[Vol. 9 

 358 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



Clark and Lubs, and in common use are difficult and laborious 

 to make, but for careful work, are apparently indispensable. 

 Using these standards and a comparator block, it is a simple af- 

 fair to compare a tube of media containing a certain indicator 

 with another tube containing the standard solution plus the same 

 indicator. This method, slightly modified by placing a tube 

 of dear water before the solution to be tested and a tube of the 

 unknown solution without indicator before the standard, as rec- 

 ommended by various investigators, has been extensively used 

 by the writer in this investigation. The procedure is fully de- 

 scribed by Clark (II, '20). 



Various devices designed to simplify the procedure in making 



standards have appeared; most of these consist in reducing the 

 number of standard solutions, which in some instances, also re- 

 duce the range of P H values (see Mcllvaine, II, '21), One of 



the simplest of these, judging from the description, is the one 

 advocated by Acree and his associates (II, '21). The writer has 

 not been able to use this. Other attempts to reduce the labor 

 involved in making standards consist in varying the number of 

 drops of indicator in a series of tubes containing simply a few 

 cubic centimeters of acid and of alkali. This was first recom- 

 mended by Barnett and Chapman (II, y \S) and later amplified 

 by Aiedalia (II, '2(1 ) and by Gillespie (II, '20), Bunker and 

 Schuber ( II, '22 ) claim good results by this method, and it is the 

 one recommended by the American Public Health Association in 

 the "Standard Methods for the Bacteriological Examination of 



Milk," L921. 1 



BUFFERS 



Standard solutions used for comparison with unknowns are 

 often called "buffers." What is a "buffer"? Any solution which 



ch 



id 



or alkali is called a "buffer." It is of course desirable that 

 standard solutions once made up should retain their calculated 



hydrogen-ion concentration as long as possible, and certain salts 



in particular are chosen for standards because of their marked 

 resistance to change in hydrogen-ion concentration, even after 

 standing for several months. Not only do a large number of or- 

 ganic and inorganic salts act as buffers but many other sub- 



1 Road j prepared Btandard solutions are advertised by La Motte Chemical Prod- 

 ucts Company, 13 W. Saratoga St., Baltimore, and by Graham Chemical Company, 



100 Rockingham Street, Rochester, N, Y. The latter concern sells the product 

 developed by Acre.' and his associates ('21). 





