The solution containing the zinc is contained in an evaporating dish, and freed from sulphuretted hydrogen by boiling, and, if necessary, from an excess of acid by evaporation. The evaporating dish must be a large one. Cautiously add sodium carbonate to the hot, moderately dilute solution, until the liquid is distinctly alkaline, and boil. Allow the precipitate to settle, decant on to a filter, and wash with hot water. Dry, transfer to a porcelain crucible (cleaning the paper as much as possible), add the ash, ignite, and weigh. The substance weighed is oxide of zinc, which contains 80.26 per cent. of the metal. It is a white powder, becoming yellow when heated. It must not show an alkaline reaction when moistened. If it contains manganese this metal will be present as sesquioxide (Mn2O3). Its amount can be determined by dissolving in dilute acid and boiling with an excess of sodic hydrate. The oxide of manganese will be precipitated, and can be ignited and weighed. Its weight multiplied by 1.035 must be deducted from the weight of oxide of zinc previously obtained. The results yielded by the gravimetric determination are likely to be high, since the basic carbonate of zinc frequently carries down with it more or less soda which is difficult to wash off.
VOLUMETRIC DETERMINATION
This method is based on the facts that zinc salts in an acid solution decompose potassium ferrocyanide, forming a white insoluble zinc compound; and that an excess of the ferrocyanide can be detected by the brown coloration it strikes with uranium acetate. The method resembles in its working the bichromate iron assay. The standard solution of potassium ferrocyanide is run into a hot hydrochloric acid solution of the zinc until a drop of the latter brought in contact with a drop of the indicator (uranium acetate) on a white plate strikes a brown colour. The quantity of zinc in the solution must be approximately known; run in a little less of the ferrocyanide than is expected will be necessary; test a drop or two of the assay, and then run in, one or two c.c. at a time, until the brown colour is obtained. Add 5 c.c. of a standard zinc solution, equivalent in strength to the standard "ferrocyanide," re-titrate, and finish off cautiously. Of course 5 c.c. must be deducted from the reading on the burette. The precipitate of zinc ferrocyanide formed in the assay solution is white; but if traces of iron are present, it becomes bluish. If the quantity of ferrocyanide required is known within a few c.c., the finishing point is exactly determined in the first titration without any addition of the standard zinc solution. Unfortunately this titration serves simply to replace the gravimetric determination, and does not, as many volumetric processes do, lessen the necessity for a complete separation of any other metals which are present. Most metals give precipitates with ferrocyanide of potassium in acid solutions. If the conditions are held to, the titration is a fairly good one, and differences in the results of an assay will be due to error in the separation. Ferric hydrate precipitated in a fairly strong solution of zinc will carry with it perceptible quantities of that metal. Similarly, large quantities of copper precipitated as sulphide by means of sulphuretted hydrogen will carry zinc with it, except under certain nicely drawn conditions. When much copper is present it is best separated in a nitric acid solution by electrolysis. The titration of the zinc takes less time, and, with ordinary working, is more trustworthy than the gravimetric method.
The standard ferrocyanide solution is made by dissolving 43.2 grams of potassium ferrocyanide (K4FeCy6.3H2O) in water, and diluting to a litre. One hundred c.c. are equal to 1 gram of zinc.
The standard zinc solution is made by dissolving 10 grams of pure zinc in 50 c.c. of hydrochloric acid and 100 or 200 c.c. of water, and diluting to 1 litre, or by dissolving 44.15 grams of zinc sulphate (ZnSO4.7H2O) in water with 30 c.c. of hydrochloric acid, and diluting to 1 litre. One hundred c.c. will contain 1 gram of zinc.
The uranium acetate solution is made by dissolving 0.2 gram of the salt in 100 c.c. of water.
To standardise the "ferrocyanide" measure off 50 c.c. of the standard zinc solution into a 10 oz. beaker, dilute to 100 c.c., and heat to about 50° C. (not to boiling). Run in 47 or 48 c.c. of the "ferrocyanide" solution from an ordinary burette, and finish off cautiously. Fifty divided by the quantity of "ferrocyanide" solution required gives the standard.
In assaying ores, &c., take such quantity as shall contain from 0.1 to 1 gram of zinc, separate the zinc as sulphide, as already directed. Dissolve the sulphide off the filter with hot dilute hydrochloric acid, which is best done by a stream from a wash bottle. Evaporate the filtrate to a paste, add 5 c.c. of dilute hydrochloric acid, dilute to 100 c.c. or 150 c.c., heat to about 50° C., and titrate. Manganese, if present, counts as so much zinc, and must be specially separated, since it is not removed by the method already given. The following method will effect its removal. To the hydrochloric acid solution of the zinc and manganese add sodium acetate in large excess and pass sulphuretted hydrogen freely. Allow to settle, filter off the zinc sulphide and wash with sulphuretted hydrogen water. The precipitate, freed from manganese, is then dissolved in hydrochloric acid and titrated.
The following experiments show the effect of variation in the conditions of the assay:—
Effect of Varying Temperature.—Using 20 c.c. of the standard zinc solution, 5 c.c. of dilute hydrochloric acid, and diluting to 100 c.c.