3704. Test for the Quantity of Free Cyanide of Potassium in Solutions

3704.    Test for the Quantity of Free Cyanide of Potassium in Solutions. It has been already mentioned that the cyanide of silver, as it forms upon the surface of the silver plate, is dissolved by the cyanide of potassium. This renders it necessary to have always in the solution free cyanide of potassium. Were we to use the pure crystalline salt of cyanide of potassium and silver, dissolved in water, without any free cyanide of potassium, wo should not obtain a deposit beyond a momentary blush, as the silver plate or electrode would get an instantaneous coating of cyanide of silver, and this not being dissolved, the current would stop. The quantity of free cyanide of potassium required in the solution varies according to the amount of silver that is present, and the rapidity of the deposition. If there be too little or it, the deposit will go on slowly; if there bo too much, the silver plate will bo dissolved in greater proportion than the quantity deposited, and the solution will consequently get stronger. The proportion we have found best is about half by weight of free cyanide of potassium to the quantity of silver in solution ; thus, if the solution contains 2 ounces of silver to the gallon, it should have 1 ounce of free cyanide of potassium per gallon. This is known by taking some nitrate of silver, dissolving it in distilled water and placing it in a common alkalimeter (see No. 82 (Acidimeter)), graduated into 100 parts. The proportion of the nitrate of silver in the solution is to be such that every two graduations of the solution should contain 1 grain. A given quantity of the plating solution is now taken - say 1 ounce by measure, and the test solution of nitrate of silver is added to it by degrees, so long as the precipitate formed is redissolved. "When this ceases the number of graduations is then noted, and the following equation gives the quantity of free cyanide. Every 175 nitrate of silver are equal to 130 cyanide of potassium in solution. Suppose 20 graduations were taken, equal to 10 grains nitrate of silver, then 175 : 130 :: 10 : 7.4 grains free cyanide of potassium. This, multiplied by 160, the number of fluid ounces per gallon, will make about 21/2 ounces. We have taken 2 graduations to 1 grain of nitrate of silver, that the solution may be considerably dilute and less liable to error. The following table is calculated at a half grain nitrate of silver to the graduation, and will be a guide to the student or workman. The quantity of solution tested is 1 ounce by measure.