434 PHYSIOLOGICAL CHEMISTRY 



sodium potassium tartrate (Rochelle salt).] This is done to determine whether 

 the solution will of itself cause the formation of a precipitate of brownish-red 

 cuprous oxide. If such a precipitate forms, the Fehling's solution must not be 

 used. Add urine L to the hot Fehling's solution, a few drops at a time, and heat the 

 mixture to boiling after each addition (never add more urine than the original 

 volume of Fehling's solution). The production of yellow or brownish-red cup- 

 rous oxide indicates that reduction has taken place. The yellow precipitate is 

 more likely to occur if the urine is added rapidly and in large amount, whereas 

 with a less rapid addition of smaller amounts of urine the brownish-red pre- 

 cipitate is generally formed. The differences in color of the cuprous oxide pre- 

 cipitates under different conditions are apparently due to differences in the 

 size of the particles, the more finely divided precipitates having a yellow 

 color, while the coarser ones are red. In the presence of protective colloidal 

 substances the yellow precipitate is usually formed. 2 



This is a much more satisfactory test than Trommer's, but even 

 this test is not entirely reliable when used to detect sugar in the urine. 

 Such bodies as conjugate glycuronates, uric acid, nucleo protein, and homo- 

 gentisic acid, when present in sufficient amount, may produce a result 

 similar to that porduced by sugar. Phosphates of the alkaline earths 

 may be precipitated by the alkali of the Fehling's solution and in appear- 

 ance may be mistaken for the cuprous oxide. Cupric hydroxide 

 may also be reduced to cuprous oxide and this in turn be dissolved by 

 creatinine, a normal urinary constituent. This will give the urine under 

 examination a greenish tinge and may obscure the sugar reaction even 

 when a considerable amount of sugar is present. According to Laird 3 

 even small amounts of creatinine will retard the reaction velocity of re- 

 ducing sugars with Fehling's solution. 



Conjugate glycuronates are formed after the ingestion of such sub- 

 stances as chloral hydrate, camphor, menthol, thymol, antipyrin, 

 phenol, etc. The chloral hydrate is excreted in the urine as trichlor- 

 ethylglycuronate, CC1 3 CH 2 OOC(CHOH) 4 CHO. This compound re- 

 duces Fehling's solution and is /e^orotatory, whereas glucose also 

 reduces but is dextrorotatory. Therefore by means of a polariscopic 

 test we may differentiate between a "chloral urine" and a "sugar 

 urine. " 



In testing urine preserved by chloroform a positive test may be ob- 

 tained in the absence of sugar. This is due to the fact that the hot 

 alkali produces formic acid (a reducing fatty acid) from the chloroform. 



Ammonium salts also interfere with Fehling's test. If present in 



1 In case doubtful results are obtained by this test owing to the presence of interfering 

 substances the urine should be clarified and the test repeated. To clarify the urine in- 

 troduce 10 c.c. into a test-tube, add i gram of pure blood charcoal, heat to boiling and allow 

 to stand with occasional shaking for five minutes. Use the filtrate in the test. 



2 Fischer and Hooker: Science, N. S. XLV, 505, 1917. 



3 Laird: Journ. Path, and Bact., 16, 398, 1912. 



