HYDROGEN -ION CONCENTRATION 



157 



the use of the familiar comparator block. A slight color or turbidity 

 of the medium may be compensated for by the use of suitable blanks. A 

 porcelain spot plate may be used instead of a comparator block with 

 considerable saving of time and materials, although the accuracy is some- 

 what less. Drops of the indicator are added to the depressions in the 

 spot plate. A drop of the medium is added to one depression, and drops 

 of standard buffers to the other depressions. The pH of the medium 

 is estimated from the pH of the buffer which yields a color matching that 

 developed in the medium. 



Table 32. The pH Range and Color Changes of Various Indicators 



(Courtesy of Eastman Kodak Company.) 



Indicator 



Bromophenol blue . 

 Bromocresol green. 

 Chloro phenol red . . 

 Bromocresol purple 

 Bromothymol blue . 

 Phenol red 



pH range 



3.0-4.7 

 3.8-5.4 

 4.8-6.8 

 5.2-6.8 

 6.0-7.6 

 6.8-8.4 



Color change 



Yellow-blue 



Yellow-blue 



Yellow-red 



Yellow-purple 



Yellow-blue 



Yellow-red 



All colorimetric methods of measuring pH require the use of standard 

 buffers (buffers of known pH) or permanent standards. Buffers may be 

 prepared in the laboratory or purchased from laboratory supply houses. 

 It is convenient to use prepared buffer tablets, which need only to be 

 dissolved in a measured amount of water before use. Potentiometric 

 pH meters also require the use of a standard buffer for calibration. 

 The easiest of these to prepare is a saturated solution of potassium hydro- 

 gen tartrate (pH 3.57). The use of this buffer was recommended by 

 Lingane (1947). It is simple to prepare, and temperature affects the pH 

 very little. 



From Table 32 it will be noted that the pH range of a single indicator 

 is less than two pH units. Much time can be saved in pH determinations 

 by the use of a wide-range indicator to determine the approximate pH 

 before using a single indicator for the final measurement. Wide-range 

 indicators (pH range 2 to 10) may be purchased or prepared by mixing 

 suitable indicators (Snell and Snell, 1948). The pH value of a medium 

 may easily be determined within 0.5 pH unit by the use of a wide-range 

 indicator. Either the comparator block or the spot-plate method may 

 be used. For detailed information about indicators, see Kolthoff and 

 Rosenblum (1937). 



Potentiometric methods. The potential difference which develops 

 between certain pairs of electrodes when they are dipped into a solution 

 is a function of the hydrogen-ion concentration. Solutions which give 



