The basic principle of pH measurement
The familiar and ancient zero current measurement method used to determine chemical reaction processes is probably pH measurement. Generally speaking, pH measurement is used to determine the acidity or alkalinity of a solution. Even chemically pure water has a trace amount of dissociation, and its ionization equation is: H2O H2O=H3O-OH - (1) Due to the fact that only a very small amount of water is dissociated, the molar concentration of ions is generally a negative power exponent. To avoid using the negative power exponent of molar concentration for calculations, biologist Sorensen suggested in 1909 to replace this inconvenient value with logarithm and define it as "pH value". Mathematically, the pH value is defined as the negative logarithm of the commonly used hydrogen ion concentration. That is, pH=one log [H]
(2) Due to the strong dependence of ion product on temperature, for the pH value of process control, it is necessary to simultaneously know the temperature characteristics of the solution. Only when the measured medium is at the same temperature can its pH value be compared. In order to obtain a reproducible pH value, potentiometric analysis is used for pH measurement. The electrode used in the potential analysis method is called a primary battery. The voltage of this battery is called electromotive force (EMF). This electromotive force (EMF) consists of two and a half batteries. One of the half cells is called the measuring electrode, and its potential is related to a specific ion activity; The other half cell is a reference half cell, commonly referred to as a reference electrode, which is usually connected to the measuring solution and connected to an industrial pH meter. The standard hydrogen electrode is the reference point for all potential measurements. The standard hydrogen electrode is a platinum wire that is electroplated (coated) with platinum chloride and surrounded by hydrogen gas. The most familiar and commonly used pH indicator electrode is a glass electrode. It is a glass tube with a pH sensitive glass film blown at the end. The tube is filled with KCI buffer solution containing saturated AgCI, with a pH value of 7. The potential difference that exists on both sides of the glass film and reflects the pH value follows the Nernst formula: E=Eo. 1n [H3oq (3) n.] In the formula, E is the potential; E is the standard voltage of the electrode; R is the gas constant; T is the Kelvin temperature; F is the Faraday constant; N is the valence of the measured ion; [HO] is the activity of the HO ion. As can be seen from the above equation, there is a certain relationship between the potential E and the activity and temperature of HO ions. At a certain temperature, measuring the potential E can calculate ln [HO] (converted into a log [HO] to obtain pH), which is the basic principle of pH detection. In the Nernst formula, temperature plays a significant role as a variable. As the temperature rises, the potential value will also increase. For every 1 ℃ increase in temperature, it will cause a potential change of 0.2 mV/pH. Represented by pH value, the pH value varies by 0.0033 per LPH per I~C. This means that for measurements around 20-30 ℃ and 7pH, there is no need to compensate for temperature changes; For applications with temperatures greater than 30 ℃ or less than 20 ℃ and pH values greater than 8 or less than 6, temperature changes must be compensated for.
