This paper describes the practical application of Truesdale and Smith's (1975) fundamental re-appraisal of the conditions that lead to the formation of α- and β-molybdosilicic acid in aqueous solutions. Two procedures, one using the α-acid and the other the β-acid, are proposed. Both procedures are designed so as to enable silicate-silicon concentrations of between 0 and 1·0 mg l^–1 to be determined, although slight adjustments to the manifold of the Technicon AutoAnalyzer II system would make both suitable for other concentration ranges. The appropriate “molybdenum blues” derived from the yellow acids are used.

Tests of the precision of both procedures were made at five concentration levels between 0 and 1·0 mg l^–1 of silicate-silicon in distilled water. The maximum value of the coefficient of variation was 1·44 per cent. In other tests 27 samples of natural water from the upland area surrounding the sources of the rivers Wye and Severn were analysed by both methods. The results suggest that for these types of water the methods yield the same value for silicate concentration.