OBSERVATIOSS ON T H E 3IECHAXISlI O F F O R l I A T I O S OF COLLOIDdL SILVER* BY H E L E N QUINCY WOODARD
The earliest theory of the formation of colloidal metals by the Rredigl method postulated a simple thermomechanical process of evaporation of the metal in the arc, and subsequent condensation of the vapor to particles of colloidal size in the surrounding fluid. The work of Beans and Eastlack’ and of subsequent workers in Beans’ l a b ~ r a t o r y ~ ~showed ~ ~ ~ ~ this ~ * ’ explanation , to be inadequate, and established the formation of a gold-anion “complex” as necessary to the formation of colloidal gold. The work of Pennycuick8 on colloidal platinum and of Eirich and Pauli9 on colloidal gold shows that in the preparation of Bredig sols of these metals complex acids are formed through the action of the electric arc. I n the present paper evidence is presented to show that the formation of colloidal silver by the Bredig method involves the reaction of silver with the electrolyte present according to the ordinary laws of physical chemistry simultaneously with the formation of a metal-anion “complex”. S o assumptions are made as to the nature of this “complex”. Apparatus and Materials These were the same as those reported in previous papers’Ogll. Method The sols were made, centrifuged, and analysed in the same manner as previously described”. pH determinations were made colorimetrically with suitable indicators to +o.z pH. Titrations for acid and alkali were made Thiocyanates XaOH or HCI, and were accurate to =t 7”;. against S,’IOO were determined by titration against standard h g S O 3 to i z c ; . Sulfates were estimated to i 10~: by the turbidity produced with S,’IO BaiYOa)?. Sitrites were determined to i 45: by titration at 35OC. against standard K l I n 0 4 . Sitrates were estimated colorimetrically to i j c C with diphenylamine reagent. Oxalates were estimated to i 1 0 5 by titration against standard K31n04. (All precisions are given for determinations made on 3 cc. of ,001j?; reagent. Khere the concentration was less the precision fell accordingly). Results As explained in a previous paper, when an arc is maintained between silver electrodes in a suitable solution, colloidal silver is formed, increases to a maximum of concentration as arcing continues, and then, upon further arcing, falls t o zero. Table I gives the results of examination of the liquid
* From the Huntington Fund for Cancer Research, Memorial Hospital, Xew York City.
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H E L E S QUINCY WOODARD
remaining after this final precipitation of the sol. The fate of the cation of the initial solution was followed by titration or pH determinations. Direct tests were made for some of the anions. Other anions were not determined directly, but are reported as silver salts because of the recovery of dissolved silver in sufficient amount to satisfy the anion originally present. Sol formawhen stable sols were formed regularly; when tion is reported as the sols formed were concentrated and stable over a long arcing time; “ - ” when sols were never formed; and “ *’’when the sols formed were extremely dilute, unstable on standing, or very sensitive to slight changes of technique. An examination of the table suggests the following theory of sol formation:-In nearly all cases when arcing takes place in distilled water or in an electrolyte solution, silver reacts with the water sufficiently to saturate the solution with AgOH. This is shown by the fact that, except in certain special cases, the liquid remaining after a sol has been arced to precipitation has a silver concentration of .ooozN - .0003KJand a pH of about 9. j . Two other processes take place at the same time. During arcing in a suitable electrolyte solution, a sol forms and precipitates according to the “complex” theory. I n addition, silver combines with the anion present to form a silver salt. If the silver salt is highly insoluble, as AgCKS, hgC1, or Ag&, then the available anion is removed so fast from the solution that little can be used to stabilize the sol, and the sol is dilute. Qualitative evidence in favor of this hypothesis is furnished by the presence of a white deposit, presumably silver