1865
V O L U M E 2 3 , NO. 1 2 , D E C E M B E R 1 9 5 1 because it is difficult to see how they can affect the reduction of silver ion by stannous chloride. SPECIAL SOLUTIONS
p-Diethylaminobenzylidenerhodanine, 0.05 gram in 100 ml. of absolute ethyl alcohol. Stannous chloride dihydrate, 15 rams in 100 ml. of 2 N hydrochloric acid. Remove any insofuble material by filtration. Prepare fresh a t reasonable intervals. Tellurium tetrachloride, 1 mg. of tellurium per ml. Treat 100 mg. of precipitated tellurium with 1 or 2 ml. of nitric acid and evaporate to dryness. Add 1 ml. of hydrochloric acid and again evaporate t o dryness. Dissolve the residue in 10 ml. of hydrochloric acid and dilute to 100 ml. with water. Nitric acid, ca. 1.25 N . Dilute 40 ml. of concentrated (16 N ) nitric acid to 500 ml. with water. The solution must be chloridefree. Standard silver solution, 0.001% silver as the nitrate in 0.050 N nitric arid, obtained by diluting a 0.01 or 0.1% solution prepared from metallic silver or silver nitrate (AgNOa/Ag = 1.57). PROCEDURE
The sample solution (0.5 to 25 micrograms of silver) may conveniently have a volume of 50 ml. and should be approximately 2 1' in hydrochloric acid. It should not contain strong oxidizing agents such as nitric acid. Add 1 ml. of tellurium solution, mix, and add with stirring 10 ml. of stannous chloride solution (or more as re uired to reduce ferric iron and copper and give a brown colloidd precipitate of tellurium, followed by an excess of 10 ml.). Heat to boiling and keep near the boiling point for 0.5 hour or until the precipitate is well-coagulated. Collect the precipitate in a small filter crucible. Wash the precipitation beaker and crucible carefully with 50 ml. of 1 N sulfuric acid, and then with 50 ml. of water. It is very important to remove all traces of chloride and other foreign substances. Dissolve the tellurium precipitate by adding three 1-ml. portions of hot concentrated nitric acid and wash the filter crucible with several small portions of water. A bell jar-type filtration apparatus is best used in this operation, eo that the nitric acid solution and the washings can be collected directly in a 25-ml. beaker or Erlenmeyer flask. Evaporate the solution to dryness. Take up the residue in 0.40 ml. of 1.25 N nitric acid and 1 ml. of water if the sample contains less than 3 micrograms of silver (determination in a final volume of 10 ml.) or 1.00 ml. of the acid and 1 ml. of water if more than 3 micrograms of silver are present (final volume 25 ml.). After transferring the solution to the appropriate volumetric flask, dilute with water just to the neck
of the flask. Add 0.40 ml. of rhodanine reagent to the 10-ml. flask or 1.00 ml. to the 25-ml. flask. Dilute to volume with water and mix by rapidly inverting several times. Obtain the transmittancy of the suspension in a 1-cm. (or deeper) cell a t about 495 mp within 30 minutes. Prepare the standard curve for the determination of silver in a final volume of 10 ml. by taking 0 to 3.0 micrograms of silver and adding sufficient nitric acid to make its concentration 0.050 N after dilution to volume, taking into account the amount of acid added with the standard silver solution. Dilute with water just to the neck of the flask and add 0.40 ml. of rhodanine reagent. Dilute to volume with water, mix, and determine the transmittancy. Prepare the standard curve for the determination of silver in a final volume of 25 ml. by taking 0 to 25.0 micrograms of silver and proceeding as described above, but increase the amount of rhodanine reagent to 1.00 ml. Determine the transmittancy of the sample and standard solutions after about the same period of standing. For less than 0.5 microgram of silver, visual comparison by the standard series method can be applied with advantage. The final volume should be kept a t or below 5 ml., so that the comparison can be made in 1.2 X 8 cm. flat-bottomed glass-stoppered tubes. The volume of rhodanine reagent is proportionately reduced from the volume given above. The final acidity is maintained a t 0.050 N . The absorption cell or tubes should be cleaned with dilute (6 N ) nitric acid after each determination to dissolve any precipitate that may have been deposited. Notes. If lead chloride is precipitated in the isolation of silver, first wash with the minimum volume of 1 N hydrochloric acid to dissolve it. When copper is present in fairly large amounts (0.2 gram) and the quantity of silver is likely to be greater than 5 micrograms, precipitation of silver is incomplete (see above). In suc$ a case, add 1 ml. of tellurium solution to the filtrate from the first tellurium precipitation, coagulate the precipitate, and proceed as before. When palladium is present (
