ammonia-selective electrode determination of nitrogen in fertilizers

8/17/2019 Ammonia-Selective Electrode Determination of Nitrogen in Fertilizers

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1096 DOY & GUIJOSA, J. ASSOC. OFF. ANAL. CIIEM, (VOL. 64, NO. 5, J9RI)

FERTILlZERS

Ammonia-Selective Electrode Dctermillation oí Nitrogcll

in Fertilizers

VICTOR M. HOY and MANUEL GUljOSA

Fertilízl1Iltes M exicallos, S.A., SlIbgerencia de IllvesfigllciólI, Chiapas 184, México 7, n.r.

The official AOAC nldgnesium ollide method

(MOM), 2.065,fOfdeterminin¡; ammnni¡ll:ai nitrogen

in fertilizen; not containing ure,l was compared with

.In alternative anllllonia-se1ective electrode rnethod

(A5EM.1). likt-wise, the official AOAC Raney

pnwJer nlelhod (l{PMI,2.063.2.064, for Jelt"fmining

, total nitrogen in fertilizers, except nitric phosphates

cOlltaining nOllsulfate S, WolScompareJ with an am.mnnia-se1eclive electrode method (A5EM-2). Each

comparison includeJ 6 s


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•

BOY &: GUIJOS": J. ASSOC. OrFo ANAL CHEM. (VOL. 1>4,NO. 5, 19f1l)

(e) Tar/arie acid soll1lioll.-lO% aqueous solu-

tion.

(d) Sodilllll 1l.lIdroxidc solulioll. -ION.

(e) Ammolliacalllitrogcll so/uliolls.-Slock so/u-

lio/l.-1400 ~g N/mL. Dissolve 5.35 g Teagentgrade NH4Cl in water in 1 L volumetric flask and

dilute lo valume with water, Workillg 50/11-

1;0115.-( J) 700.ug N¡rnL. Transfer 100 mL stock

solution lo 200 ml volurnclric f1ask, add 20 ml

tartaric acid soiution, dilute lo volume wilh

water, and mix. (2) 140.ug N{mL. Transfer 50

rnL solution 1 lo 250 rnl volumetric flask. add 20

mL tartarie acid solution. dilute lo volume with

water, and mix. (3) 70 .ug N¡mL. Transfer 100

rnl solulion 210 200 ml volumetric flask, add 10

rnL lart


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1098 HOY & : GUIJOSA: /. ASSOC. OFF. ANAL. CIiEM. (VOL. 64, NO. S, 1981)

Table 2. Comparison 01results by olficial magneslum oxide and ammonla-5elective electrode methods on 6 samples

01 !ertilizers

Ammoniacal N. %

Average Std dev, Coelf. 01var,. %Magruder

Sample results MOM ASEM-l MOM ASEM.1 MOM ASEM-I

1 7.718 :r 0.3648 7.547 0.1084 1.4362" 2.7040:t 0.0430 2.717 2.593 0.0288 0.0557 1.060 2.1483" 11.035 :t 0.0786 11.088 11.213 0,0821 0.0393 0.740 0.3504" 18.24 :1:0.17 18,267 18.058 00907 0.0454 0.497 0.2515 b.c 21.20(theor.) 21.138 21.063 0.0417 0./224 0.197 0.5666~.d 17.50(theor.) 17.527 17.232 0.0698 0.0527 0.398 0.306

~ Significant dillerences in averages al 95% leyel, t-test.1 > Ammonium sulfate. reagent grade.

e Signilicant differences in stilndard deviation at 95% level, F-test.dAmmonium nitrate. reagent grade.

Thc ammonium sulfate and ammonium nitrate

samples were reagent grade.Mean per cenl nitrogen, standard deviations,

and relative standard deviations (CV,%)for each

of the samples by both methods are given in

Table 2. No value was rejccted at the 95%sig-

nificance level by the Dixon test (11), When

standard deviations were compared by means of

the F-test, differences were statistical1y signifi-

cant only for the ammonium sulfate samplc,

which indicatcs equivalent prccision for both

mcthods:although the l-test showed significant

statistical differences in the averages for 4 of the

5 samples studied. Therefore, MOM and

ASEM-l gave comparable prccision, but average

values for MOM are in bt'tter agreement with

refcrence values than are averages for ASEM-

1.

