380 THE EXCRETIONS OF THE BODY 



the nondissociable condition is colorless; as soon however as the solution is ren- 

 dered alkaline a salt of high dissociability is formed, and the intensely red 

 color of its negative ion comes to the fore. But in order that the reaction may 

 appear with a very slight excess of hydroxyl or hydrogen ions the acid or base 

 used as the indicator must be very weak in comparison to the acid or base to be 

 tested. The acidity of weak acids obviously can only be determined by the help 

 of an indicator which itself is still weaker than the weakest of the acids to be 

 tested; in the presence of weak bases the alkalinity can only be ascertained with 

 the help of a somewhat stronger acid as indicator e. g., methyl orange. 



Now the urine contains weak acids such as C0 2 and H 3 PO 4 in considerable 

 quantities and a rather weak base, ammonium in very small quantities. In 

 order to obtain the true reaction of urine, one must, therefore, use a very weak 

 acid as indicator. Neither methyl orange nor litmus is weak enough to be 

 liberated by carbon dioxide or to detect phosphoric acid as a dibasic, much less 

 as a tribasic acid. Phenolphthalein, however, is sensitive to both, although the 

 third hydrogen atom of phosphoric acid escapes it. 



With phenolphthalem the reaction of the urine is always neutral or weakly 

 acid. A plainly alkaline reaction is never met with except in urine which 

 has suffered bacterial decomposition (Auerbach and Friedenthal). 



By titration one may ascertain the true chemical acidity of the urine 

 measured as the quantity of alkali which must be added to displace all the 

 acid hydrogen with a metal. From the physico-chemical standpoint, how- 

 ever, acidity means the concentration of hydrogen ions present in the liquid. 

 According to v. Rhorer and Hoeber, 1 1. of urine contains on the average 

 about 0.003-0.005 (minimum 0.0004, maximum 0.01) mg. of ionized hydro- 

 gen as compared with 0.0001 mg. in distilled water. This acidity corresponds 

 to an acid which in -f$ solution is dissociated to y^ per cent, and is some ten 

 thousand times less than that determined by titration. 



In view of the very complicated physico-chemical relations of the urine, it 

 is scarcely p.ossible to determine the share of the different constituents in its 

 total acidity. 



Fresh urine as a rule is perfectly clear; but on standing it sometimes 

 becomes turbid owing to the separation of urates. There also appears in it a 

 weak, flocculent precipitate (nubecula), which, according to K. A. H. Morner, 

 contains a special mucous substance (urine mucoid), probably formed in the 

 mucous membrane of the urinary passages and mixed with the urine as a weak 

 gelatinous solution. 



The color of the urine depends to a certain extent upon its concentration, 

 and varies with increasing concentration from straw yellow to dark reddish 

 yellow and reddish brown. Its taste is salty, its odor peculiarly aromatic. 



The quantity of urine depends upon many circumstances, and therefore 

 varies considerably. The average quantity for an adult man may be estimated 

 at about 1,500 c.c. per day. 



The specific gravity of the urine also varies in man from 1.017 to 1.020; 

 but it may fall as low as 1.002 and rise as high as 1.047. 



The molecular concentration ( A) of the urine, measured by the lowering 

 of the freezing point, stands in a certain relation to the specific gravity (s), and 



