As a wide range of acidity values (0.3–0.9 mol l−1) was
previously observed for this type of soil extracts, standards
containing 0.9 mol l−1 of HCl were also analised using the
conditions stated above. The calibration curve obtained presented
higher values for both slope and intercept when compared
to that provided by standards with 0.2 mol l−1 HCl
(Table 3; NaOH = 0.6 mol l−1). This situation was probably
due to an excess of NaOH in the reagent solution when
the solutions of lower acidity were used, which resulted in
an alkaline pH value. This was confirmed by collecting a
few drops from the waste tube in a pH universal indicator
paper, a few seconds after the peak appearance. For the
lower acidity standards, the pH was above 12, while for the
higher acidity standards it was about 4. Therefore, the concentration
of NaOH in the reagent solution was varied between
0.2 and 0.8 mol l−1 in order to find a concentration
value that could provide similar results for standards containing
either 0.2 or 0.9 mol l−1 of HCl. The parameters obtained
for the calibration curves are presented in Table 3.
For the lower acidity standards, higher slope values were
observed for lower concentrations of NaOH. This situation
was reversed for the higher acidity standards and it was
not possible to obtain similar calibration curves for the dif