PRACTICAL EXERCISES 821 



evaporate them to dryness on the water-bath. Rub up the residues 

 with a few c.c. of hot water. Add to each aqueous extract a small 

 quantity (say a decigramme) of finely powdered charcoal. Then heat 

 each extract to boiling in a test-tube, and filter. Evaporate the 

 filtrates to dryness, and apply 



Hopkins 's Reaction for Lactic Acid. The reagents required are (i) a 

 very dilute alcoholic solution of thiophene (C 4 H 4 S) (10 to 20 drops in 

 100 c.c.); (2) a saturated solution of copper sulphate; and (3) ordinary 

 strong sulphuric acid. 



Have ready a glass beaker containing water briskly boiling. Place 

 about 5 c.c. of strong sulphuric acid in a test-tube, with i drop of the 

 copper sulphate solution.* Add to the mixture a few drops of the 

 solution to be tested, and shake well.f 



(In the case of the muscle extracts the dry residues aro dissolved in 

 the 5 c.c. of strong sulphuric acid, the acid transferred to test-tubes, 

 and the test proceeded with by the addition of the copper sulphate 

 solution, etc.) 



Now place the test-tube in the boiling water for one to two minutes. 

 Then cool it well under the cold-water tap, and add 2 or 3 drops of 

 the thiophene solution from a pipette. Replace the tube in the boiling 

 water, and immediately observe the colour. If lactic acid is present, 

 the liquid rapidly takes on a bright cherry-red colour, which is only 

 permanent if the test-tube be cooled immediately after its appearance. 

 The tube should always be cooled as described, before addition of the 

 thiophene, as the gradual appearance of the colour on re-warming 

 makes the test more delicate. 



(The extract of the resting limb generally gives a negative, that of 

 the other a strongly positive, reaction.) 



* The copper sulphate is added to hasten the oxidation that follows. 



f For practice use a i per cent, alcoholic solution of lactic acid. The test 

 cannot be applied directly to material which chars with the strong sulphuric 

 acid used . In this case preliminary extraction of the lactic acid is necessary. 

 Alcohol should be used as the solvent, or if ether is employed it must first be 

 well washed to remove aldehyde-yielding products, since the colour-change is 

 due to an aldehyde reaction with thiophene. 



