Acid-insoluble ash is the part of the ash that acid cannot dissolve. You first obtain the total ash, then boil it with dilute hydrochloric acid and filter; what stays on the filter paper is the acid-insoluble ash.
Why it matters: if a sample is mixed with soil or stone powder, the total ash rises; acid-insoluble ash is better at exposing this external inorganic contamination, making it a good adulteration screen.
Test procedure
1) Determine the total ash first. 2) Treat the total ash with dilute HCl and heat. 3) Filter and wash until acid-free. 4) Ignite the residue with the filter paper to constant weight. 5) Calculate the acid-insoluble ash percentage.
Relation to total ash
Acid-insoluble ash is a subset of total ash. Total ash includes both soluble and insoluble inorganics; acid-insoluble ash counts only the acid-proof part (mostly silica-type), which is more sensitive to soil or sand contamination.
