Technical note: Excess alkalinity in carbonate system reference materials

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Certified reference materials (CRMs) for oceanic carbonate system measurements are critical for verifying the accuracy of laboratory protocols and the reliability of field sensors. CRMs are certified for total alkalinity and dissolved inorganic carbon, parameters that are (1) stable for a long period of time when a sample is properly stored and (2) not affected by changes in temperature and pressure. In experimentation initially designed to measure the total boron to salinity ratio of seawater, an interesting result has emerged regarding CRMs. A unique acidimetric titration method has indicated that three different batches of CRM contain excess alkalinity (i.e., alkalinity that is not attributable to inorganic bases included in the traditional definition of seawater total alkalinity) that is statistically greater than the excess alkalinity measured in open-ocean water from the Gulf of Mexico. Further, the amount of excess alkalinity appears to differ in certain CRM batches. Excess alkalinity in CRMs is likely caused by organic proton acceptors that are not completely oxidized by the ultraviolet sterilization procedure that CRMs undergo. The primary use of CRMs — to maintain the accuracy and consistency of carbonate system measurements — may be inhibited by excess alkalinity, which can cause differences in total alkalinity values determined by different titration methods. Excess alkalinity also invalidates the assumptions applied to marine CO2 system calculations, and so would produce incorrect values of CO2 system parameters calculated from certified total alkalinity and dissolved inorganic carbon values of CRMs. Finally, excess alkalinity analyses highlight the urgent need for the marine chemistry community to establish a universally agreed upon total boron to salinity ratio.