Soil and Plant Analysis

pH measurement

Principle:

A glass electrode in contact with H+ ions of the solution acquires an electric potential which depends on the concentration of H+ ions. This is measured potentiometrically against some reference electrode, which is usually a calomel electrode. The potential difference between glass electrode and calomel electrode is expressed in pH units.
Two electrodes are used in the determination of pH. One is reference electrode, which provides a standard voltage. The reference electrode is usually a saturated calomel electrode which has two layers
(1) Saturated solution of KCl and
(2) mixture of solid HgCl2 and Hg.

The outer tube is usually 5-15 cm long, 0.5-1 cm in diameter. The mixture of solid HgCl2 + Hg paste is contained in an inner tube that is connected to the saturated KCl solution in the outer tube by means of small opening. The resistance of this type of electrode is 2000-3000 ohms.

The outer electrode is glass electrode that consists of a tube enclosing a lead wire made of Ag coated with AgCl2. This wire is again enclosed in wax insulation. To the tube at the bottom is attached a glass bulb made of a special kind of glass which is sensitive to H+ ions. The thickness of the glass membrane varies from 0.03 to 0.1 mm and has a resistance of 50 to 500 mega ohms.

When these two electrodes are dipped in solution, the saturated solution of KCl comes out of reference electrode through the small holes and forms an invisible ionic bridge between electrodes through which current passes. The H+ ions are absorbed by glass electrode and depending on the amount of H ions present in the solution, an electric potential develops between electrodes. This potential difference is measured in terms of pH by suitable galvanometer.

Materials required:
i) pH meter ii) 100 ml beakers iii) Glass rod iv) Buffer solutions (pH 4.0, 7.0 and 9.2)

Procedure:

• Standardization of pH meter
• Switch on the instrument and allow it to warm up for 10 minutes. Keep the pH selector switch on zero position.
• Set the temperature compensation control to the solution temperature.
• Adjust the zero adjustment knob so that the pointer in the meter reads exactly zero, when the electrodes are immersed in distilled water.
• Lift the electrodes from distilled water and wipe it dry using filter paper and dip them in standard buffer solution of known pH (4.0, 7.0 and 9.2).
• Change the function switches to particular pH range (0-7 or 7-14) and adjust the standardization knob till the pointer reads the correct pH value of the buffer solution.
• Do not disturb the zero knob adjustment.

pH measurement

• Weigh 20 g of air dry soil and transfer to a clean 100 ml beaker. Add 50 ml of distilled water.
• Using glass rod, stir the content intermittently and allow it to stand for half an hour.
• Wash the electrodes carefully with a jet of distilled water and wipe it dry with a piece of filter paper.
• Stir the soil suspension again just before taking the reading.
• Immerse the electrodes into the beaker containing soil water suspension and change the function switch to the particular pH range.

Record the meter reading.