Zinc usually occurs in cyanide solutions as the double cyanide, but under certain conditions, e.g., in dilute solutions, a portion of the zinc may be present as zinc cyanide. It is possible that some may also exist as an alkaline zincate.

Procedure

To 500 cc of solution add 10 cc HCl, 10 cc HNO3, and 8 cc H2SO4. Evaporate on a hot plate until copious fumes of SO3 are evolved. Take down over a flame until 2 to 3 cc remain. Take up with 10 cc water and 5 cc H2SO4, and again evaporate to strong fumes of SO3 and dryness. Care should be taken at this stage to avoid spitting. Cool, add 100 cc water and 8 cc H2SO4, and boil until all soluble salts are in solution. The volume of solution should be maintained at about 100 cc by adding water as required.

Add about 6 grams of aluminum either as 30 mesh or sheet aluminum, bent up at the corners, and boil for 10 min. The aluminum precipitates copper, lead, nickel, cobalt, arsenic, antimony, and tin. Filter, and wash with hot water several times. Some extra aluminum should be present in the filter paper. Evaporate the filtrate to about 40 cc, and add 10 cc HNO3 and 1 gram potassium chlorate, KClO3. Evaporate to dryness. Do not bake the residue. Add 50 cc water and gram NaOH. Break up the cake on the bottom of the beaker, and then add 7 grams ammonium carbonate, (NH4)2CO3, and heat nearly to boiling for several minutes. Let the precipitate settle, filter, and wash several times with hot 5 per cent (NH4)2CO3, solution.

Make the filtrate acid with HCl, and add 3 cc in excess, then dilute to 200 cc. Heat to 70°C., and titrate very slowly with potassium ferrocyanide, K4Fe(CN)6, solution, using uranium acetate as an outside indicator. The end point is reached when a drop of the solution, placed on a white tile, shows a brown tinge when touched with a drop of the uranyl acetate after standing about a minute.

The solution should be kept at about 70°C. during titration and be constantly stirred.

STANDARD POTASSIUM FERROCYANIDE, K4Fe(CN)6, SOLUTION

Dissolve 21.6 grams chemically pure crystals, K4Fe(CN)6 to 3H2O, in water and dilute to 1000 cc.

1 cc K4Fe(CN)6 solution — 0.005 gram zinc, Zn

URANIUM (URANYL) ACETATE INDICATOR

Three grams uranium acetate and ½ cc acetic acid in 100 cc water.

In order to save time in titrating, the zinc solution may be divided into two nearly equal parts. Titrate one of these parts to an approximate end point, then add the remainder of the solution, and finish the titration.

For the determination of other elements .such as antimony, arsenic, cobalt, nickel, lead, soluble sulphates and sulphides in cyanide solutions the reader is referred again to “Analysis , of Cyanide Solutions” by the Canadian Industries Ltd.