Chemical speciation models based upon the Pitzer activity coefficient equations, including the propagation of uncertainties. II. Tris buffers in artificial seawater at 25 °C, and an assessment of the seawater ‘Total’ pH scale
Chemical speciation models based upon the Pitzer activity coefficient equations, including the propagation of uncertainties. II. Tris buffers in artificial seawater at 25 °C, and an assessment of the seawater ‘Total’ pH scale
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基于 Pitzer 活度系数方程的化学形态模型，包括不确定性的传播。
DOI：
10.1016/j.marchem.2022.104096
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发表时间：
2022
期刊：
影响因子：
3
通讯作者：
Dickson, Andrew G.
中科院分区：
文献类型：
--
作者：
Clegg, Simon L.;Humphreys, Matthew P.;Waters, Jason F.;Turner, David R.;Dickson, Andrew G.
Tris 物质（或 THAM，2-氨基-2-羟甲基-1,3-丙二醇，CAS 77–86-1）及其质子化形式 TrisH+，用于制备海水化学应用的 pH 缓冲溶液。需要开发含有 Tris、TrisH+ 和海水主要离子的缓冲溶液的酸碱化学形态模型，以便： (i) 可以计算介质成分变化对 pH 值的影响； (ii) 游离pH值（[H+]的测量值）和总pH值（（[H+]+[HSO4−]）测量值）可以相互转换； (iii) 总 pH 值定义中固有的近似值可以量化； (iv) H+/Cl− 和 H+/Na+ 等电极对可以更轻松地校准以测量 pH。作为实现这些目标的第一步，我们将 Waters 和 Millero 的基于 Pitzer 的形态形成模型（Mar. Chem. 149, 8-22, 2013）扩展到人造海水，以包括 25 °C 下的 Tris 和 TrisH+。通过蒙特卡罗模拟获得了附加相互作用参数的方差和协方差的估计。这使得能够估计任何模型计算量（例如 pH、形态形成）的总体不确定性，以及所有相互作用参数和平衡常数的单独贡献。这对于模型开发很重要，因为它可以识别关键交互。该模型针对 25 °C 人造海水中含有 Tris 缓冲液的细胞的测量 EMF 进行了测试，发现盐度 20 至 40 的平均偏差为 0.13 ± 0.070 mV。缓冲溶液计算电动势的总方差主要由少数相互作用参数的贡献决定，这使得该模型很容易得到改进。该模型首次用于量化 25°C 下 Tris 缓冲溶液中总 pH 值和 –log10([H+] + [HSO4−]) 的不同定义之间的差异（上文第 (iii) 项）。结果表明，使用已建立的在缓冲溶液中用 TrisH+ 替代 Na+ 的方法，可以轻松地将总 pH 范围扩展到低盐度，特别是如果使用形态模型来量化替代对 pH 值的影响。在恒定盐度的人造海水中，电动势 (EMF) 与总规模 pH 值与缓冲摩尔浓度之间的关系在约 0.01 至 0.02 mol kg−1 缓冲摩尔浓度以上呈线性。含有 TrisH+ 与 Tris 比例的 Tris 缓冲液的 pH 值（其比例不统一）可以非常简单地计算。澄清了总 pH 值范围的技术问题，例如将 pH 值外推至零缓冲液（在恒定盐度下）。建议进一步将模型扩展到0-45°C的温度范围，并提高精度，从而完全满足上述要求（i）至（iv）。
The substance Tris (or THAM, 2-amino-2-hydroxymethyl-1,3-propanediol, CAS 77–86-1), and its protonated form TrisH+, is used in the preparation of pH buffer solutions for applications in seawater chemistry. The development of an acid-base chemical speciation model of buffer solutions containing Tris, TrisH+, and the major ions of seawater is desirable so that: (i) the effects of changes in the composition of the medium on pH can be calculated; (ii) pH on the free (a measure of [H+]) and total (a measure of ([H+] + [HSO4−])) scales can be interconverted; (iii) approximations inherent in the definition of the total pH scale can be quantified; (iv) electrode pairs such as H+/Cl−and H+/Na+can more easily be calibrated for the measurement of pH. As a first step towards these goals we have extended the Pitzer-based speciation model of Waters and Millero (Mar. Chem. 149, 8–22, 2013) for artificial seawater to include Tris and TrisH+, at 25 °C. Estimates of the variances and covariances of the additional interaction parameters were obtained by Monte Carlo simulation. This enables the total uncertainty of any model-calculated quantity (e.g., pH, speciation) to be estimated, as well as the individual contributions of all interaction parameters and equilibrium constants. This is important for model development, because it allows the key interactions to be identified. The model was tested against measured EMFs of cells containing Tris buffer in artificial seawater at 25 °C, and the mean deviation was found to be 0.13 ± 0.070 mV for salinities 20 to 40. Total variances for calculated electromotive forces of the buffer solutions are dominated by contributions from just a few interaction parameters, making it likely that the model can readily be improved. The model was used to quantify the difference between various definitions of total pH and –log10([H+] + [HSO4−]) in Tris buffer solutions at 25 °C, for the first time (item (iii) above). The results suggest that the total pH scale can readily be extended to low salinities using the established approach for substituting TrisH+for Na+in the buffer solutions, especially if the speciation model is used to quantify the effect on pH of the substitution. The relationships between electromotive force (EMF), and pH on the total scale, with buffer molality in artificial seawater at constant salinity are shown to be linear above about 0.01 to 0.02 mol kg−1buffer molality. The pH of Tris buffers containing ratios of TrisH+to Tris that vary from unity can be calculated very simply. Technical aspects of the total pH scale, such as the extrapolation of pH to zero buffer (at constant salinity), are clarified. Recommendations are made for further work to extend the model to the temperature range 0–45 °C, and improve accuracy, so that requirements (i) to (iv) above can be fully met.