THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 81 



acid color in it and which give their alkahne 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 application of Eq. (5) (drop-ratio method, page 

 85). 



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). The compositions of the series of buffer 

 standards proposed by Clark and Lubs (1917) are given in Table 7. 

 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 prehmi- 

 nary 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, 6.0, 6.2, and 6.4, should suffice 

 to safely bracket the actual pH of the unknown. 



In preparing for the actual measurement, the unknown and the color standards 

 should be contained in clear glass tubes selected for uniform bore, wall thickness, and 

 inherent color. It is essential that the total concentration of indicator in the unknown 

 be exactly the same as that in each of the color standards. This is best accomplished 

 by accurately measuring, with a pipet equal amounts of indicator (e.g., 0.50 ml) into 

 equal amounts (e.g., 10.0 ml) of each of the selected standard buffer solutions. The 

 indicator may be satisfactorily measured in drops provided the dropper tip is properly 

 shaped (not too blunt) and the dropper is held vertically during the measurement. 

 The use of excessive amounts of indicator may introduce difficulties; the minimum 

 quantity necessary to produce recognizable coloration is desirable from the theoretical 

 standpoint. It is essential, of course, that the indicator be uniformly distributed 

 throughout the solutions to which it is added. 



Prepared buffer standards can be obtained from supply houses, either as solutions 

 or as powders or tablets to be dissolved as needed. They may also be obtained in 

 sealed glass tubes containing the indicator. Such commercial color standards are 

 convenient and satisfactory. They presuppose the use of comparable concentrations 

 of indicator in the solution under test, and they must be used with the understanding 

 that they are not permanent and may need to be checked or renewed at least once a 

 year. All such indicator standards should be kept in the dark when not in use. 



Color comparison. This procedure, commonly miscalled colorimetry, 

 requires intelhgent application to yield reliable results. The subject is 

 adequately discussed by Clark (1928, 1948). Accurate color comparison 

 of a standard solution with an unknown requires uniformity of the follow- 

 ing conditions: the optical path (i.e., distance through the solutions 

 traversed by the light), transparency, wall thickness and color of the 



