T H E MECHANISM OF THE FORMATION OF KOHLSCHUTTER’S SILVER SOL HARRY B. WEISER
AND
MAX F. ROY
Department of Chemistry, The Rice Institute, Houston, Texas Received M a v $8, 1959
The colloidal dispersion commonly referred to as Kohlschutter’s silver sol (1) is prepared by conducting a stream of hydrogen into a solution of silver oxide containing an excess of the solid oxide and maintained a t a temperature of 50-60°C. Kohlschutter believed that the reduction took place on the walls of the vessel, both because the color of the sol varied with the nature of the vessel, and because in glass vessels of a given kind the velocity of the reduction was apparently directly proportional to the surface of the vessel exposed to the solution and inversely proportional to the volume of the solution. Thus in Thuringen soft glass and in quartz the sols were usually yellow-brown in color, and in Jena glass they varied from red through reddish-brown to violet and blue. In every case sol formation was accompanied by the deposition of a heavy black “mirror” of metallic silver on the vessel walls. The solubility of the glass was not regarded as an important factor in determining the nature of the sol, since the color of a sol formed in a certain kind of glass was the same even though the silver oxide solution used had been previously allowed to stand for a considerable time a t 60°C. in another kind of glass. Kohlschutter found that relatively large amounts of unreduced silver oxide remained in a sol even after prolonged treatment with hydrogen. The excess oxide may be removed by passing hydrogen through the sol contained in a platinized platinum vessel protected from the air with a bell jar. By this procedure the excess oxide was reduced, the silver depositing on the platinum vessel. The conductivity of the sols was reduced t o 4 to 8 X mhos, which was approximately one-tenth that of the original sols without any appreciable change in their appearance. On analysis he found, however, that the amount of silver in the colloidal particles had decreased during the purification process, and from this he deduced that the colloidal particles in the original sol contained some oxide. Since in general the particles in the yellow-brown sols contained more oxide than the particles in the reddish to blue sols, Kohlschutter claimed that the walls of the containing vessel determine the ratio of silver to silver oxide in a sol, as well as its color. 1009
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HARRY B . WEISER AND MAX F. ROY
Erlach and Pauli (2) observed that a sol prepared by Kohlschutter’s method, using hydrogen from a Kipp generator, always contained sulfur which came from hydrogen sulfide present in the hydrogen. Starting with pure silver oxide and pure electrolytic hydrogen there was said to be no sol formation in a hard glass or a silver vessel, but if a trace of potassium carbonate, sodium hydroxide, or ammonium hydroxide was added to the silver oxide solution a stable sol resulted. As a matter of fact, sols were obtained with electrolytic hydrogen, formed by electrolysis of sodium hydroxide, unless the gas was washed with concentrated sulfuric acid which was supposed to take out “alkali mist.” The effect of alkali was attributed to the formation of an argentate, such as NaAgO, which served as the stabilizing electrolyte for the sol. Tn support of this they confirmed Kohlschutter’s observations that the colloidal particles contained up to 20 per cent of unreduced silver. The stabilizing effect of a trace of hydrogen sulfide was attributed to the formation of some silver complex, but it is not obvious what this would be or how it would form. Pauli’s explanation of the necessity for sol formation of having either alkali or hydrogen sulfide in the reduction mixture is open to question. Pauli was unable to prepare a sol with pure silver oxide and pure hydrogen, but he was also unable to prepare a stable sol by the Bredig method in pure water. Best and Cox (3) had no difficulty with the latter preparation when they hit upon the right conditions and it seemed probable that a sol would result with pure hydrogen and silver oxide if one knew how to do it. As a matter of fact, the experiments to follow will disclose that silver sol formation by Kohlschutter’s method is not determined either by the catalytic effect of the walls of the containing vessel, as stated by Kohlschutter, or by the presence of a substance capable of forming a complex negative stabilizing ion, as assumed by Erlach and Pauli. EXPERIMENTAL
Procedure and reagents The experimental procedure consisted simply in conducting hydrogen into silver oxide solutions in different kinds of glass vessels a t a temperature ranging from 50-60°C. and noting what took place. Special attention was taken in the preparation and handling of the reagents. The silver oxide was precipitated from N/10 silver nitrate with a slight excess of N/10 sodium hydroxide, and washed ten times by decantation with boiling distilled water. A saturated solution was prepared by continuous shaking of a large excess of the solid oxide with 250 cc. of water for three to four hours. The entire process was carried out in the dark and the solutions were stored in the dark. Conductivity water from block tin stills was used throughout. The hydrogen was prepared by the process of Cooke and Richards (4).
