TITRATION AND STANDARDIZATION 169 



TITRATION AND STANDARDIZATION. 



One method of titration is, in general, to prepare a series 

 of tubes of solutions of known /;H with definite amounts of 

 indicator in them. Then a measured quantity of the 

 medium is taken, the same amount of indicator is added 

 and then alkali (or acid) until the color matclies that in the 

 standard tube having the desired ^^H. This method involves 

 the preparation of a series of standard solutions of several 

 different chemicals and the making of accurate buffer mix- 

 tures from these. ^ This procedure is perhaps too difficult 

 for the ordinary bacteriological laboratory. 



Another method is that of Barnet and Chapman,^ in which 

 varying amounts of indicator are added to a series of acid 

 and a series of alkaline tubes. One of each series can be 

 so selected that wlien looking through the two tubes together 

 a color is seen which corresponds to a definite pYi. A series 

 for bromthymol blue, /jH(3.4-7.(), as given by jNIedalia,^ is 

 shown in pairs in the table below. The acid tubes con- 

 tain 10 cc of approximately X/100 HCl and the alkaline 

 tubes, 10 cc of approximately N/20 NaOH. The indicator 

 is 0.02 per cent solution in distilled water. The figures 

 are tenths of a cubic centimeter to be added to each 10 cc 

 tube: 



A similar series for phenol red 0.04 per cent solution gives 

 pH values of 7.0, 7.2, 7.4, 7.6, 7.8, 8.0, 8.2. These two series 

 cover the range for ordinary culture media. 



The titration is performed by taking 2 cc of the medium, 

 diluting to 10 cc with distilled water and adding 0.8 cc of 

 indicator. N/20 NaOH is then run in until the proper color 



1 Clark and Lubs, Jour. Bact., 1917, 2, 19-26. 



2 Jour. Am. Med. Assn., 1918, 70, 1062. 3 jour. Bact., 1920, 5, 451. 



