T H E INFLUENCE OF LYOPHILIC COLLOIDS ON T H E PRE- CIPITATION OF INSOLUBLE SALTS GELATINE AND SILVER CHROMATE. PART 11.

By 'I 'Hohus ROBERT BOLAM and MARY RUSSELL MACKENZIE.

Received April I 2 th, I g 2 6.

Introduction.

I n Part I. evidence was presented which tended to show that a consider- able portion of the silver chromate in a silver chromate-gelatine mixture containing sufficient gelatine to prevent precipitation behaved more as a crystalloid than as a colloid. I t was suggested that silver and chromate were possibly present as simple ions, in greater amount than in saturated aqueous solution. The silver ion concentration has, therefore, been in- vestigated by determining the potential developed when silver was immersed in the silver chromate-gelatine mixtures. The conductivities of the latter have also been measured to determine to what extent the silver chromate was present as electrolyte.

E.M. F. Determinations.

The compensation method was employed, two standard resistance boxes being used instead of a metre bridge. The total resistance in the two boxes together was kept at I I, I 10 ohms and the ratio between them varied. The null point instrument was an Ayrton Mather mirror galvanometer by Paul and the standard a cadmium cell by W. G. Pye. A.R. silver nitrate, potassium chromate and potassium nitrate were used, and, with one excep- tion, gelatine B.' All solutions were made up with N.P.L. calibrated apparatus and all experiments carried out in a thermostat at 25' C.

The potential of cells of the type

has been determined. X denotes solutions containing silver chromate wit6 or without other electrolytes and gelatine. In some of the earlier experi- ments a standard calomel electrode was used, but as the results obtained directly agreed with these, it was abandoned as an unnecessary complication. 10 Nammoniuni nitrate was chosen as connecting liquid since it was as- sumed that by so doing liquid-liquid junction potentials were eliminated.2

1 Part I. 2 A. C. Cumming, Trans. Far. Sot., 2 (1906).

162

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T. R. BOLAM AND M. li. MACKENZIE 163

The silver electrodes consisted of pure silver mesh threaded on to pure silver wire, which was sealed into glass tubing witti Everett wax. The mesh and wire were thoroughly cleaned with dilute nitric acid, ammonia, white sand, and distilled water before plating. The electrode was plated from a potassium silver cyanide solution, Nj’S with respect to silver and containing a slight excess of potassium cyanide. The anode, :c spiral of pure silver wire, was placed about 3 cms. distant from the cathode and current from a 2-volts battery was passed for 20-30 minutes. For the experiments in which two silver electrodes were opposed, they were thoroughly washed with distilled water and short-circuited in iVj’ I o silver nitrate until they showed no difference of potential. The electrodes were not freshly plated for each experiment but they were always equalised by short-circuiting in N/IO silver nitrate.

After a cell of the type described above had been assembled and left in the thermostat, a change in the E M E was nearly always found. This varied in quantity but was never more than four millivolts in the course of several hours, and though generally the E M.F. increased, the reverse occasionally happened. The change occurred most frequently when the gelatine mixtures were under investigation. Immediately after the final observation of the E.M.E of the cell, the electrode in the chromate-gelatine mixture was washed thoroughly with hot water and distilled water, rinsed with N / I o silver nitrate and placed in circuit with the other electrode in N/IO silver nitrate. Wherever this change in E A L E had been observed, a difference in potential between the electrodes when examined in this way was also found. Further, in the majority of cases if this were applied as a correction to the final reading the corrected result was in agreement with the first reading. The seat of the change would, therefore, appear to be the electrode, but the nature of it is not understood. Some electrodes were much more easily affected than others, and the rate at which the change took place also varied. In a few cases the difference in potential between the electrodes was greater than the difference between the first and final rtadings. It would seem that the change in these cases had set in before the first reading waz: made. \Vhenever this change approached four millivolts, the electrode was thoroughly cleaned an2 replated as described above.

The solubility of silver chromate in water was first determined. I n order to obtain a saturated aqueous solution, freshly precipitated and weli- washed silver chromate was shaken thoroughly with distilled water and placed in the thermostat for several days, during which time it n-as fre- quently s!iaken. The high resistance of this dilute solution rendered null point determinations dificuit. Some of the solution was therefore made centinormal with respect to sodium nitrate. Further, solutions were made up by mixing equivalent solutions of silver nitrate and potassium chromate at 25’ C. and allowing the mixture to remain in the thermostat ior 1 5 minutes before filling the electrode vessel. This solution should be saturated with silver chromate and will contain potassium nitrate. ‘The conditions for its production were thus exactly the same as those of the silver chromate-gelatine mixtures except for the absence of gelatine. The concentrations actually used were those for the most dilute chromate-gclatine mixture later examined. Soiutions containing silver chromate in presencc of a known excess of potassium chromate were alLo prepared by mixing equal volumes of silver nitrate and more coiicentrated potassium chromate. Iha l ly , freshly precipitated silver chromate was shaken up thoi oughly with N/I o potassium chromate. The results of the 2Z.M.R determinations using these solutions are given in Table I.

