6 4 



PHYSICAL CHEMISTRY 



shaking the solution the colloid may be precipitated in the form 

 of torn membranes, which assume a stringy appearance. In this 

 way enzymes may be inactivated, being either colloidal or pre- 

 cipitated with colloid impurities. The presence of colloids makes 

 an emulsion of oil and water permanent by forming haptogen 

 membranes around the oil drops. It is the haptogen membranes 

 on fat globules in milk that keeps it permanently emulsified, 

 but these membranes seem to be different from caseinogen. 



Measurement of surface tension is a convenient method of 

 quantitative chemical analysis provided that only one substance 

 grea'tly affecting the surface tension be variable in concentration. 

 It is only necessary to plot the graph of surface tension and con- 

 centration in order to make a table for converting surface tension 

 readings into concentrations. In mixtures, however, the problem 

 may be more difficult. 



G. D. Allen (1916) measured the surface tension of urine 

 containing different concentrations of bile salts. Bile salts greatly 

 increase the surface tension, but NaCl, which in pure solution 

 slightly raises the surface tension, increases the effect of bile 

 salts in lowering the surface tension. H ions increase the effect 

 and OH ions decrease the effect of bile salts.' Sodium. taurocho- 

 late is slightly less effective than sodium glycocholate and, hence, 

 a variation in the proportions of the two salts would slightly affect 

 the result. The following table shows the limits of error when 

 the urine is diluted to a specified specific gravity in order ap- 

 proximately to regulate the concentration of NaCl and other 

 interfering substances. 



Showing the Amounts of Sodium Glycocholate Required to Give Any 

 Observed Surface Tension in a Sample of Urine Diluted to a Standard 

 Sp. Gr. of 1.010. 



S. T. of sample diluted 

 ft 1.010 sp. gr. 



Per cent iof that of dis 

 tilled water. 



95 

 90 



85 

 80 



75 

 70 



65 

 60 



Minimum of sodium gly 



cocholate which may 



give observed S. T. 



(Sample is acid.) 



Diluted sample. 



Maximum of sodium gly- 

 cocholate which may 

 give observed S. T. 

 (Sample is alkaline.) 

 Diluted sample. 



gm. per ioo cc. 

 O.OOI 



0.004 



0.008 



0.015 



0.03 



0.06s 



0.1+ 



