334 PHYSIOLOGICAL CHEMISTRY. 



carbonates of the fixed alkalies or of such salts of vegetable acids 

 tartaric acid, citric-acid, and malic-acid salts as are easily burnt 

 into carbonates in the organism. Under pathological conditions, as 

 in the absorption of alkaline transudations, the urine may become 

 alkaline 



The degree of acidity cannot be determined by the ordinary acidi- 

 metric process, since the urine contains di-hydrogen phosphate, 

 MH 2 P0 4 , besides hydrogen di-phosphate M 2 HP0 4 . In the titra- 

 tion the di-hydrogen phosphate is changed gradually into MgHPO^, 

 and we obtain at a certain point a mixture of the two phosphates in 

 variable proportions, which mixture is not neutral but amphoteric. 

 Since it is generally admitted that the acid reaction of urine is due 

 to the di-hydrogen phosphate, it is therefore best to express the 

 degree of acidity by the amount of di-hydrogen phosphate present. 



If we wish to calculate the degree of acidity of the urine 

 as di-hydrogen phosphate or, still more simply, as phosphoric 

 anhydride, P 2 5 , contained in this salt, the titration is performed 

 according to the method of MALY and HOFFMANN, which is as fol- 

 lows : The urine (100-200 c. c.) is treated with an exactly-measured 

 quantity of normal caustic-soda solution, which is more than suf- 

 ficient to convert all the phosphate into basic phosphate, or, in other 

 words, enough to make the urine strongly alkaline. Then an 

 approximate f normal Ba01 2 solution (142.8 grms. BaCl 2 ,2H0 3 in 

 a litre) is added until no further precipitate is formed. By this 

 means all the phosphoric acid is precipitated from the urine. Filter 

 through a dry filter, measure a quantity corresponding to 50 or 

 100 c. c. of the original urine from the filtrate, and titrate with 

 J normal sulphuric acid until a neutral reaction is obtained, using 

 litmus-paper as an indicator. If the amount found by this titration 

 be subtracted from the original amount of caustic soda added to 

 this volume of urine, the difference is the amount of caustic soda 

 necessary to convert the existing di-hydrogen and hydrogen di-phos- 

 phates into normal phosphate. If we designate this by a, and the 

 quantity of total P 2 6 in milligrammes in the same quantity of 

 urine, as determined by a method which will be described later, by g, 

 then we obtain the quantity of P 2 5 in milligrammes in the di-hy- 

 drogen phosphate s by the following formula : s = 17.75 a g 



(HUPPERT). 



If, for example, in a case in which the conversion of both phos- 

 phates into normal phosphate in 100 c.c. of the urine required 20 c.c. 

 caustic soda, while the total quantity of P 2 5 in 100 c.c. urine was 

 275 milligrammes, then : s = 17.75 X 20 275 = 80 milligrammes. 

 The quantity of P 2 5 as simple acid phosphate was therefore 195 

 milligrammes. 



