The Furaduy Society is riot resfonsiblc for opiiiioiis expressed before it by Authors OY Speakers.

Cransactions OF

Che fmabay Society. F O U N D E D 1 9 0 3 .

To PROMOTE THE STUDY OF ELECTROCHEMISTRY, ELECTROMETALLURGY, CHEMICAL PHYSICS, METALLOGRAPHY, AND KINDRED SUBJECTS.

VOL. XXII. J U L Y , 1926. PART 4.

'THE INFLUENCE OF LYOPHILIC COLLOIDS ON T H E PRE- CIPITATION OF INSOLUBLE SALTS. GELATINE AND

SILVER CHROMATE. PART I.

By THOMAS ROBERT BOLAM AND MARY RUSSELL MACKENZIE.

Received M a n h 2 6th, I 9 2 6.

Introduction.

IVilliams and MacKenzie have determined the minimum concentrations of gelatine necessary to prevent precipitation of silver chromate by the interaction of equivalent solutions of silver nitrate and potassium chromate. I t was assumed that precipitation had not taken place if no red substance was discernible. Although mixtures containing sufficient gelatine were clear yellow, considerable quantities of silver chromate were present, e g . 530 x 10-5 N Ag,Cr04 in ;+*4 per cent. gelatine. Under the conditions of theie experiments clear red gels were. never obtained. I t was also found that the s.1ver chromate in the yellow gels diffused like a crystalloid. Sen and Ilhar,z however, state that they failed to reproduce some of the results of the diffusion experiments of IVilliarns and Mackenzie. The latter suggested that calcium was present in sufficient quantity to be a possible factor in causing tile apparently increased solubility of silver chromate. Recent work has empiiasised the fact that the properties of gelatine are greatly influenced by acidity. I n particular Loeb:' has shown that the behaviour of gelatine towards salts in solution depends upon the hydrogen ion concentration. This factor was not considered by either IVilliams and MacKenzie or Sen and Dhar. 'l'he objects of the present investigation were to ascertain the effect of removing the calcium impurity, to study the influence of acidity, Lind to repeat the diffusion experiments.

The Effect of Removing Calcium. The remainder of the geldtine used by ivilliams and MacKenzie was

employed in these experiments and the calcium was removed by means of

'Y.C.S., 1920, 117, 844. a Kolloid-Z., 3, 1~24, 270. See also Dhar and Chatterji, Kolloid-Z., 37, 1925, 2.

Loeb, 7. Gerr. Physiol., Vol. I. (1918-1919), 39 and 237. I 2

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I 5 2 INFLUENCE O F COLLOIDS ON PRECIPITATION

acetic acid. I t was assumed that the whole of the ash was calcium oxide and slightly more than its equivalent of acetic acid was added to a weighed quantity of gelatine. After standing for forty-eight hours the liquid was decanted and the gelatine washed, with frequent change of distilled water, for three to five days. At the end of this time the washings gave no pre- cipitate with ammonium oxalate. Throughout these operations the gelatine was kept at the temperature of tap water. In order to diminish bacterial action the wash water was boiled, and a piece of thymol suspended from the corks of flasks containing gelatine. Owing to the tendency of gelatine to grow moulds, sufficient for the whole of the work could not be washed at once, but the treatment was the same for each sample, The gelatine was dispersed and the concentration of the solution determined by drying a known volume to constant weight at 105' C. Ignition left only a trace of ash which did not colour the flame.

Exactly the same procedure was employed as in the previous work, results being obtained in triplicate.

As before, the appearance of the precipitate varied with the concentra- tion. The formation of the tiny dark crystals at low concentrations rendered difficult exact determination of the point at which precipitation failed to appear. This type of precipitation begins at those concentrations where a dip occurs in the curve. In order to reduce the chances of error a few extra concentrations in this region were examined. Again it was found that in any given series of mixtures there was a concentration of gelatine which appeared to prevent precipitation, while solutions containing more or less than this were ineffective. The phenomenon was quite repro- ducible and is worthy of further consideration. The formation of pre- cipitate in patches on the side of the test tube was very infrequent, owing to greater care being exercised in cleaning the tubes. An interesting comparison was made between a sample of such precipitation, and the normal precipitate which appears in the body of the gel. The latter dis- appeared rapidly, yielding a clear yellow solution when heated to a few degrees above 25' C. The former, on the other hand, was not affected by prolonged heating at a much higher temperature.

