PHYSIOLOGY OF FUNGI 37 



pPI or range of pH's, notable among which are a long series of sulphon- 

 plithaleins and another of azo compounds. 



There is not space here to go into the theory of indicators or even to list 

 them. Thorough and satisfactory discussions may be found elsewhere. While 

 apparently crude, this method, in addition to the obvious advantages of being 

 cheap and quick, may, in the hands of a normal, intelligent manipulator, be 

 developed to quite a high degree of accuracy. 



It must be remembered that the color perception of some individuals is 

 very imperfect so that no amount of training will give them satisfactory re- 

 sults Avith a colorimetric method in biochemistry or bacteriology. The ex- 

 tremely simple method of matching colors Avith a colored chart, such as fur- 

 nished by Clark (1920, 1922), is only a very rough approximation and is 

 practically useless Avitli even slightly clouded solutions. A more reliable 

 method, in Avhich the unknown + indicator is compared Avith a standard + 

 indicator as seen through an equal depth of solution Avithout indicator, is 

 quite commonly used and is satisfactory if the unknoAvn solution is not highly 

 colored. It is important that the vessel be of the same size, shape, material, 

 color, and thickness, and that the light intensity be equal. Simple devices 

 may be purchased quite cheaply. By carefully eliminating sources of error 

 by use of a colorimeter of the Duboscq type and varying the light intensity, 

 the Avriter (Dodge 1919, Duggar & Dodge 1919, Duggar 1919) Avas able to 

 secure as great accuracy as is usually attained by the potentiometric method, 

 and extend the range of indicators in both directions so that only half as 

 many need be used as in the usual series proposed by Clark & Lubs (1917). It 

 should also be kept in mind that the alkali sloAvly dissolves from glass, in- 

 creasing the alkalinity of solutions so that standards sealed in glass should not 

 be used indefinitely. The safest Avay is to prepare carefully one's OAvn stand- 

 ards and store them in paraffin lined bottles, for use as occasion demands. 



These standards, usually mixtures of buffers, are solutions of Aveak acids 

 and their salts, which can maintain their pH almost unaltered on the addition 

 of considerable quantities of strong acids or bases. Any substance capable 

 of remoAdng hydrogen or hydroxyl ions from the solution either physically 

 or chemically Avill act as a buffer; e.g., Bovie (1915) has shoAvn that charcoal 

 has a buffer action. We have, however, to deal only with chemical buffers, a 

 simple explanation of Avhich might not be amiss. 



Acetic acid (we will indicate the acetate group, CH3COO-, here by the simplified Ac-) 

 is a weak acid. That is, though in a normal solution there is one gram atom of ionizable 

 hydrogen per liter, most of this remains as un-ionized HAc and the reaction of the solution 

 is only slightly acid, the pH being only a little below 7. Sodium acetate, being a salt, is almost 

 completely ionized. 



NaAc :;±: Na+ + Ac- 



The Ac- ion will immediately start to take up the H* ion of the water until the dissocia- 

 tion constant for HAc is satisfied. This will liberate an excess of OH- ions which will remain 

 virtually uncombined because NaOH is a very highly ionized base. Thus a normal solution of 

 pure NaAc in pure water will give a definitely alkaline reaction. 



