{"id":1310,"date":"2015-09-02T09:30:20","date_gmt":"2015-09-02T09:30:20","guid":{"rendered":"https:\/\/www.ott.com\/blog\/?p=197"},"modified":"2021-01-10T15:20:17","modified_gmt":"2021-01-10T14:20:17","slug":"ph-measurements-in-environmental-water-quality-monitoring","status":"publish","type":"post","link":"https:\/\/blog.otthydromet.com\/en\/ph-measurements-in-environmental-water-quality-monitoring\/","title":{"rendered":"pH Measurements in Environmental Water Quality Monitoring"},"content":{"rendered":"

pH measurement in natural aquatic environments is commonly done electrochemically by measuring the voltage between a pH-sensitive glass electrode and a reference electrode. Commercial pH meters<\/a> convert voltage readings into pH units using calculations based on the Nernst equation<\/a> and sometimes other assumptions about the environment and measurement system. Because of unavoidable differences between theoretical pH measurement systems and real-world pH measurements systems, pH is often said to be defined \u201coperationally\u201d based on an accepted electrochemical method such as Standard Method 4500-H+ or similar methods from other standards organizations. Despite pH sensor imperfections, most water resource professionals accept measurements derived from glass electrode measurement systems as the \u201coperational definition\u201d of pH and use them for meaningful and defensible scientific endeavors.<\/p>\n

\"hydrolab
A Hydrolab pH sensor for use on Hydrolab multiparameter sondes.<\/figcaption><\/figure>\n

The millivolt reading from a theoretically perfect pH sensor in a pH 7 solution is zero. In nominal pH 4 and pH 10 buffers at 25 \u00b0C, millivolt readings are 177.48 and -177.48, respectively. In practice, millivolt readings will differ in an amount referred to as the asymmetry potential of the electrode. As the electrodes age and degrade with use, mV readings during calibration will change \u2013 decreasing in acidic solutions and increasing in basic solutions.<\/p>\n

Changes in mV readings observed during calibration procedures over time can be used as an indicator of electrode condition and the quality of the calibration procedure. Small deviations from theoretical mV readings or historic readings are not a concern because they are corrected during the calibration process and are generally stable between calibration events. Additional confidence in the calibration can be built by performing a linearity check. This is done by placing the sensor in a third reference standard (different from the two used in calibration) and comparing the reading to the expected reading.<\/p>\n

Reading mV directly could allow the user to derive a unique mathematical equation relating voltage to pH that is different from the one used in the pH meter. Deriving a unique equation has little purpose for most water monitoring plans.<\/p>\n

To incorporate pH-mV output into a quality assurance program<\/a>:<\/p>\n