Ashing
Under ashing means the targeted destruction of the majority of organic substances by heating under the influence of oxygen to determine the content of inorganic ( mineral ) units. To do this, the substance is heated in a crucible until the organic compounds are burned off and only ash remains. The weight loss, the so-called loss on ignition , then corresponds to the organic portion of the substance, the remaining ash, the residue on ignition , corresponds to the inorganic portion.
Experimental approach
The required amount of sample material depends on the expected inorganic content so that there is enough remaining ash to be able to weigh it precisely. In the case of high inorganic proportions (e.g. 10% by weight), even relatively small amounts are sufficient (e.g. 2 g), but very large quantities are required for small proportions (e.g. 0.01% by weight) (e.g. 200 g).
Crucibles made of quartz glass, porcelain or platinum are used for the test . These must first be heated in the muffle furnace to the test temperature until constant mass is achieved in order to remove organic residues and moisture. The crucibles are then cooled to room temperature in the desiccator and weighed precisely.
The material to be examined is dried and then precisely weighed into the crucible. The crucible is heated over an open flame and the material is slowly burned (pre-incineration). Particular care must be taken to ensure that no material is lost.
As soon as the sample is completely burned, the crucible is further heated in the muffle furnace to a specified temperature (between 500 and 1000 ° C) so that the remaining organic material that has not yet burned off. Then the crucible is again cooled to room temperature in the desiccator and then weighed.
Then the crucible is put back into the muffle furnace and annealed. This process is repeated until two successive weighings no longer lead to different results. A rapid incinerator can also be used instead of a muffle furnace .
From the mass of the dried starting sample m 0 and the mass of the remaining ash m 1 , the inorganic fraction - the residue on ignition - is calculated in percent by mass as m 1 / m 0 . According to DIN 18128 (2002), the proportion of organic matter in a sample is defined as the loss on ignition (in%) . The incineration removes the organic content in the sample, the carbon contained is oxidized and escapes as carbon dioxide . Thus, a high loss on ignition indicates a high proportion of organic matter in the sample.
application
The ashing, for example, in the analysis of sewage sludge , manure and household waste applied, or in order to characterize filled or fiber reinforced plastics. A similar provision is also used for food.
If this method is used on soils, it should be noted that it is only approximately accurate for sandy soils. If one examines soils that contain clays and sesquioxides , one obtains too high a loss on ignition, since they lose crystal water at the high temperatures . For soil analysis, potassium dichromate or a carbon analyzer is used to determine the organic substance .
Caution is required if the inorganic components can also react with oxygen at elevated temperatures . This is the case, for example, with carbonates , which convert into the corresponding oxides at high temperatures in the presence of oxygen .
Analysis of organic chemical samples
Ashing is often the first step in sample preparation when analyzing organic substances when it comes to determining the heavy metal content of a sample to be analyzed.
The oxidative incineration of organic substances can also take place wet-chemically. To do this, a 1: 1 mixture of concentrated sulfuric acid and concentrated nitric acid is poured over the substance and incinerated in the heat.
Analysis of inorganic-chemical samples
In gravimetry , chemical reactions between a sample to be analyzed and a chemical produce precipitates , which are separated from the liquid by filtration using a paper filter . In order to determine the mass of the precipitate, the precipitate and the paper filter are dried in a platinum or porcelain crucible by slow heating, then charred and finally annealed. The filter paper is incinerated. Only the precipitate remains in the crucible, which is weighed back in the desiccator after cooling.
Norms
There is a whole series of DIN standards that deal with ashing. The general procedure is always similar:
- DIN 38409-1, January 1987. German standard methods for the examination of water, waste water and sludge; Summary effects and substance quantities (group H); Determination of the total dry residue, the filtrate dry residue and the residue on ignition (H 1) 1987
- DIN 38409-2, March 1987: German standard methods for water, waste water and sludge testing; Summary effect and substance parameters (group H); Determination of the filterable substances and the residue on ignition (H 2)
- DIN EN 1879, June 1995: Products and systems for the protection and repair of concrete structures - Test methods - Determination of the residue on ignition
- DIN EN ISO 1172, December 1998: Textile glass-reinforced plastics - Prepregs, molding compounds and laminates - Determination of the textile glass and mineral filler content; Calcination process
- DIN 19684-3, August 2000: Soil investigation methods in agricultural hydraulic engineering - Chemical laboratory tests - Part 3: Determination of the loss on ignition and the residue on ignition
- DIN EN 12879, February 2001: Characterization of sludges - Determination of the loss on ignition of the dry matter; German version EN 12879: 2000 2001, 7 pp.
- DIN 18128, December 2002: Subsoil - Investigation of soil samples - Determination of loss on ignition
- DIN EN ISO 3451-1, November 2008: Plastics - Determination of ash
Individual evidence
- ^ A b Walter Wittenberger: Chemische Laboratoriumstechnik , Springer-Verlag, Vienna, New York, 7th edition, 1973, pp. 130-131, ISBN 3-211-81116-8 .
- ↑ DIN EN ISO 3451-1: Plastics - Determination of ash, November 2008.