In ocean waters, the carbonate ion is of crucial importance to benthic and pelagic organisms that build their physical support structures out of calcium carbonate (CaCO3). Marine carbonate ion concentrations ([CO32−]) are measurable through spectrophotometric observations of the ultraviolet (UV) light absorbed by lead carbonate in Pb-enriched seawater, but previous characterizations of the UV-absorption model were applicable only at a fixed temperature of 25 °C. In this paper, the model is extended to a temperature range of 3 to 40 °C and a salinity range of 20 to 40. This advancement allows for determinations of [CO32−] with temperature measurement rather than temperature control, thus decreasing the required financial investment and instrumental complexity. The extended model also represents a significant step toward the development of automated inline or in situ [CO32−] sensors and promotes the utility of [CO32−] as a fifth measured variable for inclusion in studies of the marine carbon dioxide (CO2) system. A quantitative evaluation of propagated uncertainties in CO2 system calculations based on measured [CO32−] as an input variable was also performed. The results show that total dissolved inorganic carbon (CT) and total alkalinity (AT) are the most suitable measurable variables to pair with measured [CO32−] as input to such calculations. Pairing [CO32−] with the partial pressure of CO2 yields relatively low uncertainty in calculated pH — comparable to that resulting from conventional input pairs — but relatively high uncertainties in calculated AT and CT. Pairing [CO32−] with pH results in relatively high uncertainties in all calculated variables. CaCO3 saturation states (W) determined from measured [CO32−] (alone) can circumvent some sources of uncertainty inherent to conventional (two-variable) calculations. Simpler, more direct ways of measuring [CO32−] open up new opportunities for marine researchers and others interested in monitoring CaCO3 saturation states in seawater.