The reaction between hexacyanoferrates(III) (ferricyanides) and soluble iodides is a reversible one:
2[Fe(CN)6]3-
+2I- = 2[Fe(CN)6]4- +I2
In strongly acid solution the reaction proceeds from left to right, but is reversed in almost neutral solution. Oxidation also proceeds quantitatively in a slightly acid medium in the presence of a zinc salt. The very sparingly soluble potassium zinc hexacyanoferrate(II) is formed, and the hexacyanoferrate(II) ions are removed from the sphere of action:
2[Fe(CN)6]4- +2K+ +3Zn2+ = K2Zn3[Fe(CN)6]2
The procedure may be used to determine the purity of potassium hexacyanoferrate (III).
Weigh out accurately about 10 g of the salt and dissolve it in 250 mL of water in a graduated flask. Pipette 25 mL of this solution into a 250 mL conical flask, add about 20 mL of 10 per cent potassium iodide solution, 2mL of 1M sulphuric acid, and 15 mL of a solution containing 2.0 g crystallised zinc sulphate. Titrate the liberated iodine immediately with standard 0.1M sodium thiosulphate and starch; add the starch solution (2mL) after the colour has faded to a pale yellow. The titration is complete when the blue colour has just disappeared. When great accuracy is required, the process should be conducted in an atmosphere of carbon dioxide.