salt, in the sludge thrown down when sols prepared in XaCNS and NaCl or solutions are centrifuged. If the silver salt is sparingly soluble, as AgzCr04,then the anion is removed slowly from the system, and is available in considerable concentration long enough to stabilize concentrated sols. (Data for silver sols stabilized by K2Cr04 are not given in the table because the chromate ion is so good a stabilizing agent for colloidal silver that very prolonged arcing was necessary to precipitate the sols. This rendered quantitative work impossible). If the silver salt is moderately to highly soluble, as AgNOs, CH3C00Ag, or Ag2S04,then the disintegrated silver reacts with the electrolyte instead of dispersing, and no sol, or an exceedingly dilute sol, is formed. In this case the anion remains approximately constant in concentration during arcing (Pu’oa-),or decreases slowly (Sod-). The decrease is probably due to adsorption on the sludge. This is evidently an electrolytic process, since only minute traces of silver (.oooo4N~are dissolved when the electrodes are left in contact with the arcing solution with the current not flowing. Interesting results are obtained when arcing takes place in the solution of an electrolyte whose anion is capable of forming a highly soluble silver salt. When the electrolyte is an acid, as H2SOd or CHsCOOH, silver salt forms. The final solution is slightly alkaline, and contains sufficient silver to neutralize the acid and to saturate the solution with AgOH. When the electrolyte is an ammonium salt, replacement takes place, the silver goes into solution as silver salt, and the ammonium appears as hydroxide (NH4N03 and (XH4)&304). This is in contrast to the condition when arcing takes place in
“+”
“++”
THE MECHANISM O F F O R Y A T I O S O F COLLOIDAL SILVER
42 7
solutions of K a S O s or NazS04. Here the silver can not replace the sodium as it does the ammonium, and only dissolves sufficiently to saturate the solution with AgOH. As the anion is removed from the solution as salt or precipitated “complex”, the cation is set free to form hydroxide almost wholly (KOH, KaOH, KaCSS, Na2C03,KH4C1,KaC1, ” S O 3 , ( S H 4 ) 8 0 4 K2Cz04, , CH3COONa), or in part (XasSOd, SazS). The sodium from KasS04which is not recovered as hydroxide apparently remains in the solution as NaHS04, since there is
wt. A b d , > i n : c $ r a t r d p a r
IOOLL.
201
f o r m e d -$MS.
Change in Electrolytes during Arcing A -‘S$ stabiljzed k y ,001s-N XaCNS. B ,001.5N HC1. O = concentration sol silve; in percent. x = ” thiocyanate in normality. 0 = Alkalinity in Fig. I A; acidity in Fig. I B.
sufficient sulfate ion in the solution to account for this. The fate of the portion of the sodium from S a z S which is not recovered as hydroxide remains unexplained, although it might possibly be accounted for by preferential adsorption of NaHS on the sludge. If the electrolyte in the initial solution is an acid, the released H+ disappears as HzO (HCl, HzS04, CH3C‘OOH). This process was followed directly with two electrolytes, S a C S S and HCl. Samples were withdrawn at intervals during sol formation. The samples stabilized by N a C S S were precipitated by solid r\’aS03, centrifuged, and the thiocyanate in the supernatant liquid was determined. The samples stabilized by HCl were titrated against S i 1 0 0 KOH, the sols being diluted sufficiently so that a fair end-point was obtained. As no other acid was present, this gave a good indication of the concentration of HCl in the sol. A few direct determinations of the chloride content of the supernatant liquid remaining after
428
H E L E S QUISCY WOODARD
the precipitation of these samples with KaXO3 were also made. These checked the results of direct titration within the limits of experimental error. The results are given in Fig. I. I t will be seen that the anion of the original solution disappears a t about the same time as the precipitation of the sol in each case, while the alkalinity of the solution reaches its maximum a t about the same point. This confirms the findings of Shear:, Amsden7, and Eirich and Pauli9 for colloidal gold.
-
-v
0
A = SEIS stabtjzed ),y B =
c
D
= =
,001j ,001j
S SaCSS.
N HCI.
”
”
”
,0015 N H?SO,.
’I
”
”
,001j
N CH3COONa.
