College of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg, FL, 33701, USA.
Anal Chim Acta. 2022 Aug 8;1220:340008. doi: 10.1016/j.aca.2022.340008. Epub 2022 Jun 6.
This work describes a novel conductometric/spectrophotometric method to determine the ionic strength (I) of dilute aqueous solutions (e.g., natural waters from rivers and lakes). Because I ≤ 0.01 mol kg in such waters, precision as well as accuracy is of paramount importance. In current practice the ionic strength of natural waters is determined almost exclusively with conductometric measurements. We used solutions of artificial freshwater to assess the performance of two commonly used types of conductometric instruments and found that a conductivity probe systematically overestimated I while a salinometer systematically underestimated I. We therefore recommend here an empirical correction that can be easily implemented to improve the accuracy of both types of conductivity measurements. Additional improvement in measurements of I can be achieved by using that high-quality conductometric measurement as input to a hybrid conductometric/spectrophotometric procedure that makes use of robust quantitative characterizations of the influence of ionic strength on the dissociation characteristics of phosphate pH buffers and sulfonephthalein pH indicators. This approach mitigates systematic conductometric errors associated with solution composition, thus yielding measurements of ionic strength with substantially improved accuracy and precision. The method was validated by testing on a broad suite of artificial freshwaters (n = 64) with compositions that include the major ions present in dilute natural waters (Na, K, Mg, Ca, Cl, HCO, and SO). This new hybrid method is applicable to waters of 0 ≤ I ≤ 0.01 mol kg (i.e., electrical conductivity of up to 900 μS cm at 25 °C), with an accuracy of ±0.0003 and a precision of ±0.0003.
这项工作描述了一种新颖的电导/分光光度法来测定稀水溶液（例如来自河流和湖泊的天然水）的离子强度（I）。由于此类水中的 I≤0.01molkg，因此精度和准确性至关重要。目前，天然水的离子强度几乎完全通过电导测量来确定。我们使用人工淡水溶液评估了两种常用类型的电导仪的性能，发现电导探头系统地高估了 I，而盐度计系统地低估了 I。因此，我们建议在这里采用一种经验修正方法，可以轻松实现对这两种类型的电导测量的准确性的提高。通过将高质量的电导测量作为输入，将其应用于混合电导/分光光度程序中，可以进一步提高 I 的测量精度，该程序利用对磷酸盐 pH 缓冲剂和磺基萘酚 pH 指示剂的离子强度对离解特性的影响进行稳健定量描述。这种方法减轻了与溶液组成相关的系统电导误差，从而实现了离子强度的测量，具有显著提高的准确性和精密度。该方法通过对包括稀天然水中存在的主要离子（Na、K、Mg、Ca、Cl、HCO 和 SO）的组成的广泛人工淡水（n=64）进行测试得到了验证。这种新的混合方法适用于 0≤I≤0.01molkg 的水（即在 25°C 时电导率高达 900μScm），其准确度为±0.0003，精密度为±0.0003。
