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Practical pH: Theory and Use Page 3 TEXT SIZE: A A A    PRINT THIS PAGE   EMAIL THIS PAGE

Electrode Construction

There are many ways to construct a glass indicator electrode and a reference electrode. Refer to the figure below for a typical representation of each.

Figure 2. Typical Electrode Constructions.
Figure 2 - Typical Electrode Construction

The potential which will develop for the glass electrode is a function of both the indicator electrode's pH sensitive glass and the composition of the inner solution. The potential of the silver chloride coated silver wire, the inner electrode, depends on the chloride concentration in the inner solution and will remain constant in each electrode.

Response of the electrode may be described as the voltage developed between the inside and the outside of the glass membrane. It is proportional to the pH difference in the sample and in the inner solution. The ion exchange, controlled by the concentration of the H+ in both solutions, determines the electrode response caused by an exchange at both surfaces of the membrane between the ions of the glass and the H+ ions of the solution. In addition, owing to the fact that the glass membrane is rarely uniform, an asymmetry potential may develop even if the pH is the same on both sides of the membrane.

The figure above shows a saturated calomel electrode, the reference electrode, consisting of a chamber containing mercury and calomel ( Hg2Cl2 ) which are in contact with each other. Contact between the mercury and the calomel is made by a platinum wire while the small internal chamber is surrounded by a solution of saturated KCl. Contact between the saturated KCl and the measured solution, the liquid junction, is made through a porous ceramic pin. The potential which occurs at this point is constant and is determined by the solubility product of the calomel and the concentration of the KCl solution.

A silver-silver chloride reference electrode is constructed in much the same way. The small internal chamber should be constructed of red glass to prevent photochemical reactions from occurring from exposure to light. A Ag/AgCl reference electrode is used for temperatures that vary from sample to sample and for high temperature measurement.

The potential of the reference electrode should not depend on the sample solution. Ideally, only K+ and Cl- ions are transported through the porous pin and move at the same speed. This occurs when the pH range of the samples is in the range of 1 to 13 and when a saturated or 3M KCl internal filling solution is used. A liquid junction potential may occur if there is deviation from this optimal situation.

Liquid junction potentials in different samples obtained with saturated KCl as the internal filling solution are listed in Table 1. Dependence on sample composition and especially on pH is obvious by checking the liquid junction potentials.


Table 1. Liquid junction potentials in different samples.


Sample Liquid Junction Potential
1M HCl 14.1 mV
0.1M HCl 4.6 mV
0.01M HCl 3.0 mV
0.1M KCl 1.8 mV
pH 1.68 buffer 3.3 mV
pH 4.01 buffer 2.6 mV
pH 4.65 buffer 3.1 mV
pH 7.00 buffer 1.9 mV
pH 10.01 buffer 1.8 mV
0.01M NaOH 2.3 mV
0.1M NaOH -0.4 mV
1M NaOH -8.6 mV


Table 2 shows the equivalent conductivity in infinitely dilute solutions of the ions commonly used in internal filling solutions. The lowest liquid junction potential results from equal conductivity of the cation and anion, a measure to their mobility in solution.

Table 2. Equivalent conductivity of ions
in infinite dilutions (S*cm2 / equivalent) at 25oC.


Cation λ Anion λ
Li+ 38.7 CH3COO- 40.9
Na+ 50.1 ClO4- 67.4
K+ 73.5 NO3- 71.5
NH4+ 73.6 Cl- 76.4
    Br- 78.1
    1/2 SO4-2 80.0
H+ 349.8 OH- 198.3


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