416 PHYSIOLOGICAL CHEMISTRY. 



have been ' strongly diluted with water. In urines poor in sugar 

 which contain the normal amount of urea and which have not been 

 strongly diluted, a rather abundant formation of ammonia from 

 the urea may take place on boiling the alkaline liquid. This am- 

 monia dissolves the suboxide in part, which easily passes into oxide 

 thereby, and besides this the dissolved suboxide gives a red color 

 with potassium ferrocyanide. In just those cases in which the 

 titration is most difficult this end-reaction is the least reliable. 

 Practice also renders it unnecessary, and it is therefore best to 

 depend simply upon the appearance of the liquid. 



To facilitate the settling of the copper suboxide and thereby 

 clearing the liquid, MUNK has lately suggested the addition of a 

 little calcium-chloride solution and boiling again. A precipitate of 

 calcium tartrate is produced which carries down the suspended 

 copper suboxide with it, and the color of the liquid can then be 

 better seen. This artifice succeeds in many cases, but unfortunately 

 there are urines in which the titration with FEHLING'S solution in 

 no way gives exact results. 



The necessary conditions for the success of the titration under 

 all circumstances are, according to SOXHLET, the following: The 

 copper-sulphate and Rochelle-salt solution must, as above, be diluted 

 to 50 c. c. with water; the urine must only contain between 0.5$ 

 and \% sugar, and the total quantity of urine required for the re- 

 duction must be added to the titration liquid at once and boiled 

 with it. From this last condition it follows that the titration is 

 dependent upon minute details, and several titrations are required 

 for each determination. 



It is best to give here directions for the titration. The proper 

 amount of copper-sulphate and Rochelle-salt solution and water 

 (total volume = 50 c. c.) is heated to boiling in a flask: the color 

 must remain blue. The urine diluted five times is now added to 

 the boiling-hot liquid, 1 c. c. at a time; after each addition of 

 urine boil for a few seconds, and look for the appearance of the 

 end-reaction. If you find, for example, that 3 c. c. is too little but 

 that 4 c. c. is too much (the liquid becoming yellowish), then the 

 urine has not been sufficiently diluted, for it should require be- 

 tween 5 and 10 c. c. of the urine to produce the complete reduc- 

 tion. The urine is now diluted ten times, and it should now 

 require between 6 and 8 c. c. for a total reduction. Now prepare 

 four new tests, which are boiled simultaneously to save time, and 

 add at one time respectively 6, 6, 7, and 7J- c. c. of urine. If it is 

 found that between 6J and 7 c. c. are necessary to produce the 

 end-reaction, then make four other tests, to which add respectively 

 6.6, 6.7,6.8, and 6.9 c. c. of urine. If in this case the liquid is 

 still somewhat bluish with 6.7 c. c. and completely decolorized with 

 6.8 c. c., we then consider the average figure 6.75 c. c. as correct. 



