A mass balance model of total soluble sulfide and free zinc with a second-order reaction term is theoretically able to reconstruct the zinc effluent concentration and the kinetic parameter (k). However, under real conditions this model predicts the zinc effluent concentration four orders of magnitude higher than the measured ones. The applied error analysis, based on linearization of the model followed by first-order variance propagation, showed that the accuracy of several of the input variables (flows and influent concentrations) jeopardized the estimation of the Zn concentration in the effluent, which is a phenomenon expected for every fast reaction with low product concentration. In order to overcome the inaccuracy issue, an "apparent solubility product'' as a function of pS (11-20) was calculated from the experimental data, allowing for the subsequent determination of an "apparent kinetic parameter'' (kA), that excluding parallel reactions was between 1.7 X 10(23) - 6.2 X 10(24) L=(mol.h). This allowed for further tuning of the model such that the estimates of the Zn effluent concentration became of the same order of magnitude as the measured ones (10(-7) M Zn).