time is only slightly dependent on the thickness of the
membrane layer.(iii) The response time decreases if the solution contains non-interfering
ions in large excess.(iv) The response time of the iodide-based cyanide electrode in cyanide
solutions is the same as in iodide solutions. This strongly suggests that thepotential-determining processes are identical for the two responses.
Using a sulphide electrode, rather than a halide electrode, Light92 founda response time of about one millisecond.
PRACTICAL SECTION
Measuring techniquesThe measuring techniques applied to the precipitate-based electrodes are
the same as those used for every other kind of electrode.The measurements may be carried out either in cells with liquid—liquid
junctions or without liquid—liquid junctions. In the latter case we mustchoose a reference electrode which is reversible to one of the components inthe system—one which does not interfere with the ion-selective electrode.At the same time this component may also be used to maintain the constancyof the ionic strength. We have used in this way, as a reference electrode, theion-exchange based perchlorate electrode1 23 Such an experimental systemis especially suitable for the investigation of complex equilibria, the ionicstrength being maintained by perchlorate.
Cells with liquid-liquid junctions have to be constructed in such a waythat the electrolyte in the salt bridge does not interfere with the measuringelectrode.
If both the measuring and reference electrodes have high internal resist-ances, then the leads connecting them to the measuring instrument shouldbe shielded and both instrument inputs should have high impedances,rather than just one as with a normal pH meter. To meet these requirements,
123
F. PUNGOR AND K. TOTH
Brand and Rechnitz' ' have designed a device with two, very high impedance,matched and symmetrical inputs using operational amplifiers.
Measurement with ion-selective electrodes can be carried out in severaldifferent ways. The activity of the ion can be estimated directly, as whenusing a glass electrode for pH determination, or alternatively the standardaddition technique8 16 22, 39, which is also a direct method, can be used.In many cases this latter technique gives better results than the simple direct-measuring procedure. A further elaboration is the so-called multi-additiontechnique. This technique has the advantage, compared to the simple calibra-tion procedure, that it is not necessary to know anything other than that theelectrode response is linear with the logarithm of the ionic activity in themeasurement range. This multiple standard addition technique gives veryaccurate results, particularly when combined with a computer.
Precipitate-based electrodes can be used as indicator electrodes in potentio-metric titrations. One method where they may be particularly used toadvantage is null-point potentiometry. Durst37 has devised an arrangementusing this technique for the measurement of nanogramme quantities involumes of 50 tl.
ErrorsThe errors arising from the direct application of membrane electrodes
have recently been studied. Krull82 studied generally the influence of errorsof the selectivity constant on the analytical results and his findings are alsovalid for the precipitate-based electrodes, if the measurement of the selec-tivity constant is incorrect.
In direct potentiometry an error of 1 mV leads to an error of nearly 4 percent in the determined concentration. Moreover, this error increases if themeasurement is carried out in a non-Nernstian part of the calibration curve.
If precipitate-based electrodes are used for end-point indication in atitration, the precision of the determination is higher than in direct measure-ment. Schultz1 ° has suggested that in ideal conditions the titration curveshould be symmetrical about the end-point. However, these ideal conditionsare only assured if, among other things, the system to be titrated does notcontain ions for which the selectivity constant is appreciable (< 108).Schultz has written the equation of the titration curve when the titrandcontains interfering ions, as follows:
n(E—E0) / c.V(= — logc +
If the interfering ion is in the titrant:
n(E — E0) ( cV— log ci + k 0 + tand if both the titrand and titrant contain interfering ions, the expressionbecomes
n(E—E0) / cl'0 cV= — log + k 01++ k
v0z)124
PRECIPITATE-BASED ION-SELECTIVE ELECTRODES
Carr26 discusses the question of errors in detail and points out that theend-point error of a precipitation titration is only negligible if interferingions are absent. Schultz'5' studied the same question for chelatometrictitrations and found that the titration error increases as a function of theselectivity constant.
The difficulties increase if the composition of the titration product isuncertain, as was found, for example, by Lingane93 when using metal—fluoride complexes in determinations.
