Rl. Aitken et Pw. Moody, THE EFFECT OF VALENCE AND IONIC-STRENGTH ON THE MEASUREMENT OF PH BUFFER CAPACITY, Australian Journal of Soil Research, 32(5), 1994, pp. 975-984
Although the measurement of pH buffer capacity (pHBC) is used to deter
mine lime requirement and acid addition rate in acidification studies,
the experimental conditions under which pHBC is determined have not b
een studied. The effect of valence and ionic strength on the measureme
nt of pHBC was investigated on a range of soils. The effect of the mon
ovalent or divalent accompanying ion was examined by establishing sepa
rate titration curves for each of 100 soils by adding incremental amou
nts of either Ca(OH)2, NaOH, HCl or H2SO4 to soil suspended (1 : 5) in
water. Linear regressions were fitted to the linear portion of each t
itration curve and the slopes of these lines were used as a measure of
pHBC. For each soil, the pH buffer capacities were statistically comp
ared. The pHBC determined with Ca(OH)2 was significantly (P = 0.05) gr
eater than that determined with NaOH in 92 soils and, on average (all
soils), was 2.2 times the pHBC in NaOH. The effect of ionic strength o
n pHBC was investigated in each of 20 soils by titrating with HCI in w
ater and suspensions at nominal ionic strengths of 0.006, 0.03 and 0.3
m. In all soils there was a trend for increasing pHBC with increasing
ionic strength (I) and, for I less-than-or-equal-to 0.03 m, there was
a marked increase in pHBC with increasing I. The results are discusse
d in relation to the effect of valence and ionic strength on pH buffer
capacity mechanisms, and the implications with respect to calculating
acidification rates and lime requirements.