PH

DETERMINATION OF SOIL pH AND SOIL ELECTRICAL CONDUCTIVITY                                                                                                                (JACKSON, 1973)

Aim

         To determine the pH and electrical conductivity of the given soil sample.

Soil pH

         pH  is defined as the negative logarithm of hydrogen ion activity at 25ºC (or) the reciprocal of the hydrogen ion concentration (Sorenson, 1906).

                                 pH = -log (H+) = log 1/(H+)

         Two electrodes are used in the determination of pH. One is reference electrode which provides standard voltage. The reference electrode is usually a saturated calomel electrode which has two layers (i) saturated solution of KCl and (ii) mixture of solid HgCl2 and Hg. The outer tube is usually 5-15 cm long and  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 a small opening. The resistance of this type of electrode is 2000-3000 ohms.

         The outer electrode is a glass electrode that consists of a tube enclosing a lead wire made of Ag coated with AgCl. This wire is again enclosed in a  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 the reference electrode though 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 the electrodes. The potential difference is measured in terms of pH by suitable galvanometer.

Principle

         Glass electrode in contact with H+ ions of the solution acquires an electrode 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.

Materials required

i.                    pH meter

ii.                  100 ml beakers

iii.                Glass rod

iv.                Buffer solution (pH 4.0, 7.0 and 9.2)

Procedure

·         Switch on the instrument and allow it to warm 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 switch to a 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 passed through 2 mm sieve and transfer to a clean 100 ml beaker

·         Add 50 ml of distilled water

·         Using glass rod, stir the contents intermittently and allow it to stand for half an hour

·         Wash the electrodes carefully with a jet of distilled water and wipe 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 both in supernatant solution and suspension .

Precautions

·         Electrodes should not be rubbed strongly with hand or any other material as they are too fragile

·         Electrodes should not be allowed to dry. They should be kept in distilled water when not in use

·         Electrodes should not be allowed to remain in the test solution for a long time. After testing it should be washed with distilled water immediately.

Rating

<4.5            :     Extremely acidic

6.6-7.5        :     Neutral

4.5-5.0        :     Very strongly acidic

7.6-7.8        :     Mildly alkaline

5.1-5.5        :     Strongly acidic

7.9-8.4        :     Moderately alkaline

5.6-6.0        :    Moderately acidic

8.5-9.0         :     Strongly alkaline

6.1-6.5         :   Slightly acidic

>9.0            :    Very Strongly alkaline

Estimation of pH in Aqueous Solution and Electrolyte Solution

Aim

To estimate the pH of the soil in aqueous solution and electrolyte solution.

Principle

The pH is defined as the negative logarithm of hydrogen ion concentration or the negative logarithm of activity of hydrogen ion.

i.e. pH          = -log [H+] (or)

= -log a H+ [a = activity]

 The activity of H+ indicates effective H+  and  this effective H+ comes from

-          Dissociation of organic acids in  soil .

-          Soil micelle.

Quantification of pH is done by pH meter. In pH meter two electrodes are present. i) glass electrode (ii) reference electrode. A glass electrode in contact with H ions of the solution acquires an electrode potential which depends on the H+ ions. This is measured potentiometrically against some reference electrode (calomal electrode). The potential difference between glass electrode and calomal electrode is expressed in pH units.

Materials Required:

•

100 ml beaker

•

Glass rod

•

Distilled water

•

O.IN KCl

•

O.OIM CaCl2 solution

•

pH meter

 Procedure

i)                    Saturated Soil Paste

Weigh 20g of soil sample and add a small quantity of distilled water stir well and make a paste. Stir once in 10 minutes for one hour.

(ii) 1:1 Soil-Water Solution

Weigh 20g of soil sample and transfer to a 100 ml beaker and add 20 ml of distilled water and stir for about one hour with an interval of 10 minutes.

(iii) 1:2.5 Soil Water Dilution

Weigh 10g of soil and transfer to a 100 ml beaker and add 25ml of distilled water and stir for about one hour with an interval of 20 - 30 minutes.

(iv) 1:5 Soil Water Solution

Weigh 20g of soil sample and transfer to a 100ml beaker and add 100ml of distilled water and stir for about one hour with an interval of 10-15 minutes.

(v) 1:10 Soil Water Solution  

Weigh 10g of soil and transfer to a 100 ml beaker and add 100 ml of distilled water and stir for about one hour. It is called as hydrolytic soil pH. The pH will increase with the degree of sodium saturation.

Soil Electrolyte Solution

(vi) Soil:  0.01M CaCl2  (1:2 dilution)