Ocean alkalinity (A) critically affects the buffering capacity and the precipitation and dissolution of calcium carbonate minerals in the ocean. However, our knowledge of A is still incomplete because of limited investigations into the contributions of dissolved organic matter (DOM) to alkalinity (A) across diverse ocean environments. Here, to improve the accuracy of seawater A measurements, we have implemented three key modifications to the conventional method based on a second-HCl-titration. These modifications include: (1) determining concentrations and pK spectra of A using the same NaOH-titration data; (2) routinely evaluating residual alkalinity (A) remaining in seawater; and (3) validating the accuracy of A against known concentrations of acetate ions in sodium acetate (CHCOONa) solution, where only acetate ions contribute to A. In experiments involving modification (3), the A values obtained through NaOH-titration agreed with those obtained through standard HCl titration, showing an average difference of −0.4 ± 2.7 μmol kg. Modification (3) represents a critical improvement incorporated into the NaOH-titration method proposed here and can also improve the accuracy of A obtained by the conventional second-HCl-titration method. Modification (2) would also enhance the A accuracy because the uncertainty in A can be directly translated into A values. When applied to certified reference materials for A and C, aged local seawater, and deep waters of the East Sea, the NaOH-titration method consistently yielded A values approximately 5 μmol kg higher than those obtained for artificial seawater containing zero A. This small yet significant A value was attributed to the recalcitrant DOM component, which is widespread in the global ocean.

中文翻译：

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海洋中的持久有机碱度


海洋碱度（A）严重影响海洋中碳酸钙矿物的缓冲能力以及沉淀和溶解。然而，由于对不同海洋环境中溶解有机物 (DOM) 对碱度 (A) 的贡献的研究有限，我们对 A 的了解仍然不完整。在这里，为了提高海水 A 测量的准确性，我们对基于第二次 HCl 滴定的传统方法进行了三项关键修改。这些修改包括：（1）使用相同的 NaOH 滴定数据确定 A 的浓度和 pK 谱； (2)定期评价海水中残留的残余碱度(A)； (3) 根据乙酸钠 (CH3COONa) 溶液中已知乙酸根离子浓度验证 A 的准确性，其中只有乙酸根离子对 A 有贡献。在涉及修改 (3) 的实验中，通过 NaOH 滴定获得的 A 值与那些一致通过标准 HCl 滴定获得，平均差异为 -0.4 ± 2.7 μmol kg。修改（3）代表了本文提出的 NaOH 滴定方法的关键改进，并且还可以提高通过传统的第二次 HCl 滴定方法获得的 A 的准确性。修改（2）也会提高 A 的准确性，因为 A 的不确定性可以直接转化为 A 值。当应用于 A 和 C 的有证标准物质、老化的当地海水和东海深水时，NaOH 滴定方法始终产生比含有零 A 的人造海水获得的值高约 5 µmol kg 的 A 值。显着的 A 值归因于顽固的 DOM 成分，该成分在全球海洋中广泛存在。