Thiocyanate ions hexacyanoferrate

It is difficult to filter this precipitate, as it tends to form a colloid. The reaction can be used to distinguish hexacyanoferrate(II) ions from hexacyanoferrate(III) and thiocyanate, which do not react. 

Ammonium molybdate solution from a solution of potassium hexacyano-ferrate(II), acidified with dilute hydrochloric acid, a brown precipitate of molybdenyl hexacyanoferrate(II) is formed. The exact composition of the precipitate is not known. The precipitate is insoluble in dilute acids, but soluble in solutions of alkali hydroxides. The test can be applied to differentiate hexacyanoferrate(II) ions from hexacyanoferrate(III) and thiocyanate, which do not react. 

Bromides and iodides interfere because of the liberated halogen the test is not trustworthy in the presence of chromates, sulphites, thiosulphates, iodates, cyanides, thiocyanates, hexacyanoferrate(II) and (III) ions. All of these anions may be removed by adding excess of nitrate-free Ag2S04 to an aqueous solution (or sodium carbonate extract), shaking vigorously for 3-4 minutes, and filtering the insoluble silver salts, etc. 

Cyanides, thiocyanates, hexacyanoferrate(II)s, and hexacyanoferrate(III)s also yield ammonia under these experimental conditions. The reaction is somewhat slower for these anions up to 5 minutes may elapse before ammonia can be detected from hexacyanoferrate(II)s and hexacyanoferrate(III)s. If these are present, or are suspected as a result of the preliminary tests, particularly that with concentrated sulphuric acid, they must first be removed as follows. Treat the soda extract with excess of nitrate-free silver sulphate, warm the mixture to about 60°, shake vigorously for 3-4 minutes, and filter from the silver salts of the interfering anions and excess of precipitant. Remove the excess silver ions from the filtrate by adding excess sodium hydroxide solution and filter off the precipitated silver oxide. Evaporate the filtrate to about half bulk and test with zinc, aluminium or Devarda s alloy. If cyanides alone are present, they may be rendered innocuous by the addition of a little mercury(II) chloride solution. 

---