The quantitative results obtained are given in Tables I. and 11. and curve II., Figs. I and 2 . In this paper all weights of gelatine are expressed in terms of gelatine dried to constant weight at 105' C. Curve I., Figs. I and 2, is that of the untreated gelatine.

TABLE I. TABLE 11.

Volume.

5000

800 600 550 530 5 0 0 450 400 300 250

1000

200 I00

Gela tine, Per Cent.

0'0 0'001 0'01 0'01 0'10 0'20 0'20 0'20 0'20 0'20

I '00 1-00

0.50

C.C.

0'0

0-9 1'1 2'1

1-4

1.4 1'7

3'8

1'9 5'35

1'2

2'1

2'2

Ag2Cr04 (N x 10- 5).

20

42 51 59 71 76 7s 83 88 95 141 181 242

Gelatine, Per Cent.

0'0 0~00015 WCOI8 0.0030

0.03 9 0'044 0.051

0.068 0.15

0'022

0'0 jQ

Ge1atinelE.W.

0'0 3.6

35'3 50.6 309'7 513.0 564.0 614.5 670.5 713.6 1064.0

1547-0 2149.0

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T. R. BOLAM AND M. R. MACKENZIE I 5 3

Table 111. provides a comparison with the results of Williams and. MacKenzie.

TABLE 111.

Gelatine Treated.

AgfCrO, ( N x 10-5).

Ge1atinelE.W.

3'6 35'3 50%

309'7 513.0 564.0 614.5 670'5

1064.0 1547'0 21,49-0

Gelatine Untreated.

GelatinelE. W.

2-64 26.4 45'3

271.9 494'4 494'4 708.6 531.1

2060'0 2 5 5 4 '4 3872'8

43 5 2 57 70 77 77 h I $6

154 197 297

A4t the higher concentrations much less gelatine is required after treat- ment. The amount ot' the difference falls off with decreasing concentration. Below 80 x 10-j Nsilver chromate, it would appear as if the reverse state of affairs prevailed, but experimental error may quite well account for this apparent anomaly. I t is obvious that the calcium salt in the untreated gelatine was not responsible for preventing the precipitation of silver chromate.

Cone. Ag,Cr04 in gram-equivalents/litre + FIG. I.

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INFLUENCE OF COLLOIDS ON PRECIPITATION

800 500

I00 200

P

0 01 0 ' 2 0 2 '00 3-00

Cone. Ag2Cr04 in gam-equivalent/litre - + Cone. Ag2Cr04 in gam-equivalent/litre - + FIG. 2.

The Influence of Acidity.

The effect of alteration in the acidity of gelatine on its power of preventing precipita- tion was therefore next studied.

We are indebted to Mr. Bruce of Messrs. Cox, Ltd., for the trouble he took in producing a second batch of gelatine as nearly as possible the same as that used in the previous work. This second batch will be referred to as gelatine B, and the first batch as gelatine A. Gelatine A gave 17-6 per cent. of moisture when dried to constant weight at Iojo C. and 1.2 per cent. of an ash consisting mainly of calcium oxide \vith some calcium sulphate. Gelatine B gave 17.65 per cent. moisture and 1.0 per cent. ash which contained less sulphate. Gelatine B was darker in colour than gelatine A, but this was iiot due to iron as the ash was pure white.

Despite the similarity in composition gelatine J3 proved to be less effective than gelatine A in pi-eventing precipitation. Determinations were made at four representative concentrations. The results are shown in Tables IV. and V. and in curve HI., Figs. I and 2.

The gelatine after treatment was found to be more acid.

I t became necessary at this point to procure more gelatine.

TABLE IV. TABLE V.

I

C.C.

1'1 1'2

2'7 5'6

Gelatine Per Cent. Gelatine1E.W.

29.1 393'6

3567'0 7713.0

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T. R. BOLAM AND M. R. MACKENZIE 155

AgLCr04 ( N x 10-5).

Some gelatine B was treated with acetic acid and washed free from calcium and more acetic acid added. The inhibitive influence of this gelatine on the precipitation was determined for four concentrations. The appearance of silver chromate in the dark crystalline form occurred at higher concentrations than previously. The results are given in Tables VI. and VIX. and curve IV., Figs. I and 2.

TABLE VI. TABLE VII.

Gelatine. Per Cent.

