A thermodynamic model is developed for the precipitation of copper oxalate from aqueous solutions of copper nitrate and sodium oxalate in a small well-mixed batch reactor. This model is then used to explain the discrepancy between experimental pH evolution and the predicted evolution for the precipitation of copper oxalate as the only solid phase. The experimental behaviour is simulated by taking into account the possible precipitation of small quantities of copper hydroxyl carbonates from adventitious CO2. Experiments with controlled CO2 partial pressures illustrate the power of these types of simulations in helping elucidate precipitation reactions. The thermodynamic model can help simulate the kinetics of precipitation by linking precipitation yield with real time pH measurements.