A hybrid conductometric/spectrophotometric method for determining ionic strength of dilute aqueous solutions

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^-1 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⁺, Mg²⁺, Ca²⁺, Cl^-, HCO₃^-, and SO₄^2-). This new hybrid method is applicable to waters of 0 ≤ I ≤ 0.01 mol kg^-1 (i.e., electrical conductivity of up to 900 μS cm^-1 at 25 °C), with an accuracy of ±0.0003 and a precision of ±0.0003.