THE AMINO ACIDS 



It is 



Properties. 



Taurine crystallises in colourless four or six-sided prisms (Fig. 32). 

 soluble in 15-16 parts of cold water, 

 more easily in hot water. It is insoluble 

 in absolute alcohol and ether, but is 

 slightly soluble in cold dilute alcohol, 

 more easily in hot. 



It is decomposed on boiling with 

 caustic alkali yielding acetic and sul- 

 phurous acids. 



It is identified by its crystalline form, 

 sulphur content, and the formation of 

 a white insoluble compound when its 

 solution is boiled with freshly precipi- 

 tated mercuric oxide. It is not precipi- 

 tated by metallic salts. 



Estimation of Amino Acids. 



(i) By Titration. 



FIG. 32. Taurine. (After Funke.) 

 As the ammo acids are neutral 



in reaction, they cannot be titrated by means of standard alkali like 

 ordinary acids. They combine, however, with formaldehyde and 

 yield an acid which can be titrated in this way : 



CH 2 . NH 2 



CH . N= 



+ OHC . H = 



I 

 C 



H 2 0. 



COOH COOH 



The process of estimation is carried out as follows : 

 10 c.c. of commercial formalin are diluted with two volumes of 

 water and neutralised with -iN alkali, using 6 to 8 drops of phenol- 

 phthalein in the solution as indicator. This neutralised formalin is 

 added to 20 or 25 c.c. of the amino acid solution measured out with 

 a pipette into a small flask or beaker. The pink colour disappears. 

 iN sodium hydroxide solution is run in from a burette until a dis- 

 tinct pink colour again appears. The number of c.c. of alkali used is 

 noted. 



If the amino acid be known its amount in the solution can be 

 calculated : 



CH 2 . (NH 2 ) . COOH NaOH 

 - --- v - - ' + *<> = CH 2 . (NH 2 ) . COONa + H 2 O. 



75 40 



i c.c. *iN NaOH = i c.c. 'iN glycine = 0-0075 g m - glycine. 



If the amino acid be unknown, or if a mixture of amino acids be 

 present, the amount is best expressed in terms of *iN acid, or in terms 

 of nitrogen. 



