ii2 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Detection. 



(1) Lactic acid in concentrated solution is decomposed by heating 

 with concentrated sulphuric acid with the formation of carbon mon- 

 oxide ; the gas may be ignited at the mouth of the test tube : 



CH 3 . CHOH . COOH = CH 3 . CHO + H 2 O + CO. 



Weak solutions of lactic acid are neutralised with sodium carbonate 

 and evaporated to a small volume before heating with sulphuric acid. 



(2) The formation of acetaldehyde may be detected by Denigh' method. 

 The dilute solution of lactic acid is heated for 2 minutes in a boiling water- 

 bath with 10 times its volu re of concentrated sulphuric acid. The liquid is 

 cooled, and 2 or 3 dro s of a 5 per cent, solution of guaiacol in alcohol are 

 added. On m'xing, a rose-red colour formed from the a'dehydeand guaiacol 

 is produced which increases in intensity on standing. 



(3) Lactic acid forms a soluble deep yellow ferric salt. If a dilute 

 solution of ferric chloride (scarcely coloured) be treated with a few 

 drops of a dilute solution of lactic acid, the colour becomes yellow. 



Other hydroxy acids and oxalic acid also give a sirrylar colour. 



(4) Uffelmanris Test. Uffelmann's reagent a I or 2 per cent, 

 solution of phenol treated with ferric chloride solution till of a dis- 

 tinctly violet colour is changed to yellow on the addition of lactic 

 acid. 



Note. Mineral acids decolorise the reagent ; other organic acids 

 also give a yellow or brownish colour. 



(5) Lactic acid gives the iodoform reaction (p. 67). 



(6) Thiophene Test. A few drops of a I percent, solution of lactic 

 acid in alcohol are added to 5 c.c. of concentrated sulphuric acid 

 containing 3 drops of saturated copper sulphate solution and heated 

 in a boiling water-bath for 5 minutes. Two drops of a -2 per cent, 

 alcoholic solution of thiophene are added to the cooled solution, and 

 on again warming a cherry-red colour is formed (Hopkins). 



These tests for lactic acid are not easily observed in extracts of 

 organs, etc., which contain lactic acid. The lactic acid should be ex- 

 tracted with ether, the ethereal solution evaporated and the residue 

 then tested for lactic acid. 



Estimation. 



Lactic acid is sometimes estimated by converting it into its zinc salt by 



boiling the solution with excess of zinc carbonate, filtering, evaporating and 



we ghing the zinc lactate which crystallises out. It is more usually estimated 



by oxidising it with permanganate in acid solution ; acetaldehyde is formed : 



CH 3 . CHOH . COOH + O = CH 3 . CHO + CO 3 + H 2 O. 



The acetaldehyde is distilled off during the oxidation, collected and 

 determined by treatment with excess of standard potassium bisulphite solution, 

 the excess of which is estimated by titration with standard iodine solution 

 (see p. 85). 



The estimation of lactic acid in tissues is described on p. 590. 



