Determination of effective cation exchange capacity ECEC

Method 5.3. Determination of effective cation exchange capacity (ECEC) 

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 KCI 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 KCI. Combine the leachates from the total addition of 200 ml (some will be retained in the soil) and mix. 

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 AP+). 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 HCI to convert the above pink colour back to colourless, then add 10 ml 4% NaF solution. Titrate with 0.1 M HCI, 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  

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