##Principle of pH measurement:
Electrochemical pH measurement is based on the Nernst equation, which describes the electrical potential created by ions migrating through a permeable membrane. Nernst equation:
V = Voltage produced across membrane due to ion exchange (volts)
R = Universal gas constant (8.315 J/mol·K)
T = Absolute temperature (Kelvin)
n = Number of electrons transferred per ion exchanged (unitless)
F = Faraday constant, in coulombs per mole (96485 C/mol e−)
C1 = Concentration of ion in measured solution (moles per liter of solution, M)
C2 = Concentration of ion in reference solution (moles per liter of solution, M)
##Working & Construction:
The equation describes the electrical potential created by ions migrating through a permeable membrane. The example of this is a device called a concentration cell, where two halves of an electrochemical cell are filled with solutions having different concentrations of ions.
As ions naturally migrate through this membrane in an attempt to equalize the two concentrations, a voltage corresponding to the difference in ion concentrations between the two cell halves will develop between the two electrodes. The greater the difference in concentrations between the two sides, the greater the voltage produced by the cell. The Nernst voltage may be used to infer the concentration of a specific type of ion if the membrane is selectively permeable to that one type of ion.
The reference and measurement electrode is combined. The measurement and reference electrodes provide a voltage-generating element sensitive to the pH value of whatever solution they are submerged in the solution. This combination of electrodes is connected to a pH meter, which gives the pH value according to the current generated across the electrodes.
Special pH-measurement electrodes are manufactured with a closed end made of this glass, a small quantity of buffer solution contained within the glass bulb:
Any concentration of hydrogen ions in the process solution differing from the hydrogen ion concentration in the buffer solution will cause a voltage to develop across the thickness of the glass. Thus, a standard pH measurement electrode produces no potential when the process solution’s pH value is exactly 7.0 pH. Thus, the Nernst voltage produced by a glass pH electrode is directly proportional to the difference in pH value between the measured solution and the probe’s internal 7.0 pH buffer.
The glass is selected as permeable to hydrogen ion.
Practical structure of pH meter is shown:
The fig A shows the internal diagram of the pH meter with internal preamplifier. Preamplified pH probes have multi-conductor cables with extra wires used to conduct DC power from the pH transmitter to the pH probe to power the preamplifier. A feature seen in the fig A amplified probe is an RTD sensor for detecting the temperature of the liquid process solution. This is important because the Nernst equation contains a term for membrane temperature, which means the Nernst potential depends just as much on temperature as it does on ionic concentration.
fig B is photograph of the pH meter, there is weep hole near the bulb electrode for process liquid to enter the reference electrode assembly and a metal solution ground electrode.