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Chemical Fundementals
Working Medium
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The working medium is the solvent or solvent mixture in which the sample is dissolved and the Karl Fischer reaction takes place. A suitable working medium assures the stoichiometry of the Karl Fischer reaction and must be able to dissolve the sample and the products of the titration reaction as well as allow confident end point determination [1]. Only few solvents can fulfill all these requirements.
Methanol is the preferred choice for the working medium as it allows for a rapid and stoichiometric course of reaction. Most samples dissolve easily in methanol and it gives a sensitive and reliable indication of the end point. Often, other solvents are added although the methanol content should never fall below 25 %.
For water determination in aldehydes and ketones, several less reactive alcohols as 2-methoxyethanol (ethylene glycol monomethyl ether), 2-chloroethanol and 1-methoxy-2-propanol were found to be suitable alcohol solvents. They are generally preferred if side reactions (esterification, acetal/ketal formation) occur in the presence of methanol. However, titrations in these solvents are sometimes slower than in methanol and the endpoint may be sluggish. Using mixtures of these alcohols, we developed HYDRANAL® solvents that ensure rapid titrations.
1-Propanol, ethanol and other long-chained alcohols are better solubilizing agents than methanol for lipophilic molecules and can be added to improve the solubility of the sample. Occasionally, they can be used alone as a solvent while developing methods for these types of compounds.
Chloroform is a good solvent for solubilizing fats and can be used with methanol. The methanol content should comprise at least 25 % of the total volume, preferably 50 %. Using 100 % chloroform as the working medium solvent will change the stoichiometry of the Karl Fischer reaction and is therefore not recommended.
Formamide improves the solubility of polar substances and can be mixed with methanol for the determination of water in proteins. The added formamide volume should not exceed 50 % [1]. A 30% portion of formamide usually suffices and is preferred.
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