The Theory of pH Measurement Emerson（pH值测量爱默生的理论）.pdf

Application Data Sheet ADS 43-002/rev.C Theory November 2010 The Theory of pH Measurement pH measurement has a wide variety of industrial applications in virtually every industry. These range from water conditioning to specific process related measurements to waste treatment. WHAT IS pH? pH is a measure of the acidity or alkalinity of a water solution. The acidity or alkalinity of a water solution is determined by the relative number of hydrogen ions + - (H ) or hydroxyl ions (OH ) present. Acidic solutions have a higher relative number of hydrogen ions, while alkaline (also called basic) solutions have a higher relative number of hydroxyl ions. Acids are substances which either dissociate (split apart) to release hydrogen FIGURE 2. pH Values of Acids and Bases ions or react with water to form hydrogen ions. Bases vary over 15 orders of magnitude in water solutions. are substances that dissociate to release hydroxyl Even common household items can reflect this wide ions or react with water to form hydroxyl ions. variation (Figure 2). In water solutions, the product of the molar concentra- pH is strictly defined as the negative logarithm of the tions1 of hydrogen and hydroxyl ions is equal to a hydrogen ion activity (aH): dissociation constant (K ). Knowing the value of the w constant and the concentration of hydrogen ions makes it possible to calculate the concentration of pH = -log10 aH hydroxyl ions, and vice versa. At 25°C, the value of Kw is 10-14 (see Figure 1). The hydrogen ion activity is defined as the molar concentration of hydrogen ions mu