178 C L. BRIGHTMAN, MEACHEM AND ACEEE 



not large and the " corrections" are small. If there is any change 

 in this "correction," it fortunately becomes smaller with in- 

 creasing color. These facts have been known qualitatively to Acree 

 from preliminary work ever since this study of the sulf onphthalein 

 series was suggested by him and undertaken by Slogle, White, 

 Lubs, Clark, Guy, Birge, Hopfield and the present authors; it was 

 one of the chief reasons, along with the intense color 12 of these indi- 

 cators and the lack of "fading" in excess 13 of alkali, for choosing 

 this as the most promising series of indicators to develop for col- 

 orimetric work. Bacteriologists not familiar with the history of 

 the work are referred to the articles in the Journal of the Ameri- 

 can Chemical Society. Now that we have the present study 

 completed, we can express quantitatively in the following table 

 the "corrections" which workers must use to compare their P H 

 values for the same color in different phosphate concentrations. 

 Any worker can calculate the "correction" to be applied to his 

 own data in order to compare his results with those for another 

 concentration of phosphate or even for cases where no phosphate 

 is present. The table gives the corrections in P H which must be 

 added to the observed values in order to give the P H in solutions 

 free from phosphates and "salt effects" and having the same 

 indicator transformation. The corrections are calculated from 

 the above curves and the unpublished ones and can be considered 

 accurate within the usual experimental errors. There may be 

 some changes for the dilute solutions. As it requires careful 

 work to determine hydrogen ion concentrations within 0.02- 

 0.03 P H it is clear that the "salt effects correction" for these 

 phosphates below N/10 or N/25 concentrations is within the 

 usual experimental error. As the writers have obtained excel- 

 lent results in growing fungi on media buffered with M/25 and 

 M/50 solutions of phosphates mixed with asparaginates, acetates 

 and phthalates we recommend in general the use of the more 

 dilute buffers in order to obviate these "salt" errors. Of course 

 the concentration and "buffer range" required depends not only 



12 See the articles by White, Lubs, Birge and Acree, loc. cit., for the relation 

 of the color intensity to the tautomeric equilibrium and ionization constants. 



13 See Brightman, Hopfield, Meacham and Acree, loc. cit. 



