What is the titration of Mohr salt

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

With this titration, the solution containing chloride or bromide is titrated directly with standard silver nitrate solution. Potassium chromate is used as an indicator. Silver chromate is formed at the equivalence point. This is sparingly soluble and, in contrast to silver chloride, which is also sparingly soluble, red-brown.

$ \ mathrm {Cl ^ - _ {(aq)} + Ag ^ + _ {(aq)} \ longrightarrow AgCl \ downarrow} $
When adding silver nitrate solution, white silver chloride precipitates up to the equivalence point.
$ \ mathrm {CrO_ {4 (aq)} ^ {2-} + 2 \ Ag ^ + _ {(aq)} \ longrightarrow Ag_2CrO_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 7.2, as 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 changes 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.

miscellaneous

This verification procedure is used in the Austrian Pharmacopoeia (├ľAB).

Other detection methods for halide ions 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.