An analysis of some Bredig silver sols showed them to be unsuitable for some investigations recently begun. The precipitated colloid was found to contain from 26 to 44% of silver oxide depending, among other things, on the amount and kind of electrolyte used for stabilizing. Approximately a liter of the gases given off during the preparation of Bredig silver sols was collected and analyzed. The gases consisted of 22.1% oxygen and 77.9% hydrogen by volume. There was, therefore, an equivalent of 11.2% oxygen united with the silver to form silver oxide.A silver colloid comparable in constitution and properties to Zsigmondy gold sols seemed desirable, so a method of reduction with formaldehyde without the use of a protective colloid was developed.

The qualitative separation of zinc from chromium by the systems of analysis in general use requires considerable time. The production of the green cobalt zincate used as a confirmatory test is dependent upon the comparative amounts of cobalt nitrate and zinc present. Properly adjusting these amounts offers considerable difficulty to inexperienced analysts. The employment of diphenylamine and diphenylbenzidine as indicators in the titration of zinc' suggested their utilization in a qualitative test for zinc. A 0.5% solution of potassium ferricyanide and one gram of diphenylamine dissolved in 100 cc. of glacial acetic acid are the solutions used in this test. MethodAfter the separation of the iron and aluminum groups by sodium hydroxide and sodium peroxide, the filtrate containing the aluminum group is acidified with hydrochloric acid, ammonium chloride is added, and the aluminum precipitated with ammonium hydroxide. The filtrate containing the chromium and zinc is acidified with acetic acid and divided into two equal parts. One part is tested for chromium and the other for zinc. To the part to be tested for zinc, five drops of the diphenylamine acetate solution and 5 cc. of the 0.5% potassium ferricyanide solution are added. The immediate appearance of a dark brown, green or purplish-black turbidity indicates the presence of zinc.To test for any zinc that might be carried over into the iron group, the sulfides of zinc, cobalt and nickel are stirred for a few minutes with cold, dilute hydrochloric acid and filtered. The hydrochloric acid solution is boiled until all of the hydrogen sulfide is expelled, sodium hydroxide is added to alkalinity, the solution cooled and one gram of sodium peroxide added in small portions, with stirring. The mixture is boiled to decompose the excess sodium peroxide, cooled and filtered. The filtrate containing the zinc is acidified with acetic acid and the diphenylamine acetate and potassium ferricyanide are added as above.Knop2 states that in the absence of iron salts no coloration is produced by bichromate in a dilute solution of diphenylamine sulfate. This was found to be true only when the amount of bichromate present was small, less than .05 mg. per cc., a blue color being produced immediately if the concentration of bichromate was .05 mg. per cc. or greater. By using diphenylamine acetate instead of diphenylamine sulfate no color 1

The rate of reduction of peroxydisulfate by chromic ion,1 by vanadyl ion2 and by manganous ion and hydrazine3 has been shown to be independent of the concentration of the reducing agent. The reaction rate could be represented by the equation, 2.3/f(Ag+) log Co/C, where Co is the initial concentration of peroxydisulfate and C is the concentration of peroxydisulfate at time t. The experiments described in this paper were performed to see if the reduction of peroxydisulfate by cerous sulfate followed the same equation, and to study the effect of temperature on the reaction rate. ExperimentalCerous Sulfate.-The cerous sulfate was freed of chloride by adding a small amount of silver nitrate to a nearly saturated solution which was acid with sulfuric acid. This solution was filtered and crystals of cerous sulfate obtained by evaporation. The crystals were washed w'ith dilute sulfuric acid to remove traces of silver ion and then dissolved in approximately molar sulfuric acid.Potassium Peroxydisulfate.-Reagent grade peroxydisulfate was used without purification but was assayed by

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