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The generator which contains the system, zinc amalgam-dilute hydrochloric acid-platinum, has been found to give hydrogen whose only impurity is a small amount of gaseous hydrogen chloride. To remove the latter the gas was passed over solid potassium hydroxide, then washed through distilled water and, as a final check on its purity, it was passed through a wash-bottle containing silver oxide solution before being conducted into the silver oxide solution in the reduction vessel. The reduction vessels were test tubes with a hydrogen inlet tube of the same material. This type of vessel was chosen since it required the use of less of the pure reagents and assured thorough mixing and more intimate contact between the reacting compounds and the vessel walls. All connections in the gas train were glass to glass with one exception. Since a quartz reduction vessel and inlet tube was employed in some cases, the end of the quartz inlet tube was held against the end of the glass tube from the wash train by a short piece of gum rubber tubing which had previously been cleaned in a boiling sodium hydroxide solution and soaked in melted paraffin in a, vacuum so that as the pressure was released into the evacuated space, the pores of the rubber were filled with the paraffin. This treatment of rubber has been found quite satisfactory in microanalytical work to prevent the rubber from giving off organic vapors and hydrogen sulfide. The same method of connection was used for all types of inlet tubes, for the sake of uniformity of procedure. The reduction mixture was kept a t the desired temperature by immersing in a water bath consisting of a 1-liter beaker of water heated by the flame from a microburner. The temperature was readily maintained within the desired limits and observations were easily made. A few drops of paraffin oil on the surface of the water decreased evaporation and this facilitated the maintenance of a constant temperature for long intervals. Between runs the reduction vessels and the hydrogen inlet tubes were thoroughly cleaned as follows: The vessels were (1) soaked in concentrated nitric acid for six to twelve hours, (2) boiled with concentrated nitric acid for 5 minutes, (3) washed with distilled water and soaked for twelve to fifteen hours in several changes of distilled water, and (4) steamed for 5 minutes just before using.
Reduction of saturated silver oxide in the presence of excess solid In order to test the effectof the containing vessel on the nature of the sol, mirror formation, and the action on solid silver oxide, observations were made with vessels of soft glass, Pyrex, and quartz. The results are summarized in table 1. At the end of each of the above experiments the excess solid silver oxide was collected and, after washing several times with distilled water, it was found to be only partially soluble in dilute nitric acid (0.01 N ) and in aque-
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ous ammonia. The residues were washed and found to dissolve in concentrated nitric acid. From this it is apparent t.hat the solid particles were partially reduced to metallic silver, the process being retarded gradually as the surface became coated with a silver film. Kohlschutter's statement that the solid silver oxide was not acted upon is obviously not in accord with the facts. Although the differences in the colors of the sols were quite apparent, they were not as marked as would have been expected from Kohlschutter's report. TABLE 1 Reduction of saturated silver oxide solution containing a n excess of solid silver oxide Temperature 55-58°C. OBSERVATIONS REDUCTION VESSEL
After a few minutes
Soft glass
Weak sol; silver mirror on walls.
Pyrex
Weak sol; silver mirror on walls.
Quartz
Weak sol; no mirror.
After 3 to 1 hours
After 12 hours
Sol light yellow-brown in Dense sol, deep yellow-brown in (T) and d a r k - g r a y to (T)*and yellow in (R)t. Deposit on wall inbrown-green in (R). Heavy black mirror on walls of increasing. let tube and containing vessel. Sol light yellow-brown i n Dense sol, deep brown with a reddish tinge i n (T) and (T) and grayish yellowgrayish yellow-green in (R). green in (R). Deposit Heavy black mirror on walls. on walls increasing. Sol yellow-brown in (T) Dense sol, deep yellow-brown in (T) and dark brownishand yellow-green in green in (R); no mirror. (R); no mirror.
* (T) = transmitted light. t (R) = reflectedlight. Reduction of silver oxide solution in the absence of excess solid Since solid silver oxide is reduced by hydrogen at 55-60°C., observations were made of the effect on the sol formation process of eliminating the excess suspended solid. The results are given in table 2. The above sols attained their maximum color after three to four hours, showing no perceptible darkening thereafter. The black mirror deposit likewise deepened very little after the first few hours. In every case the sols contained appreciable amounts of silver in solution even after prolonged treatment with hydrogen. Reduction of ultrafiltered solutions of silver oxide Since reduction of filtered silver oxide solutions with hydrogen gave only weak yellow sols in spite of the fact that they contained considerable unreduced silver in solution, it seemed probable that the substance reduced
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was not the silver in solution but colloidally dispersed silver oxide that was not removed by filtration through an ordinary filter. I n support of this it was found that the saturated solutions, such as used in the preceding experiments, always contained appreciable amounts of colloidally dispersed oxide. To remove this in so far as possible, the saturated solution was ultrafiltered before use. For this purpose a filtering membrane was prepared by impregnating No. 40 Whatman filter paper with collodion from a 4 per cent solution TABLE 2 Reduction of filtered saturaled silver oxide solution Temperature 55-58°C. REDUCTION VESSEL
Soft glass Pyrex Quartz
OBSERVATIONS
After 1 hour
Very weak sol; thin silver mirror on walls. Very weak sol; thin silver mirror. Extremely weak sol; no mirror.
1
After 5 to 6 hours
After 12 hours
Light yellow sol; some darkening of mirror.
Almost the same as after 5 t o 6 hours
Light yellow sol; some Almost t h e same as darkening of mirror. after 5 to 6 hours Light yellow-brown sol; Almost the same as after 5 to 6 hours no mirror.
TABLE 3 Reduction of ultrafiltered solutions of silver oxide REDUCTION VESSEL
Soft glass Pyrex Quartz Platinum (quartz +platinum foil) Silver (quartz silver foil)
+
I
OBBERVATIONS
Silver mirror starts to form after 1 hour and darkens gradually; little or no sol formation in 35 hours; silver ioninthesolution. Silver mirror starts to form after 1to 2 hours and darkens slowly; no sol formation in 35 hours; silver ion in the solution. No sol formation and no mirror formation even after 100 hours; nephelometric analysis shows no loss in silver ion concentration. No sol formation; no deposit on quartz tube; after 12 hours all the silver deposited on the platinum foil in the form of minute hexagonal platelets. No sol formation or deposition of silver on either the walls of the quartz vessel or the silver foil.
in glacial acetic acid. The membrane was used in a gold-plated ultrafilter in which the wire gauze usually employed was replaced with a sheet of perforated gold foil. At a pressure of 40 lbs. per square inch, 125 cc. of solution was filtered in 5 minutes. Filtration with a cellophane membrane was unsatisfactory, since it reduced the silver solution completely. The observations with ultrafiltered solutions in various vessels are given in table 3.
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HARRY B. WEISER AND MAX F. ROY
The results shown in table 3 furnish conclusive evidence that silver oxide in true solution is not reduced in a quartz or silver vessel by hydrogen. Since there is no action in a quartz container, silver or platinum foil in a quartz vessel is equivalent to a container of the metal in question. The reduction of dissolved silver oxide in the presence of platinum is due to catalytic activation of the hydrogen at the surface of the metal. As a result, hydrogen goes into solution as hydrogen ion and an equivalent amount of silver ion leaves the solution, giving a minute nuclear deposit of silver at certain points on the platinum surface. As the process continues, these nuclear deposits grow to relatively large hexagonal platelets of silver. This phenomenon does not take place at a silver surface because hydrogen is not catalytically activated to any appreciable extent by silver, under the conditions of the experiment. The ease with which solid silver oxide, in suspension or on the walls of the vessel, is reduced by hydrogen indicates that hydrogen is activated at the surface of silver oxide or a t the interface silver-silver oxide (5). The reduction a t the surface of the glass vessels with the formation of a mirror-like deposit is probably preceded by the formation of a film of silver oxide, which is subsequently reduced. The deposition of a silver oxide film is due either to adsorption from solution or to precipitation by means of alkali dissolved from the glass. I n support of the latter view it has been found by H. L. Johnston1 at Ohio State University, that the solubility of silver oxide in water, 2.2 X equivalents per liter,2 is reduced enormously by the presence of very small amounts of alkali. Thus 0.02 N alkali reduces the solubility to 6 X lo-@equivalents per liter, which is approximately 3 per cent of the solubility in pure water. At a temperature of 5060°C. sufficient alkali is extracted from the glass to cause the deposition of a film of oxide which is