Determination of free acid in the presence of hydrolysable cations by titration

Abstract

A simple method is described for the determination of free acid in process solutions containing iron(III), aluminium(III), iron(II) and manganese(II). The solutions analysed are those resulting from the leaching of Witwatersrand gold and uranium ores with sulphuric acid. The method involves titrating the sample solution with sodium hydroxide solution to the hydrogen sulphate end-point, the detection of which is facilitated by the use of the first derivative of the pH response.Preliminary results have indicated that the method is accurate and free of interferences from aluminium(III)(up to 2 g l–1), iron(II)(up to 2 g l–1) or manganese(II)(up to 5 g l–1). The hydrolysis of iron(III) at a concentration of 5 g l–1 reduced the magnitude of the signal, which made detection of the end-point difficult. This problem was overcome by the addition of a neutral salt, e.g., sodium sulphate.The derivative method, which is applicable to all strong mineral acids in the presence of sulphate anions, has been compared with the potassium oxalate method and has been found to be more accurate, even though it is subject to errors caused by the differentiating circuit.