The ash remaining following ignition of herbal materials is a measure of the total amount of the inorganic material left after burning. For some drugs this value gives an indication of the care taken in the preparation of the drug. Adulteration with mineral matter will give higher ash values than for pure unadulterated drug. Determination of ash is useful for detecting low-grade products, exhausted drugs and excess of sandy or earthy matter, especially in powdered drugs. Different types of ash values such as total ash, acid-insoluble ash, sulphated ash and water- soluble ash are measured.

The total ash is determined by incinerating 2–4 g of air-dried drug by gradually increasing heat up to 450° C. The objective is to remove the carbon at as low a temperature as possible. This is because volatile alkali chlorides may be lost at high temperatures without leaving an ash. EP and BP use sulphated ash which involves treatment of the drug with dilute sulphuric acid before ignition. All oxides and carbonates are thus converted to sulphates and the ignition is carried out at 600° C.

For water-soluble ash, the total ash is boiled with water and filtered. The insoluble matter is further ignited in an ashless filter paper. The weight of the formed ash subtracted from the total ash value gives the water-soluble ash.

Total ash is treated with dilute hydrochloric acid, filtered and the insolubles on ignition give acid-insoluble ash. This measures the amount of silica present, especially as sand and silicious earth.

Total ash consists mainly of carbonates, phosphates, silicates and silica. It includes both ‘physiological’ ash which is derived from the plant itself and ‘non-physiological’ ash, which is the residue of extraneous matter such as sand, soil, chalk or lime (used for coating drugs such as nutmeg, ginger etc.) that may be present.

Total ash is not useful in detecting adulteration with earthy matter when it varies within a wide range as it does for rhubarb (8% to 40%), because of the variable calcium oxalate content. In such cases acid-insoluble ash which removes the variable calcium oxides or carbonates formed will indicate the presence of earthy matter likely to be present in roots, rhizomes and pubescent leaves such as digitalis and henbane.

Water-soluble ash detects material exhausted by water. E.g., tea and ginger, if admixed with exhausted drugs will show much greater reduction in water-soluble ash than total ash. It is therefore an important indicator when exhausted material is substituted for the genuine drug.