86 



MANUAL OF MICROBIOLOGICAL METHODS 



The fraction on the right side of the above equation is called the drop 

 ratio. The values of the standards for seven of the indicators are given 



* If the standards are prepared by the method suggested in the footnote to Table 8 

 (i. e., measuring the indicator in tenths of 1 ml and diluting to 10 ml) it is well to use 

 O.IA'^ instead of 0.05 AT NaOH to assure proper strength. The exact concentration or 

 the exact number of drops used is of no great importance. 



t Use approximately 0.05iV HCl (or O.liV if the method is modified as indicated in 

 the footnote to Table 8) except in the case of cresol red and thymol blue. In the case 

 of these two indicators a weaker acid must be used. Gillespie recommends 2 per cent 

 KH2PO1, or in the case of thymol blue no acid need be used, water alone having a suffi- 

 ciently high pH value to bring out the full acid color. 



in Table 9. They may be computed for the other indicators by using the 

 above equation and the pK' values in Table 6. 



For approximate work it is often possible to compare the Gillespie 

 standards with the unknown by merely holding the two tubes of the 

 standard in the hand between the eye and a source of light. For accurate 

 work, however, a comparator block must be used, but one with six holes 



