Methods of test for soils, Part 25: Determination of silica sesquioxide ratio
1982 Edition

The silica sesquioxide ratio (SSR) represents the proportion of silica (SiO2) to sesquioxides, primarily aluminium oxide (Al2O3) and iron oxide (Fe2O3), within the clay fraction of soil. This ratio is essential in classifying clay minerals and understanding their behavior. Higher SSR values typically indicate clays with greater cation exchange capacities and more reactive properties, while lower values suggest less reactive soils. The SSR is calculated using gram molecular percentages derived from the oxide contents after careful sample preparation that removes exchangeable cations, organic matter, and iron oxides.

Soil samples should first be dried and the clay fraction smaller than 2 microns isolated via centrifugation. Exchangeable cations are eliminated through dialysis or ion exchange resins. Organic and colloidal impurities are oxidized using hydrogen peroxide, followed by dissolving iron oxides with oxalic acid. The clay fraction is then thoroughly washed to remove oxalate ions and dried before proceeding with chemical estimations of silica, aluminium oxide, and iron oxide. This preparation ensures accurate and reliable measurement of the silica sesquioxide ratio.

Essential chemicals include fusion mixtures (5-6 times the clay weight), concentrated hydrochloric acid, distilled water, hydrogen peroxide, oxalic acid, and bromine water. Required apparatus consists of platinum dishes for fusion and ignition, a muffle furnace capable of reaching approximately 900°C, beakers with watch glasses, sand baths for evaporation, Whatman No. 42 filter papers or equivalents, pre-weighed crucibles, centrifuges for particle separation, and dialysis or ion exchange setups for cation removal. These materials enable accurate processing and quantification of silica and sesquioxide components.

The oxide percentages are expressed as gram molecular percentages relative to the clay sample weight. This involves measuring the weight of each oxide in the treated sample, then dividing by their molecular weights to convert to molecular proportions. Specifically, silica (SiO2) uses a molecular weight of 60, aluminium oxide (Al2O3) approximately 102, and iron oxide (Fe2O3) about 159.7. The formula is: (weight of oxide × 100) divided by (weight of clay × molecular weight). These values form the basis for calculating the silica sesquioxide ratio.

To ensure precision and reproducibility, use the clay fraction smaller than 0.002 mm obtained by centrifugation, remove exchangeable cations via dialysis or resin treatment, and oxidize organic and colloidal matter with hydrogen peroxide. Iron oxides must be dissolved using oxalic acid, followed by thorough washing to eliminate oxalate residues. During silica estimation, accurately weigh about 1 gram of dried clay, fuse with the fusion mixture, ignite at around 900°C, treat with concentrated hydrochloric acid until effervescence ceases, and evaporate to dryness on a sand bath to remove acid traces. Use distilled water for dilution, filter with appropriate paper, and ignite the residue until a constant weight is achieved. Repeat tests and average results to reduce random errors and follow strict temperature and reagent purity controls.