Formol titration

In the formol titration is a quantitative determination method for ammonium salts and primary aliphatic amines

Since ammonium compounds are not sufficiently acidic (pK _a value 9.25) to be determined directly with sodium hydroxide solution , the fact that formaldehyde (formol) is added leads to the weakly basic urotropin (pK _a value 5, 2) arises:

${\displaystyle \mathrm {4\ NH_{4}^{+}\ +\ 6\ HCHO\ +\ 4\ H_{2}O\longrightarrow \ Urotropin\ +\ 6\ H_{2}O\ +\ 4\ H_{3}O^{+}} }$

The oxonium ions released after the above equilibrium reaction can be titrated with sodium hydroxide solution against phenolphthalein as an indicator. Formol titration is used to determine ammonium salts and organically bound nitrogen after it has been converted into water-soluble ammonium compounds by means of Kjeldahl digestion . Formol titration was developed by SPL Sørensen in 1907 . Sørensen titrated amino acids with potassium hydroxide in the presence of formaldehyde.

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• Harry Auterhoff : Textbook of Pharmaceutical Chemistry , 5th edition, Wissenschaftliche Verlagsgesellschaft mbH Stuttgart, 1968.