As the solubility of most silver salts increases markedly with temperature, it was possible to make direct tests of the influence of the solubility of the salt which silver is capable of forming with the anion of the stabilizing electrolyte, as is shown in Fig. 11. Silver sols were made a t the temperature ranges 15’-35’C., and 4o0--6o0~., in H 2 S 0 4and CHSCOOSa, and in 5’-15’C., X a C S S and HCl. The first two yielded anions whose silver salts are above the most favorable solubility for sol formation; the silver salts with the anions of the second two electrolytes are below the most favorable solubility for sol formation. I n Fig. I1 the maximum concentrations of the sols formed are plotted against the average temperature. TWO effects are evident in the curves. Rise of temperature in itself evidently favors sol formation, as is shown by the initial rise in all the curves. The favorable effect of rise in temperature is largely offset by the increasing solubility of the highly soluble
THE JlECH.4SISY O F FORMATIOX O F COLLOIDAL S I L T E R
429
h g 2 S 0 4and CH3COOXg. On the other hand, the increase in the low solubility of XgCSS and AgCl causes a marked rise in the curves for sols stabilized by S a C S S and HC1. The difficulty of maintaining an arc below 5°C. or above 60°C. rendered it impossible to follow these curves further. Several points in the data remain unexplained by the theory presented above. The most important of these relates to the behavior of silver sols stabilized by KOH and S a O H . The alkalinity of these sols remains nearly
.L
wr, ~b
3
d i i ~ n t e g r a t e hper
,1
.9
,b
ioo c c s o l f o r m e d - g m s
FIGSIIIa and IIIb Successive Sols made in the same Solution a = fist sols stabilized by ,001j N NaOH. b = second sols stabilized by .oor5 N NaOH 0 = sols in NaOH remaining after arcing in .0015 N KaOH. 0 = sols in SaOH remaining after arcing in . o o q N NaC1. x = sols in XaOH remaining after arcing in .oorg N KazC03. constant during arcing and after the precipitation of the sol. If the cation is released during arcing and reforms the hydroxide as fast as released, then the electrolyte remains essentially unchanged. There is thus a constant supply of hydroxyl ion available for sol stabilization. One would therefore expect that the concentration of sol silver would remain constant a t its maximum even after prolonged arcing. This is not the case, however, since silver sols stabilized by KOH or S a O H may readily be arced to precipitation. Further, KOH is formed during arcing in solutions of K2C204, and NaOH
43 0
HELES QLXNCY WOODARD
c. .3 C
T H E MECHAXISM O F FORMATIOX O F COLLOIDAL S I L V E R
43'
remains in solutions of XaCSS, NaCl, and Na2COafrom which the anions have been removed by sol formation and precipitation, yet prolonged arcing in these alkaline solutions does not result in the formation of sols stabilized by hydroxyl ion. I t seemed possible that the explanation of this anomaly lay in the presence of temporarily suspended sludge in the solution after the precipitation of the sol. This might provide nuclei for the condensation of metal vapor from further arcing, and so prevent the formation of stable colloid. That this explanation is correct is shown by the following experiment. Sols were prepared in . O O I S?\' S a O H , .OOISN SaCl, and . O O I S ~ SanCOs, and were arced to precipitation. The liquid remaining was then centrifuged to remove sludge. After centrifuging, the liquid was colorless, or retained a faint green color from traces of colloidal silver. This liquid was then used for the preparation of further sols. Sols so prepared were poorly reproducible, probably owing to the presence of traces of suspended silver. They had, however, approximately the same maximum of concentration and position of maximum as original sols made in ,00159 XaOH. This is shown in Fig. 111. I t was possible to repeat the process a third time with the liquid remaining after the precipitation of the second sols. Further evidence in support of the above explanation was obtained by arcing to precipitation sols stabilized by ,0015 X HC1. The liquid remaining after the precipitation of these sols only contained sufficient hydroxyl ion to bring the pH to 8.3-8.8, and no further sols could be formed in it even after prolonged arcing. An observation which remains unexplained is that KzCZOd and NaKOs are not good stabilizing agents for colloidal silver, although their anions form salts with silver which are in the range of solubility favorable to sol formation. The theory in its present form is inadequate for these cases.
summary Evidence has been presented to show that the formation of colloidal silver by the Bredig method involves the effect of the laws of physical chemistry.
Bibliography Bredig: 2. angew. Chem., 11, 951 (1898). 2 Beans and Eastlack: J. Am. Chem. SOC., 37, 2667 (1915). 3 Beaver: Dim Columbia (1921). 4 Davidson: Diss., Columbia (1924). Shear: Diss., Columbia (1925). a Layton: Dim, Columbia (1926). ' Amsden: Diss., Columbia (1926). 8 Pennycuick: J. Chem. SOC., 1927, 2600. 9 Eirich and Pauli: Kolloidchem. Beihefte, 30, 113 (1930). 10 Woodard: J. Am. Chem. SOC.,50, 1835 (1928). l1 Woodard: J. Phys. Chem., 34, 138 (1930). 1