Anfält and Jagner2 studied various titration methods using ion-selectiveelectrodes from the point of view of the errors, and concluded that the mostprecise data were achieved by use of the Gran54 method, corrected byIngman and Still70. Liberti and Mascini87 have published a paper describingan application of the Gran method.
A further measuring error arises if the electrode becomes polarized, ashas been pointed out by Buck25. Hence, because of the high resistance ofprecipitate-based electrodes, the input impedance of the measuring devicemust be high, normally not less than 1012 ohms.
SOME ANALYTICAL APPLICATIONS OFPRECIPITATE-BASED ION-SELECTIVE ELECTRODES
Some of the more recent analytical applications of precipitate-basedelectrodes are collected in Table 9. We should like to briefly discuss someof these applications.
One type of application is in the determination of very low concentrations.As examples we can mention the determination of cyanide in sewage, someresults of which are presented in Table 10, and of cyanide in leaves, as shownin Table 11. In this estimation of cyanide in leaves, a particular advantageis that the usual preliminary distillation step is unnecessary. A further appli-cation is shown in Table 12, which contains results of chloride determina-tions in potassium hydroxide; such determinations are of especial importancein semiconductor technology. In this method the sample is initially passedover a cation-exchange resin, and the eluted chloride measured with theelectrode.
Another type of application is in the titration of such compounds thatreact to produce components to which the electrode is reversible, as forexample in the determination of thiourea' Thiourea is only slightlyhydrolysed in aqueous solution, as may be demonstrated by potentiometricmeasurements using a sulphide-selective electrode. The approximate con-centration of sulphide ion, as calculated by the Nernst equation, in alkaline10' M thiourea solution is extremely small—about iO M. On titrationof the solution with silver nitrate in the presence of 0.1 N sodium hydroxide,the following reaction takes place:
H2N—C—NH2 + 2AgNO3 H2N—CN + Ag2S + 2HN03S
H2N—CN + 2AgNO3 Ag2N—CN + 2HN03125
Tab
le 9
. Som
e re
cent
app
licat
ions
of p
reci
pita
te-b
ased
elec
trod
es
CN
D
irec
t C
N
pCN
= 1
-6
Hg2
C
u2
Ni2
Was
te w
ater
pH
> 1
1 17
3
Ag
Dir
ect
Ag
pAg=
0—17
—
—
H
g2
Cm
pise
nt
Mea
suri
ng
Inte
rfer
ing
Non
- R
efer
- or
ion
Met
hod
Ele
ctro
de
rang
e T
itran
t E
rror
io
ns
inte
rfer
ing
Sam
ple
Med
ia
Com
men
ts
ence
s ill
s
92
('I —
2°3
Res
pons
e tim
e S
Oj -
and
tem
pera
ture
de
pend
ence
dat
a H
alid
es
Indi
rect
A
g0
0.l—
l0tc
q.
AgN
O3
0.35
—
—
In
orga
nic
mat
eria
ls
80, a
cetic
aci
d A
fter
bur
ning
, ha
lf-a
utom
atic
Hg2
0 In
dire
ct
Ag0
0.
l—4O
ppm
D
ithio
oxam
ide
0.3°
, C
o2
Ag0
Zn2
C
d20
Pb2
0 Fe
30
—
Buf
fer
—
CN
In
dire
ct
Ag0
m
mol
A
gNO
3 0.
4 —
—
—
- —
C
onse
cutiv
e C
l —
pote
ntio
met
ric
titra
tion
CL
Dire
ct
Cl
pCI
= 1—
5 —
±
0.05
pC
I —
—
U
rine,
blo
od
—
In th
e pr
esen
ce
Indi
rect
C
L
pCi
= I
-3
AgN
O3
<1
seru
m
of p
rote
ins
Cl
Indi
rect
C
1 pC
I =
1—
3 A
gNO
3 2.
1 B
r, L
—
P
lant
s A
cidi
c D
irect
CL
dete
rmin
atio
n ca
n no
t be
use
d
CL
D
irect
C
L
pCi =
1—
4 —
±
0.05
pC
i B
r, I.
