Fundamentals of Soil Science (1+1)

Exercise: 6

Aim: Determination of Soil pH

• pH is a measure of the acidity or a alkalinity of the soil It is defined as the negative logrithm of hydrogen ion activity.

Mathematically pH = - log10aH⁺.
Where; aH⁺ is activity of hydrogen ions in g ions/litre.
Pure water is dissociated into H+ and OH- ions according to the following equations:
H₂O = H⁺ + OH^-

• According to law of dissociation

[ H⁺] x [ OH^-] / H₂O = K
Where: H⁺ etc are the concentrations and K is the dissociation constant. Since the concentration of undissociated water remains practically the same, because of very little ionization of H₂O molecule the above relationship becomes
[ H⁺] x [ OH^-] = K_w = 10-14 at 20⁰C
Kw: ion product constant of water.

• At neutrality H⁺= OH^- and H⁺ = 10-7 or pH 7.Pure water has a pH value of 7.
• As the hydrogen ion activity increases, the pH will decrease. On the other hand, if the hydrogen ion activity decreases, the pH value will increase.. The pH scale extends from 0 to 14 and mid point on this scale is neutral. The pH value above 7 denotes alkalinity and below 7 acidity.

Principle

• The principle underlying potentiometric determination of pH is based on the measurement of electrical potential developed by an electrode (whose potential depends on the hydrogen ion activity of the solution) in conjunction with a second electrode (reference electrode) dipping into the same solution.

Material Required:

• Beakers (100 ml)
• Glass rods
• Electrical or analytical balance
• Cylinder, graduated (50 ml)
• pH meter fitted with glass and reference electrodes
• Buffer solutions of pH 4 and 10 or tablets
• Ordinary filter paper
• Wasing bottle with distilled water.

Procedure:

• Weigh out 20 g of soil in a 100 ml beaker, add 40 ml of distilled water and stir at least four times within a period of half an hour. This time is required for the soil and water to attain equilibrium.
• After half an hour again stir the soil suspension and measure the pH on a pH-meter.
• In the meantime, switch on the pH meter and set the temperature compensation knob at buffer solution tempertaure and the range selector to zero.
• After 10 minutes warming up peroid, dip the electrode in to a buffer solution of known pH, and adjust the reading to the pH of the buffer solution.
• Rinse the electrodes with distilled water and carefully wipe the filter paper.
• Shake the soil solution and insert the electrode in to it.
• Read the pH on the instrument.
• Rinse the electrodes with the distilled water and than lower them back into a beaker containig distilled water.

Observations

• pH of the soil (1:2 ratio) = ------------------

Precautions in the use of pH meter

• The electrodes should not be allowed to remain in the test solution or suspension for a longer period than necessary.
• Immediately after testing, the electrodes should be washed off with a gentle stream of distilled water.
• For storage, after cleaning, the electrodes are suspended in distilled water and the system is protected from evaporation.
• Drying out of the electrode should be avoided.

Interpretation:

pH	Interpretation
< 6.5	Acidic- requires liming
6.5-7.5	Normal- No treatment; optimum for most crops.
>7.5	Alkaline