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With the help of emf of the cell measured we can find out no of properties or functions. One of the important application of emf is to find out the pH of the solution. The pH of solution defined as $$ pH = -\log { \left[ { H }^{ + } \right] } $$ In 1909 Firtz Haber and Z. Klemensiewiuz introduced the glass electrode with the help of which they said we can find the pH of unknown solution very correctly. Whenever two different \(\left[ { H }^{ + } \right] \) ion concentrations are separated by a very thin glass membrane. There develops a difference of potential between two surfaces of the glass membrane. By keeping the pH of one of the solution inside electrode as constant, the pH of the other solution can be determined. This electrode differ from the other type of electrode the sense no electrons are involved in the electrode reaction. When electrode is in contact with the acid of unknown pH, then the potential of this electrode is given by, $${ E }_{ G } = { E }_{ G }^{ 0 } - 0.059 \log { \left( \frac { 1 }{ { a }_{ { H }^{ + } } } \right) } = { E }_{ G }^{ 0 } - 0.059 pH $$ where \({ E }_{ G }\) = Potential of Glass electrode and \( { E }_{ G }^{ 0 }\) = Standard electrode potential. This value depends on the quality of glass i.e. nature of the glass.
Construction: The electrode consist of special type of glass bulb or tubing with composition (72% \(Si{ O }_{ 2 }\), 22% \({ Na }_{ 2 }O\) and 6% CaO). The thickness of the glass tube is 0.001mm. Bulb contains either a buffer solution of pH = 4 of 0.1M HCl. Silver wire coated with AgCl is dipped for electrical contact. The electrode is represented as, $$Ag, AgCl(s), 0.1M HCl | Glass | Sample \ Solution, \ pH = ? $$
Working: The glass electrode is then coupled with reference electrode e.g. SCE then we get cell and we will find out the emf of this cell. This cell is, $$ Ag, AgCl(s), \underset { (OR pH = 4) }{ 0.1M HCl } | Glass | \underset { (pH = ?) }{ Sample \ Solution } \parallel SCE $$ We can now find \( \underset { Glass }{ { E }_{ cell } } \) from the equation $$ { E }_{ cell } = \underset { SCE }{ { E }_{ cell } } - \underset { Glass }{ { E }_{ cell } } $$ and also pH of solution using the equation $$ { E }_{ cell } = \underset { SCE }{ { E }_{ R } } - { E }_{ G }^{ 0 } + 0.0592 pH $$ Here the problem is that \({ E }_{ G }^{ 0 }\) value is not constant, it depends on the type (nature) of glass and as the raw material for preparation of glass changes from time to time. Therefore \({ E }_{ G }^{ 0 }\) is not constant. It is important to find out \({ E }_{ G }^{ 0 }\) value for the particular glass electrode. This can be done by coupling two glass electrodes to form a cell, $$ Glass \ electrode | 0.1M \ HCl | Glass \ electrode $$ Theoretically potential of such combination must be zero. But in practice, small difference is always observed. This is called asymmetric potential of glass electrode. Thus indicating \({ E }_{ G }^{ 0 }\) value of two glass electrodes is not same. Now immerse a glass electrode in a standard buffer and couple with suitable reference electrode and find out the emf of the cell. From emf of cell we can find out the \({ E }_{ G }^{ 0 }\) value of electrode and then by using Nernst equation we can find out the standard electrode potential i.e. \({ E }_{ G }^{ 0 }\).
Advantages: 1) It attains equilibrium very quickly.
2) No foreign substance is added to experimental solution.
3) It can be used for oxidising and reducing agents which do not react with glass.
4) It can also be used for turbid and colloidal solutions.
Limitations: 1) Glass electrode has very high internal resistance of \({ 10 }^{ 8 }\Omega\) hence ordinary potentiometer which draws some amount of current cannot be used for pH determination by using glass electrode. Therefore either quadrant electrometer or vacuum tube potentiometers are used.
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Shared publicly - 2019-08-23 00:00:00
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