S2
—
Phar
mac
eutic
als
Cal
cula
ted
activ
ity
indi
rect
co
effic
ient
A
myg
dalin
D
irect
C
N -
pAm
= 3
-.5
—
Cu2
C
d20
Hg2
0
—
Buf
fer
pH =
10—
11
Imm
obili
zed -
gluc
osid
ase
on
theC
N
elec
trod
e su
rfac
e,
lifet
ime
200h
C
yano
- D
irect
C
N
pB1,
> 6
±
0.O
4pC
N
—
Pha
rmac
eutic
als
Alk
alin
e pH
> 7
C
N
was
co
bala
min
lib
erat
ed b
y (v
itam
in
redu
cing
age
nts
B12
) an
d lig
ht
CN
D
irect
C
N
pCN
5
—
Fru
its,
bran
dy
60 alc
ohol
—
in
dire
ct
CN
pC
N
4 A
gNO
3 pH
> 1
1
pCI =
I
3 \g
NO
81
73
31
112
85
113
139 35
59
Indl
r.ct
C
N
pCN
>4
AjN
O1
1%
—
—
—
—
CN
D
irect
C
N
>ip
pm
2%
Plan
t it
extr
actio
ns
CN
in
dire
ct
CN
N
i-sa
lt —
K
3P04
N
aOH
or
At th
e en
d-po
int
166
KSC
N
NH
3 so
lns
is {
Ni(
CN
)4]2
N
aCl
Na2
SO4
at
4 x•
102M
P1
so
ins
CN
D
irec
t C
N
pCN
= 5
—
S2,
Cl
Air
, dri
nkin
g In
the
pres
ence
30
w
atcr
, m
d. w
aste
, or
cr
biol
. m
edia
H
CN
mus
t be
isol
ated
C
N
Dir
ect
CN
pC
N=
2-6
—
—
--
—
Cl
Dir
ect
Cr
pCI
= 1
—5
Hg2
,Cu2
tNi2
t-
Was
te w
ater
—
t2
7 In
dire
ct
Cl -
pCI =
1—3
AgN
O3
1—13
1 D
irec
t 1
p1 =
1—
6 M
ilk
18
Hg2
In
dire
ct
1 pH
g2 <
6 N
at
±0.
4%
—
—
Hgc
ompl
exin
g 6
ligan
ds c
an b
e 2
mas
ked
with
m
etal
s fr
i Ph
arm
a-
Dir
ect
Cl
pCI =
1—4
—
0.O
t—0.
04
Bas
ic m
ater
ials
O
rgan
ical
ly
34
ceut
ical
s pX
of
pha
rmac
euti-
bo
nded
hal
ogen
s In
dire
ct
1 p1
= 1—
3 A
gNO
3 ca
ls
can
be m
easu
red
—
afte
r bu
rnin
g,
half
-aut
omat
ic-
IT
ally
H
alid
es
Dir
ect
Cl —
Dire
ct
62
Br
pX =
1—6
—
52
dete
rmin
atio
n —
1
of h
alid
es
Indi
rect
1
AgN
O3
—
S2
p.a.
KO
B
(a)
part
ial
C
oxid
atio
n nI
(b
) ion
-exc
hang
e H
alid
es
Dir
ect
X
pX =
1—6
—
± 0.0
5—
—
Stu
dy o
f "p-
sel"
96
0.
15 p
X
NaB
Ph4
Indi
rect
H
alid
es
0.2
mM
A
gNO
3 0.
38—
0.49
%
—
pH =
5
K +
In
dire
ct
Ag
or
4—20
mg
AgN
O3
—
—
—
Bac
k titr
n.
159
hahd
es
exce
ss N
aBPh
4
NT
A
Indi
rect
C
u2
pCu
= 3—
4.3
NT
A
Al3
In
dire
ct
Cu2
pA
l =
2.5
—3.
5 D
CT
A
Ca2
+ M
g2In
dire
ct
Cu2
0.
03—
0.41
M M
e
Cs
Indi
rect
C
s 6.
