Ar. Coscione et al., REVISITING TITRATION PROCEDURES FOR THE DETERMINATION OF EXCHANGEABLEACIDITY AND EXCHANGEABLE ALUMINUM IN SOILS, Communications in soil science and plant analysis, 29(11-14), 1998, pp. 1973-1982
The use of 1 mol L-1 ammonium chloride (NH,CI) or ammonium nitrate (NH
4NO3) 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) measure
ments while the exchangeable acidity results, using either NH4Cl or NH
4NO3 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) s
alts 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 addi
tion, the reaction media also seems not to be appropriate for precise
and accurate determinations of the exchangeable Al by back-titration a
fter the addition of sodium fluoride (NaF), due to the formation of a
complex chemical system composed of multiple equilibria, such as hydro
xo, chloride, and fluoride complexes with Al at relatively high ionic
strength. This strongly contributes to poor indicator color changes. T
hus, the extraction of soils with 1 mol L-1 KCl solutions is still the
best way to extract exchangeable acidity and exchangeable Al for dete
rmination by titration. The use of phenol red as indicator for the det
ermination of exchangeable acidity in KCl extracts is recommended in p
lace of the commonly used bromothymol blue or phenolphthalein to incre
ase reproducibility and precision. Other procedures, such as spectroph
otometric ones, have to be considered for the determination of exchang
eable Al, principally if NH4 salts are used as extractants.