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TA

BL

E I

. U o\

P

No.

Solu

tion

in C

ell.

Satu

rate

d A

g,C

r04

Satu

rate

d A

g,C

rO,

-t- N

/IO

O NaN

O,

Satu

rate

d A

g,C

rO,

+ 162

x IO

-~

N

KN

O,

Satu

rate

d A

g,C

rO,

+ 162

x I

O-

~N

K

NO

, + 162

x I

O-

~ N

K,C

rO,

(a =

1.0

).

Satu

rate

d A

g,C

rO,

+ 162

x I

O-

~ N

KN

O,

+ 162

x IO-~N

K,C

rO,

(a =

0-8

6).

Satu

rate

d A

g,C

tO,

+ N

/IO

K,C

r04

(a =

0'7

3).

Firs

t R

eadi

ng.

Tim

e.

E.M

.F.

(Vol

ts.)

0.1462

0'1456

-

0.1 469

._

0.1491

0'14

55

0.1448

0'1756

0'1756

0'21

11

0.2247

Seco

nd R

eadi

ng.

Tim

e.

E.M

.F.

(Vol

ts.)

0.1463

0'1 483

0.1449

0.1469

0.145 5

0'1.424

0.1459

0.1471

0'1767

o 1760

0.2107

0.2251

Silv

er Ion C

oncn

. (N

x 1

0-5)

.

} 8.602

2204

1'276

9.908

6.311

6.440

5'937

H

Solu

bilit

y.

Z A

g~C

r04(

N x

lo-6

).

0

Z 23'28

23-42

22.82

a =

dtg

ree

of d

isso

ciat

ion

of K

2Cr0

,; J

ones

and

Jac

obso

n ;

Am

cr. C

hcnr

. you

r.l 40 (Igo8), 374.

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T. R. BOLAM AND M. R. MACKENZIE 1 65

The silver ion concentrations were derived from the E.MF.'s by the- usual formula

C E.M.3 = 0.0591 log -l Cf)

where C, = concentration of silver ion in N/IO AgN03 (0*0814N),~

11.1 the cases of 4, 5, and 6, the solubility of silver chromate in water was. calculdted from the solubility product equation [2 Ag'I2 x [CrO4"] = con- stant.

Tt:e presence of potassium nitrate and sodium nitrate does not appear to affect the value of the E.M.fi Abegg and C O X , ~ in an endeavour to deter- mine the solubility of silver chromate, used an i-lectrode of silver plated with silver chromate and immersed in N/I o potassium chromate. They state that the E . M R measured was ver! uncertain, sometimes decreasing, suddenly by as much as twenty millivolts, and suggest that the potential involved may not be that of Ag/Ag" but is possibly an oxidation poreiitial. I t will be seen that, contrary to their experience, the silver electrode in presence of silver chroniate and potassium chromate gave a very steady arid reproducible potential difference. Since the value of the solubility prodcct is constant within the limits of error, even when the amount of the excess of potassium chromate is considerably varied, it seems certain that in these expel iments the silver potential is measured.

Kohlrausch 3 by conductivity obtained 14.8 x I O - ~ N f o r the solubility of silver chromate at 17.07~ C. and 21.9 x I O - ~ N at 30.76" C., values lower than any obtained by the authors.

The silver ion concentration in mixtures containing the minimum amount of gelatine necessary to prevent precipitation was next investig3,ted and the results are given in Table 11. All mixtures were made up in exactly the same way as in the experiments in Part I., and throughout the whole of the subsequent E M f i work, except where stated otherwise, the mixtures were left in the thermostat for 15-30 minutes after mixing and before filling the electrode vessel. Three readings were made, the first fifteen minutes and the final two hours after assembling the cell, this time- limit being fixed quite arbitrarily. The second reading showed that any change which had taken place was gradual and always in the same direction for any single experiment. I n the tables only the first and final readings are. given and the latter are the values after correcting for the change in the electrodes referred to above. Only the most dilute solution was meahured after standing for seventy-two hours in the thermostat as the others set to a jelly in that time and could not be transferred to the electrode vessel.

'The influence of potassium nitrate and potassium chromate in excess on the 23.M.R of the cell was tested and the results are given in Tables 111. and IV. respectively. I n the former case the gelatine solution used was made. N/75 with respect to potassium nitrate. I n the latter, the same procedure was adopted as in cases 4 and 5, Table I. Two solutions, A and B, were examined.

A = 25 C.C. N/2co silver nitrate + 13.5 C.C. 2 per cent. gelatine were added to 2 5 C.C. N/IOO potassium chromate + 13-5 C.C. 2 per cent.. gelatine. Silver chromate did not precipitate from this at the time of mixing.

1; = The same as A except that N/IO potassium chromate took the. place of N/IOO.

C, = concentration of silver ion in X.

The brackets denote molecular concentrations.