5—72
.4 m
g C
s F
0.03
8-19
ng
ml
met
ry
F In
dire
ct
F pF
= 5
—6
La(
N03
)3
Non
-aqu
eous
m
etha
nol
acet
one
acet
onitr
ile
Zn,
Cd,
Ni,
Pb,
Ca.
Mg
indi
rect
de
term
inat
ion
with
cop
per-
ch
elat
e in
dica
tor
Zr,
Th,
La,
Sm
, Fe
, C
a, H
g in
dire
ct de
ter-
m
inat
ion
with
C
u-E
DT
A
indi
cato
r
Indi
rect
po
tent
iom
etri
c tit
ratio
n W
ater
anal
ysis
C
u-E
DT
A
indi
cato
r met
hod
The
end-
poin
ts
wer
e de
term
ined
fr
om G
ran
plot
s W
ater
anal
ysis
The
sol
ubili
ty
prod
ucts
of
LaF
3, E
uF3
have
be
en m
easu
red
PKH
Y h
as b
een
dete
rmin
ed
Tab
le 9
(con
tinue
d)
Com
pone
nt
or io
n M
etho
d M
easu
ring
E
lect
rode
ra
nge
Titr
ant
Inte
rfer
ing
Err
or
ions
N
on-
Ref
er-
inte
rfer
ing
Sam
ple
Med
ia
Com
men
ts
ence
s io
ns
Me2
In
dire
ct
Cu2
pM
e =
3—
5 T
EPA
, ED
TA
, —
C
l, pE
, pH
= 3
—11
E
GT
A, 1
, 10
- N
a ph
enan
thro
line
Cu2
+
Indi
rect
C
u2 +
pC
u =
2—5
ED
TA
, T
EPA
, 5,
6-di
met
hyl-
1,
10-p
hena
nth-
ro
line
Me4
In
dire
ct
Cu
5—7J
.tM M
e F
DT
.\ pH
= 5
Na-
acet
ate
Me3
bu
ffer
M
e2
pH=
10
0.1M
NaO
H
—
—
Por
, SO
R
-S0
Na5
P3O
10
—
Mg2
N
a,N
H
Ca2
+
—
—
NaC
l
F L
inea
r nu
ll-po
int
pote
ntio
-
ED
TA
H3P
Mo1
2O40
—
1%
Wat
er
pH
9.6
dete
rgen
ts
0.1
M
NH
3-N
H4N
O3
buff
er
—
pH=
5.5
Wat
er
144
133
C) 0 >
135
-I
0'
160
- 99
29
37
94
174
Li
Na
NaB
Ph4,
C
sNO
3,
LiN
O3
—
Wat
er
Dtr
ect
F pF
=3—
5
pH =
9.2
bor
ate
buff
er p
H =
10.
0
phen
olat
e buf
fer
Aqu
eous
6% al
coho
l
Aci
dic
F D
irec
t F
pF <
6
±10
%
Prot
eins
B
iolo
gica
l 17
1 sa
mpl
es
F D
irec
t F
>0.
1 pp
m
—
—
—
Rep
rodu
cibi
lity
90
0.05
ppm
D
irec
t F
1 m
g m
l' 1%
N
atur
al w
ater
D
istil
led w
ater
F
dete
rmin
a-
91
Indi
rect
F
—
0.00
5 M
4%
—
W
ater
—al
coho
l tio
n af
ter i
on-
, T
h(N
03)4
ex
chan
ging
pr
oces
ses
F D
irec
t F
pF <
6.3
in
1.7%
N
aCI
TIS
AB
Se
wag
es
179
IMN
aCI
Indi
rect
F
pF =
1—
6 2.
3%
Nat
ural
wat
er
F D
irec
t F(
mic
ro)
pF =
2—5.
3 —
In
mic
ro-s
ampl
es
38
F
Dire
ct
F 2 p
pm
—
Fe2
, B
iolo
gica
l A
fter
dist
illat
ion
76
D
Fe3
÷
sam
ples
F -
S
tand
srd
F -
0.07
34 m
ss
>0.
