130 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



solution. Fehling's solution should always be freshly prepared, as, on stand- 

 ing, racemic acid is formed from the tartaric acid, and this substance itself 

 reduces the cupric to cuprous oxide. Fehling's solution should, therefore, 

 always be tested by boiling before it is used. If it remains unaltered by 

 boiling, it is in good condition. 



4. Quantitative Determination of Sugar in Urine. — Fehling's solution is pre- 

 pared as follows : — 34'639 grammes of copper sulphate are dissolved in about 

 200 c.c. of distilled water ; 173 grammes of Rochelle 

 salt are dissolved in 600 c.c. of a 14-per-cent. solution 

 of caustic soda. The two solutions are mixed and 

 diluted to a litre. Ten c.c. of this solution are 

 equivalent to 0-05 gramme of dextrose. Dilute 10 c.c. 

 of this solution with about 40 c.c. of water, and boil it 

 in a white porcelain dish. Run into this from the 

 burette (see fig. 54) the urine (which should be 

 previously diluted with nine times its volume of 

 distilled water) until the blue colour of the copper 

 solution disappears — that is, till all the cupric hydrate 

 is reduced. The mixture in the basin should be boiled 

 after every addition.^ The quantity of diluted urine 

 used from the burette contains 0'05 gramme of sugar. 

 Calculate the percentage from this, remembering that 

 urine has been diluted to ten times its original 

 volume. 



The following formula will be found useful in con- 

 verting grammes into grains : — 



X = number of grains of sugar in the 24 hours, 

 a = number of ounces of urine in the 24 hours. 

 1 ounce = 28"396 c.c. 



b = number of c.c. of urine used from the burette 

 to decompose 10 c.c. of Fehling's solution (equivalent 

 to 0*05 gramme = 0-77 grain of sugar). 

 Then 



Fia. 54.— Two burettes on 

 stand. (Sutton.) 



x= ~ X 28-396 X 0-77 = , x 21-865. 



Pavy's modification of Fehling's solution is some- 

 times used. Here ammonia holds the copper in solution, and no precipitate 

 forms on boiling with sugar, as ammonia holds the cuprous oxide in solution. 

 The reduction is complete when the blue colour disappears ; 10 c.c. of Pavy's 

 solution = 1 c.c. of Fehling's solution = 0-005 gramme of dextrose. In some 

 cases of diabetic urine where there is excess of ammonio-magnesic phosphate, 

 the full reduction is not obtained with Fehling's solution, and when the 

 quantity of sugar is small it may be missed. In such a case excess of soda 

 or potash should be first added ; the precipitated phosphates filtered off, and 

 the filtrate after it has been well boiled may then be titrated with Fehling's 

 or Pavy's solution. 



5. Picric Acid Test. — The work of Sir George Johnson and G. S. Johnson 

 has shown the value of this reagent in detecting both albumin and sugar 

 in the urine. The same reagent may be employed for the detection of 

 both substances. The method of testing for albumin has been already studied 

 with Esbach's tubes. To test for sugar do the following experiment. Take 

 a drachm (about 4 c.c.) of diabetic urine ; add to it an equal volume of 

 saturated aqueous solution of picric acid, and half the volume (i.e. 2 c.c.) of 

 the liquor potassse of the British Pharmacopoeia. Boil the mixture for about 



' On cooling the blue colour reappears, owing to re-oxidation. 



