THE MEASUREMENT OF pH 



1X48-5 



To determine the pH of an unknown solution, proceed as above ex- 

 cept that the unknown solution is substituted for the standard HCl. 



The "glass electrode". Under suitable conditions, a properly pre- 

 pared thin membrane of special glass separating two solutions of 

 different pH exhibits an electric potential that is proportional to the 

 difference in pH of the solutions. Based on this property, a device 

 called the glass electrode is now widely used for the comparative 

 determination of pH. 



The glass probably most generally employed is that known as Corning No. 015; 

 Beckman type E glass has been advocated for alkaline solutions (pH 9 to 14) because 

 of its low sodium error as compared with that of glass 015. 



One of the common forms of the glass electrode consists of a tube 

 of the glass terminating in a thin-walled bulb which contains an 

 electrode of definite potential in a solution of fixed pH. A combina- 

 tion of electrode and buffer solution frequently employed is a plati- 

 num wire, silver-plated and then coated with AgCl, in a half-cell 

 containing 0.1 M HCl. For the construction, operation, and theory 

 of the glass electrode, consult Dole (19-il). 



The carefully rinsed bulb of the electrode, after seasoning in water 

 or buffer solution, is immersed in the solution to be tested and coupled 

 through a saturated KCl liquid junction with the saturated calomel 

 half-cell as indicated schematically below, 



Ag; AgCl; HCl (0.1 M) I Glass membrane I Solution X 1 KCl (sat.); HgzCla; Hg 



all parts of the cell being maintained at a uniform temperature. 

 The potential difference between the terminals of this cell can be 

 related to the pH of solution X if the glass electrode has been stand- 

 ardized in buffer solutions of known pH. 



Standardization of the glass electrode. The potential of a properly 

 functioning glass electrode should vary linearly with pH, from about 

 pH 1 to 9, in solutions of low salt content (up to 0.1 M). For this 

 range, therefore, the electrode requires standardization in buffer 

 solution at one point of pH, but preferably at two, within this linear 

 range. Standard buffer solutions convenient for this purpose may 

 be selected from Tables 1 and 3. 



TABLE 1 



SOME STANDARD BUFFER SOLUTIOX9 



Solution 



pH 



25" 



0.1 MHCl 



0.01 M HCl, 0.09 M KCl 



0.05 M Acid potassium phthalate 



0.025 M KH2PO4. 0.025 M NaaHPOi'gHoO. 

 0.05 M Na2B4O7-10H..O 



1.085 

 2.075 

 4.000 

 6.855 

 9.180 



1.082 

 2.075 

 4.015 

 6.835 

 9.070 



Such standardization should be performed at least daily; preferably, 

 it should be done immediately before a measurement. As occasion 

 requires, a series of buffer solutions of known pH should be used to 

 establish more carefully the linearity of response of the electrode. 