promptly reduced. Continuation of the process causes a gradual thickening of the mirror thus deposited. The process is in no sense a result of the catalytic activation of hydrogen by the glass surface. No film formation occurs on a quartz vessel because of the insolubility and non-alkaline character of the surface. Further evidence in support of the above conclusions is furnished by the results of three experiments in which very dilute alkali, colloidal silver oxide and colloidal silver, respectively, are added to the ultrafiltered solution in a quartz vessel before conducting in the hydrogen. The silver oxide sol was formed by saturating and filtering a solution at 100°C. and cooling. The results are given in table 4. These observations support the view that Private communications. Kohlschiitter believed the solubility of silver oxide to be twice this value. The most probable explanation is that Noyes and Kohr (J. Am. Chem. SOC.24, 1143 (1902)) reported a value df 2.16 X moles per liter, but their data shows that they meant to say 2.16 X 10-4 equivalents per liter. 1
2
FORMATION OF KOHLSCHUTTER’S
SILVER SOL
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sol formation and mirror formation by Kohlschutter’s method result from reduction of solid silver oxide and not of the dissolved compound. Colloidal silver, like silver in mass, does not catalytically activate the hydrogen and effect reduction. Because of the precipitation of silver oxide by the alkali extracted from glass, an ultrafiltered solution stored in a Pyrex flask for two weeks gave a light yellow sol on reduction in a quartz vessel. The precaution was therefore taken to store the stock solutions in quartz. The very slow formation of a weak sol in a glass vessel with the ultrafiltered solution may result from the breaking-off of pieces of the mirror first deposited on the walls, or of silver oxide itself before the latter is reduced. TABLE 4 Reduction in a quartz vessel of ultra$ltered silver oxide solutions ajter certain additions ADDITIONS
Solution made 2 to 5 x 10-4 t o SOdium hydroxide Colloidal oxide
silver
Colloidal silver
1l-
OBSERVATIONS
Weak sol after 1hour; light yellow to yellow-brown sol after 4 t o 5 hours; no mirror; colorimetric examination shows that t h e depth of color increases with the sodium hydroxide concentration but not in direct proportion. Sols are formed, the depth of color of which was found by colorimetric analysis to he directly proportional t o the amount of colloidal silver oxide added; no mirror formation. No reduction of the solution and no mirror formation even after 30 to 40 hours.
Reduction of silver oxide solutions formed by diluting the saturated solution I n the experiments considered in the last section there is one possible source of error. In the ultrafiltration process to remove all silver oxide nuclei, the solution came in contact with both the ultrafiltration membrane and the rubber gasket. The possibility that some foreign substance was introduced which would inhibit the reduction process was not excluded. To get around this possible source of error, the colloidal silver oxide was removed from the saturated solution by diluting with enough water to dissolve the colloidal oxide. The dilutions were made in quartz vessels and the resulting solutions were allowed to stand for three to four days to establish equilibrium conditions. Some observations with such solutions are given in table 5. I t is apparent from the above experiments that a filtered saturated solution diluted in the ratio of 6 of solution to 4 of water is not reduced a t all by hydrogen in a quartz vessel a t 50-60°C. The solution diluted in the ratio of 7:3 showed barely perceptible sol formation. The obvious explanation is that the dilution in the ratio of 6:4 is sufficient to remove all col-
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HARRY B. WEISER AND MAX F. ROY
loidal oxide, thus eliminating reduction with the formation of silver sol. I n soft glass at this dilution, mirror formation proceeds slowly and sol formation is extremely slow. The common ion effect of the alkali from the glass is localized a t the surface, depositing a film of oxide which is subsequently reduced. If the dilution is in the ratio of 1:5, the alkali from the glass is insufficient to precipitate the oxide and no mirror formation results. This indicates that the precipitating action of alkali is a more important factor in the formation of a film of silver oxide on the glass than adsorption of silver oxide from solution by the glass wall. For if the latter were the important thing, one would expect some mirror formation from the diluted solution, especially since the adsorption is in general proportionately greater from more dilute solutions. The absence of mirror formation in quartz and silver with saturated solutions likewise indicates that the precipitating TABLE 5 Reduction of silver oxide solutions .formed b y reducing saturated solutions REDUCTION VEEBEL
Quartz Quartz Quartz Soft glass Soft glass
DILUTION
Solution: water
1
OB8ERVATIONR
5:5
7:3 6:4 6:4 1:5
'
No sol and no mirror even after 30 to 40 hours. Very light yellow sol in 10 to 12 hours; no mirror. No sol and no mirror even after 30 to 40 hours. Faint mirror after 2 hours, darkening gradually; very light yellow sol in 30 hours. No sol and no mirror even after 30 hours.
