158 THE SIMPLER NATURAL BASES 



The use of zinc chloride alone was introduced by Pettenkofer 

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

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 as such it has been entirely superseded by Folin's colorimetric estima- 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

 OT 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 monoclihic prisms of the composition C 4 H 7 O 2 N 3 , 

 H 2 O. The 12*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 18; 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 1*81 x io~ u at 40*2, Wood 

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

 Creatine is precipitated from aqueous solution by mercuric nitrate, 

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