Precipitate formed immediately. Noyes and Falk, Y . A . C . S . , 33 (1911). Zeit. p h y s . ckent., 46 (1903). Ibid. , 64 (1908).

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I 66 INFLUENCE OF COLL0II)S ON PRECIPIl'ATION

E.M.F. (Volts).

I n these two instances the experiments were carried on through a proloiiged period. No correction has been made for change in the electrodes since this had not been observed at the time of these experiments.

Ag' Concn. E.M.F. Ag' Concn. ( N x 10-5). (Volts). ( N x 10-5).

TABLE 11.

162 x 10 - NAg,CrO, + 0.70 per cent. geiatine + 623 x I O - ~ KNO,

1 s t Reading.

0.1116 105 '3 0.1116 I05 '3 0'1134 98.1 0.1132 99'0 -

Mean 101.7 Mean 102.2

2nd Reading. AgaCr04

(A x 10-5). Gelatine, Per Cent.

Ag' Concn. ( N x 10-6).

Ag' Concn. ( N x 10-5) .

E.M.F. (Volts), C.M.F. (Volts).

333 9 9

? 7

3 Y I

9 9

0-1141 o.rog1 0.1115

Mean

95'4 I I j 'Y

105-5

105.6 -_____

o*r119 0.1094 O'IIIq

Mean 108.4

236 9 7

I

0.1 I 13 0.1097 0.1 148

Mean

0-1 137 0- I 097 0.1 I 14

iMeati

97'2 115.5 106.0

106.2

0'1 I22 - 0-1 I 17

+t 0'1102 * 0'1 109

Mean

102.7 -

I O j ' I 111'1 108.2

106.8 ---

0.1123 0.1116 0.1105

o-*og7 0'1107

Mean

102.4 105.3

113-2 109.0

110'1

108.0

* Solutions marked thus we;e left in the thermostat 72 hours before transferring to the eIectrode vessel.

TABLE 111.

THE EFFECT OF EXCESS KNO,.

I First Reading. I Final Reading.

Solution.

The presence of potassium nitrate then does not affect the E.M.R. of the cell and potassium chromate produces an increased E. Af.R The appearance of, and increase in quantity of, red solid silver chromate in the presence of excess potassium chromate was easily observed. Precipitation and the accompanying increase of E.ME proceeded remarkably slowly !with the more dilute potassium chromate. From these observations there appears little doubt that here also the E M F . measured was due to the Ag/Ag' potential.

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rr. R. BOLAM , 4 m M. R. MACKENZIE 167

The conclusion that in the silver chromate-gelatine mixtures of Table 11. the silver ion concentration was much greater than in a saturated aqueous solution of silver chromate appears justified. Since nothing like the total amount of silver present is accounted for as ion, the question of the

TABLE IV.

THE EFFECT OF EXCESS K,CrO,.

Experiment.

A

2

3

'4

5

t 6

Solution. Time.

20 mins. ~ h r . 25 ,,

3 hrs. o ,, 3 50 l 9

Pptn. -+

E.M.F. (Volts).

0.1203 0.1241

O'IZSI 0.12s I 0.1246 O ' I 2 j I

0.I.SI 0.1291 0.1366

0'1 I sq 0.1191

0.130'i 0'1316 0' I j 3 ,< o.13S1 0*1414 0'1430

0.Iqofi 0.1412 0' I 460 0.1467

0.1813 0'1821 0*1829

0'1804 0'1807 0.1811

Ag' Concn. (A' x 10- 5).

75'1

55'4

63'5

55'4

40.0

so.:.

43 '2

31-0

3 4'0

2'5-9

7'0

6-6

7 '2

7'0

* Solution left in thermostat 15 hours before filling cell. t 9 , ¶ I 9 9 IS* Y f Y Y I 1 9 9

relationship between the ionic concentration and the relative proportions of silver chromate and gelatine next presents itself.

Two series of experiments were carried out and the results are given in Tables V. and VI. I n the former the Concentration of the gelatine was kept constant and that of the silver chromate varied, and in the latter the silver chromate concentration was kept constant. The mean values of the results already obtained have been introduced in both tables for comparison.

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168 INFLUENCE OF COLLOIDS ON PRECIPITATION

T A B L E V.

First Reading. Final Reading. Gelatine,

Per Cent. A:* Concn.

( N x 10-5). Ag' Concn.

( N x 10-5). E.M.F. (Volts). E.M.F. (Volts).

333

250 Y 9

105'6

70-1 65 -6

108.4 Mean

0'1220 0'1 23 8

Mean

0'1 624 0'16.25

Mean

Mean

0.1393 0'1408

Mean

Mean

0'1220 0.1237

Mean

0'1621 0.1628

Mean

Mean

0.1397 0.1387

Mean

70.1 65 '9

68.0 67-8

14.5 14'5

14.7 14'3

14.5

106.8

35'2 36.6

236

118 Y t

1'59

1-59 9 9

34'8 35'9

*324 9 9

162

0.70 1 9

0.70

0'1206 0'1233

Mean

Mean

65'3 66.6

74'2 66-9

0.1239 0'1234

Mean

Mean

70-6

106.8

66.0

108.0

* Precipitates immediately on mixing.

