Chapter 7 Reference
7.1 pH measurement
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HORIBA
depending on the degree of contamination of the reference solution and
the state of the glass membrane. Also, if the electrode membrane dries
out, a large asymmetric potential will occur, giving rise to measurement
errors.
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Temperature compensation
The electromotive force generated by the glass electrode changes
depending on the temperature of the solution.
Temperature
compensation is used to compensate for the change in electromotive
forces caused by temperature. There is absolutely no relation between
the change in pH caused by the temperature of the solution and
temperature compensation. This is often misunderstood. When pH is to
be measured, the temperature of the solution when the pH is measured
must be recorded along with that pH value, even if a meter that has
automatic temperature compensation is used. If the solution
temperature is not recorded, the results of the pH measurement are
relatively meaningless.
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Types of pH standard solutions
When measuring pH, the pH meter must be calibrated using a standard
solution. There are several kinds of standard solutions. For normal
measurement, three standard solutions—with a pH of 4, 7, and 9—are
sufficient to accurately calibrate the meter.
・
pH 1.68 standard solution: Oxalate
0.05 mol/L tetra-potassium oxalate aqueous solution
・
pH 4.00 standard solution: Phthalate
0.05 mol/L potassium hydrogen phthalate aqueous solution
・
pH 6.86 standard solution: Neutral phosphate
0.025 mol/L potassium dihydrogen phosphate, 0.025 mol/L sodium
dihydrogenphosphate aqueous solution
・
pH 9.18 standard solution: Borate
0.01 mol/L tetra-sodium boric acid (boric sand) aqueous solution
・
pH 12.45 standard solution: Saturated calcium hydroxide solution
