PH value The pH value is the negative logarithm of the hydrogen ion activity in the solution When the concentration of H + is small, the concentration of H + can be used to calculate pH instead of activity. PH value is one of the most commonly used water quality indicators. Generally, the pH value of drinking water should be between 6.5 and 8.5; The pH value of surface water is in the range of 6.5-8.5; In order to ensure the normal operation of pipelines and equipment, the pH value of some industrial water must be kept between 7.0 and 8.5; In the biochemical treatment of wastewater, pH value is also one of the important indexes to evaluate the toxicity of toxic substances. In addition, pH value has great influence on the migration and transformation of pollutants in water, so it should be controlled. PH value is not only related to but also different from acidity and alkalinity. The pH value indicates the strength of the acidity and alkalinity of water, and the acidity or alkalinity is the content of acid or alkali substances in water. For solutions with the same acidity, such as 0.1mol hydrochloric acid and 0.1mol acetic acid, the acidity of both solutions is 100mmol / L, but their pH values are quite different. Hydrochloric acid is a strong acid, which is almost 100% ionized in water with pH of 1; Acetic acid is a weak acid, its ionization degree in water is only 1.3%, and its pH is 2.9. The pH value of water is mainly determined by glass electrode method (gb6920-86) and colorimetry. 1. Colorimetry Different pH aqueous solutions show different colors in various acid-base indicators, and each indicator has a certain range of color change. Add appropriate indicator into the standard buffer solution with known pH value to prepare the standard color column, and compare with the solution to be tested to determine the pH value of water sample. This method is not suitable for colored, turbid or high free chlorine, oxidant and reductant water samples. If the pH value of water sample is roughly determined, pH test paper can be used. 2. Glass electrode method The pH glass electrode was used as indicator electrode and the saturated calomel electrode as reference electrode Where:    Calomel: the electrode potential of saturated calomel electrode does not change with the activity of hydrogen ion (ah +) in the measured solution, so it can be regarded as a constant value; The electrode potential of glass pH glass electrode changes with the activity of hydrogen ion in the solution.   Glass can be expressed by Nernst equation, so the above equation is (at 25 ℃): E battery =    Calomel-（   0+0.0591gaH+）＝K+0.059pH It can be seen that as long as the E-Cell is measured, the pH of the solution to be measured can be obtained. In the actual determination, it is difficult to obtain the K value accurately, so the calculation method is not used. Instead, the solution with known pH value is used as the standard for calibration, and the pH of the solution to be measured is directly measured with a pH meter. The internal resistance of pH glass electrode is usually as high as tens to hundreds of megohm, so the matching pH meters are transistor millivoltmeter or electronic potentiometer with high impedance input. In order to correct the influence of temperature on pH measurement, the pH meter is equipped with temperature compensation device. In order to simplify the operation and make it convenient to use and suitable for field use, a variety of pocket pH meters and pen pH meters have been made by widely using composite pH electrodes. At present, more advanced pH meters at home and abroad can measure - 2.00 ～ + 19.00, some can even measure the range of - 9.00 ～ + 23.00, and the measurement accuracy is 0.01 pH value. The glass electrode method is accurate, rapid, and less affected by the factors of water color, turbidity, colloidal substances, oxidant, reducing agent and salinity.