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MANUAL OF MICROBIOLOGICAL METHODS 



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

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

 its low sodium error as comparsf. 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 combination of electrode 

 and buffer solution frequently employed is a platinum 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 (1941). 



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) \ glass membrane | solution X | KCl (sat.); HgsCla; 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 standardized 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 1-9, 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 

 5 and 7. 



Table 5. Some Standard Buffer Solutions 



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 estab- 

 lish more carefully the linearity of response of the electrode. In solutions 

 more alkaline than about pH 9, the 015 glass electrode responds also to 