T A B L E VI.

Firs t Keading. Final Reading. AgrCrO4

( N x 10-5). Gelatine,

Per Cent. E.M.F. (Volts.)

0.1401 0'1388

Mean 0'1277 0'1262

Mean Mean

0-1069 0.1027

3 0'1012

Mean 4 O'IIyg

0'1165

Mean

E.M.F. (Volts.)

0'1398 0.1389

Mean 0'1262 0'1262

Mean Mean

0.1128 0'1 I 18

2 0'1 Iog

Mean 0.1205 0'1206

Mean

Ag. Concn. ( N X 10-5).

34'5 36'4

35 '5 56.2 59'7

5S.o 10j-6 126'4 I 48 -8 158%

--

Ag' Concn. (N x 10-5).

35 '0 36'3

35'7 59'7 59'7

59'7 10s-4 100.6 104.6 I0%'2

104's

74'9 74'2

74'6

--_

---

6 9 9

4 '5 9 9

3 1 '5

9 1

9 9

I Y Y

153'4 82.3 86.5

84.6

[For footiiotes see next pagc.

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T. R. BOLAM AND M. R. MACKENZIE 169

Ag9Cr04 ( N x" I 0 -5).

Gelatine, Per Cent.

2'10

9 9

1-40 ?,

0-70

0'35 9 9

9 ,

9 9

TABLE VI (continued).

First Reading.

E.M.F. (Volts.)

0.1317 0.13 I I

Mean 0'1185 0'1172

Mean Mean

0'1043 0-1064

Mean 5 0.1 170 0.1176

Mean fi 0'1240

0'1216

Mean

0.1361 0'1349

Mean 0'1237 0.1233

Mean Mean

3 0 ~ 0 8 0 0.1068

Mean ' 0'1247 'i 0'1224

Mean

Ag' Concn. ( N x 10-5).

48-2 49 '3 --_ 48'7

84.5 80'5

82.5

104'3 140'2 128.7

134'5 Sj'2 83 '3

84'3 64 '9 71'4

68.2

--

40.6 42'5

41'6

66'9 65 '9

66'4 106.8 121.3 127.0

124.7 63 '2 69 'I

66.2

Final Reading.

E.M.F. (Volts.)

0.1312 0.1301

Mean 0'1 I90 0'1 163

Mean Mean

0.1038 30*1071

Mean 0'1175 0.1 179

Mean 0'1230 0.1228

Mean

0.1360 0'1357

Mean 0.1230 0.1233

Mean Mean

0'1094 3 0.108 I

Mean 0'1245 0.1235

Mean

Ag' Concn. ( N x 10-5).

49'1 51'4

50'3 79'7 87.6

83'7 106.2 142%

---

125'5

134.2 83'8 82.3

82.6

68 o

67'8

67'5

---

40'7 41'2

41.0

66.9

67'2 105.0 I 14'8 120.7

117'7 63-8 66-3

67'5

_-

65.1

1 Precipitation started. 3 No precipitation. s Solutions kept in thermostat for 24 hours before filling cell.

2 Precipitation throughout. 4 Precipitated on mixing.

Precipitated. 6

9 7 9 , 9 9 9 ) 9 9 48 1 9 > 9 9 9 9 9 9 9

7 9 9 9 9 9 1 9 ( 1 174 9 9 1 9 9 7 9 9 9 9

I t seems possible that where precipitation occurred immediately the method of mixing is not entirely satisfactory. I n actual experiment 30-40 C.C. potassium chromate and gelatine would be poured into a similar quantity of silver nitrate and gelatine. Precipitation in the upper part of the tube took place before the lower portion of the silver nitrate was reached. Thus for a short but quite perceptible space of time one portion of the silver nitrate was in contact with excess of potassium chromate. This lack of homogeneity may produce irregularities in the results.

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I 70 INFLUENCE OF COLLOIDS ON PRECIPITATION

Whenever dealing with silver salts the effect of light cannot be over- looked. The majority of these determinations were made either in daylight or in artificial light. Owing to recoiistruction in the lighting system of the laboratories, a number of the readings had to be made in the dark, but there was no difference in the results obtained.

remained and it was used to prepare some mixtures containing the minimum amount of gelatine and the silver ion concentration of these was measured. There was not sufficient to examine more than one concentration. The results are given in Table VII.

A very small quantity of gelatine A

TABLE VII.

Solution. Experiment. I

334 x IO- N Ag,CrO, + 334 x 10 - - 6 N KNO, 1 + 1-97 per cent. gelatine A + I o - 2 N N a \ 0 ,

3

Time.

Immediately Y 7

7 3

6 hours later

E.M.F. Ag' Concn. (Volts.) ( N X 1fd-5).

