Method 53 Determination of effective cation exchange capacity ECEC

Reagents.

• Phenolphthalein indicator, 0.1% (m/v) - dissolve 0.1 g phenolphthalein in 100 ml 95% ethanol.

• Potassium chloride, 1 M - dissolve 74.55 g KCl and make up to 1 l with water.

• Sodium fluoride, 4% (m/v) - dissolve 40 g of NaF in water and make up to 1 l with water.

Procedure. Weigh 20 g of air-dry soil sieved to ^2 mm into a 250-ml wide-mouth high-density polyethylene screw-cap bottle. The square type bottles fit best the square box of the reciprocating shaker. Add 100 ml 1 M KCl and shake for 15 min. Transfer all the suspension to a filter funnel holding a Whatman No. 6 paper, and collect the filtrate. When leaching has ceased, add two successive 50-ml aliquots of 1 M KCl. Combine the leachates from the total addition of 200 ml (some will be retained in the soil) and mix.

Determination of hydrogen and aluminium

Transfer 100 ml of filtrate to a 250-ml conical flask, add 5 drops of phenolphthalein indicator and titrate to a permanent pale pink colour using 0.1 M NaOH and with alternate swirling and standing. Record the titre that is equivalent to the total acidity (H+ plus Al3+). The associated equations are:

The aluminium hydroxide appears as a hazy white precipitate, and is titrated as described below.

Add one drop of 0.1 M HCl to convert the above pink colour back to colourless, then add 10 ml 4% NaF solution. Titrate with 0.1 M HCl, stirring constantly, until the pink colour just disappears. Next add two more drops of indicator, and if a pink colour returns, titrate again until it disappears and remains colourless for 2 min. This second titration is equivalent to the amount of exchangeable aluminium. The associated equations are:

Determination of exchangeable calcium and magnesium for ECEC

This is carried out by AAS as for CEC above, except that the blank and all standards must be made up using M KCl solution.

Calculation. The ECEC is the sum of the Ca2+ + Mg2+ + H+ + Al3+ in units of cmolc kg-1 soil (giving the same numerical value as milliequivalents per 100 g soil).

The titration is interpreted as follows:

200 ml M KCl solution = 20 g soil, therefore 100 ml M KCl solution = 10 g soil

1 ml 1.0 M HCl = 1 x 10-3 mol = 0.1 cmol, therefore 1 ml 0.1 M HCl = 0.01 cmol which is per 10 g air-dry soil, therefore 1 ml 0.1 M HCl = 1.0 cmolc analyte ion per kg air-dry soil

Thus the titre in ml 0.1 M HCl is identical to the concentration of analyte ion(s).

The first titre corresponds to H+ + Al3+, and by subtracting the second titre which corresponds to the exchangeable cmolc Al3+, one gets the cmolc H+.

The Ca2+ and Mg2+ readings (y) are obtained as pg ml-1. The ratio of soil to extractant is 10:100, therefore 1 kg soil corresponds to 10,000 ml extractant.

y pg Ca2+ ml-1 = 104 x y pg Ca2+ kg-1 air-dry soil = 10-2 x y g Ca2+ kg-1 air-dry soil

The molar mass of Ca2+ is 40.1 g mol-1, thus we get

{(10-2 x y)/40.1} x y mol Ca2+ kg-1 air-dry soil = y/40.1 cmol Ca2+ kg-1 air-dry soil

Now 1 cmol Ca2+ contains 2 cmolc Ca2+, so the above expression becomes 2y/40.1 cmolc Ca2+ kg-1 air-dry soil = 0.050 y cmolc Ca2+ kg-1 air-dry soil

Correct for soil moisture content as in Method 5.2, Calculation (2), and any other dilution factors.

The expression for exchangeable magnesium is obtained in the same manner, thus

2y/24.305 cmolc Mg2+ kg-1 air-dry soil = 0.082 y cmolc Mg2+ kg-1 air-dry soil Take into account any correction factors as above.

Sum the values of Ca2+ + Mg2+ + H+ + Al3+ to obtain the ECEC in units of cmolc kg-1 soil.

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