INDICATORS AND THEIR EMPLOYMENT 121 



and Chapman have introduced a very simple method that 

 appears to meet all the requirements. The following are 

 the essentials of that method: For the preparation of cul- 

 ture media, which are to be neutral or nearly so, the bacteri- 

 ologist is concerned only with reactions falling between pH7 

 and pH8, and as phenolsulphonephthalein (phenol red) indi- 

 cates best at about these points it is the indicator used. 

 In this method use is made of the principle of superimposing 

 the two extreme colors of the indicators in determining the 

 so-called "half transformation" point. Within the range 

 of its transition from red to yellow -we may regard the 

 observed color of a phenol red solution as composed of a 

 definite amount of red plus a definite amount of yellow, and 

 such a color may be exactly duplicated by superimposing 

 the extreme red and the extreme yellow of the indicator 

 in proper concentrations. Thus, if to one test-tube we add 

 5 c.c. of dilute acid and to another 5 c.c. of dilute alkali and 

 to each add 5 drops of phenol red solution, a bright yellow 

 will be produced in the acid tube and a bright red in the 

 alkaline. But if we look toward the light through both 

 tubes, a color will be observed that is half-way between 

 yellow and red. In fact, it will be identical with the color 

 produced by 10 drops of the indicator solution in 5 c.c. of a 

 standard solution having a pH value of 7.9. This is the 

 ''half transformation" point, and is a definite constant for 

 this indicator. But if instead of using equal amounts of 

 indicator in each of the two tubes we vary the partition 

 of the 10 drops of indicator between them, then by super- 

 imposing each pair and viewing them by transmitted light, 

 a series of colors will be observed which will cover the range 

 of usefulness of this particular indicator. Once such a 

 series is standardized ("calibrated") against solutions of 



