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Generally, the slope is ~59 mV at 25°C. Slope is determined during calibration (see Calibrating the electrode).
The function F, when plotted as a function of the volume of acid added (v), is linear when sufficiently removed from the equivalence point. We measure mV instead of pH to determine the endpoint because this method offers better precision. The optimum range of millivolts for linearity is 220–240 mV. The value of v at F = 0 is the equivalence point from which the alkalinity is evaluated.
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- | 0.1 N HCl solution (premade from Fisher, AMS# CH5009) |
- | 3 M KCl solution (223 224 g KCl in 1 L reagent water) |
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Figure 2. Main instrument panel.
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Before an electrode can be used, it must be calibrated against pH buffers in the range expected in samples. Generally, calibration at pH 4, 7 and 10 covers the necessary range.
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