THE MEASUREMENT OF pH 



1X48-11 



TABLE S—{Conti7iued) 



COMPOSITION OF MIXTURES GIVING PH VALUES AT 20°C AT INTERVALS OF 0.2. 



From Clark {1928) p. 200-1 



Notes. Overlapping members of the above series should be checked for consis- 

 tency, i.e., phthalate "5.8" to "6.2" should match phosphates of the same pH numbers 

 when tested with bromcresol purple; likewise for phosphate and borate "7.8" and 

 "8.0" when tested with cresol red. 



According to more recent assumptions used in standardization, the pH values given 

 in the above table are too low by about 0.03 to 0.04 unit of pH. 



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 cotyiparison. This procedure, commonly miscalled colorim- 

 etry, requires intelligent 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 following conditions: the optical path (i.e., dis- 

 tance through the solutions traversed by the light), transparency, 

 wall thickness and color of the containers, concentration of indicator 



