THE EFFECT OF VALENCE AND IONIC-STRENGTH ON THE MEASUREMENT OF PH BUFFER CAPACITY

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.