Determination of Calcium with Ion-Selective Electrode in Black Liquor from a Kraft Pulping Process

A solid-contact Ca(2+)-selective electrode was used to study speciation of calcium in black liquor. During extended time of operation the standard potential was found to change, but the sensitivity of the electrode, and the selectivity coefficients remained unchanged. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) measurements showed sodium sorption to the electrode membrane during the measurement in black liquor. Linearization of the complexometric titration curves were used to determine the concentrations of Ca(2+), CaOH(+) as well as weak and strong calcium complexes. With inductively coupled plasma optical emission spectrometry (ICP-OES), similar total calcium concentrations were obtained as by the EDTA titration.