Talk:Chemical methods

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Author: Hans-Jürgen Schwarz
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Abstract

This article contains an explanation of the calcium carbide method and the Karl-Fischer method [Kupfer:1997]Title: Materialfeuchtemessung: Grundlagen, Messverfahren, Applikationen, Normen
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. Both investigation methods require sampling of a building material. The sample size is based on the estimated amount of water in the sample.

Calcium carbide method

Figure 1 - The CM- device
Figure 2 – Diagram: the determination of moisture using the CM- device. Grafik: Fa Satorius GmbH

When using the calcium carbide-method<ref>http://de.wikipedia.org/wiki/Calciumcarbid-Verfahren, gesehen am 20.7.2010</ref>, a crushed sample has to be introduced into the CM- device. The CM-device is a pressure vessel, which contains a steel sphere. The vial with the sample material in it has to be broken by vigorously shaking the device, the sample material is then mixed with calcium carbide, and the following reaction will take place:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2}

The water contained in the sample reacts with the calcium carbide, producing calcium hydroxide and acetylene. The amount of gas inside the container is a measure for the amount of water contained in the sample. The pressure increase can be read on a built-in pressure gage. Data from tables, which are provided by the supplier, are used for the interpretation of the results. Choosing a sample of an appropriate size requires some experience, because it depends on the moisture content of the sample. The lower the moisture content of a sample, the more material is needed, in order to prevent measurement errors. Also during the crushing of the sample a measurement errors occurs, that results in disparities of 1-3%. The measuring values of this method are often too low, if the sample was not crushed finely enough to cause a reaction that includes all the moisture from the capillary bonds.

Karl Fischer titration

The Karl Fischer titration is a reaction between the Karl Fischer solution (J2 und SO2) and the sample (contains H2O). The following equation applies:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle J_2 + SO_2 + 2 H_2O \rightarrow H_2SO_4 + 2 HJ}

The brown iodine water is discolored. When the end of the titration is reached, the sample takes on the brown color (free J2 is present again), making a visual determination of the point in time possible. The moisture content of the sample is determined by the amount of Karl Fischer solution needed. With this method even very small traces of water can be detected. However, the crushing of the sample can lead to a measurement error (similar to the carbide method).

Weblinks

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Literature

[Kupfer:1997] (1997): Materialfeuchtemessung: Grundlagen, Messverfahren, Applikationen, Normen, Expert VerlagLink to Google Scholar