Total Nitragen

Six samples (7-12, Table 1) were .:malyzcd in

6 replica tes for lotal nitrogen by RPM and

ASEM-2. Samples 7905 B, 7907 B, 7807, 7709,

7717, and 7607were analyzed by 5, 5, 6, 7, 5, and

7 participant laboratories, respectively, in tlw

Magruder check sample program (lO).In a preliminar)' cxperiment with ASEM-2,

nitrogen was measured wilh the ammonia-se-

lective electrode directly a£ter digestion; howw

ever, values were very high. These high values

mal' be due to the increascd ionic strength of the

solution caused by solids forml'd during the ad-

dition of NaOH to evolve ammollia. Such re-

sults an' in accordanc{' with observations by

other authors (2, 7) who recommended that

samples ano standards have the same ionic

strength. Further studies are necessary to con-

sider this eHecL

Mean per cent nitrogen standard deviations,

and coefficienls of variation for t'ach ()f the

S Significant diflerences in standard deyiations at 95% level. F.test.


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BOY & GUIJOSA: J - ASSOC. OH. ANAL. CHEM. (VOL. (,4, NO. S, 1981) 1099

In general. the average valucs fOf arnmoniacal

and total nitrogen by MOM and RPM are in

better agreement with the referenee values than

are lhe average valucs ohtained byASEM-l and

ASEM.2. This means that the ac(uraey of the

magnesium oxide method and RalH'Y powder

rnethod are better Ihan those obtained by am-

monia-selective e¡c(trode tcchniques. 00 the

other hand, Ihe average vaJues ohtained by

ASEM.l and ASEM-2 are higher than those ob-

lained by MOM Uf RPM.

The accuracy and precision of Ihe ammonia-

selective electrode technique is suitable fOf

confine determination of ammoniacal nitrogen.

It simplifies Ihe normal procedure because Ihe

distillation step is omitted. In the determinatian

o C total nitrogen with the Raney powder method,

the final titration can be substituted by an elec-

trode measurement, hut with no improvemento C results.

REFERENCES

(1) Hoove-r, W. L., Colvin, B. M., Me!ton, J . R., Hanks,

A. R., & Howard, P. A. (1977) De-terminalion of

nilrogen in fertilizers and protein in feeds by the

Orion ammonia ('¡('ctrode, Bull. Prpl. AXri •. A/lul.

Srrvirrs, TeXilSA&M University, ColI('g(' Slation,

TX

(2) Thomas, R. F., & Booth, I{. L. (1973) £I1l'iro/l. Sci.

TccJl/Illl. 7,523-526

(3) Cilbert, T. R.,& Clay, A, " ' 1 . (1973) Aual. Clre1/!. 45,1757-1759

(4) Renfro, L.J., &Patel, Y.(I974) f. AI'I,I. PII.IISilll.37,

756-757

(5) Barbera, A., &Cant'pa, D. (1977) 1 - AS."Ilf. Off. Al/al.

OCIII. (,0,708-709

(6) Woodis, T. c., Jr, &Cummings, J. M., Jr (1973)'. Assoc. 0lf. Allal. Chem. 56,373-374

(7) Eagan, M. L., &Dubois, L. (1974) Al1al. CJ¡jm. Aria

70,157-167

(8) 0l/icial Metlwds o/ Al1/1I.II';S (1980) 13th Ed., AOAC,Arlington, VA, secs 2.065, 2.063-2.064

(9) 'Ilstmclioll Manual Irlr Ammon;a Elec/rode i\ltJdl'1

95-10 (1975) Orion Researeh, Ine., Cambríd~e,

MA

(10) Statistical Evalualion on 7607, 7709, 7712, 7807,7905 n, 7907 n , 7908, and 8003 M


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COROMINAS ET Al..: l. ASSOC. OFF. ANAL. CHEM. (VOl. 63, NO. 3. 1980)

Preliminary ComparisoH of Methods for Determining

Sodium in Fertilizers

LUIS F. COROMINAS, VICTOR M. BOY, and MANUEL GUIJOSA

i=erfilizllntes Mexicanos, S.A., Resellrcll Division, Zacatecas 80, México 7, D.1=.

