This report, the second in a series describing the collaborative work of ICG Sub-Committee AII, considers briefly the requirements for determining trace amounts of silica and discusses the merits or otherwise of the two most frequently used colorimetric procedures, the measurement of the colour intensity of the blue or yellow silico-molybdate complexes. The more sensitive "blue" method is then examined in detail and is applied to the determination of silica in synthetically prepared known solutions and in durability extract solutions prepared using the ISO 720 grain method and the European Pharmacopoeia whole article method. The procedure recommended eventually, given in the Appendix, is shown to produce acceptably reproducible results on both kinds of solution.