2%
Stab
le F
- S
ea w
ater
H
alf-
auto
mat
ic
3 v,
ad
ditio
n kg
1 co
mpl
exes
tit
ratio
n, en
d-
poin
t is
calc
ulat
ed by
6
com
pute
r Z
D
irect
F
2—6%
0.
05—
0.1%
A
l34
—
Oxi
des,
T
ISA
B
Fast
met
hod
115
cn
Fe34
fl
uors
par,
C
a2 +
op
al g
lass
rn
Dir
ect
F 0.
1—1%
F
—
Pho
spha
te
pH =
6.5
—
11
6
rock
, ap
atite
N
H4O
AC
buff
er
F D
irec
t F
l—lO
ngF
—
B
Org
anic
H
OA
C +
NaC
I +
In b
urnt
sam
ple
68
I fl
uori
ne
+ N
a-ci
trat
e +
+N
aOH
pH=
5.5
r In
dire
ct
F 0.
1 pp
m
Pollu
ted
air
Na-
mon
ocitr
ate
Aut
omat
ic
89
NaO
H b
uffe
r co
ntro
l, th
e ai
r be
ing f
ilter
ed
thro
ugh
a su
itabl
e m
ediu
m
rn
F Po
tent
io-
F 10
—20
00 p
pm
5.7%
In
veg
etat
ion
Aci
d ex
ts. f
rom
71
'
met
ric s
td.
9.3
%
pulv
eriz
ed p
lant
tis
sue
F D
irec
t F
0.01
—1.
0 pp
m
—
Cya
nam
ide
Co,
Mg,
Fe
ln fe
rtili
zers
—
F
was
dis
tr.
182
H3P
O4,
A
l, C
r, T
i an
d pl
ant
(cal
ibra
tion f
or
silic
tc a
cid,
si
licof
luor
ide)
H
2CO
1
Tab
le 9
(co
ntin
ued)
Com
pone
nt
or io
n M
etho
d E
lect
rode
M
easu
ring
ra
nge
Titr
ant
Inte
rfer
ing
Non
- E
rror
io
ns
inte
rfer
ing
ions
Indi
rect
Pb
24
pPb
= 3
P
b2 +
Dire
ct
PlY
÷
pPb
= 1
—5
Indi
rect
Pb
24
K2S
04
K2C
rO4
PO
Indi
rect
Pb
24
0.5—
1.5m
g
H2 S
O4
Aqu
eous
in
the
pres
ence
of
com
plex
ing
ions
, T
ISA
B
In th
e pr
esen
ce
pH =
5.3
of
com
plex
ing
ions
det
n. F
in
op
al g
lass
—
pH
=3,
8 /2
= 0.
3
Was
te C
aSO
4 an
d ga
ses
Was
te w
ater
R
iver
wat
er
TIS
AB
Aft
er di
still
atio
n In
terp
reta
tion o
f th
e re
sults
usin
g a c
ompu
ter
Th(
N03
)4
Sam
ple
Med
ia
Indi
rect
F
—
F
Dir
ect
F pF
= 6—
11
pF
5 or
0.
19 p
pm
F D
irec
t U
1.
84m
g
Al3
D
irec
t U
pA
l =
4—6
—
HF
D
irect
U
0.
41—
7.72
%
CaF
, C
N
CN
0.
lmgC
N-r
'—
U
Dir
ect
U
Cl -
Dire
ct
Pb24
Ref
er-
Com
men
ts
ence
s
—
Cry
olite
A
1F,
—
—
0.6—
2.5%
B
e. F
e M
g24.
Cu2
P0
4 >
0.2
Pb
24
/2M
mr
0.01
-0.1
9%
—
H2S
04,
BaS
O4
SO,,
Cu5
04
0.05
—I
mg
U
pPb=
2—6
—
—
Ni2
1
Zn2
4 E
DT
A
Bio
logi
cal
sam
ples
B
iolo
gica
l sa
mpl
es
—
Ag%
TP
, N
H:
Mic
ro a
nd
sem
imic
ro
dete
rmin
atio
n
In b
lood
un
succ
essf
ul
Sele
ctiv
ity
dete
rmin
ed
140
52 6
72
84
109
tb
C 2
134
-i
86
z
152
153
143
Pb(C
l04)
2 1-
2%
C1,
U.
