Cerimetry

The Cerimetry is a chemical analysis method for quantitative determination of reducing agents .

principle

During a titration cerium by a (IV) compounds reducing agent to cerium (III) compounds reduced :

${\displaystyle \mathrm {Ce^{4+}+e^{-}\longrightarrow Ce^{3+}} }$

The substance to be determined is either itself a reducing agent or it has previously been converted into such a chemical reaction.

Customized solutions used

The European Pharmacopoeia uses for the determination of the content, for example of paracetamol , cerium (IV) sulfate solution . However, other cerium (IV) salt solutions are also common, e.g. B. the ammonium cerium (IV) sulfate or the ammonium cerium (IV) nitrate used as a primary substance for redox titrations .

Adjustment of the customized solutions

The European Pharmacopoeia prescribes adding excess potassium iodide as a base titer to adjust cerium (IV) standard solutions and titrating the resulting iodine back with sodium thiosulphate (see iodometry ).

Arsenic (III) oxide is also suitable as a primary titer, as it reacts in alkaline solution to arsenite , which is quantitatively oxidized by cerium (IV) to arsenate . However, because of the toxicity of arsenic compounds , their use is not recommended.

Indication of the end point

The endpoint is indicated electrochemically or by a redox indicator such as ferroin . The iron (II) contained in the ferroin complex is oxidized to iron (III), whereby its color changes from red to blue.

Cerimetric determination of the content of paracetamol

Paracetamol is heated with dilute sulfuric acid . The hydrolysis of the carboxamide yields 4-aminophenol and acetic acid .

After adding ferroin solution and dilute hydrochloric acid , titration is carried out with cerium (IV) sulfate solution, during which parahydroxyaniline is oxidized to parachinonimine .

At the transition point of ferroin, two equivalents of Ce ⁴⁺ correspond to one molecule of paracetamol.

literature

• European Pharmacopoeia . Deutscher Apotheker Verlag, Stuttgart 2005 (basic work 5th edition; supplements 2005–2008).
• Eberhard Ehlers: Analytics, Vol. 2: Quantitative and instrumental pharmaceutical analytics . 9th edition. Deutscher Apothekerverlag, Stuttgart 1999, ISBN 3-7692-2488-4 .

Individual evidence

1. ↑ Entry on cerimetry. In: Römpp Online . Georg Thieme Verlag, accessed on March 24, 2016.
2. ↑ Entry on cerium compounds. In: Römpp Online . Georg Thieme Verlag, accessed on March 24, 2016.