PH METERS

General consideration

• The hydrogen ion concentration or pH is a measure of the acidity or alkalinity of a solution. It is expressed as follows
• PH=1/log10(H⁺) =-log of hydrogen ion concentration.
• (H⁺) is the hydrogen ion concentration of the solution in moles per liter.
• At a given temperature, in an aqueous solution, the product of hydrogen ion concentration and hydroxyl ion concentration is constant.
• Hence, a measure of hydrogen ion concentration gives a measure of hydroxyl ion concentration.
• At a temperature of 22°C, this product conveniently happens to be exactly 10^-14 (expressed in gram- molecules per liter.
• The pH scale is introduced to avoid awkward small numbers.
• The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration, in an aqueous solution.
• If a solution is described as pH 6, its hydrogen ion concentration is 10^-6 and its hydroxyl ion concentration is 10^-8 at room temperature.
• The value of using pH can be seen in the case of human blood which has an extremely low hydrogen ion concentration. Blood H+ 0.398 x 10^-7
• Blood pH log (0.398 x 10^-7) = 7.4
• The scale of the pH meter is normally taken as extending from 0 to 14 pH.
• An acid solution has a pH value less than 7.0 and a basic solution has a PH value greater than 7.0 A neutral solution has a pH value of 7.0.
• A change of one pH meter unit corresponds to a 10-fold change in the hydrogen ion concentration of the solution.
• The most reliable and convenient method for measuring pH is by the use of a pH meter.
• It measures the e.m.f. of a concentration cell formed from a reference electrode, the test solution and a glass electrode, expressed diagrammatically as follows:

PH meter

• Ag.Agcl
• (0.1M) Hcl/glass
• Test solution
• KCl( saturated)
• Hg₂cl₂

principle of pH meter

• When the pair of electrodes or a combined electrode (glass electrode and calomel electrode) is dipped in an aqueous solution, a potential is developed across the thin glass of the bulb (of glass electrode).
• The electromotive force (e.m.f.) of the complete cell (E) formed by the linking of these two electrodes at a given solution temperature is therefore –
• E = E_ref – E_glass
• E_ref is the potential of the stable calomel ref electrode, which at normal room temperature (25°C ± 5°C) is + 0.250V.
• E_glass is the potential of the glass electrode which depends on the pH of the solution under test.
• The resultant small e.m.f. can be recorded potentiometrically by using a vacuum tube amplifier.
• Variations of pH with E may be recorded directly on the potentiometer scale graduated to read pH directly.

Important components of a pH meter

• glass electrode
• calomel electrode

Glass electrode

It consists of a very thin bulb about 0. 1 mm thick blown onto a hard glass tube of high resistance.
The bulb contains 0. 1 mol/liter HCL connected to a platinum wire via a silver-silver chloride combination.

Calomel electrode

It consist of a glass tube containing saturated KCL connected to platinum wires through mercury mercurous chloride paste.