EXCRETION BY THE KIDNEYS 



473 



The substances insoluble in alkaline urine are thrown down, the 

 deposit containing ammonio-magnesic or triple phosphate, formed by 

 the union of ammonia with the magnesium phosphate present in fresh 

 urine, and precipitated as clear crystals of ' knife-rest ' or ' coffin-lid ' 

 shape (Fig. 179), along with amorphous earthy phosphates, and often 

 acid ammonium urate in the form of dark balls occasionally covered 

 with spines (Fig. 182). Calcium phosphate (CaHPO 4 ) is another phos- 

 phate found in sediments deposited from alkaline or faintly acid urine. 

 It is usually amorphous, but sometimes in the form of long prismatic 

 crystals arranged in star fashion, and hence spoken of as stellar phos- 

 phate (Fig. 181). It is not pigmented. 



It is only in pathological conditions that the alkaline fermentation 

 takes place within the bladder. The reaction of the urine can readily 



o 



Fig. 179. Triple Phosphate. Fig. 180. Cystin. 



Fig. 1 8 1. Stellar Phos- 

 phate Crystals. 



be made alkaline by the administration of alkalies, alkaline carbonates, 

 or the salts of vegetable acids like malic, citric, and tartaric acid, which 

 are broken up in the body and form alkaline carbonates with the alkalies 

 of the blood and lymph. It is not so easy to increase the acidity of the 

 urine, although mineral acids do so up to a certain limit. If the admin- 

 istration of acid be pushed farther, ammonia is split off from the pro- 

 teins, and is excreted in the urine as the ammonium salt of the acid. 



Determination of the Acidity. A titration method is described in 

 the Practical Exercises (p. 508). In speaking of the reaction of blood, 

 it has already been mentioned (p. 25) that we can- 

 not determine by titration the true acidity or alka- 

 linity of a liquid in the physico-chemical sense i.e., 

 the concentration of the dissociated hydrogen and 

 hydroxyl ions respectively. E.g., when we titrate 

 equal quantities of decinormal* acetic acid and deci- 

 normal hydrochloric acid with decinormal potassium 

 hydroxide, using, say, phenolphthalein as the indi- 

 cator, nearly the same volume of the potassium Fig. 182. 

 hydroxide solution will be needed to neutralize each 

 acid. Yet it can be shown by physico-chemical 

 methods that the acetic acid in the strength used is 

 only dissociated to the extent of a little more than i per cent., while 

 about 80 per cent, of the hydrochloric acid is dissociated. The concen- 

 tration of the hydrogen ions is therefore eighty times as great in the 

 hydrochloric as in the acetic acid solution. What we determine by the 

 titration is not the true acidity, but the total amount of hydrogen which 

 can be replaced by metal. The concentration of the hydrogen ions in 

 normal urine is very small, on the average only about 0-003 milli- 



* A normal solution of a substance contains in a litre a number of grammes 

 of the substance equal to the number which expresses its equivalent weight 

 a decinormal (usually written ^) solution one-tenth of this amount, a 

 centinormal one-hundredth, etc. Thus, a normal solution of potassium 

 hydroxide contains 56 grammes of KOH, and a decinormal solution 5-6 

 grammes in 1,000 c.c. 



Ammo- 

 nium Urate (after 

 Milroy). 



