158 THE SIMPLER NATURAL BASES 



The use of zinc chloride alone was introduced by Pettenkofer 

 [1844], the discoverer of creatinine ; Neubauer [1863] and Salkovvski 

 [1886, 1890] attempted to make this method a quantitative one, but 

 as such it has been entirely superseded by Folin's colorimetric estima- 

 tion. The use of picric acid for the precipitation of creatinine from 

 urine was introduced by Jaff [1886]; other precipitants are mercuric 

 chloride (Maly [1871]) and phosphotungstic acid (Hofmeister [1880]). 



Quantitative conversion of creatine to creatinine. Benedict and 

 Myers [1907, 2] heated a dilute creatine solution containing 6-7 per 

 cent, hydrochloric acid (i.e. \ volume of the concentrated acid) in an 

 autoclave to 117 for forty-five minutes. Dorner [1907] warmed a 

 o*l per cent, creatine solution for 3-4 hours on the water bath with 

 twice its volume of normal hydrochloric acid (hence concentration of 

 acid = 2-44 per cent.). Thompson, Wallace and Clotworthy [1913] 

 recommend adding an equal volume of normal hydrochloric acid and 

 heating on the water bath for 3 hours or in the autoclave to 117-1 20 

 for 25 mins. 



According to the last named authors pure dextrose, up to 10 per 

 cent., does not affect the estimation of creatine, although 3 per cent, 

 phosphoric acid has been recommended instead of hydrochloric acid, 

 in order to avoid the formation of coloured products. Creatine figures 

 for diabetic urine may come 5 per cent, too low, probably owing to the 

 presence of aceto-acetic acid. The darkening of the urinary pigment 

 by treatment with acid may increase the creatine readings in human 

 urine by \-2\ per cent., in dog's urine by 10 per cent. 



According to Folin and Blanck [1910] creatine crystals may be 

 converted quantitatively into creatinine by heating without a solvent 

 in an autoclave for three hours at 4-5 atmospheres ; the water of 

 crystallisation appears to be the active agent. 



Physical and chemical properties of creatine. This substance forms 

 lustrous transparent monoclinic prisms of the composition C 4 H 7 O 2 N 3 , 

 H 2 O. The 1 2 -08 per cent, of water of crystallisation is given off 

 quantitatively at 100-110, and the crystals become opaque (a deter- 

 mination of the loss of weight may be used for identification). 



Creatine dissolves in 74 parts of water at 1 8 ; it is much more 

 soluble in hot water, but hardly at all in absolute alcohol (i : 9400). 

 The aqueous solution is neutral. The basic properties of creatine 

 are very feeble (dissociation constant i'8i x io~ n at 4O'2, Wood 

 [1903]) and its salts with mineral acids are hydrolysed by water. 

 Creatine is precipitated from aqueous solution by mercuric nitrate, 

 but not by phosphotungstic acid, nor by basic lead acetate crystal- 