Cr0
C
20 -
Indi
reet
Pb
24
1—25
mg
Pb(C
104)
2 0.
06%
C
ompl
exin
g C
20
and
ppt-
ing
spec
ies
S0
Indi
rect
Pb
24
pPb
<4.
3 Pb
(C10
4)2
±1%
A
g4.
Cu2
4 H
g2 P
O
TIS
AB
Wat
er al
coho
l
dim
ethy
l su
lfox
ide
Buf
fer
pH =
8.25
—8.
75
40 %
p-di
oxan
e pH
= 3
.5—
10.5
SOy/
v % d
ioxa
ne—
w
ater
S0, N
0
Ac,
form
ate
prop
iona
te,
phth
alat
e
Dir
ect
S2
p5 =
0-2
0 —
S
2O -,
—
A
lkal
ine
Res
pons
e tim
e 92
SO
. SO
pH
13
sn
d te
mpe
ratu
re
Indi
rect
52
A
gNO
3 I,
CIT.
—
depe
nden
ce da
ta
Br,
F, C
N
Indi
rect
52
pS
<4
Na2
Pb(O
H)4
—
—
C
IT B
r, IT
—
—
—
10
8 SC
N, S
O
Inor
gani
c In
dire
ct
S2
AgN
O3
Alk
alin
e 1
M
Dir
ect m
etho
d 16
1 Q
pu
lpin
g liq
uor
NH
4OH
w
as a
lso
stud
ied
Org
anic
B
ack
S2 -
—
AgN
O3
-—
-1
titra
tion
S2
Indi
rect
S2
Sp
pm
AgN
O3
<lp
pm
S2O
C
O, S
O
—
Aqu
eous
soln
. —
57
rn
SS
gro
up
Indi
rect
52
A
gNO
3 7%
In
pro
tein
s -—
60
ex
cept
lys
osym
e T
hiol
s In
dire
ct
52
—
AgN
O3
—
Hg2
—
56
rn
H
alid
es
indi
rect
S2
0.
04—
1.Sg
eq.
AgN
O3
1%
-—
75%
ace
tic ac
id
Aut
omat
ic
80
NH
4_
Dir
ect
S2
—
±0.
1 N
a2S2
O3
Red
ox-c
oupl
ei
flot
atio
n m
ill
—
Met
allu
rgic
al
141
diet
hyl
pS2
- so
lns
indu
stry
; z
dith
io-
sele
ctiv
ity is
ph
osph
ate
disc
usse
d T
hiou
rea
Dir
ect
52
pS =
1—
6 —
—
-—
—
In
vest
igat
ion
and
114
Indi
rect
52
pS
= 1
—3
AgN
O3
—
—
—
Alk
alin
e an
d id
entif
icat
ion
C
of th
e pr
oduc
t by
elem
enta
ry
anal
ysis
and
i.r.
N
a2S
Indi
rect
52
pS
= 0—
17
AgN
O3
In th
e pr
esen
ce
—
Mea
sure
d an
d 14
7 of
diff
eren
t ca
lcul
ated
sel
ectiv
ity
ITt
anio
ns an
d co
nsta
nts s
how
14
8 ca
tions
go
od ag
reem
ent
H2S
—
—
—
A
lkal
ine
Sel
ectiv
e gas
- 77
0
HC
I —
C
IT
l05M
—
eh
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atog
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S2
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)
F. PUNGOR AND K. TOTH
In the first step, one sulphur splits off to form silver sulphide and at thesame time cyanamide is formed. The resulting cyanamide reacts with a furthertwo molecules of silver nitrate and a silver cyanamide precipitate is formed.
