ANALYSIS OF TISSUES 591 



solution poured over a strip of filter paper. The flask is washed out with 

 small quantities of water and the washings poured upon another strip of paper. 

 The strips are rolled whilst wet, placed in a Soxhlet extractor and extracted 

 with washed ether for 3-4 hours; 100 c.c. of water are added and the ether 

 removed by distillation. 



(Embden performs two estimations each with 500-600 c.c. of solution ; the 

 solution is neutralised with strong caustic soda, then slightly acidified with hydro- 

 chloric acid and concentrated in vacua to 100 c.c. at a temperature below 50 C. 

 The liquid must be clear and nearly colourless and of neutral or slightly acid 

 reaction. If it has become alkaline, the extraction cannot be continued as the 

 alkali may have produced lactic acid from glucose. The solution is transferred 

 to an extractor, acidified with 15 c.c. of 60 per cent, phosphoric acid and 

 saturated with ammonium sulphate. The ether extraction is continued for 30 

 hours and subsequently for another 10 hours to ascertain if the whole has 

 been extracted during the first period. 



The contents of the flask are transferred to. a conical flask and the remains 

 washed into it with ether ; 20 c.c. of water are added, the ether evaporated off 

 and then 200-300 c.c. more water.) 



The solution is filtered and heated on a boiling water-bath for i hour 

 with excess of lead carbonate. The contents are allowed to stand at o for 

 12-16 hours and filtered from the precipitate which consists mainly of lead 

 phosphate and which is thoroughly washed with water. The lead is re- 

 moved with hydrogen sulphide, the precipitate washed and the hydrogen 

 sulphide removed by a current of air. The solution is boiled with an excess 

 of zinc carbonate, again filtered, concentrated to 20 c.c. and any precipitate 

 filtered off. The filtrate is evaporated to dryness in a weighed basin, the 

 white crystals dried at no and weighed. 



The purity of the salt should be ascertained by a determination of the 

 water of crystallisation and the zinc content. 



The estimation of lactic acid as aldehyde in the zinc salt is effected by the 

 method of von Fiirth and Charnass as follows : 



The zinc lactate is dissolved in hot water and washed into a round 

 bottom flask of 800 c.c. capacity. The volume of water should not exceed 

 50 c.c. and the amount of lactic acid '2 gm. 300 c.c. of *5 per cent, sulphuric 

 acid and about '5 per cent, of talc are added. The flask is fitted with a 

 dropping funnel and safety tube leading to a condenser, placed most con- 

 veniently in a vertical position and the end of which dips under 150 c.c. of 

 water and a measured quantity of - 2N bisulphite solution contained in a 

 litre flask. The solution is distilled and when boiling has commenced - oiN 

 permanganate is dropped in at the rate of 90-120 drops per minute. The 

 oxidation is continued until the solution appears brown and then for 10 

 minutes longer, during which time the permanganate is run in more slowly. 



The contents of the receiver are titrated with 'iN iodine solution using 

 starch as indicator : 



CH 3 . CHOH . COOH + O = CH 3 . CHO + CCX, + H 2 O 

 CH 3 . CHO + NaHS0 3 = CH 3 . CH(OH) . SO 3 Na 



I 2 + S0 2 + 2H 2 6 = H 2 S0 4 + 2HI. 



go gm. lactic acid = 44 gm. acetaldehyde = 2 x 127 gm. iodine, 

 i c.c. of 'iN iodine solution = -0022 gm. acetaldehyde 

 = '0045 gm. lactic acid. 



The bisulphite solution is titrated with iodine just before use, since its 

 strength alters on keeping, but it does not alter during the time taken in the 

 oxidation process, 



