162 THE SIMPLER NATURAL BASES 



is due to the reduction of the picric acid and is also caused by acetone, 

 acetaldehyde, hydroxylamine and titanium chloride in the cold, and 

 by dextrose, maltose, laevulose, and urea on warming. 



(<:) Maschke's reaction [1878]. The creatinine solution is satur- 

 ated with sodium carbonate ; on warming with Fehling's solution 

 the blue colour is discharged and a white precipitate of creatinine 

 cuprous 6xide appears, which is readily soluble in water, but only 

 slightly so in sodium carbonate solution. 



Foliris method [1904]. Since the coloration produced by picric 

 acid and sodium hydroxide gradually fades, a half normal solution of 

 potassium bichromate (24 -54 grm. per litre) is employed as a permanent 

 standard of colour ; this accurately matches the creatinine coloration. 

 Since the intensity of coloration is further influenced by dilution, it is 

 necessary to work within certain limits and the solution to be ex- 

 amined should contain 7-15 mg. of creatinine in 500 c.c. 



Folin adds to 10 c.c. of urine in a 500 c.c. measuring flask 15 c.c. 

 of saturated (i'2 per cent.) aqueous picric acid solution and 5 c.c. of 

 ip per cent, sodium hydroxide; after shaking, the solution is allowed 

 to stand for five minutes to let the colour develop fully, and is then 

 made up to 500 c.c. The solution thus diluted is now matched with 

 a column of the O'5N bichromate solution 8 mm. high. If the column 

 of creatinine solution required to do this has a height of x mm. there 



8'i 

 are present in the 10 c.c. of urine employed - - x 10 mg. of creatinine. 



X 



If more than 15 mg. of creatinine is present, only 5 c.c. of urine are 

 taken, if less than 7 mg. 20 c.c. are employed. For substances which 

 interfere with the test, see above, under JafTe's reaction. 



According to Thompson, Wallace and Clotworthy [1913] the 

 maximum colour develops in 5 minutes at 17-20; at 15-17 seven 

 minutes are required, at 10-15 eight minutes. 



The necessity of a constant temperature has been emphasised by 

 Mellanby [1908], Chapman [1909] and others. Mellanby has 

 plotted a curve showing the variation of colour with dilution and 

 Cook [1909] has suggested a correction for dilution, namely the 

 addition of 0-19 mg. to the value found for every 10 c.c. of dilution 

 above the original 10 c.c. ; thus for a 100 c.c. solution 9 xo'19 mg. 

 should be added. For factors influencing the estimation in urine 

 consult Taylor [1910] who considers that the variation in the light 

 and in the pigmentation of the urine constitute the chief sources of 

 error, and also Thompson, Wallace and Clotworthy [1913]. Under 

 ideal conditions 10 mg. of creatinine may be estimated to within 0*1 