551

The AOAC offidal hrst action method 2.147-

2.150 for the f1ame emission speclrophotometrk

(fES) determinalion of sodium in ferlilizers was

compared wHh the atomic absorption spectro-

photometric (AAS) method and Ihe sodium selec-

tive electrodc (SSE) method. Two synthetic NPK

samples, 3 commercial samples (urea, DAP, and

superphosphate), 2 Magruder NPK check samples,

and one Magruder muria te check sample were

used for the study. Slatistically significant differ-

ences were obtained in both averages and standard

deviations for all samples. In general, the preci-

sion for all methods was acceptable; the AAS

method seems to be the more accurale¡ the AOAC

method needs lo be revised; and the SS[ melhod

has the tendency lo give higher values. Addilional

experimental work is necessary to define which

method is the most convenient.

Previous experimental work (1-3) on the

detcrmination oE sodium in Eertilizers resulted

in the adoption o E the AOAC officia.l Hrst ac-

tion method 2.147-2.150 (4), which is based

on a fl.1mc cmission spectrophotomelric peoce-

dure (FES). It seems thal this method is not

used in many laboratorics. The Magruder re-

port for Samplc 7804 (5) showcd Ihat 23 Ia.bo-

ratories repoded valucs Eor sodium and o E

these. only 5 used the FES method .1nd 17 used

an atomic absorption spectrophotometric pro-

cedure (AAS). The availability o E a sodium

selective electroue (SSE) suggested the possi-

bility o E applying this tcchnique to the de-

termination of sodium in fertilizers, thus

greatly simplifying the analysis.

'"Ve dccided to perEorm a preliminary study

comparing the official AOAC procedure (FES

method) with the 2 aIternative methods. TheFES method was tcsted using 2 different types

o E equipment and flamesj the AAS rnethod

was adapted frorn a procedure for phosphate

rock (6)j and the SSE mcthod was established

Eollowing the recommendations from the elcc-

trode 5upplicrs (7, 8).

Comparative Sludy

Table 1 shows the 8 samplcs used E o r com-

parative evaluation. Thrcc samples were com-

rncrcial fertilizers of unknown sodium con-

centration, 2 samples were Magruder NPK fer-

tilizer check samples, 2 were synthetic NPK

samples, and one was 7804, Magrudcr muria te

check sample, mentioned earHee. Samples were

selected to test the applicability of the peocc-

dures to different types o E products and levels

of sodium content. Ten replica te determina-

Iions were performed on each sample for each

method.

METHODS

1. F1ame Emission Spectrophotomelric

Method 1 (FES Methud-l)

See 2.147-2.150 (4). Readings were made using

Model Mark 11flame photometer (available fraIn

Evans Eledroselenium Ltd), wilh air-propane

flame.

n . Flame Emission Spectrophotometric

Method 2 (FES Method-2)

See 2.147-2.150 (4). Readings were made using

Model AA6 atomic absorplion spectropholometer

(available from V••rian Techlron Ply Ud) in emis-

sioo mode. wilh air-acetylene flameo

111. Atomic Absorption Spectropholometric

Method (AAS Method) (5)

Apparatus and Reagents

(a) Atomic absorptioJl spectropllOfometer.-

Model AA6 (Varian Techlron Ply Ud), or equiv-

alent.

(b) SodiulIl so{utiolls.-(l) Stock SOIIl/íOlI.-

1000 IIg Na/mL Disso]ve 2.5421 g rure NaCI in

ca 3-5 rnl HCI (1 + 1). Di]ule lo 1 L (2) Workingstandard solutioIlS.-O, 20, 40, 100. and 200 M~

Na/ml. To 250 ml volumelric flasks, add O. 5.

10, 25, and 50 ml Na stock solulion. Adjusl each

standard to ca o.lN in HCI (aboul 0.8 ml HCII100 ml) and dilule lo 250 mL.

