151 



aliquots allow errors as great as 10 per cent of the 5 cc. aliquot taken. 

 An error of .5 cc, which is easily made with a graduate, means an 

 error of 10 cc. per 100 cc. of media. Again an error of .05 cc. (one drop) 

 of N/10 alkali in titrating means an error of plus or minus 0.1 per 

 cent in the calculated acidity. 



Indicator. 



Large amounts of indicator are used. In the literature and in the 

 standard methods (12) 1 cc. of a Vz per cent solution of phenolphthalein 

 is specified. In accurate chemical work the amount the mass of indi- 

 cator affects the accuracy of the determination is taken into considera- 

 tion. One or two drops of indicator have proven sufficient. Anthony 

 and Ekroth (6) give a list of shades of color called suitable or correct 

 end-points with phenolphthalein. The colors listed vary from "first trace 

 of pink" to "brilliant red." Clark (1) presents a table showing that 

 the variations in acidity of a 1 and a 5 per cent peptone media when 

 these media were titrated by four chemists and four bacteriologists. 

 The acidities calculated from the titrations of the different workers 

 varied from 0.58 cc. to 1.40 cc. N/40 alkali for the 1 per cent and from 

 2.68 cc. to 7.40 cc. N/40 alkali for the 5 per cent media. 



Clark and Lubs (4) describe indicators which undergo rapid color 

 changes at certain definite hydrogen ion concentrations. They give 

 Brom thymol blue as undergoing color changes between Ph 6.0 and Ph 7.6. 

 These indicators are new and have been manufactured (and there, 

 almost under protest) by only one chemical supply house. Their sta- 

 bility and the exactness with which they can be used under the crude 

 conditions phenolphthalein has been used are unknown. At the present 

 time it is fair to assume that these new indicators will come into gen- 

 eral use, but as long as different investigators do not agree on a definite 

 value for the contact potential between 0.1 N. HCl. and 0.1 N. KCl. 

 phenolphthalein is not to be discarded for use under exactly defined and 

 proper conditions. 



A further evidence that phenolphthalein (properly used) is satisfac- 

 tory for determining neutrality of media is found in Itano's work on 

 the proteolysis brought about by certain bacteria when put under known 

 initial hydrogen ion concentration. The reaction of all the media (19) 