I I

so0 5 30

I00 200

0’001 2.4 0’010 3’0 0’200 I ‘5 0-500 2.6

42 62 192 329

I

0.0033 0.0038 0’0462 0.1711

Gt1atinelE.W.

7 ‘6 613

240’6 520.1

With one concentration of the reactants, viz., ~ V / I 00, determinations

I. Gelatine B treLited with acetic acid and washed, 2 . Geiatine €3 treated with acetic acid, 3. Gelatine B purified by precipitation with alcohol (Bogue),l 4. Coignet’s Gold Label Gelatine, 5. Coignet’s Gold Label Gelatine treated with acetic acid and washed. The results will be found in Table IX. The determination of the P, of the various gelatines was carried out

(a) Walpole’s Sodium Acetate-Acetic Acid Mixtures :-Methyl Red. (6) Clark and Lub’s Acid potassium phthalate Mixtures :-Methyl Red.

(c) Sgrensen’s Phosphate Mixtures :-{ (d ) Sdrensen’s Sodium Citrate-Hydrochloric Acid Mixtures :-Methyl

Red and Methyl Orange. t and d were standardised by the quinhydrone electrode and a and b by

colorometric comparison with them. The values for P, given in Table VIII. are those of the standard

solutions between which the gelatine lay, except in the cases where the colour of the gelatine was identical with that of a standard solution. The experiments were done in duplicate.

I t will be observed that dilution does not appreciably alter the P, except in the case of the very acid gelatine B. This is in accordance with the amphoteric nature of gelatine. The exception would appear to be due to alteration in the dissociation of the acetic acid.

Table IX. shows the relation between P, and concentration of gelatine necessary to prevent precipitation of approximately equal concentrations of silver chromate.

I t is obvious from these data that the efficiency of a gelatine in pre- venting precipitation is much increased by increase in acidity. Loeb has. shown that P, = 4.7 is critical for many physical properties of gelatine. I t appears to be without significance in the experiments here described.

The question arises whether the apparent increase in solubility is not due entirely to the acidity of the medium. Solution of silver chromate in

were made with

colorirnetrically, using the following buffer solutions and indicators :-

Brom-cresol-purple. Methyl Red.

‘‘ The Chemistry and Technology of Gelatine and Glue ” (1922), p. 62.

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I 56 INFLUENCE OF COLLOIDS ON PRECIPITATION

.acid is generally attributed to the formation of the more soluble dichromate, but this should be indicated by a change in colour on addition of gelatine t o the potassium chromate. Only in the case of the very acid gelatine was

TABLE VIII.

Gelatine.

Gelatine A. Curve I. . . . . 9 , 9 9

9 , ,t

9 , 9 9

. . . . . . . . . . . . Gelatine A. Curve 11. . . .

9 , 9:

9 , 9 9

. . . . . . . . Gelatine B. Curve 111. . . .

?, ,, 9 , > v

?, 9 ,

. . . . . . . . . . . . Gelatine €3. Curve IV. . . .

9 , ,, 9 , 9 9

9 9 9 3

I , 9 ,

. . . . . . . . . . . . . . . . Gelatine B treated with H S and washed

,, B acidified by HAc . . ,, B precipitated by alcohol .

Coignet's Gelatine . . . . 9 9 9 ,

7 1 9,

. . . .

. . . . Coignet's Gelatine treated with H x c and

washed . . . . . . . Ditto. . . . . . .

-

TABLE IX.

Gelatine.

Gelatine B acidified Hxc . ,

,, ,, treated HAcand washed ,, A (Curve 11.) . . .

Coignet's treated HAcand washed . Gelatine A (Curve I.) . . . Coignet's . . . . . Gelatine B precipitated by alcohol .

,, ,, (Curve 111.) . . .

,, ,, (Curve IV.) . . . 347 329 281 242 29 4 I97 217 258 236

Concentrat ion Per Cent.

2'000 1'000 O'KOO 0'010

1'000 0'100 0'010

1'000 0'100 0'010 0'001

1'0000 O'IOOO 0'0100 0'0010 0 '000 I

1'0

1'0 1'0

3'0 1'0 0'01

1'0 0'01

3 '9 3 '9 4'5 4 '6 4'7 5'0 5 '45 5 '7 5 '75

'h'

5'0 4'9-5'0 4'9 5 -0-5 * 1

4'6-4'7 4'6 4'7-4'8

5'7-5'8 $ 9 9 ,

7 ) 9 9

,, 1 I

3.8-3.9 3 '8-3 '9 4-1 6-42 4'6-4'9 5.1-5'3

4 '5 3 '9 5'7 5'4 5'4 5 '3

4'7 4'85

I' I Per Cent. Soh tion.

h, Gelatinel E. W.

