146 



alkalosis it is advisable not to continue the administration of bicar- 

 bonate without evidence from blood analysis showing an alkali 

 deficit.) The urine should be voided by the subject before each 

 administration of bicarbonate. Test each specimen of urine with 

 litmus, boiling those samples which are only faintly acid so that 

 any bicarbonate present will be converted to carbonate. Note 

 the number of grams of bicarbonate necessary to produce an 

 alkaline urine. 



NEPHELOMETRIC METHODS 1 



The Nephelometer. The nephelometer is an instrument for 

 measuring the density of precipitates and thus determining the 

 amount of any substance which can be obtained in the form of 

 a suitable suspension. It is somewhat similar in form and principle 

 to a colorimeter. It differs from the latter in that the light which 

 reaches the eye is not transmitted light, which, on the contrary, 

 is excluded, but light reflected from the particles of the suspension. 

 The brightness of the two fields is compared instead of their colors. 

 It is adapted particularly for the determination of substances 

 that in very dilute solution may be precipitated in the form of 

 suspensions which do not agglutinate appreciably in the time 

 required for making readings (ten to twenty minutes). The 

 method has been adapted to the determination of proteins in 

 digestion mixtures, milk, urine, etc.; nucleic acids; chlorides, 

 phosphates, and phosphatides in blood, etc.; fats in milk, blood, 

 etc.; acetone bodies in urine and blood; uric acid and purine bases; 

 ammonia; calcium; silver, etc., and is continually finding new 

 applications. It is possible to determine very minute amounts 

 of substances, entirely outside of the range of gravimetric methods 

 of analysis, and hence the procedure may be used where the 

 amount of material is very limited. If properly carried out the 

 limits of error of the method are not greater than those of the 

 colorimetric methods commonly used. 



Nephelometric Calculations. The amounts of precipitate in 

 solutions examined nephelometrically is not exactly inversely 

 proportional to the readings of the scale. When the concentration 

 of the unknown and of the standard are within 10 per cent of each 



1 Hawk: Practical Physiological Chemistry, Sixth Edition, 1918. 