Preparation of Sample Solution

Weigh 1.00 g sample into 250 ml beaker. add

15 ml HCI (1+ 1) and digesl 15 min on hol plale.Filter Ihrough fas! paper inlo 250 ml volumelric

f1ask. washing paper and residue Ihoroughly with

waler. Measure absorption of soIution directly or

dilule with o.IN HCI to oblain soIulion withinrange of inslrumenl.

Rece;ved AUlI:u,1 30. 1979. Accet>led Dl'cl'mber U. 1979.

000"-'7'6/80/6303--0"1-03$01 ,00~ A••o(iolion of Offieiol Anolylicol Chemi,h, In(.


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552 COROt.UNAS ET AL., J . ASSOC. OFF. ANAl. CHEM. (VOL. 63, NO. J, 19~O)

Table l. Description 01lertili;ter samples studled

This "'po.1 01 the As~odale Referce wa, p.e,enled al lhe

9Jrd Annual Meeting of lhe AOAC, Oct. 1.5--1&,1979, ilt

W,)shin¡:lon, OC.

Determinationo

5et wavelength al 5890A. Adjust burner hcad

900 to bcam oc anolher position if preferred loreduce sensitivity, using air~acetylene flameo As-

pírate standards and unknown samples. PJol curve

from standard values and determine Na content of

unknown samples from plot of absorplion against

¡.¡g/mL

IV. Sodium Selective £Iectrode Method

(SSE Me.hod) (7, 8)

Apparatus and Reagents

See 2.091(a)-(b) and (e) pH meter.-Model 701 Digital (available

from Orion Research, lnc., Cambridge, MA

02139), or equivalent.

(d) Eleetrodes.-Sodium ion seleetive glass elec-

trode and reference eleclrode Models 39278 and

39402 (Bcckman Instrumcnts, Inc., Fullerton, CA

92634), or equivalent.

(e) Sodium solutjOllS.-Stock SOllaioll,-IOOO flg

Na/roL. Dissolve 2,5421 g pure NaCl in 800 ml

water. Adjust to pH 10 wilh concentrated am-

monium hydroxide and dilute lo 1 L. Workü¡g

solutiolls.-(l) 500 ¡.¡g Na/mL. Transfer 100 ml

stock solution to 250 ml beaker, add ca 50 ml

water, and adjust to pH 10 with coneentrated NH¡OH. Transfer to 200 ml volumetric flask,

dilute lo volume with waler, and mix. (2) 100 ¡.¡g

Na/mL. Transfer 50 ml 50lution 1 to 250 rol

beaker, add ca 150 ml water, and adjust to pH 10

with concen!rated NH40H. Transfer lo 250 rol

volumelric flask, dilute to volume with water, and

mix. (3) 50 ¡.¡gNa/roL. Transfer 25 ml Solution 1

to 250 ml beaker, add ca 175 ml water, and ad.

Sample Oescriplion

A rnurlate, 0.0.60, Magruder 1804cheCk s8mple

B DAP, 18.46.0,fertilizer grade dium.

monium phosphate

e superphosphate, 0.20-0,lertillzer grade

normal superphosphale

O 19'19-19,Magruder I¡quid check sample

E 12.}5-10,Magruder sol id check sample

F urea, 46.0-0,ferlilizer grade urea

G 20.20.20, synlhetic sample, prepd with

diammoniurn phosphate, potassium

chloride, and sodium chloride.

reagent grades

H same as Sample G

Approx.sodiumcontenl,

%N.

1.0

0.1

just to pH 10 with concenlraled NH,¡OH. Transfer

to 250 ml volumelric f1ask, dilule to volume wilh

water, and mix. (4) 10 ¡.¡gNa/roL. Transfer 25 rol

Solution 2 to 250 rol beaker, add ea 175 rol water,

and adjus! lo pH 10 with concentrated NH,OH.