441 520

1559 2149 2803 2554 4087 5430 7713

there any deepening in colour and then only to a slight extent. I t must also be emphasised that while the amount of silver chromate which is not precipitated increases with increase in gelatine concentration, the P, remains constant. I t seemed, however, desirable to find what was the action of an aqueous solution of acetic acid of the same P, as the gelatines.

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T R. BOLAM AND M. R. MACKENZIE

Volume.

200 I00

I5 7

Acetic Acid. Normality.

' h . Normality. C.C. Ag2Cr04. Acetic Acid.

0'0001 6 '5 33-3 x 10 - 8-7 x 10 - 4-3-4.45 0'001 4'6 89.3 x IO- 8.2 x 10-* 3-7-3'8

Two very dilute solutions, approximately O*OOOI N and 0.001 N, of acetic acid were prepared. The same procedure was followed as in experi- ments with gelatine, using, however, I C.C. of each of the reactants, silver nitrate and potassium chromate (N,200 in the case of 0-0001 N H A c and T/IOO in the case of 0.001 N H Z ) , and varying the volume of the acetic acid. The mixtures were placed in the thermostat and readings taken at the end of seventy-two hours, Here also precipitation was often delayed for a considerable length of time and the silver chromate always appeared in the form of small dark crystals. Results were reproducible within 0.2

C.C. acetic acid. The P h values of the acetic acid solutions just preventing precipitation were determined colorimetrically and the values are given in Table X.

TABLE X.

If we compare these figures with those already obtained we find-

Gelatine B-0.171 I per cent., Ph 3-65, prevents precipitation Of 329 x 10 - N Ag,CrO,. Gelatine A-0.1648 per cent., P h 5.0, prevents precipitation of 115 x IO- N Ag,CrO,.

Increase 214 x I O - ~ N Ag,CrO,. Acetic acid-8.2 x I O - ~ N, Ph 3-75, prevents precipitation of 89-3 x 10 - N Ag,CrO,.

It would appear from these figures that the greater acidity produces in t h gelathe some change which increases its power of preventing the pre- cipitation of silver chromate.

Diffusion Experiments.

Sen and Dhar' found that five per cent. gels of Nelson's photographic gelatine containing N/2 jo or N/330 silver chromate were yellow and stated that these mixtures did not turn red when potassium chromate diffused into them. On the other hand, Williams and MacKenzie2 carried out many similar experiments which resulted in the formation of banded precipitation.

All the experiments were conducted at room temperature, and the gels protected from the strong sunlight. Precipitation always occurred on the side towards the window if this precaution were not taken. I t was found that, at room temperature, 3 per cent. and 1-82 per cent. gelatine B just pre- vented precipitation of N/300 and A7434 silver chromate respectively and these concentrations were selected for investigation.

I t appeared desirable to repeat this work, using gelatine B.

'l'ypical results are set out in Tables XI. and XII.

1 L O C . c i t . a LOC. cit .

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158 INFLUENCE OF COLLOIDS ON PRECIPITATION

Expt.

Expt.

TABLE XI.

Gel.-o'oo23 N Ag,CrO, in 1'82 per cent. gelatine B. Duration of Experiment.-3 days.

Solution above Gel.

N / 2 AgNO,

N / I O AgNO,

N l P &9O,

N / 2 K,CrO,

N/IO K,CrO,

N/50 K,CrO,

N/IOO K,CrO,

Result of Diffusion beneath Surface (in order of depth).

Precipitate at interface : 0'2 cm. general precipitation : 3 crns. ordinary close rings : haze.

Precipitate at interface : I cm. close rings : general pre- cipit at ion.

Precipitate at interface: 0.5 cm. close rings: bands of dense precipitation alternating with bands where precipi- tation is less dense.

Precipitate a t interface : deep band general precipitation : deep band of much less dense precipitation : deep band of dense precipitation: deep band of much less dense precipitation : deep band of dense precipitation.

Precipitation a t interlace : deep band general precipitation faintly marked with bands: banded structure as in expt. 4.