0-0915 230.4 0*08gg 244.6 W O ~ I I 234-2 o'og20 22j-7

Mean 233-7 - .--

Conductivity Determinations. A conductivity cell of the ordinary type, with platinised gold electrodes,

was employed in conjunction with the usual Wheatstone bridge arrange- ment, using the telephone as null point instrument. ,411 solutions were made up with conductivity water and the small correction for its con- ductivity has been made for all results given.

The chief sources of error were three in number, viz., the measurement of the small quantities of all solutions used, the viscosity of the gelatine solutions and the lack of homogeneity of the gelatine powder. With regard to the first, measurements of the conductivity of aqueous solutions of potassium nitrate, potassium chromate, and silver nitrate, prepared in the same way as the corresponding gelatine solutions, gave values in good agreement with those of other workers.2 In general, two observations were made within forty-five minutes of the preparation of the solutions, i t . before they had set. Table VIII.. shows that the large increase in viscosity on setting has very little influence on the conductivity. No appreciable error was therefore introduced by making measurements as soon as the solution had attained the temperature of the thermostat :-

TABLE VIII.

Solution Examined.

3 per cent. gelatine 250 x IO-~NKNO,

3 , 9 , AgNO, + 9 , $ 9

333 x 1o-~NAg,Cr0, + ,y 9 ,

250 x 1o-~NAg,Cr0, + ,, 9 ,

1 2 5 x 1 0 - ~ ,, 9 , Y ,

+ 3 per cent. gelatine

3 , 1 , 9 9 9 ) 1 ,

Spec. Cond. while Liquid r.0. x 10-3.

0.2820 0'5946 0.4712 0'8407 0.6884 0.6852 0.4669

Spec. Cond. when Set

r.0. x 10-3.

0'2815 0'5923 0.4676 0.834 9 0'6863 0'6825 0.4640

Part I. !a Mellor, '' Treatise on Inorganic and Theoretical Chemistry," Vol. II., 819 ; Jones

and Jacobson, Am. Ckcm. ~ ' o H Y . , 40 (1~08)~ 374; Lob and Nernst, 2. physikal Chenz., 2 (1888), 948.

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T. R. BOLAM AND M. R. MACKENZIE

Table IX. shows the extent to which the conductivity varied with It will be seen that the variation is ,different samples of gelatine powder.

quite small :- TABLE IX.

Gelatine, Per Cent. 0.5.

- 0.0598 0*0600 0.0602 -

0.70.

- 0'0800 0.0784 0.0804 0.0808

1'0.

- 0-1062 0.1097 0.1127 -

1.59.

- 0.1662 0.1674 0.1659 -

3.0.

02751 0.27qo 0.2783 0.2753 0'2708

Spec. Cond. r.0. x 10-8.

Mean ~ 0'0600 0.1664 0.109 j 0.0799 0.27 57

The conductivity of a silver chromate gelatine mixture will be due to gelatine, potassium nitrate, and silver chromate. I t was assumed that the conductivity due to the gelatine and potassium nitrate would be the same as in the absence of the silver chromate. The conductivities of mixtures of potassium nitrate and gelatine of the same concentrations as in the silver chromate-gelatine mixtures were therefore determined. By subtracting these from the total conductivities of the corresponding silver chromate- gelatine mixtures the conductivity due to silver chromate (" excess " con- ductivity) was obtained. The values found for three concentrations of silver chromate-gelatine mixtures which contain the minimum amount of gelatine to prevent precipitation, are given in Table X. The conductivities of saturated aqueous solutions of silver chromate without and with gelatine are given in Table XI. To prepare those containing gelatine, 25 C.C. samples of the saturated solution were rapidly pipetted on to accurately weighed portions of gelatine in dry flasks and the whole warmed till the gelatine dispersed.

TABLE X.

Spec. Cond. Corresponding

Concn. KNO:t + Gel. r.0. x 10-~*

Excess Spec. Cond.

r.0. x 10- 3.

Gelatin:, Per Cent.

Spec. Cond. r.0. x lo-'*

0.8373

c.8327 0'8324

0'6866 1 7

7 9

333 9 .

7 9

0.1507 0.1458 0.1461

Mean 0.8341 Mean 0-1475

o*:gSg 0.5859 0-5900

Mean o.jg16 --

1-59 9 9

7 9

0.1217 0 ' 1 ~ 8 7 0.1 I 28

Mean 0.1144

I 6 2 7 )

7 7

7 7

9 9

0.70 $ 7

7 7

7 )

7 i

0.1166 0'1120 0' I I 68 0'1206 0'1214

Mean o.11g5 Mean 0-4207

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I 72 INFLUENCE O F COLLOIDS ON PRECIPITATION

Saturated solution + 3 per cent. gel.

Saturated solution + 1'59 per cent. gel.

Saturated solution + 0.70 per cent. gel.

Solution.

0'2801 0.285 I

Mean 0.2833

0'1 743 0.1775

Mean 0'1759

0.09 5 5

Mean o-og44

0.2848 ---

--

0.0932 --

Spec. Cond. r.o.xxo-3

TABLE XI.

