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The following method is given by Clennell to determine the Ammonia and Ammonium Salts content by assaying cyanide leach solutions. “Add excess of AgNO3, i.e., sufficient to precipitate all cyanogen compounds, then a little NaCl to remove excess of AgNO3, filter, wash, evaporate filtrate on a water bath to a moderate bulk, distil with caustic soda or sodium carbonate and collect distillate in N/10 acid. Titrate residual acid with N/10 alkali and methyl orange. If free ammonia only is to be estimated use water instead of NaOH or Na2CO3 for distilling. In this case also the preliminary evaporation on water bath must of course be omitted.”
Estimation of Formates in Leach Solution
The presence of formates in a cyanide solution is usually taken as a measure of the amount of ammonia and ammonium compounds formed by decomposition of the cyanide, according to the equation,
KCN + 2H2O = HCO2K + NH3
The following method of determination is a modification by the writer of a method suggested by Clennell, and depends on the reduction of silver from the nitrate by a formate, in the sense of the equation,
2NaCO2H + 2AgNO3 = 2Ag + CO2 + HCO2H + 2NaNO3
It has to be assumed that formates are the only substances present capable of reducing the nitrate of silver to the metallic state.
Procedure for Ammonia and Ammonium Salts Analysis
Take 100 cc of the solution to be tested. Add decinormal H2SO4 to neutralize the alkali exactly, using the amount required in the protective alkali test. Add AgNO3 in slight excess and bring to boil. A blackening of the white precipitate first thrown down may be taken to indicate the presence of formates. Care should be taken to see that the filtrate contains free silver in solution. The precipitate is then washed to free it from the excess of silver and treated with ammonia or sodium thiosulphate to remove the silver cyanide which would otherwise dissolve to a sufficient extent in the subsequent nitric acid treatment to vitiate the results. The remaining blue-black precipitate is well washed to remove every trace of soluble silver and dissolved in 10% boiling HNO3 solution. This solution is then titrated for its silver content with decinormal potassium sulphocyanate (Volhard’s method).
1 cc N/10 KCNS = 0.010788 gm. Ag = 0.00450 gm. CO2H
This method may tend to give a result somewhat too low since precipitated metallic silver is slightly soluble in both ammonia (Prescott & Johnson, page 46, sixth edition) and hypo (H. F. Collins. The Metallurgy of Silver, page 187).