No precipitate at interface : 0.3 cm. clear yellow gel : r i n g 0.1 cm. deep (started at centre of gel and grew out): 0-2 cm. clear yellow gel : ring (started at wall of tube) : band of clear yellow gel deeper than first: two mare rings formed with clear yellow space between still deeper.

Precipitate at interface: verv fine ring : 0.5 cm. clear yellow gel : ring 0'2 cm. deep : 0.55 cm. clear yellow gel : ring 0.2 cm. deep.

TABLE XII.

GeZ.-om0333 N Ag,CrO, in 3 per cent. gelatine B. Duration of Experiment.-3 days.

;ohtion above Gel.

N/2 AgNO,

N / I O AgNO,

N/50 &NO,

N / 2 K,CrO,

N/IO K,CrO,

Result of Diffusion beneath Surface (in order of depth).

Precipitation at surface: 3-3 cm. close rings: general precipitation.

Precipitation at surface: I cm. very close rings: general precipitation.

Precipitation at surface : wider rings : bands of dense and less dense precipitation (as in expt. 4, Table XI.)

No precipitation at interface : 0.2 cm. clear yellow gel : 2.1 crns. general precipitation : three bands (0'2 cm.) less dense precipitation alternating with bands dense precipitation, cf. Expt. 4, Table XI.

No precipitation at interface : 0.3 cm. clear yellow gel : ring 0.1 cm. (started in centre of gel and grew out): 1.5 crns. clear yellow gel : 2nd ring . . .

Altogether 5 rings form with clear yellow interspaces gradually widening, very like expt. 6, Table XI, but corresponding rings and spaces thinner.

No precipitation at interface: 0-3 cm. clear yellow gel: ring 0.1 cm. deep: 0.4 cm. clear yellow gel : 2nd ring : 0.5 cm. clear yellow gel : 3rd ring.

Slight precipitation at interface : 0-7 cm. clear yellow gel : ring about 0-1 cm. deep.

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T. R. BOLAM AND M. R. MACKENZIE I 5 9

Fig. 3 shows diagramatically the type of ring formation produced by silver nitrate and Fig. 4 that produced by potassium chromate.

Precipitation was more rapid in the case of silver nitrate. N/2 solutions of sodium nitrate, potassium nitrate and zinc nitrate

superposed on 3 per cent. gels containing N/300 silver chromate caused no precipitation even after keeping for several weeks.

Sen and Dhar found chromate but failed to find any silver in layers of gelatine or water superimposed on their silver chromate gels even after the lapse of a week. Williams and MacKenzie detected the presence of both silver and chromate.

The next step, therefore, was to repeat the experiments on diffusion into gelatine filters both in U tubes and test-tubes. Gels containing 3 per cent. gelatine B and N/300 silver chromate, and two types of filter, ziiz., (a) 3 per cent. gelatine, (6) 3 per cent. gelatine containing Ny300 potassium nitrate, weie used. In the test-tube experiments the gelatine filter (2.5 cms. deep) was added in two portions, the first being allowed to set before pouring on the second. The whole operation was conducted at as low a temperature as possible. This procedure minimised any disturbance of

AgNO, superposed. K,CrO, superposed. F I G . 3. FIG. 4.

the silver chromate gel. I n a very few hours the presence of chromate throughout the filter was evident from the colour, which deepened with time but in the course of an experiment never reached the same intensity as that of the silver chromate gel. After five days N/2 silver nitrate, N/z potassium chromate and N/2 potassium chloride were poured on the top of the filters. Immediately on addition of the silver nitrate a red precipitate formed at the surface of the filter. I t extended down for I cm. and below this formed patches which developed into broken rings. The potassium chromate produced no precipitate until it had penetrated some distance below the filter-silver chromate gel interface. Some time after the addition of the potassium chloride a white precipitate began to form a short distance (0.5 cm.) below the surface of the filter. This grew into a band of continuous precipitation below which a number of fine white rings appeared. That the precipitate was silver chloride was confirmed by its rapid darkening when placed in bright sunlight. The presence of potassium nitrate in the filter appeared to be without influence.

A set of tests was made, in test-tubes only, with filters 2 cm. deep. When to these was added, after five days, N,z potassium chloride precipita- tion of silver chloride occurred immediately and at the surface of the filter. Silver nitrate and potassium chromate gave the same results as before.

