ELEMENTARY CHEMICAL PHYSIOLOGY 253 



Urea, p. 255). For this reason stale urine always reacts alkaline. 

 If the alkaline reaction of freshly passed urine is due to ammonia, 

 decomposition must be taking place in the bladder. 



A very simple method of determining the distribution of the phosphate 

 between the acid and alkaline salts has been introduced by Leathes. In a 

 solution of sodium phosphate, if all the phosphate is acid phosphate, methyl 

 orange turns pink : if any of it is dibasic this indicator is yellow ; when it is 

 all dibasic, and not before, phenolphthalein turns pink. To determine the 

 nature of the phosphates present in the urine two 10 c.c. samples of urine well 



diluted with distilled water are titrated, the one with j-QH 2 SO 4 , using methyl 



N 

 orange as indicator, and the other with NaOH, using phenolphthalein as 



indicator. The first titration gives the amount of dibasic phosphate present 

 and the second the amount of acid phosphate. The ratio of the amounts of 

 acid and alkali used in the two titrations gives the ratio in which the two 

 phosphates are present. As the phosphates are the principal constituents of 

 the urine which determine its reaction, the reaction of the urine may be 

 expressed in the same way as that of a pure phosphate solution, in acidity or 

 alkalinity per cent. 



Colour. The colour of urine varies in health from pale amber 

 or straw to deep red brown. 



(a] Nearly colourless due to dilution or diminution of pigments, 

 as in polyuria from any cause, diabetes insipidus, certain diseases 

 of the kidney. 



(6) Dark yellow or brown red due to an increase of pigment or 

 increase of concentration of urine as in acute febrile diseases or 

 after profuse sweating. 



(c) Red or reddish due to the presence of haemoglobin or pigments 

 used in colouring foodstuffs. 



(d) Brown or brown black due to the presence of haematin, 

 methaemoglobin, melanin or after carbolic acid poisoning. 



(e) Brown yellow to red brown due to the presence of various 

 colouring matters of vegetable origin, as after the administration 

 of santonin, chrysophanic acid, senna, rhubarb, etc. Note the 

 addition of NaOH turns urine which contains many of these 

 vegetable dyes, red. 



(/) Greenish yellow or yellow brown due to the presence of bile 

 pigments, as in various hepatic diseases. 



As regards the actual pigments, apart from those of definitely 

 pathological origin or derived from substances administered to the 

 subject, the principal pigment would seem to be urochrome. After 

 the removal of this pigment from the urine the colour is largely 

 lost. It is a sulphur containing pigment and has an acid reaction. 

 It shows no absorption bands, but it cuts off the violet end of the 

 spectrum. It does not fluoresce with zinc salts. Urobilin, mostly 

 in the form of a colourless chromogen, is also present in very small 

 amount in normal urine. In many pathological conditions urobilin is 

 present in large amount. The chromogen is readily converted into 

 urobilin by oxidation. Uroerythrin is also present in traces. (See 

 p. 297.) 



