Pharmaceutical Analysis
Ash Value Calculator
Works out total, acid-insoluble or sulfated ash as a percentage of a crude drug or excipient sample, from three weighings.
Type of ash
Total Ash
Enter the crucible weight, sample weight and crucible + ash weight.
Weighings
All three in grams, on the same balance.
Empty crucible, dried to constant weight.
Accurately weighed sample, usually 2–4 g of powdered drug.
After incineration to constant weight.
Incineration conditions
Try a reference sample (monograph limit in brackets)
Pharmacopoeial limits
Total ash
Herbal drugs: ≤ 15%
Plant extracts: ≤ 10%
Senna leaves: ≤ 12%
Digitalis leaves: ≤ 10%
Acid-insoluble ash
Most herbs: ≤ 5%
Vegetable drugs: ≤ 2%
Purified extracts: ≤ 1%
Indicates silica/sand contamination
Sulfated ash
Organic substances: ≤ 20%
Cellulose derivatives: ≤ 0.5%
Pharmaceutical excipients: varies
% Ash = ((W₃ − W₁) ÷ W₂) × 100
W₁ = crucible weight (g), W₂ = sample weight (g), W₃ = crucible + ash weight (g). W₃ − W₁ is the weight of the ash itself.
Working mechanism
1. Weigh the empty crucible (W₁) after drying to constant weight.
2. Add the sample and weigh crucible + sample (total = W₁ + W₂).
3. Heat gradually to 100°C to avoid spattering, then incinerate at 450–600°C.
4. Cool in a desiccator and weigh crucible + ash (W₃).
5. Calculate: % Ash = ((W₃ − W₁) ÷ W₂) × 100.
For acid-insoluble ash, treat the total ash with HCl, filter and ignite the residue. For sulfated ash, moisten with H₂SO₄ before ignition.
Sources: Testronix Instruments, Nikopharmed, ASTM, Kintek, PubMed, European Pharmacopoeia, Precisa.
Common questions
Total ash is everything inorganic left after burning — the plant's own minerals plus any soil or sand. Acid-insoluble ash is the part of that which survives boiling in dilute HCl, which is mostly silica, so it points specifically at earthy contamination. Sulfated ash (residue on ignition) is ignited with sulfuric acid so metals end up as stable sulfates; it is the usual test for synthetic drugs and excipients.
Crucible + ash (W₃) was entered as lighter than the empty crucible (W₁). Either the weights were swapped or the crucible lost weight on ignition — heat the empty crucible to constant weight before you start.
A general one per ash type: total ash ≤ 15%, acid-insoluble ash ≤ 5% and sulfated ash ≤ 20%. Individual monographs are often much stricter — senna leaf total ash ≤ 12%, cellulose sulfated ash ≤ 0.5% — so always compare with the monograph for your material.
The ash should be white or grey and free of carbon, and two successive weighings after further heating and cooling should agree (constant weight). Black specks mean unburnt carbon, which inflates the result.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy