Amino-acids formol titration

An alternative, simpler, procedure for improving the inflexion in the neutralization of an amino-acid is to add formaldehyde to the solution although this does not affect the acid-titration curve, the one for alkaline titration is changed, as seen in Fig. 107. The effect of the formaldehyde is to increase the strength of the ammonium ion acid which is being titrated, and so the pH inflexion at the equivalence-point becomes much more obvious. This is the basis of the formol titration of amino-acids discovered by Sorensen (1907) approximately 10 per cent of formaldehyde is added to the solution which is then titrated with standard alkali using phenolphthalein as indicator. In the presence of thii concentration of formaldehyde the pH-neutralization curve has a sharp inflexion in the region of pH 9, and so a satisfactory end-point is possible with the aforementioned indicator. 

Formol titration and other methods of determining terminal amino groups are also used to measure the average molecular weight of protein hydrolysates, but they are imprecise because —NH2 groups of amino acid side chains also react and the molecular masses are usually underestimated. 

Formol titration after the addition of formaldehyde to a solution of the sample at pH 8-9, the free amino acids ate determined by titration with sodium hydroxide solution. 

Another kind of titration after transformation is provided by the formol titration of a-amino acids (Sprensen s method). We have already seen that it is impossible to titrate them in water with sodium hydroxide, because of their macroscopic ionization constant on the order of 10 -. The reaction with the hydroxide ion is not sufficiently quantitative to be used as a titration reaction ... 

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