Formalin solution estimation

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. 

Davies et al. (27) also extended their studies to distillation of formalin solutions, that is the analysis of a distillation coluOTi involving reactions of formaldehyde with water and methanol. The problem was complicated by the estimation of equilibrium data for a system of five-components of which one (hemiformal) does not exist in a pure state. Although good agreement is claimed between theory and experiment, some of the assumptions are rather doubtful, e.g. the use of the A.I.Ch.E. method for prediction of plate efficiencies. 

Estimation of Ammonia.— Neutralise 10 ml. of urine to phenol-phthalein, using N/10 NaOH, as in the estimation of the total acidity of urine. Neutralise 2-3 ml. of commercial formalin solution (30-40 per cent, formaldehyde) in the same manner. Mix the two neutral solutions. The mixture becomes acid, and the pink colour of the indicator is discharged. This is because the H-ions previously combined with the ammonia have been liberated by the formaldehyde. Titrate this increase in acidity as before with N/10 NaOH. Each ml. of alkali required corresponds to 1 7 mg. of NHg in the original urine. The ammonia value obtained by the formaldehyde method is usually 10-25 per cent, too high owing to the fact that the —NHj groups of other urinary solutes also interact. The discrepancy is usually neglected in clinical estimations. 

Although ammonium acetate, C2H302(NH4), Mol. Wt. 77-09, can be estimated by either of the usual methods (a) by distillation of the ammonia in sodium hydroxide solution into excess of standard acid, or (b) by adding excess standard alkali, boiling until free from ammonia and back titrating it is most conveniently determined by utilising the reaction of ammonia with formalin neutral hexamethylenetetramine is produced and the acetic acid is titrated. 

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