Spec. Cond. Gelatine,

r.0. x 10 - 3.

Saturated aqueous solution 0*0250 0.0223 0.0252

Mean 0.0242

0.2757

0.1664

0.0799

Spec. Cond. AgaCrO4 in Presence of

Gelatine, r.0. x 10-3.

0.0076

0*0095

0-0145

From Table XI. it appears that the conductivity of silver chromate is reduced by the presence of gelatine.

The conductivities of silver chromate-gelatine mixtures containing less than the maximum amount of silver chromate which can be prevented from precipitating were next determined. The results are given in Table XII.

TABLE XII.

Gelatine Per Cent.

Spec. Cond. r.0. x 10 - 5.

0.6850 0.6866 0.6811

Mean 0.6842

0'4635 0'4645 0,4628

Mean 0.4636

0-3680 0.3642 0'3746 0.3745

M a 0'3705

Spec. Cond. Corresponding

Zoncn. KNO3 + gel. r.0. x 10-3.

0'5920

0.4327

0.3177

Excess Spec. Cond. r.0. x 10-3 .

0.1475

0.0922

0'0309

0.1 144

0.0526

0.1 195

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T. K. BOLAM AND M. K. MACKENZIE I 7 3

along with the mean values for the mixtures containing the maximum amount of silver chromate.

TABLE XIII.

KNO,,.

333 I I

2 j0 5 9

125

2;)6

I 18

162

7 9

I I

5 9

- 0.6866 - 0'5920 -

0.4327

0'4772

0'3177

0.3012

-

-

-

K,CrO,.

0'464 I 0*4109 0.3515 0.3 I 63 0.1786 0.1570 0.3285 0.3 I 08 0.1678 0.1513 0'2283 0.2213

0'0532

0.0352

0'0216

0.0177

0.0165-

0*0070.

1000

I 9

9 )

500 9 )

I f

333

250

125

2;)6

I )

I 9

P I

118

162 9 3

I 9

1000

I 9

P I

500 I 9

> I

333

250

125

2;'6

118

162

I *

7 9

7 f

9 )

Y I

- 1'312 1.336

0.7024 0.6972

0.6022

0.5134

-

-

-

- 0'3921 -

0.4179

0'2865

0.2693

-

-

AgN 0,.

- 1.223

0.6652

1'222 -

0'6479

0'5545

0.4695

- - -

0.3542

0.3881

0.2636

0.2566

- - -

- 0'1095 0'0600 -

0'1095 0~0600

0.2757

0.275 7

0'2757

0.1664

0.1664

0.0799

-

- -

-

-

-

- 0.1095 0'0600 -

0.1095 0-0600

0.2757

0.2757

0-2757

0.1664

0.1664

0'0799

-

-

-

-

-

-

1'354 1-203 1'276 0'6950 0.5929 0'6372 0.4748 0.3265 0.3571

0'1801 0.1164 0'3371 0.2515 0.1684

0.2343

0'2387

0'1201

0.1894

1.243 1.114 1'1 62 0.6299 0'5557 0.5879 0'4288 0.2788 0'3239 0.1938 0.1641

0.3086 0.2217

0-2146 0.1767

0.0785

0.1551 0'0972

0.15 I 0.078

0'1021 0.05 78

0'1483

0.1184

0.063 7

0.0856

0.0483

0'0449

0'129 0.081

0'074~ 0'0420

0.1~00

0.1301

0.0856

0.0869

0.05 79

0.03 79

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T 74 INFLUENCE OF COLLOIDS O N PRECIPITATION

333 333 324

E M F . determinations indicate an increased concentration of silver ion This could occur without change in the value of the solubility product if by some means the concentration of the chromate ion were reduced. Such a view does not accord with the results of diffusion experiments, described in Part I., where the chromate appeared to be the more free of the two ions. Some light may be thrown on the question by determining the effect of gelatine on the two ions when apirt. Hence the effect of gelatine on the conductivities of silver nitrate, potassium chromate, and potassium nitrate was examined. I n Table XIII. will be found the values for the decrease in conductivity which was observed in all cases. I n obtaining these values it has been assumed that potassium nitrate, silver nitrate and potassium chromate do not affect the conductivity of the gelatine.

1 '5 2 hrs. 15 mins. 104.8 1'0 1, 9 , 74'6 0.7 9 1 9 9 66.0

Discussion of Results. 'The 2Z.M.E: measurements show that in the mixtures of equivalent

amounts of silver nitrate and potassium chromate, containing just sufficient gelatine to prevent precipitation, the silver ion concentration was much greater than in a saturated solution of silver chromate. I n the three different mixtures selected for investigation the silver ion was found to be approximately constant although the total concentration of silver varied considerably. The amount of silver other than ion is directly proportional to the gelatine concentration (Table XIV.) :-

TABLE XIV.

Ag - Ag' Gelatine, Ag - Ag' (1%' x 10 - 9. Per Cent. Gel. concn.

