Mohr titration

The Mohr titration is a method for the quantitative determination of chloride and bromide ions . It is counted to argentometry . The method is named after the German pharmacist Karl Friedrich Mohr .

Titration

In this titration , the chloride or bromide solution directly with silver nitrate - Standard volumetric solution titrated. As an indicator it is potassium used. At the equivalence point here is silver chromate formed. This is sparingly soluble and in contrast to silver chloride, which is also sparingly soluble, red-brown.

{\ displaystyle \ mathrm {Cl _ {(aq)} ^ {-} + Ag _ {(aq)} ^ {+} \ longrightarrow AgCl \ downarrow}}

When adding silver nitrate solution, white silver chloride precipitates up to the equivalence point.

{\ displaystyle \ mathrm {CrO_ {4 (aq)} ^ {2 -} + 2 \ Ag _ {(aq)} ^ {+} \ longrightarrow Ag_ {2} CrO_ {4} \ downarrow}}

From the equivalence point onwards, sparingly soluble silver chromate forms, which precipitates out as a red-brown precipitate.

conditions

With this titration it is important to maintain a pH value between 6.5 and 10.5, otherwise incorrect results will be obtained. This is usually achieved with acetic acid or sodium hydrogen carbonate solution. If the solution is too acidic, the potassium chromate is predominantly in the form of dichromate and the color change occurs too late. In the basic range, sparingly soluble silver (I) oxide precipitates , which also falsifies the results. Iodide cannot be titrated according to Mohr, because the chromate used as an indicator oxidizes the iodide to iodine or higher oxidation states. On the one hand, this would result in lower results; on the other hand, the indicator would wear out and could no longer function as such.

Others

This determination method is used in the Austrian Pharmacopoeia (ÖAB).

Similar methods of determination for halide ions that can do without the toxic chromate are titration according to Fajans and titration according to Volhard .

swell

Jander, Blasius: Introduction to the inorganic-chemical internship. 14th edition. S. Hirzel Verlag, Stuttgart, Leipzig 1995, ISBN 3777606723

Mohr, Friedrich: Textbook of the chemical-analytical titration method. 1st edition. Friedrich Vieweg and Son, Braunschweig 1855, 2nd section, p. 10 ff.

Individual evidence

↑ Friedrich Mohr: Textbook of the chemical-analytical titration method. Based on our own experiments and presented systematically . For chemists, doctors and pharmacists, mining and Metalworkers, manufacturers, agronomists, metallurgists, mint officials, etc. Second revised edition. Friedrich Vieweg and Son, Braunschweig November 1862, VII Precipitation analyzes § 138 Chlorine, p. 317–320 ( online on the pages of the Bayerische Staatsbibliothek [accessed on January 9, 2015]).
↑ Gerhard Schulze, Jürgen Simon, Jander Jahr Maßanalyse , 17th edition, de Gruyter, Berlin, 2009, p. 140.

[1] Friedrich Mohr: Textbook of the chemical-analytical titration method. Based on our own experiments and presented systematically . For chemists, doctors and pharmacists, mining and Metalworkers, manufacturers, agronomists, metallurgists, mint officials, etc. Second revised edition. Friedrich Vieweg and Son, Braunschweig November 1862, VII Precipitation analyzes § 138 Chlorine, p. 317–320 ( online on the pages of the Bayerische Staatsbibliothek [accessed on January 9, 2015]).

[JanderJahr-2] Gerhard Schulze, Jürgen Simon, Jander Jahr Maßanalyse , 17th edition, de Gruyter, Berlin, 2009, p. 140.