Assessment of pH dependent errors in spectrophotometric pH measurements of seawater
Assessment of pH dependent errors in spectrophotometric pH measurements of seawater
复制标题
海水分光光度 pH 测量中 pH 相关误差的评估
DOI：
10.1016/j.marchem.2020.103801
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发表时间：
2020
期刊：
影响因子：
3
通讯作者：
Warren, Joseph K.
中科院分区：
文献类型：
--
作者：
Takeshita, Yuichiro;Johnson, Kenneth S.;Coletti, Luke J.;Jannasch, Hans W.;Walz, Peter M.;Warren, Joseph K.
最近对全水柱水文数据的分析显示，使用纯化的间甲酚紫进行的pH测量与从溶解无机碳（DIC）和总碱度（TA）计算的pH之间存在pH依赖性差异。在pH 7.4和8.2时，差异（pHspec- pHTA，DIC）分别约为−0.018和0.014。这种差异对海洋无机碳测量有广泛的影响，例如为在剖面浮标上工作的pH传感器制定可靠的校准协议。在这里，我们进行了一系列基于实验室的实验，通过直接比较其性能与已知具有能斯特行为的离子敏感场效应晶体管（ISFET）pH传感器测量的pH值，来评估海水中分光光度法pH值测量的pH值相关误差的大小。用高CO2海水滴定天然海水，同时使用分光光度法和ISFET传感器在pH（7-8.5）和温度（5-30 °C）的大范围内测量pH。两个pH测量值在所有温度下一致，优于±0.003（范围），除了在5和10 °C以及非常低和高的pH下，其中差异大至±0.005。这些结果表明，分光光度法pH测量中的pH依赖性误差可以被拒绝为pH specand pHTA，DIC之间pH依赖性差异的原因。这种差异的原因可能是由于我们对海洋无机碳模型的不完全理解，其中可能包括热力学常数的误差，海水中主要离子的浓度，TA或DIC测量的系统偏差，或总碱度定义中未考虑的有机化合物的贡献。这应该是无机碳社区的研究重点。
A recent analysis of full water column hydrographic data revealed a pH-dependent discrepancy between spectrophotometrically measured pH using purified meta-cresol purple and pH calculated from dissolved inorganic carbon (DIC) and total alkalinity (TA). The discrepancy (pHspec– pHTA,DIC) is approximately −0.018 and 0.014 at pH 7.4 and 8.2, respectively. This discrepancy has a wide range of implications for marine inorganic carbon measurements, such as establishing robust calibration protocols for pH sensors operating on profiling floats. Here, we conducted a series of lab based experiments to assess the magnitude of pH-dependent errors for spectrophotometric pH measurements in seawater by directly comparing its performance to pH measured by an Ion Sensitive Field Effect Transistor (ISFET) pH sensor known to have Nernstian behavior. Natural seawater was titrated with high CO2seawater while simultaneously measuring pH using spectrophotometry and an ISFET sensor over a large range in pH (7–8.5) and temperature (5–30 °C). The two pH measurements were consistent to better than ±0.003 (range) at all temperatures except at 5 and 10 °C and very low and high pH, where discrepancies were as large as ±0.005. These results demonstrate that pH-dependent errors in spectrophotometric pH measurements can be rejected as the cause of the pH-dependent discrepancy between pHspecand pHTA,DIC. The cause of this discrepancy is thus likely due to our incomplete understanding of the marine inorganic carbon model that could include errors in thermodynamic constants, concentrations of major ions in seawater, systematic biases in measurements of TA or DIC, or contributions of organic compounds that are not accounted for in the definition of total alkalinity. This should be a research priority for the inorganic carbon community.
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