The carbon dioxide (CO2) system is the primary buffer in seawater which controls oceanic pH. Changes in the marine CO2 system affect a number of processes such as metal speciation, mineral saturation states, auditory responses in fish, and primary productivity rates. Increased atmospheric concentrations of CO2 from human activities (e.g. burning of fossil fuels, deforestation, and cement production) has led to a global decrease in surface ocean pH termed anthropogenic ocean acidification. One particular concern in response to increased oceanic CO2 is a substantial decrease in the calcium carbonate (CaCO3) saturation states, ΩCaCO3. The long-term physiological effects of ocean acidification and decreased ΩCaCO3 on marine biota are currently subjects of intensive global investigation. Consequently improved methods are needed to facilitate evaluations of the evolving CO2 system chemistry and the responses of marine organisms to those changes. Currently two of four measureable chemical parameters (pH, dissolved inorganic carbon, fugacity of CO2, and total alkalinity) are required for full characterization of the inorganic CO2 system; carbonate ion concentrations ([CO3 2-]) can, for example, be calculated from paired measurements of pH-DIC and pH-TA. The primary objective of this dissertation is to refine a method for directly determining [CO3 2-] using a single measurement, the distinctive ultraviolet absorbance spectra of Pb(II) species in seawater. The technique is fast, methodologically simple, and suitable for routine use in laboratory and shipboard studies. It is, as well, suitable for analyses using autonomous instrumentation. My studies began with an investigation of lead carbonate (PbCO3 0) complexation in synthetic media (at 25 °C between 0.001 to 5.0 molal ionic strength) to evaluate factors that control Pb(II) speciation, and thereby Pb(II) spectra, in seawater. Since laboratory investigations of Pb(II) speciation in seawater require potentiometric measurements of seawater pH, my dissertation includes development of a novel spectrophotometric method for calibrating pH electrodes directly in seawater. This spectrophotometric electrode calibration enables improved assessment of the extent to which electrode behavior is Nernstian and the influence of salinity on electrode calibrations. In addition, for the first time at sea, [CO3 2-] was directly determined in the Arctic and the Eastern Pacific Oceans using the Pb(II) method. These field studies allowed assessment of the consistency between direct [CO3 2-] determinations and carbonate determined using conventional CO2 system measurements. Finally, using techniques from my evaluation of lead speciation, as well as my electrode calibration development and field studies, additional laboratory studies were used to increase carbonate measurement sensitivity and applicability over a wider range of salinity.