2
The products of the titration were separated by filtration and were analysedgravimetrically and also for their elemental composition. In addition theirinfrared spectra were taken. For comparison the spectra of pure cyanamideand thiourea were also recorded. These experiments proved the proposedreaction path. The titration allows an accurate determination of thioureaand a fully detailed method has been worked out with the help of ourco-worker Mrs Kucsera. The reaction, which takes place rapidly as the titra-tion proceeds, is followed conveniently by a suiphide ion-selective electrodein 10 '--iO M thiourea solutions.
Table 11. Determination of cyanide in various materials by direct potentiometric measurementwith the pCN electrode
Type of Designation Cyanidematerial or variety content (ppm)
Bitter almond 280Spicy almond 92
9598
86
Sweet almond 2231
Bt 1355
Bt 1362
Bt 13 65
Bt l366
Bt 70
Bt I
lit 11
I. A
BC
2. ABC
Type of Designation Cyanidematerial or variety content (ppm)
Peach leaves Alexander 39Sunbeam 63May Flower 72Mariska 74Champion 76Elberta 91Ford 120
Sudan grasses 5. A 130B 49C 16
6.A 71B 45C 36
7.A 123B 62C 34
175B 70C
132
successive developmeniai stages (capiial
Table 10. Determination of cyanide in sewage
Cyanide content of sewageSample Dilution (ppm) Direct method*
1:1 13.11:5 14.61:10 13.1
24.71:1 23.4
* The values are calculated back to the undiluted solution,
Sudan grasses*54
29886
89
27
14
231270
12
17
27
2
3. ABC
4. ABC
* Sudan grasses are altered by the fertili7aiion practices lsample numbers) andletter.sj ul the plants in the respective sample.
PRECIPITATE-BASED ION-SELECTIVE ELECTRODES
Table 12. Direct determination of chloride in KOl-! using a chloride membrane electrode
[Cl ], (M) CLcontent
(Q/)Error(%)
- -Caic. Found
1.OOx i0' 1.02x i0 7.24x 10 21.OOxlO 1.04x104 7.38x103 4l.OOx iO 1.05x iO 7.45x iO 5
Another case where ion-selective electrodes may be used indirectly forthe determination of compounds to which there is no direct response, is incombustion analysis. In organic microanalysis, combustion is frequentlyemployed and ion-selective electrodes may be conveniently used to deter-mine halides in the combustion products. Some results are collected inTable 13.
Table 13. Potentiometric determination of pharmaceuticals using a chloride ion-selectiveelectrode
Compound
2,6-Dinitro-chlorohenzene
Sample taken(mg)
Halide coCaIc.
ntent (%)Found A( %)
18.105 17.21 17.43 + 1.224.084 17.21 17.42 + 1.221.074 17.21 17.67 +2.624.018 17.21 17.71 +2.920.757 17.21 17.42 +1.2
Chlorpromazine UCI 20.20320.78717.29019.95017.782
19.96199619.9619.9619.96
20.44200319.9819.9920.03
+ 2.4+0.3+0.1+0.1+0.3
SOME OTHER APPLICATIONS OF ION-SELECTIVEELECTRODES
As well as the above analytical applications, ion-selective electrodes canbe used in the field of physical chemistry for investigating equilibria andkinetics of various reactions. In this article we have referred to many paperswhere the electrodes have been used for determining acid-base dissociationconstants and formation constants of complexes. Another interesting newapplication, which is furnished by the short response time of the electrodes,is the study of oscillating reactions. Woodson and Liebhafsky18' used theiodide electrode in a study of the ClOH2O2 reaction and Körös et a!.(personal communication) are using our bromide electrode for investigatingsuch reactions as that between BrO and malonic acid.
CONCLUSION
In the last decade the field of ion-selective electrodes has undergone anastonishingly rapid development with many new sensors being developed
I 33
E. PUNGOR AND K. TOTH
and applied in a wide range of analytical and physicochemical research.But can we predict future developments in this branch of chemistry? In ouropinion the number of precipitate-based electrodes will be limited, but inthe application of these electrodes to the monitoring of chemical processesthere will be a rapid expansion. However, future major developments cannotbe expected unless we can discover the exact relations between the propertiesof the precipitates used for the electrodes and their electrochemical behaviour.
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137