Berthelot reaction

The Berthelot reaction named after the French chemist Marcelin Berthelot (1827–1907) is the reaction of ammonia to form the blue colored indophenol ion. The reaction is used for the quantitative detection of ammonia or ammonium in aqueous solution.

Reagents

The following reagents are required for the reaction:

• Base:
  The reaction takes place in an alkaline medium . Therefore, the ammonia solution has to be brought to a pH of approx. 13 beforehand
• Chlorinating agent: Hypochlorite (OCl ^- )
  has proven useful here. According to DIN, however, dichloroisocyanuric acid must be used.
• Phenol :
  For the mechanism, it does not matter which phenolic compound is used. In practice, thymol (2-isopropyl-5-methylphenol) has proven itself due to its higher stability and speed. Here the DIN requires sodium salicylate .
• Catalyst:
  The complex nitroprusside is usually used to catalyze the reaction . It is possible to use Mn (II) ions as a less toxic alternative, but the temperature optimum is then around 80 ° C, which cannot be achieved with many standard measuring devices.

mechanism

Since the reaction takes place in an alkaline medium (pH = 13), all ammonia is present as NH ₃ , because the equilibrium of the reaction is strongly shifted to the side of ammonia according to the principle of Le Chatelier :

${\displaystyle \mathrm {NH_{3}+H_{2}O\rightleftharpoons NH_{4}^{+}+OH^{-}} }$

In the second step, ammonia reacts with hypochlorite to form monochloramine :

${\displaystyle \mathrm {NH_{3}+OCl^{-}\longrightarrow NH_{2}Cl+OH^{-}} }$

In the third step, the monochloramine reacts with thymol to form N-chloro-2-isopropyl-5-methylquinone monoimine. (This reaction is catalyzed by nitroprusside .)

In the fourth step, N-chloro-2-isopropyl-5-methylquinone-monoimine reacts with another thymol molecule to form the corresponding indophenol . In an alkaline medium, the indophenol molecule formed is in its blue base form:

Concentration determination

The concentration can be determined photometrically . The limits of quantification for this method are approx. 0.01 mg / l and approx. 3.5 mg / l ammonium / ammonia.

Individual evidence

1. ↑ B. Känel, K. Mez, Microbial Ecology Group of the University of Zurich, Laboratory Methods, Ammonia colorimetric  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 8 kB).  
2. ↑ Tsuboi, T .; Hirano, Y .; Shibata, Y .; Motomizu, S .: Sensitivity Improvement of Ammonia Determination Based on Flow-Injection Indophenol Spectrophotometry with Manganese (II) Ion as a Catalyst and Analysis of Exhaust Gas on Thermal Power Plant . In: Analytical Sciences , October 2002, Vol. 18, pp. 1141-1144. doi : 10.2116 / analsci.18.1141