當使用一般的pH meter來檢測超純水時，其讀值是非常不穩定的，其原因如下：

結論

補充閱讀資料

pH Theory

pH is the universally accepted scale for the concentration of hydrogen ions in aqueous solution. It is an indication of acidity (pH<7)， alkalinity (pH>7) or neutrality (pH=7). pH is defined as the negative logarithm of the molar concentration of the active hydrogen ions (activity).

pH = -log₁₀[a_H+]

Pure water undergoes autopyrolysis to yield equal numbers of hydrogen and hydroxide ions in very low quantities.

H₂O <–> H⁺ + OH^–

This is an equilibrium reaction， for which an equilibrium constant has been determined.

K_w = [H⁺] x [OH^–] = 10^-14 at 25^oC

If pOH is defined as the negative logarithm of the hydroxide concentration， then the equilibrium expression can be rewritten as:

pK_w = pH + pOH = 14

This equation applies for any system containing water and explains the balance between acidity and alkalinity and the reason for the pH scale ranging from 0 to 14.

pH is probably the most common of all routine measurements with extensive application in laboratories， industries of all kinds and the environment. The most common mode of measurement is the electrode method. It requires measuring the voltage developed between two electrodes immersed in the sample and comparing that measurement to a calibration derived from the same electrode pair and known standards. The voltage developed by the electrode pair has very low power and requires a special， high impedance voltmeter.

The two electrodes have special qualities that enable them to work together to specifically measure pH. Most electrode pairs are enclosed in a single electrode body. Figure 6 shows the components of the IJ Combination pH electrode.

The glass half cell electrode consists of a pH sensitive lithium glass membrane attached to a sealed insulating tube containing a solution of fixed pH in contact with a silver-silver chloride element (Figure 7). It develops a voltage across the thin pH sensitive glass proportional to the activity of hydrogen ions in the solution. The relationship between the voltage and the hydrogen activity follows the Nernst equation:

E = E₀ – (RT/nF) ln[H⁺].

This equation can be rewritten in linear form by substituting in the definition of pH and grouping all the constants to give:

E = E₀^’ + SLOPE (T) x pH

E₀^’ is referred to as the offset， zero potential point or isopotential point， since theoretically， it is defined as the pH which has no temperature dependence. Most pH electrode manufacturers design their isopotential point to 0mV at pH 7 to correspond with the temperature compensation software in most meters. The offset potential is often displayed after calibration as an indication of the condition of the electrode. The IJ44/64 should read 0 +/- 30mV in a pH 7 buffer. In reality， E₀^’ is composed of several single potentials， each of which have slight temperature coefficients， and are sources of error in temperature compensation algorithms. For greatest accuracy， it is advisable to calibrate at the same temperature as the sample measurement.

The SLOPE(T) factor is a function of temperature and contains the conversion of the natural logarithm to the base ten logarithm. It is defined as the number of mV per unit of pH and is the factor which is adjusted in temperature compensation algorithms in pH meters. The slope is another electrode status indicator often displayed after calibration on most pH meters and should read 58+/-3mV per pH unit at 25^oC for the IJ44/46. Table 1 shows how the ideal SLOPE(T) factor varies with temperature.

The potential developed across the membrane requires a reference electrode to complete the circuit. The reference half cell ideally maintains a constant potential， regardless of other species in solution. Stability and non-selectivity are maintained by making electrical contact between the sample and reference half cell via an inert salt bridge. Typically the salt bridge is composed of concentrated potassium chloride， the same salt used to form the Ag/AgCl half cell， but since the IJ is a double junction design it has the option of using a variety of inert salts. This electrical contact must allow uninhibited movement of electrolyte between the sample and reference half cell to assure a repeatable constant reference potential. At the same time it must not grossly contaminate the sample with electrolyte. Therefore， a restriction (typically a porous ceramic or plastic frit) is used to slow the flow.

If the restriction becomes clogged and movement of ions becomes inhibited， the electrode system will appear to be stable in buffer solutions， but produce errors in non-ideal samples (e.g. low ionic strength samples) (1). The IJ reference system addresses this problem by allowing free movement of electrolyte past the restriction and by allowing the junction to be easily cleaned and refreshed when needed. The result is a reference electrode system of assured reliability.

References:
(1)John A. Illingworth “A common source of error in pH measurement”， Biochem. J.， (1981) 195， p. 259-262.