action of alkali from glass is more important than adsorption of silver oxide from solution, as the initial step in mirror formation. Samples of solutions diluted in the ratio of 6: 4,which are not reduced at all in quartz, were treated with alkali, colloidal silver oxide, and colloidal silver in the same way as the ultrafiltered solutions considered in the last section. The results were identical with those recorded in table 5 and are therefore not repeated here. These observations merely confirm the observations with ultrafiltered solutions and show that no complication was introduced by the ultrafiltration process. I n this connection it may be mentioned that the solubility of silver oxide determined by chemical methods is approximately 2.2 X while that by conductivity methods is 1.38 X 10-4 equivalents per liter (6). From this it is assumed that the silver hydroxide which is formed in solution is approximately 60 per cent dissociated. In view of the fact that 4 parts of water must be added to 6 parts of saturated silver oxide as ordinarily prepared in order to get rid of all colloidal silver oxide, it is suggested that the higher values of the solubility as determined by chemical methods may be
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due in part to the presence of some colloidally dispersed oxide in the solution. This question is now being investigated.
Efect of purity of the hydrogen o n sol formation
It will be recalled that Pauli and Erlach failed to obtain silver sols in Pyrex vessels with pure solutions and electrolytic hydrogen freed from “alkali mist” with sulfuric acid. Since we always obtained sols with hydrogen, free from alkali, provided the saturated solution was not ultrafiltered or diluted to eliminate colloidal silver oxide, the failure of Pauli and Erlach to obtain sols must be explained on some basis other than purity of hydrogen. There are two possibilities: either the silver oxide solutions which they used were too dilute to give sols or mirrors with glass (see table 5 ) , or the sulfuric acid used in the washing train to remove alkali mist substituted a sulfuric acid mist in its place. Since they doubtless worked with saturated solutions, the latter explanation appears the more probable. At any rate it was shown that a filtered saturated silver oxide solution which contained colloidal silver oxide gave no mirror or sol on treating with hydrogen for forty hours provided the solution was made N with sulfuric acid. With low4N acid very slight sol and mirror formation was observed after twelve hours. Sulfuric acid added directly or with the hydrogen has the same effect as dilution in removing silver oxide particles and in preventing the deposition of a silver oxide film on the walls of the vessel. The results of this investigation may be summarized briefly as follows: 1. The formation of Kohlschutter’s silver sol by the reduction of silver oxide with hydrogen at 50-6OoC. is accomplished only in the presence of solid silver oxide. Saturated solutions that have not been ultrafiltered ordinarily contain appreciable amounts of the colloidally dispersed oxide. 2. I n a quartz or silver vessel an ultrafiltered silver oxide solution undergoes no reduction with pure hydrogen a t 50-60°C. ; in a glass vessel the reduction is confined to the surface of the glass giving a thin mirror of metal; in a platinum vessel the reduction is at the surface of the metal, depositing relatively large’hexagonal platelets of silver. 3. The reduction of silver oxide solution in a platinum vessel results from catalytic activation of the hydrogen a t the platinum surface. There is no catalytic activation of hydrogen and no reduction at the surface of quartz or silver. 4. Mirror formation by reduction in glass vessels is not due to catalytic activation of hydrogen a t the surface of glass as implied by Kohlschutter. The mirror formation is preceded by the deposition of a film of oxide which is subsequently reduced. The oxide film results chiefly from precipitation by means of alkali dissolved from the glass, but may be due in part to adsorption of the oxide from solution.
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5 . The ease with which solid silver oxide, in suspension or on the walls of the vessel, is reduced by hydrogen, indicates that hydrogen is readily activated a t the surface of silver oxide or at the interface silver-silver oxide. 6. Pauli’s view that sol formation will not take place except in the presence of alkali or sulfide which can furnish a complex stabilizing electrolyte, is not in accord with the experimental evidence herein recorded. REFERENCES (1) (2) (3) (4) (5) (6)
KOHLSCH~TTER: Z. Elektrochem. 14,49 (1908). ERLACH AND PAULI:Kolloid-Z. 34, 213 (1924). BESTAND Cox: J. Chem. SOC.1929,2727. COOKE AND RICHARDS: Am. Chem. J. 10, 102 (1888). Cf. PEASEAND TAYLOR: J. Am. Chem. SOC.43,2179 (1921). BOTTGER: Z. physik. Chem. 48, 602 (1903).
