ADVANCED CHEMICAL PHYSIOLOGY 



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sheet of white paper pinned behind it, when tilted slightly backwards 

 serves quite well. For more accurate work a colorimeter or a blackened 

 box called a comparator may be made use of. 



The indicators commonly employed are as follows : 



The amount of indicator added is important. Speaking generally, 

 5 drops of any of the foregoing solutions added to 10 c.c. of the solution 

 to be tested give a good result. 



A series of standard solutions may be prepared with different pH. 

 values such as the convenient standards of Clark and Lubs. (An 

 excellent account of the methods, indicators and various standard 

 solutions and their preparation is to be found in The Determination of 

 Hydrogen Ions, by W. M. Clark, Baltimore, 1920.) The various salts 

 required for the preparation of the standard solutions can now be 

 purchased in a pure state. 



A range of pR from 1-3 may be obtained with 0-1 N HC1 (1-04), 

 0-02 N HC1 (1-72), 0-1 N acetic acid (2-89). 



A range of >H from about 3-5-6 may be obtained by combining 

 various volumes of 0-1 N acetic acid and 0-1 N sodium acetate. 



A range of pH. from 6-8, the most important physiological range, is 

 best prepared from standard Sorensen phosphate solutions. (1) An 



Y 5 solution of primary potassium phosphate which contains 9-078 gms. 



KH 2 PO 4 per litre, and (2) an j-g solution of secondary sodium phos- 

 phate which contains 11-876 gms. Na 2 HPO4, 2H 2 O per litre. 



Brom-thymol blue and phenol red are satisfactory indicators at this 

 range of pH. concentrations. 



In order to test the turning point of the various indicators at varying 

 hydrogen ion concentration measure 5-7 c.c. of distilled water into a 

 clean beaker, add 5 drops of one of the indicators,, and then run in care- 

 fully from a burette 1 c.c. of decinormal sodium hydrate. Note the 

 colour. Then add from another burette, drop by drop, decinormal 

 acetic acid until the colour of the indicator changes. Note the colour 

 and the amount of acid required. Repeat this (a) with the whole 

 series of indicators, and (6) using decinormal hydrochloric acid in place 

 of the acetic acid. 



