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,[1] such as chromatography, or optical analysis, such as spectroscopy.

Overview
A crucible and tongs, on a green mat.

The residues after a sample is completely burnt - in contrast to the ash 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.

Fuels and Consumption

Ash content represents the incombustible component remaining after a sample of the furnace oil is completely burned. The ash content of petroleum products is generally low. It is defined as inorganic residue that remains after combustion of the oil in air at specific high temperature. Ash ranges from 0.1–0.2%. The ash content of a fuel is a measure of the amount of inorganic noncombustible material it contains. Some of the ash forming constituents occur naturally in crude oil; others are present as a result of refining or contamination during storage or distribution.

Knowledge of the amount of ash-forming material present in a product can provide information as to whether or not the product is suitable application. The ash content of petroleum products came from the carbon residue of oil. As established on the american standard testing method D-482, the mass of ash is 16% of the mass of its carbon residue.

Apparatus

Some necessary apparatus include:

  • crucible (or similar porcelain or metal dishes)
  • muffled furnace
  • hot plate
  • the sample

Procedure

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.

Analysis

The analysis of honey shows:[2]

Typical honey analysis

In this example the ash would include all the minerals in honey.

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See also

References
  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "ashing". doi:10.1351/goldbook.A00468
  2. Sugar Alliance Archived December 3, 2005, at the Wayback Machine
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