Method for estimation of CO2(aq) plus H2CO3°, HCO3− and pH in soil solutions collected under field conditions

A method for the collection of soil solution and the determination of pH, H₂CO₃* (= CO₂(aq) plus H₂CO₃°), HCO₃^? and CO₃^2?, was developed which excluded atmospheric gases during the entire procedure. The soil solution was collected by tension lysimeters without exposure to the atmosphere. Using a closed system, the sample was transferred to a titration beaker for the analysis of pH, H₂CO₃* and HCO₃^?. The analysis of CO₂-acidity was done by titration with 0.0454 N Na₂CO₃ to the end point pH of 8.3. It was immediately followed by an acidimetric titration for the determination of alkalinity using 0.005 N H₂SO₄ under gentle N₂ flow; the equivalence point was determined graphically from the titration curve. In standard solutions, this method gave nearly 100% recovery of H₂CO₃* and HCO₃^?. In soil solutions, the pH markedly increased and H₂CO₃* decreased upon exposure to the atmosphere. The values of the sum of CO₂-acidity and alkalinity in soil solutions at a depth > 5 cm agreed well with the values of total inorganic carbon obtained by CO₂ infrared detection following CO₂ degassing. For solutions obtained from 100 cm and 300 cm depth (limestone) the measured distribution of H₂CO₃* and HCO₃^? was in agreement with the calculated values based on pH-measurement and total inorganic carbon. This comparison was unsatisfactory for the concentration of H₂CO₃* in solutions of the surface (0–15 cm) soil, possibly because the mathematical model as well as the interpretation of the titration curves did not consider any organic compounds in the solution.