(333 - 108 (236 - 1061 (162 - 108)

3'0 1-59 0.70

75 x 10-5 82 x I O - ~ 77 x 10-5

'The conductivity data are in agreement with these results but show that t he '' excess " conductivity of the most concentrated mixture is definitely greater than that of the other two. The quantity of silver ion did not change in seventy-two hours, the arbitrary time limit.

When, however, less gelatine was present than could hold up precipi- .tation for seventy-two hours but enough to delay it for a time, the value of the silver ion concentration was at first higher than the constant value, but !fell below this and steadily decreased as precipitation proceeded. Precipi- tation, whether immediate or delayed, was always accompanied by decrease in silver ion. The period of delay before precipitation diminished quite

rapidly with increase of the ratio for a given concentration

of chromate, and at the same time the speed of precipitation increased proportionately. That the silver ion concentration alters correspondingly is shown in Table XV. :-

silver chromate gelatine

TABLE XV.

1 Gelatine, Pe r Cent. Time. Ag' concn. I ( N X LO-5).

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‘l’. K. BOLAM ANI) M. R. MACKENZIE I 7 5

The extent to which precipitation will progress has still to be settled. ‘The most protracted E M I R determination in this connection was made with 236 x I O - ~ Nsilver chromate in 1.06 per cent. gelatine (Table VI.). At the end of fifty hours there remained much more silver ion than in a saturated solution and it was still uncertain whet3er equilibrium had been reached.

There can be little doubt that whenever preciFitation was delayed zi large part of the silver chromate was kept in a state of supersaturation and everything points to the same being true of those s p e m s which showed no change in seventy-two hours. Presumably, accc rding to Ostwald’s view, in the latter we have ‘( metastable ” supersaturation and in those containing less gelatine ‘( labile ” supersaturation. \irkether his distinction is valid or not, it is quite in accord with observations on ordinary supersaturated solutions to have a region of high rate of precipitation separated by a small concentration range from one where precipitation is imperceptible.

I t should be pointed out that the increase in E.ME which went with precipitation was of a much greater order of masnitude than that due to change in the electrode. That the effect was due to change in the inzxtzire is also shown by the fact that, if precipitation took place before the solution was poured into the electrode vessel, the 2Z.M.R measured after fifteen minutes was always greater and might be much greater than that observed with a mixture in which precipitation had not occurred.

If more gelatine was present than was just sufficient to prevent pre- cipitation in seventy-two hours, the silver ion concentration was again found to have less than the constant value. I n accordance with this the “ excess ” conductivity was smaller. If similar concentrations of chromate (‘Table XII.) are compared, 236 x I O - ~ N with 250 x 10‘~ N, and I 18 x I O - ~ N with 1 2 j x I O - ~ N; it is evident that the “excess” was greater in the presence of 1 * j 9 per cent. gelatine than in the presence of 3 per cent. There is thus an optimum concentration of gelatine; more or less leads to disappearance of silver ion.

Precipitation by excess of potassium chromate reduced the amount of silver ion. I n the case of mixture A (‘Table IV.) which contained the smaller excess precipitation was delzlyed for an hour. The silver ion con- centration was diminished from the start. Apparently the gelatine hindered the formation and growth of solid nuclei. The amount of silver ion de- creased very slowly but steadily. On the other hand, in the case of mixture B, it was immediately reduced to a much smaller value which altered but slightly. These experiments are comparable with those of Langdon who measured the silver ion concentration in mixtures of silver nitrate and excess of potassium chloride in the presence of gelatine. It map be that the difference between chloride and chromate is one of time only and is to be expected from the difference i n their solubilities.

The following table shows the paralle!ism between the values of “ excess )’ conductivity and the silver ion concentration as deduced from E.M.E :-

TABLE XVI.

Ag&r 0 4 (1V x I 0 - 5 ) .

333 250 125 23 6 I18 162

Gelatine, Per Cent.

Ag’ by E.M.F. (s X 10-5).

I08 63 I5 106 35

I 08

Excess Sp. Cond. r.0. x 10-3.

O‘I475 0.0922

0.1144 0.0526

0’0309

O ’ I I g j

Ag‘ by conductivity ( N x 10- 5 ) .

Tram. Far. SOL., XIX. (I923), 28j.

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I 76 INFLUENCE O F COLLOIDS ON PRECIPITATION

.

The figures in the last column are calculated from those in the fourth on the assumption that the ionised silver chromate has the same conductivity a.i silver nitrate and that the specific conductivity of the latter is directly proportional to the concentration. The two sets of values for the silver ion concentration are of the same order of magnitude and in some cases agree closely. The differences are no greater than might be expected from, the experimental error involved in the E.M I;: determinations.

