162 MASS. EXPERIMENT STATION BULLETIN 167. 



Table III. — Standard Solution of Different Ph = (6.£4-7.So). 



Ph=x. 



m= molecular. 



Before an indicator ^ is added to the standard solution, it is important 

 to adjust the color of the standard solution itself to the identical color of 

 the medium. If this adjustment is neglected it is impossible to compare 

 the color produced in each, upon the addition of the indicator. For this 

 purpose 4 drops of tropajolin 0, as previously suggested, and the same 

 amount of methyl violet, gave an almost identical color upon very careful 

 comparison. Finally, 10 drops of neutral red^ were added to both the 

 standard solution and the medium. Then the color was compared by the 

 usual method, and it was found that the color of the medium corresponded 

 to the color which was intermediate between tubes No. 6 and No. 7, or 

 approximately Ph = 6.35. Therefore Medium IV., without any addition 

 of either acid or alkali, contained Ph = 6.35 according to the findings of 

 this method. 



An attempt was next made to prepare a series of Ph = 2, 4, 8 and 10, 

 after finding the Ph in the medium by adding various amounts of n/5 

 HCl and n/5 NaOH, so that other Ph can be obtained by interpolation. 

 For this purpose 10 cubic centimeters of the medium was taken and 

 treated as in the preceding processes, except that a suitable standard 

 solution and indicators for the different Ph were used. 



1 In this case neutral red which gives a rosy red to orange colors within the range from Ph=6 

 toPH=10. 



2 Prepared according to S. P. L. Sorensen. 



