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0.6896 - ?o6, - (0.0617 X 1.082) (4) from which the value of Ecai can b6 calculated after substitution of the experimentally determined value of Eobs- To determine the pH of an unknown solution, proceed as above except 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 Purchase Sildenafil Citrate called the glass electrode is now widely used for the comparative determination of pH. 76 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 Purchase Sildenafil Citrate 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 Purchase Sildenafil Citrate 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 Purchase Sildenafil Citrate this Purchase Sildenafil Citrate 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 Solution pH 25 38 0.1 M HCl 1.085 2.075 4.000 6.855 9.180 1.082 01 M HCl, 0.09 M KCl 2.075 0.05 M acid potassium phthalate 0.025 M KH0PO4, 0.025 M Na2HP04-2H20 4.015 6.835 0.05 M Na2B4O7-10H2O 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 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
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