You can perform a Zinc determination using this titration method with methyl orange as your indicator.
- To 0.5 grams of sample add 10 cc’s HCl and boil for several minutes. Add 3 drops of NH3 and boil until the NH3 is expelled.
- Add 40 cc’s NH4Cl solution and 15 cc’s NH4OH and bring to a boil.
- Filter off and wash well with hot NH4Cl solution (2 parts water to 1 part NH4Cl).
- Add methyl orange indicator. Add 3 cc. of extra HCl to acidify.
- Add water and test lead to precipitate the copper. Boil.
When the solution has boiled until it is clear, add 5 cc. HCl and cool slightly with water, and titrate with standard solution of potassium ferro-cyanide using ammonium molybdate as outside indicator.
When brownish tinges appear on adding a spot of solution to a drop of the ammonium molybdate indicator, the end point is reached.
Take your reading on the pipette and multiply by the standard.
VOLUMETRIC METHOD FOR THE DETERMINATION OF ZINC
Several schemes have been advanced for the estimation of zinc by titration with potassium ferrocyanide, but so far no one of them has found universal application. The methods which have found favor are either so complicated or so difficult in manipulation that a large personal error is always introduced and it seldom happens that very close checks are obtained by different operators, even when the same procedure is followed. Nearly a year ago I became interested in the volumetric determination of this metal with the idea of substituting it for the rather tedious gravimetric method which has always been in use in our laboratory.
At first, and for quite a long time in fact, I was not very successful and encountered numerous difficulties. The samples which I used in this preliminary work were spelters, the zinc content of which had been very carefully determined by difference. Finally, after a trial of all the methods which seemed reliable, with varying successes, I concluded that changes might be made to good advantage in nearly all of them, so with what experience I had already gained, I attempted to work up a scheme which would embody the good points of all of these methods and as far as possible none of the bad ones. The method which I am about to describe, therefore, is not new, but is rather a re-modeling of the older ones. The scheme is one which is identical in certain parts for almost every condition, but there are some slight variations which are necessary’ for different products and, if accuracy is desired, they should be observed. Accordingly, I will describe the method, applying it to typical cases.
Preparation of the Ferrocyanide Solution.—Dissolve crystals of c. p. potassium ferrocyanide in water in the proportion of 22 grams to the liter. If the solution is not clear, it should be filtered before it is diluted to the desired volume. It is a good plan to make up several liters at a…