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III. PRECIPITATION METHODS; of which the titration for silver with potassium thiocyanate solution is an illustration. Examples of the latter class will be pointed out as they occur in the procedures.
The ferric thiocyanate differs from the great majority of salts in that it is but very little dissociated in aqueous solutions, and the characteristic color appears to be occasioned by the formation of the un-ionized ferric salt. If the ammonium thiocyanate is used, the amount is 76.08 grams.
To prepare the solution for this determination, which should be approximately 0.05 N, dissolve about 5 grams of potassium thiocyanate, or 4 grams of ammonium thiocyanate, in a small amount of water; dilute this solution to 1000 cc. in a liter bottle and mix as usual. About 5 cc. of this solution should be used as an indicator.
The addition of a solution of potassium or ammonium thiocyanate to one of silver in nitric acid causes a deposition of silver thiocyanate as a white, curdy precipitate. If ferric nitrate is also present, the slightest excess of the thiocyanate over that required to combine with the silver is indicated by the deep red which is characteristic of the thiocyanate test for iron.
Run in the thiocyanate solution from a burette, with constant stirring, allowing the precipitate to settle occasionally to obtain an exact recognition of the end-point, until a faint red tinge can be detected in the solution. From the data obtained, calculate the relation of the thiocyanate solution to the normal.
If the nitrate has come into contact with organic bodies it suffers a reduction and blackens during the heating. PROCEDURE. Weigh out two portions of the coin of about 0.5 gram each. From the corrected volume of the thiocyanate solution required, calculate the percentage of silver in the coin. Above that percentage it is necessary to add silver in known quantity to the solution.
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