Volhard titration

The Volhard titration is a method for the quantitative determination of silver , chloride , bromide , iodide , cyanide and thiocyanate ions . It is counted to argentometry . The method is named after the German chemist Jacob Volhard .

Determination of silver by direct titration

This titration is a direct titration of silver ions with a standard thiocyanate solution. The sparingly soluble silver thiocyanate precipitates . The titration is carried out in a nitric acid solution, which must not contain any nitrite ions.

${\displaystyle \mathrm {Ag^{+}(aq)+NO_{3}^{-}(aq)+SCN^{-}(aq)\longrightarrow AgSCN\downarrow +NO_{3}^{-}(aq)} }$

Titration of silver with standard thiocyanate solution

Solutions containing iron (III) ions, usually ammonium iron ( III) sulfate solutions, are used as indicators . If there is an excess of thiocyanate, the bright red compound Fe (SCN) _{3 is formed} .

${\displaystyle \mathrm {3\ NH_{4}SCN(aq)+NH_{4}Fe(SO_{4})_{2}(aq)\longrightarrow Fe(SCN)_{3\ (rot)}+4\ NH_{4}^{+}+2\ SO_{4}^{2-}} }$

Reaction of the indicator at the equivalence point

Determination of halides

Silver can be determined directly in this way; the back titration method must be used for the halides, cyanide and thiocyanate . To do this, the sample is mixed with a known amount of silver nitrate solution and the remaining amount of silver nitrate is titrated with standard thiocyanate solution.

${\displaystyle \mathrm {Br^{-}(aq)+Ag^{+}(aq)+NO_{3}^{-}(aq)\longrightarrow AgBr\downarrow +NO_{3}^{-}(aq)} }$

Titration of the halide solution with standard silver nitrate solution

application

This detection method is often used in a wide variety of areas, for example for the determination of chloride in food or animal feed or in concrete. It is also listed in the Austrian Pharmacopoeia (ÖAB).

Alternatives

Other detection methods for halide ions are titration according to Fajans and titration according to Mohr .

Historical

Jacob Volhard (around 1900)

Jacob Volhard , who was appointed professor in Munich in 1869, published the method named after him for determining silver for the first time in 1874. In 1878 he published a detailed description.

swell

• Jander, Blasius: Introduction to the inorganic-chemical practical course , 14th edition, 1995
• Schweda, Eberhard: Jander / Blasius Inorganische Chemie II - Quantitative Analysis & Preparations , 16th edition, Hirzel Verlag, 2012 , p. 132f.

Individual evidence

1. ^ S. Suzanne Nielsen: Food Analysis . Fourth edition / Fourth ed.Springer Science & Business Media, 2010, ISBN 978-1-4419-1477-4 , doi : 10.1007 / 978-1-4419-1478-1 .
2. ↑ Concrete, hardened: Chloride Content by Volhard Titration. (PDF (55.4 kB)) Nordic Council of Ministers, 1996, accessed on January 9, 2015 . 
3. ↑ Jacob Volhard: About a new method of mass analytical determination of silver . In: Bavarian Academy of Sciences, mathematical-natural science class (Hrsg.): Session reports of the mathematical-physical class of the royal. Bavarian Academy of Sciences . tape 4 . Munich 1874, p. 54–62 ( online at archive.org - The Internet Archive ).
4. ↑ Jacob Volhard: About a new method of the analytical determination of silver . In: Journal for Practical Chemistry . tape 9 , no. 1 , 1874, p. 217–224 , doi : 10.1002 / prac.18740090117 ( online on the Gallica - Bibliothèque numérique website ).
5. ↑ Jacob Volhard: The application of sulfur cyanammonium in mass analysis . In: Justus Liebig's Annals of Chemistry . tape 190 , no. 1 , 1878, p. 1-61 , doi : 10.1002 / jlac.18781900102 .