Assay percentage is usually the single most important number on a chemical spec sheet, but it's also one of the most commonly misread — here's what it actually measures and how to use it.
What Assay Actually Measures
Assay (or purity, or active content) is the proportion of the labelled active chemical actually present in the product, by weight — a 98% assay caustic soda flake means 98% of that flake's weight is genuinely NaOH, with the rest being water, minor impurities, or other components. It's typically measured by titration or another analytical method specific to the chemical, referenced on the product's Certificate of Analysis.
Why It Affects Dosing
Two products with the same name but different assay percentages aren't interchangeable at the same weight-based dose — a 90% assay product needs proportionally more weight dosed to deliver the same active chemical as a 98% product. When comparing suppliers, always normalise to cost per unit of active chemical delivered, not just cost per kilogram of product, or a lower-assay product can look cheaper while actually costing more per unit of real performance.
A Worked Example
If Supplier A quotes ₹X/kg for 98% assay material and Supplier B quotes a lower price per kg for 90% assay material, the fair comparison is (price ÷ assay%) for each — not the raw price per kilogram. This simple normalisation is worth doing on every quote comparison, especially for chemicals like caustic soda, hydrated lime and bleaching powder where assay varies meaningfully between suppliers and grades.