Analysis of a global hydrographic data product showed a clear pH-dependent discrepancy between pH on the total scale measured spectrophotometrically (pH_spec) using purified meta-Cresol Purple (mCP) and pH calculated from total alkalinity and dissolved inorganic carbon. However, this was based mainly on US cruises, and three recent Japanese cruises do not show this pH-dependent discrepancy. One potential explanation is that purified mCP batches obtained from different institutions lead to significantly different pH_spec. Here, we tested this hypothesis by comparing the performance of purified mCP obtained from four different institutions. We demonstrate that consistent pH of ±0.0012 (95% C.I.) can be achieved regardless of the institution when impurities are properly removed. However, there was at least one batch from three of the four institutions that had significant pH-dependent errors that were as large as −0.008. The presence of impurities that led to pH-dependent errors was identified using HPLC and, for 8 out of the 9 cases, by spectrophotometry (although issues still remain for the latter). We conclude that pH-dependent errors due to impurities that remain after the purification process are, by themselves, too small to account for the differences observed between the recent set of cruises. Identifying the source of this difference should be a top priority. This study also highlights the importance of establishing robust quality assurance and quality control protocols to ensure consistent behavior with previously published equations to compute pH. We recommend a centralized system where one or a handful of institutions distribute purified mCP for the community, as this distribution approach will lead to lower prices and simplify quality assurance.

对全球水文数据产品的分析表明，使用纯化的间甲酚紫 (mCP)分光光度法测量的总标度（pH_规格）上的 pH 值与根据总碱度和溶解的无机碳计算的pH 值之间存在明显的 pH 依赖性差异。然而，这主要基于美国游轮，最近三次日本游轮没有显示出这种依赖于 pH 值的差异。一种可能的解释是从不同机构获得的纯化 mCP 批次导致显着不同的 pH_规格. 在这里，我们通过比较从四个不同机构获得的纯化 mCP 的性能来测试这一假设。我们证明，当杂质被正确去除时，无论机构如何，都可以实现 ±0.0012 (95% CI) 的一致 pH 值。然而，来自四个机构中的三个机构中至少有一批具有与 -0.008 一样大的显着 pH 依赖性误差。使用 HPLC 确定了导致 pH 相关误差的杂质的存在，对于 9 个案例中的 8 个，通过分光光度法（尽管后者仍然存在问题）。我们得出结论，由于纯化过程后残留的杂质导致的 pH 相关误差本身太小，无法解释最近一组巡航之间观察到的差异。确定这种差异的来源应该是重中之重。这项研究还强调了建立稳健的质量保证和质量控制协议的重要性，以确保与先前发布的计算 pH 值的方程一致。我们建议采用集中式系统，由一个或少数机构为社区分发纯化的 mCP，因为这种分发方法将降低价格并简化质量保证。