Apparatus, Reagents.—The usual apparatus. For the preparation of the standard solution pure K4FeC6N6 (free from K6Fe2C12N12) is used, and pure ZnO is employed for checking the standard. As an indicator a solution of uranium acetate is employed. In the preparation of the ore for analysis the following solutions are required :—About 100 c.cs. of a solution of KClO3 crystals in strong HNO3, prepared by shaking the crystals and acid in a flask. Solid NH4Cl and 20E. NH4HO are also required, and a litre of E/5 NH4Cl. If copper is present, a number of small strips of thin sheet aluminium are required for its precipitation.
For analysis, the student may take a zinc ore containing a fair percentage of zinc blonde (ZnS).
For fuller details regarding the technical application of this and other volumetric methods the student is referred to Furman’s Manual of Assaying.
Method, Reactions.—If to a hydrochloric acid solution of a zinc ore, free or freed from elements such as copper, iron, manganese, etc., a solution of potassium ferrocyanide be added, the zinc is precipitated as zinc ferrocyanide.
2ZnCl2 + K4FeC6N6 = Zn2FeC6N6 + 4KCl
The end point of this reaction is determined by testing on a porcelain plate a drop of the solution with a drop of uranium acetate solution. As long as there is not an excess of K4FeC6N6 the drop of uranium acetate remains yellow, but as soon as the ferrocyanide is in excess it turns a light brown. Before the solution of the ore is ready for titration iron and copper must be removed, if present, as they interfere with the reaction.
Preparation of the Standard Solution. — Dissolve 42.2 gms. pure K4FeC6N6,3H2O in water and make up to 1 litre at 16° C.
Checking the Standard.—Weigh out in duplicate .2 gm. pure ZnO, recently ignited to free it from any carbonate. Dissolve in 5 c.cs. 10E. HCl, and add 50 c.cs. water, using a 300 c.c. beaker. Add 20E. NH4HO in slight excess, and neutralise with HCl, using litmus paper as an indicator, a small piece of the paper being floated in the liquid. Add an excess of 10 c.cs. 10E. HCl and dilute with distilled water to 250 c.cs. The solution is now titrated with the ferrocyanide from a burette, the solution being run in rapidly, at first a few c.cs. at a time, until nearing the finishing point. A drop is tested, after each addition, by a drop of uranium acetate on the porcelain slab. Finish carefully, and as soon as the drop of uranium solution turns brown read the burette.
Treat the duplicate in the same way, and calculate the value of one c.c. of the ferrocyanide solution.
The neutralisation and addition of acid are for the purpose of having the conditions of the solution on standardisation similar to those in the actual analysis.
The Analysis.—Weigh out .5 gm. of the sampled and finely powdered ore. Transfer to a porcelain dish or casserole. Add 15 c.cs. aqua regia. Cover, and when dissolved rinse the cover and evaporate to dryness. If particles of the ore remain undecomposed add gradually 15 c.cs. of the KClO3,HNO3 solution. Cover, and warm gently until greenish fumes cease to be given off. Then boil just to dryness on the hot plate. Cool. Add 7 gms. solid NH4Cl, 15 c.cs. 20E. NH4HO, and 25 c.cs. hot water. Boil for a minute, stirring with a rod to break up any lumps or clots. Filter into a 250 c.c. flask and wash several times with hot E/5 NH4Cl.
If a considerable precipitate has been formed it should be gently scraped off the paper with the assistance of the wash bottle and transferred to a porcelain dish, the excess of water evaporated and 15 c.cs. HNO3KClO3. Treat as before with the NH4HO,NH4Cl and water. Filter and combine the first and second filtrates.
This treatment of the first precipitate is for the purpose of removing any traces of zinc hydrate that may have been precipitated with the iron.
If the combined filtrate is blue in colour, copper is present and must be removed as follows:—Neutralise with HCl and then add 10 c.cs. 10E. HCl. Heat to about 70° C. Now add a number of the aluminium strips. Shake till the copper is all precipitated.
If, however, the copper is absent, neutralise the combined filtrates with HCl, add then 10 c.cs. 10E. HCl, and the solution, if cool, is ready for titrating.
Titrate as before, and note the volume used. Repeat the determination on a fresh sample of the ore. Duplicates should agree within .2%. Report the results in percentage of zinc.
Note.—The student should note the importance, when dealing with complex ores, of standardising the solution under similar conditions to those existing in the solution to be analysed.