Revisting titration procedures for the determination of exchangeable acidity and exchangeable aluminum in soils

The use of 1 mol L-1 ammonium chloride (NH4Cl) or ammonium nitrate (NH4NO3) solutions as alternative soil extractants were evaluated for the determination of the exchangeable acidity and exchangeable aluminum (Al) in twelve soil samples by the traditional titration/back-titration method. The exchangeable Al results were compared with those obtained by inductively coupled plasma emission spectrometry (ICP-AES) measurements while the exchangeable acidity results, using either NH4Cl or NH4NO3 solutions as alternative soil extractants, were compared against those using 1 mol L-1 potassium chloride (KCl) and bromothymol blue as the indicator. The results indicated that the use of ammonium (NH4) salts in these determinations is not recommended since the exchangeable acidity found using these extractants was systematically higher than that found in KCl extracts due to the additional acidity caused by the presence of comparatively higher concentrations of NH4+ ions. In addition, the reaction media also seems not to be appropriate for precise and accurate determinations of the exchangeable Al by back-titration after the addition of sodium fluoride (NaF), due to the formation of a complex chemical system composed of multiple equilibria, such as hydroxo, chloride, and fluoride complexes with Al at relatively high ionic strength. This strongly contributes to poor indicator color changes. Thus, the extraction of soils with 1 mol L-1 KCl solutions is still the best way to extract exchangeable acidity and exchangeable Al for determination by titration. The use of phenol red as indicator for the determination of exchangeable acidity in KCl extracts is recommended in place of the commonly used bromothymol blue or phenolphthalein to increase reproducibility and precision. Other procedures, such as spectrophotometric ones, have to be considered for the determination of exchangeable Al, principally if NH4 salts are used as extractants.