I t will be seen that gelatine did not react in the same way with all the salts. On potassium nitrate the effect was slight in comparison with t h e others. The depres- sion in the conductivity of electrolytes in the presence of gelatine has been, observed by other workers1 Of the theories advanced to explain this de- pression the most applicable to the present case appears to be that elec- trolyte is removed from the solution by the gelatine either through adsorption: or chemical inceraction. Since the diffusibility of electrolytes is not in general appreciably affected by gelatine in the concentrations used, it is unlikely that the mobilities of the ions will be altered. Potassium nitrate and potassium chromate have the same conductivities in aqueous solution, yet the depression in the case of the latter is much greater than in the case of the former. I t seems improbable that this could be caused by changes. in mobility. The same reasoning applies to the silver nitrate. found that when gelatine is added to silver nitrate solution the silver ion concentration, as indicated by E M E , is reduced. This was repeated with silver nitrate and silver chromate and the same effect observed but systematic investigation was not undertaken. Dumanski ascribes the depression to diminution in the area of cross-section of the electrolyte. He, however, deals with much higher concentrations of gelatine. Moreover, such an effect would not account for the difference between potassium nitrate and the other two salts.

The foregoing makes it appear probable that in the silver chromate- gelatine mixtures that part of the silver and chromate which was not present as ions was combined in some fashion with the gelatine. Table XVII. shows that the conductivity of any of these mixtures was almost equal to the sum of the conductivities of its constituents, taking into account no other effect than the depressive action of the gelatine. The numbers in the third column are calculated from : spec. cond. (AgNO, + gel.) +spec. cond. (K2Cr0, + gel.) - spec. cond. of the corresponding concentration of gelatine.

TABLE XVII.

The depression in their cases was almost identical.

Audubert

Spec. Cond.

Calculated, Found, r.0. x 10 - 3. r.0. x 10 - 3.

--_____ 0.8810 0-8341 0.7072 0.6842 0.4706 0.4636 0.6396 0.5916 0'3837 0.3703 0.4460 0'4207

333 250 125

I I8 162

236

Gelatine, Per Cent. Difference, I r.0. x 1 o - 3 .

0.0469 0*0230 0-0070 0.0480 0'0134 0'0253

The differences between the calculated and actual values may possibly be due to the formation of molecular silver chromate which, it is reasonable to suppose, would also be increased in amount by supersaturation.

Rettig, Kolloid-Z., 27 (1920)~ 165 ; Dumanski, 2. physikal Chem., 60 (1907)~ 553. Compfes rcndus, 12 (1923), 838.

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T. R. BOLAM AND M. R. MACKENZIE I 7 7

Gelatine A was able to promote a higher degree of supersaturation (Table VII.) and it was decidedly the more acid (Part I.). If the super- saturation were due to viscosity only the reverse state of affairs would have been expected.l

The results of the present investigation obviously support Wi. Ostwald’s theory of Liesegang ring formation for the case of silver chromate. Hatschek 2 considers that the theory is disproved by showing that when lead nitrate was allowed to diffuse into agar gels, containing potassium iodide and rings of lead iodide, a second independent series of rings was formed. The authors found that inoculation seemed to be .incapable of initiating precipitation in chromate-gelatine mixtures containing just sufficient gelatine to prevent spontaneous precipitation. I t may be that at these particular concentrations the rate of precipitation is so slow as to be imperceptible, or a layer of gelatine may be adsorbed on the added solid which interferes with its action. Thus it cannot be assumed that pre-existing solid d l always eliminate supersaturation in such systems.

The hardening action of bichromate on gelatine under the influence of light raises the question of whether some sort of decomposition of the chromate in the mixtures may not take place. According to Lumiitre and Seyewetz,3 however, gelatine is affected only with extreme slowness by potissium chromate.

Summary. ( I ) The silver ion concentration in mixtures of silver nitrate, potassium

chromate and gelatine has been measured electrometrically. ( 2 ) Its value may be m ~ c h greater than in saturated aqueous solution

of silver chromate. There is an optimum concentration of gelatine for each concentration of silver chromate. More or less gelatine leads to decrease of silver ion.

(3) I n mixtures where precipitation occurs the silver ion concentration falls off at a rate which may be very slow and which depends upon the re- lative concentrations.

(4) The conductivities of the mixtures have been determined and con- firm the E.M.F. results.

(5) I t has been found that gelatine reduces the conductivity of aqueous solutions of silver nitrate, potassium chromate and potassium nitrate, the effect being much greater in the first two than in the last.

(6) The conclusion is that when equivalent solutions of silver nitrate and potassium chromate are mixed in the presence of gelatine to give a clear yellow system, the gelatine removes a portion of the salts from solution and maintains the remaining silver chromate in a state of supersaturation,

The authors wish to thank Professor Sir James Walker for his constant interest and support, Messrs. Brunner Mond Sr Co. for a grant enabling them to obtain necessary apparatus and Messrs. Cox, Ltd., for the gelatine,

Bogue, 7. A.C.S . , 4 (1922), 1349. ‘‘ Second B.A. Report on Colloid Chemistry ” (IgIg), 21. Bull. SOL. c h k . , 33 (I905), 1033.

Chemistry Bfartment, Kin,.’s Buildings,

Edinburgh University.

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