This test contributes to one of the characterising analytical methods in Soil Taxonomy (Soil Survey Staff, 1975). The property refers to acidity associated with release of Al3+ and displaced H+ at pH 8.2 and its neutralisation with TEA. Excess TEA is measured with standard acid. When added to exchangeable bases, exchange acidity provides an estimate of CEC at pH 8.2, which reflects maximum net negative charge of the soil.
The method described involves overnight shaking, clarification, and titration of an aliquot of the equilibrated buffer solution (Blakemore et al. 1987).
TEA Buffer Solution To 61.07 g barium chloride (BaCl2.2H2O) add 28 mL triethanolamine [N(CH2CH2OH)3] and make to 1 L with CO2-free deionised water. Adjust solution pH to 8.2 with ≈10 mL 5 M HCl. Protect the solution by attaching a drying tube containing a CO2 adsorbent (e.g. soda-lime) to the air intake.
0.10 M Hydrochloric Acid
Use a commercial standard solution or prepare by adding 10 mL 10 M HCl to deionised water and make volume to 1.0 L. Standardise against sodium tetraborate as in Method 7A1.
0.1% Bromocresol Green
Dissolve 0.1 g bromocresol green (C21H14Br4O5S) in 100 mL CO2-free deionised water.
Mixed Indicator
Dissolve 1.250 g methyl red (C15H15N3O2) and 0.825 g methylene blue (C16H18ClN3S) separately in a small quantity of 96% ethanol (C2H5OH). When dissolved, combine and make to 1 L with 96% ethanol.
Generally, weigh 2.5 g of air-dry soil (<2 mm) into a 50 mL, screw-cap, polypropylene centrifuge tube; weigh 1.0 g for soils high in OM and/or in short-range-order minerals. Add 25 mL of buffer solution and shake for 16 h on an end-over-end shaker. Centrifuge at 2000 rpm for 15 min. Concurrently, shake two blanks of 25 mL of buffer solution.
Transfer 10 mL aliquots of supernatant solution to 125 mL conical flask, add about 20 mL deionised water and mix. Add 1 drop Bromocresol Green and 5 drops Mixed Indicator. Titrate with 0.10 M HCl until solution reaches first full purple colour. Titrate blank solutions first then titrate sample solutions to the same point of colour change.
For 2.5 g soil, 25 mL buffer solution and 0.10 M HCl:
TEA Exchange Acidity (cmolc/kg) = {[Blank titre – Sample titre (in mL)] × 10}
For 1.0 g soil, 25 mL buffer solution and 0.10 M HCl:
TEA Exchange Acidity (cmolc/kg) = {[Blank titre – Sample titre (in mL)] × 25}
Report TEA Exchange Acidity (cmolc/kg), expressed on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to convert to an oven-dry concentration. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.