Sorensen’s reaction

Sorensen s reaction. First read carefully the Estimation of Glycine, p. 463. Dissolve 0 2 g. of glycine in a few ml. of water in a test-tube A, add 2 drops of phenolphthalein and then very dilute NaOH solution drop by drop until the solution just turns pink. In a second test-tube B place 2 ml. of 40% formalin solution, add 2 drops of phenolphthalein solution and then the dil. NaOH solution until the solution just turns pink. Pour the contents of B into A and note the immediate decolorisation of the phenolphthalein, the solution now being acid. Observe also that several drops of dil. NaOH solution can now be added before the pink colour is restored. 

Formaldehyde Condensation.—When an amino acid in neutral solution is mixed with excess of neutral formaldehyde solution, the mixture becomes acid, and can be titrated (Sorensen s reaction), thus affording an important method for the estimation of amino acids and ammonium salts. The mechanism of the change has been studied by Levy (1937). According to the older view, formaldehyde condenses with the amino group to form an unstable methylene-imino derivative, thereby destroying the basicity of the group. 

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