Ash (analytical chemistry)
In analytical chemistry, ashing or ash content determination is the process of mineralization for preconcentration of trace substances prior to a chemical analysis, such as chromatography, or optical analysis, such as spectroscopy.
The residues after a sample is completely burnt - in contrast to the ashes remaining after incomplete combustion - consist mostly of metal oxides. Ash is one of the components in the proximate analysis of biological materials, consisting mainly of salty, inorganic constituents. It includes metal salts which are important for processes requiring ions such as Na+ (Sodium), K+ (Potassium), and Ca2+ (Calcium). It also includes trace minerals which are required for unique molecules, such as chlorophyll and hemoglobin.
A crucible can be used to determine the percentage of ash contained in an otherwise burnable sample of material such as coal, wood, oil, rubber or plastics. The ISO mandates ash content determination for most foodstuffs. Examples include
- ISO 2171: Cereals, pulses and by-products — Determination of ash yield by incineration;
- ISO 3593: Starch — Determination of ash;
- ISO 928: Spices and condiments - Determination of total ash; and
- ISO 936: Meat and meat products - Determination of total ash.
Some necessary apparatus include:
A crucible and its lid are pre-weighed after thorough drying. The sample is added to the completely dry crucible and lid and together they are weighed to determine the mass of the sample by difference. The sample is placed in the hot furnace long enough so that complete combustion of the sample occurs. The crucible, lid and ash then are re-weighed.
- Typical honey analysis
In this example the ash would include all the minerals in honey.
- Oxides, e.g. Al2O3, CaO, Fe2O3, MgO, MnO, P2O5, K2O, SiO2
- Carbonates: Na2CO3 (soda ash), K2CO3 (potash)
- Bicarbonates, e.g. NaHCO3 (baking soda)
- Sulfates: sulfate ash according to Ph. Eur.