Emerson Process Management 3081 pH/ORP instruction manual Liquid Junction Potential, 116

MODEL 3081 pH/ORP	SECTION 13.0
	pH MEASUREMENTS

13.4 LIQUID JUNCTION POTENTIAL

The salt bridge (see Figure 13-4) is an integral part of the ref- erence electrode. It provides the electrical connection between the reference electrode and the liquid being meas- ured. Salt bridges take a variety of forms, anything from a glass frit to a wooden plug. Salt bridges are highly porous, and the pores are filled with ions. The ions come from the fill- ing solution and the sample. Some bridges permit only diffu- sion of ions through the junction. In other designs, a slow outflow of filling solution occurs. Migration of ions in the bridge generates a voltage, called the liquid junction poten- tial. The liquid junction potential is in series with the measur- ing and reference electrode potentials and is part of the over- all cell voltage.

FIGURE 13-4. Reference Electrode.

The fixed concentration of chloride inside the electrode keeps the potential constant. A porous plug salt bridge at the bottom of the electrode permits electrical contact between the reference electrode and the test solution.

Figure 13-5 helps illustrate how liquid junction potentials originate. The figure shows a section through a pore in the salt bridge. For simplicity, assume the bridge connects a solution of potassium chloride and hydrochloric acid of equal molar concentration. Ions from the filling solution and ions

from the sample diffuse through the pores. Diffusion is driv- en by concentration differences. Each ion migrates from where its concentration is high to where its concentration is low. Because ions move at different rates, a charge separa- tion develops. As the charge separation increases, electro- static forces cause the faster moving ions to slow down and the slower moving ions to speed up. Eventually, the migra- tion rates become equal, and the system reaches equilibri- um. The amount of charge separation at equilibrium deter- mines the liquid junction potential.

Liquid junction potentials exist whenever dissimilar elec- trolyte solutions come into contact. The magnitude of the potential depends on the difference between the mobility of the ions. Although liquid junction potentials cannot be elim- inated, they can be made small and relatively constant. A small liquid junction potential exists when the ions present in greatest concentration have equal (or almost equal) mobilities. The customary way of reducing junction poten- tials is to fill the reference electrode with concentrated potassium chloride solution. The high concentration ensures that potassium chloride is the major contributor to the junction potential, and the nearly equal mobilities of potassium and chloride ions makes the potential small.

13.5 CONVERTING VOLTAGE TO pH

Equation 1 summarizes the relationship between meas- ured cell voltage (in mV), pH, and temperature (in Kelvin):

E(T) = E°(T) + 0.1984 T pH (1)

The cell voltage, E(T)—the notation emphasizes the dependence of cell voltage on temperature—is the sum of five electrical potentials. Four are independent of the pH of the test solution and are combined in the first term, E°(T). These potentials are listed below:

1.the potential of the reference electrode inside the glass electrode

2.the potential at the inside surface of the glass mem- brane

3.the potential of the external reference electrode

FIGURE 13-5. The Origin of Liquid Junction Potentials.

The figure shows a thin section through a pore in the junction plug. The junction separates a solution of potassium chloride on the left from a solution of hydrochloric acid on the right. The solutions have equal molar concentration. Driven by concentration differences, hydrogen ions and potassium ions diffuse in the directions shown. The length of each arrow indicates relative rates. Because hydrogen ions move faster than potassium ions, positive charge builds up on the left side of the section and negative charge builds up on the right side. The ever-increasing positive charge repels hydrogen and potassium ions. The ever-increas- ing negative charge attracts the ions. Therefore, the migration rate of hydrogen decreases, and the migration rate of potassi- um increases. Eventually the rates become equal. Because the chloride concentrations are the same, chloride does not influ- ence the charge separation or the liquid junction potential.

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