PHYSIOLOGICAL CHEMISTRY 455 



The extract is evaporated to small bulk, acidified with phosphoric acid, and 

 extracted repeatedly by shaking in a funnel with many times its volume of 

 ether. The residue after the evaporation of the ether extract is dissolved in 

 water, boiled with zinc carbonate, filtered and evaporated. Crystals of the 

 zinc lactate are thus obtained, which are weighed either after drying in air, 

 when they contain their water of crystallisation, or after complete drying at 

 110 C. The product is liable to be impure. This is partly obviated by washing 

 the crystals with alcohol. Frequently the aqueous solution obtained after ether 

 extraction is treated with lead carbonate, filtered, treated with hydrogen 

 sulphide to remove lead, and again extracted, but this procedure involves 

 loss. In any case the percentage of zinc in the zinc salt must be determined 

 in order to prove its identity as zinc lactate. 



The method, though cumbrous, is sufficiently satisfactory when relatively 

 large amounts of lactic acid are present, but, when the amount of lactic acid 

 is small, the zinc lactate is almost certain to be impure, especially if the 

 determination is made on uriiie. Moreover, in the extraction of lactic acid 

 from a watery solution by means of ether there is always some loss, due to 

 incomplete extraction and to oxidation of lactic acid by impurities in the ether. 

 When the amount of lactic acid is small, this loss is relatively very considerable. 



The Thiophene Test for lactic acid introduced by Hopkins has already 

 been described (see p. 354). It is very sensitive and is distinctive for 

 a hydroxy acids, but cannot be used quantitatively and is difficult to apply to 

 urine. 



The Distillation Method. This method depends on the fact that lactic 

 acid, when heated above 140 C. with sulphuric acid, yields acetaldehyde 

 quantitatively according to the following equation : 



CH 3 . CHOH . COOH = CH 3 . CHO + H . COOH. 



40 c.c. of the liquid, which must be free from sugar and nearly free from 

 protein, are placed in a 500 c.c. Jena distillation flask. 45 c.c. pure sulphuric 

 acid are rapidly added from a dropping funnel, the flask being shaken and 

 cooled under the tap. The flask is then fitted with a rubber cork carrying 

 an inlet tube for steam and a thermometer, so arranged that both dip well 

 below the surface of the liquid. It is then placed in a slanting position on 

 wire gauze on a retort stand and attached to a good vertical condenser. (For 

 this purpose the exit tube of the flask must be bent at a suitable angle.) A 

 flask of about 300 c.c. capacity, immersed in cold water, is placed as the 

 receiver of the condenser with its mouth just touching the jacket of the 

 condenser, so as to prevent loss of aldehyde by evaporation. A gentle 

 current of steam from an ordinary steam generator is then passed into the 

 distillation flask, which is vigorously heated with a Bunsen burner. Distillation 

 will generally begin at about 140 C., but the heating is continued till the 

 temperature reaches 155 C., when the current of steam is increased, and the 

 heat applied to the flask adjusted so that the temperature is kept between 

 153 and 157 C. When about 100 c.c. have collected in the receiver, or the 

 distillation has lasted nearly 30 minutes, the decomposition is complete. The 

 contents of the receiver are rendered just permanently alkaline by the addition 

 of 2 per cent, caustic soda solution and a little litmus solution, diluted to 

 about 150 c.c., and redistilled into a flask with a 100 c.c. mark in the neck, 

 using the same precautions to prevent loss as before, until about 50 c.c. have 