For purposes of comparison 3 grams gelatine B were dispersed in

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I 60 INFLUENCE OF COLLOIDS ON PRECIPITATION

I 00 C.C. of saturated aqueous solution of silver chromate and allowed to set. N/z silver nitrate and N/2 potassium chloride were poured on different portions. The former produced only a red coloration and the latter a very faint opalescence. Thus the amount of silver chromate which had diffused into the gelatine filter was much greater than that present in the saturated aqueous solution.

Discussion of Results.

These results confirm those of Williams and MacKenzie in most respects. The exceptions are that potassium chromate in some cases did not produce precipitation at the interface when superposed on silver chromate gel, and in no case gave a precipitate in a gelatine filter. How- ever, the presence of silver in ttie gelatine filters was shown by the formation of silver chloride.

Sen and Dhar suggest that the silver detected by Williams and Mac- Kenzie was due to the silver nitrate solution, used in preparing the gels, being slightly more concentrated than the potassium chromate. Conduc- tivity determinations showed that the solutions used by us were accurately .equivalent. Further, it was observed that although in every case the filtering gelatine became yellow, showing chromate, yet silver chloride was precipi- tated in it on addition of potassium chloride. In the U tube experiments portions of the sume gel were used, and the filters gave evidence of the presence of both ions by precipitation with silver nitrate and potassium chloride.

The gels showing diffusion of silver ion contained just sufficient gelatine .to prevent precipitation (N/300 silver Chromate in 3 per cent. gelatine), whereas those of Sen and Dhar probably contained more than sufficient (N/zjo and N/330 silver chromate in 5 per cent. gelatine). Further ex- perimental evidence (see Part 11.) suggests that this may account for the differences observed. The gels used by Williams and MacKenzie con- tained a still higher proportion of silver chromate than those used in our woi k.

Our observations support the view that a large proportion of the silver chromate diffuses like a crystalloid. Now, in filters 2 - 5 cm. in depth silver chloride was precipitated 0.5 cm. below the surface when potassium chloride was superposed, whereas the more soluble silver chromate was precipitated a t the surface by potassium chromate. I t thus seems thzt the silver does not diffuse into or through the gelatine filter so rapidly as the chromate, which suggests that silver and chromate are moving independently, possibly as simple ions. Loebl has shown that the more alkaline a gelatine the greater is the proportion of silver which is not removed from it by washing. Williams and MacKenzie obtained a precipitate of silver chromate in the filtering gelatine when potassium chromate was added (the amount of pre- cipitate being, however, not so great as that produced by silver nitrate). We could not detect silver except by precipitation of the less soluble silver chloride. Gelatine B is more alkaline than gelatine A and it is possible that some effect related to that observed by Loeb is accountable for the difference. Sen and Dhar do not state the P, of their gelatine.

Contrary to the experience of Sen and Dhar precipitation followed the diffusion of potassium chromate into the silver chromate gels. This again may be due to the different proportions of gelatine employed. Rings,

1 L O C . cit.

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T. R. BOLAM AND NI. R. MACKENZIE 161

separated by gradually widening interspaces, were formed when the concen- tration of the superposed potassium chromate was not too great. The importance of concentration in the production of rings is shown by the unusual formation of silver chloride rings in the gelatine filter. This is probably due to the very low concentration of silver.

'The yellow gels were examined with the ultramicroscope which revealed only an occasional bright particle. The particles were observed with gelatine in the absence of silver chromate and were probably due to dust. Hence if any colloidal aggregates were present they must have been amicronic. However, if precipitation by silver nitrate and potassium chromate be due to coagulation of a colloid, then sodium nitrate, potassium nitrate and zinc nitrate should also have precipitated the silver chromate, which they did not do. So far we have obtained no evidence that the silver chromate in these gels was in the colloidal state.

This problem of the condition of the silver chromate will be further dealt with in Part 11.

Summary .

not dependent on the presence of calcium. I . The power of gelatine to prevent precipitation of silver chromate is

2. The efficiency of the gelatine increases with increasing acidity. 3. The silver chromate present in gels containing sufficient gelatine to

4. The silver and chromate diffuse independently. j. There is no evidence of the presence of silver chromate in the

prevent its precipitation diffuses like a crystalloid.

colloidal state.

Chem is t ty Department, King's BuiZdiqgs,

University, Edinburgh.

Publ

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01

Janu

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1926

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by U

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