Transfcr to 250 ml volumetric flask, dilule lo vol-

ume with water, and mix. (5) 5 ¡.¡gNa/mL. Trans-

fer 25 ml Solution 3 lo 250 ml beaker, add ca

175 ml waler, and adjust to pH 10 with concen-

!rated NH10H. Transfer to 250 ml volumetric

f1ask, dilute to volume wilh waler, and mix. (6) 1

¡.¡g Na/ml. Transfer 25 ml Solution 4 to 250 ml

beakcr, add ca 175 ml water, and adjusl to pH JO

wilh concentratcd NH,OH. Transfer to 250 ml

volumetric f1ask, dilute to volume wilh water, and

mix.

Preparation of Solulion

Prepare solution as in 2.093(a), using sample

containing ca 25 mg Na, in 250 ml beaker. After boiling adjust lo pH 10 wilh NH,OH. Transf('r lo

250 ml volumetric f1ask, dilute to volume wilh

waler, mix, ami pass through dry filter.

Determination

Conlled Na and reference elcctrodes lo pH

meter, place electrodes in low concentration Na

solution, and warm IIp pH meter. Transfer C.1 100

ml stock, working, and unknown sample solulions

into sep.1fale 250 ml I;.eakers. Place electrodes in

each solution and while stirring wilh magnetic

stirrer al constan! rate, read mV of the stock

working and unknown sample solutions. Rime

and blol c1ectrodes between solutions. Plot Ihe mVrcadings (linear axis) against concentralion (Iog

axis) on standard 3- or 4~cycle semilo¡¡;arithmic

paper. Determine Na content of unknown samples

from slandard curve.

Results and Discussion

The averages D E the 10 determinations madI"

on eaeh s.lrnple with eaeh method, and the cor-

rcsponding standard deviations and eoefficients

o E variation, are shown in Table 2. In general,

the standard deviations for the 4 methods

showed precisiom that are aeeeptahle, al-

Ihough the F-Iest showed significanl s!atisticaidiffcrenecs in 50% o E thc total pairs that can

be formed. Comparing the averages by mean"of the t-test showed signiricant statistical dif-

ferences in 70% of the total pairs.

Differences are evident between the aver-

ages for the 2 versions of Ihe FES method, anJ

this can only be attrihutcd to thc use of differ-

cnt equipment and different Aame Ch.1file-

teristics¡ thus, the AOAC official method need ••

to be revised. In the Magruder report for

Sarnple 7804 (Sample A), the avcrage for tite


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COROMINA5 f.T AL.: J. AS50C. orF. ANAL. CHEM. (VOL. 63, NO. J, 1980) 553

Table 2. Comparative results lor the det••rmlnatlon 01!!Iodlum(% Na) In 8 samples 01 ferUlizers

Method Melhod

Slatislic 1 1 1 1 1 IV 1 1 1 1 1 IV

Sample A, Muriate Sarnple B, Diamrnonium Phosphate

Average 1.538 1.447 1. 449" 1. 809 0.080 0.09l" 0.098 0.111Std dev. 0.030 0.010 0.013 0.014 0.007 0.004 0.006 0.009Coer!.of

var., % 1.951 0.691 0.897 0.774 8.750 4.301 6.122 8.109

Sample C, Normal Superphosphate Sample D. 19.19.9

Average 0.130 0.159 0.232" 0.211 0.602" 0.605 0.512 0.714Std dev. 0.008 0.008 0.004 o.on 0.007 0.009 0.008 0.027Coeff.ol

var.. % 6.1S4 5.0]1 1. 724 6.161 1.167 1. 488 1.56] ].782

Sarnple E. 1l.15.1O Sample F. Urea

Average 0.200' 0.230" 0.173 0.260 NO' 0.00246 0.00103 0.0079Std dev. 0.008 0.007 0.004 0.005 0.00040 0.00013 0.0010Coell.ol

var .•% 4.000 3.043 2.]12 1.9lJ 16.260 12.621 12.658

Sample G. lO-lO.lO. Synlhetle Sample H. lO.20.20. Synthetie

Average 0.861~ 1.312 1.108 0.790 0.133 0.117 0.119 0.084Std dev. 0.058 0.116 0.008 0.027 0.005 0.003 0. 000 0.004Coefl.ol

var .•% 6.736 8.841 0.712 3.418 3.759 2.564 0.000 4.76l

ammonia-selective electrode determination of nitrogen in fertilizers

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