THE MEASUREMENT OF pH 1x43-9 



of bacterial nutrition, the kind of cation and even the small amounts 

 thus added may be of significance. In such cases, it is advisable to 

 learn from the maker what cations (Na, NH4, etc.) are present in the 

 indicator salt in order to make due allowance for their possible effects. 

 The colorimetric method of pH determination depends on matching 

 the color of a suitable indicator in the unknown solution with that of 

 the same indicator in a standard. The standards can be set up in 

 two different ways: by means of buffer standards or by means of 

 "drop-ratios". These will be considered in detail presently. In 

 brief outline, the colorimetric method includes these major steps: 



1. Selection of the appropriate indicator. 



2. Preparation of color standards. 



3. Color comparison for pH determination. 



Later paragraphs will outline essential specifications that must 

 be observed in each of these steps in order to assure reliable results. 



Selection of the appropriate indicator. Test successive small por- 

 tions (1 ml.) of the unknown with a drop of bromthymol blue (BTB). 

 If the color produced is orange or red then the unknown is probably 

 in the range of pH covered by thymol blue (acid range). If the BTB 

 color is yellow, repeat the test with the indicators of successively 

 lower pK' (see Table 2) until that indicator is found which gives a 

 color within its sensitive or useful range. If the BTB color is blue, 

 proceed in like manner with indicators of higher pK' until the ap- 

 propriate indicator is found. Of course, if the unknown is more acid 

 than pH 1 or more alkaline than pH 10, none of the indicators listed 

 in Table 2 will serve. 



If the unknown solution is unbuffered (e.g., water or saline) or 

 very weakly buffered, the buffering effect of the added indicator may 

 prevail and significantly change the pH of the unknown. In such 

 cases, special methods are required (see Clark, 1928). 



It is plain that a rough idea can be obtained as to the pH value 

 of a sufficiently buffered solution by simply finding which indicators 

 give their acid color in it and which give their alkaline color. Indeed, 

 the intelligent employment of indicators with overlapping pH ranges 

 can be made to define the upper and lower limits of a relatively 

 narrow zone of pH within which lies the pH of the solution under 

 study (Small, 1946). Accuracy, however, can be obtained only by 

 actual comparison with the colors produced by the indicators in 

 solutions (buffers) whose pH values are known, or produced by ap- 

 plication of equation 5 (drop-ratio method, p. 12-14). 



Buffer solutions and color standards. A considerable variety of 

 buffer solutions have been proposed; and many of them are discussed 

 and described by Clark (1928). Thecompositionsof the series of buffer 

 standards proposed by Clark and Lubs (1917) are given in Table 3. 

 Preparation of the stock solutions is described by Clark (1928). 



After finding the appropriate indicator, prepare or select a series 

 of properly graded standard buffer solutions sufficient in number to 

 bracket the estimated pH of the unknown solution as determined in 

 the preliminary trials. If, for example, the indicator selected is 

 bromcresol green and the estimated pH of the unknown is near 6.0, 

 then not more than five standards, namely buffers of pH 5.6, 5.8, 



