ELECTRICAL CONDUCTIVITY TEMPERATURE COMPENSATION - A NOVEL APPROACH FOR ACID WATERS

Electrical conductivity (κ) measurements of natural waters are typically referenced to 25 °C (κ₂₅) using standard temperature compensation factors (α). For acidic waters (pH<4), this can result in a large κ₂₅ error (δκ₂₅). The more the sample temperature departs from 25 °C and the lower the pH, the larger the potential δκ₂₅. For pH < 4, the hydrogen ion transport number becomes substantial and its mode of transport is different than most other ions resulting in a different α. Hydrogen ions migrate by what has been termed a “jump" of a proton from one water molecule to the next as H₃O⁺, whereas most other ions move through the solution as individual entities surrounded by waters of hydration. Therefore, not only is the hydrogen ion temperature compensation factor lower, but its ionic conductivity is about 5 times larger than that of most other ions. Common temperature factors do not account for the unique mobility of the hydrogen ion. A new approach to determining α as a function of pH and temperature is presented. Samples with varying amounts of H₂SO₄ and NaCl were used to develop the new α, which was then applied to 65 natural water samples including acid mine waters, geothermal waters, sea water, and stream waters. For each sample, the κ and pH were measured at several temperatures from 5 to 90 °C and κ₂₅ calculated. The δκ₂₅ ranged from -11 to 9% for the new approach as compared to -42 to 25% and -53 to 27% for the constant α (0.019) and ISO-7888 methods, respectively. As an example of the error that can be introduced using the ISO 7888 method for natural waters, the field measured κ₂₅ was 8300 µS/cm at 74 °C and the laboratory measured κ₂₅ was 14,700 µS/cm at 25 °C (δκ₂₅ = -44%) for an acid-sulfate hot spring sample from Yellowstone National Park (pH 1.45). Using the new approach, the field measured κ₂₅ was 14,300 µS/cm (δκ₂₅ = -2.7%) – resulting in a much smaller δκ₂₅ than the ISO 7888 method. This approach is a substantial improvement for acidic waters and performs as well as or better than the standard methods for circumneutral waters.