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pH Theory
pH Measurement
The measurement of pH using a pH Electrode is an age old method that centres around the perception of a substance as acidic or alkaline and is dependent on the concentration of the hydrogen Ion (H+) within the substance.
The use of a pH electrode allows this measurement to be expressed in a meaningful way.
Derived from the Sorensen Equation the pH value is defined as a negative logarithm of the H+ concentration in a given solution.
- A high H+ Concentration equals: 1mol/L = 10o pH = 0 (ACIDIC)
- A low H+ Concentration equals: 10-14 mol/L pH=14 (ALKALINE)
It is therefore simple to measure the pH of a substance and compare it with other substances. pH 0 is extremely acidic, pH 14 is extremely alkaline and pH 7 neutral.
The second part of the circuit is the reference electrode. This is a stable point that has a defined potential and is independent of the solution to be measured. The reference electrode fig is made up of a reference element that is commonly a Silver/Silver Chloride wire encased in a known electrolyte. The reference electrode then has a junction, which is the contact between the stable internal reference electrode and the solution to be measured. This is commonly a porous ceramic pin.
The evolution of pH electrodes came with the joining of the two separate electrodes to produce the COMBINATION pH ELECTRODE.The formation of the combination electrode can be seen in fig. The formation of the combination electrode still with the pH Glass membrane acting in the same way. The reference electrode is continually encased around the pH electrode. As the pH Glass comes into contact with an aqueous substance to measure, a gel layer forms on the membrane. This also happens on the inside of the glass layer. The pH value of the aqueous solution will either force Hydrogen Ions out of the Gel layer or into this layer. The Internal buffer in the glass electrode has a constant pH value and this keeps the potential at the inner surface of the membrane constant. The membrane potential is therefore the difference between the inner and outer charge. If you then factor in the reference electrode with its stable potential you have a combination pH electrode that encapsulates the measuring electrode and reference electrode.
How the pH Electrode Works
As the pH Glass comes into contact with an aqueous substance to measure, a gel layer forms on the membrane. This also happens on the inside of the glass layer.
The pH value of the aqueous solution will either force Hydrogen Ions out of the Gel layer or into this layer. The Internal buffer in the glass electrode has a constant pH value and this keeps the potential at the inner surface of the membrane constant.
The membrane potential is therefore the difference between the inner and outer charge. If you then factor in the reference electrode with its stable potential you have a combination pH electrode that encapsulates the measuring electrode and reference electrode.
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