In chemistry, pH is a scale used to specify the acidity or basicity of an aqueous solution. The pH value of the acidic solution is lower, while the pH value of the alkaline solution is higher. At room temperature (25°C or 77°F), purified water is neither acidic nor alkaline, with a pH of 7. The pH is logarithmic and instead indicates the concentration of hydrogen ions in the solution (a lower pH value indicates a higher hydrogen ion concentration). This is because the formula used to calculate pH approximates the negative base-10 logarithm of the hydrogen ion molar concentration in the solution. More precisely, pH is the negative base-10 logarithm of hydrogen ion activity. At 25 ° C, solutions with a pH value of less than 7 are acidic, while solutions with a pH value greater than 7 are alkaline. The neutral value of pH depends on the temperature, and if the temperature rises, the pH value is lower than 7. For strong acids, the pH can be less than 0, and for strong bases, the pH can be greater than 14. The pH can be traced back to a set of standard solutions whose standard solutions are determined by international agreements. Determine the original pH standard value by measuring the potential difference between the hydrogen electrode and a standard electrode (such as a silver chloride electrode), using a concentration cell with transfer properties. The pH of the aqueous solution can be measured with a glass electrode and a pH meter or a discoloration indicator. pH measurement is important in chemistry, agronomy, medicine, water treatment and many other applications.
By taking advantage of the fact that its color changes with pH, an indicator can be used to measure pH. a visual comparison of the color of the test solution with a standard color chart provides a way to measure pH to the nearest whole number. If you use a colorimeter or spectrophotometer to measure color spectrophotometrically, you can make more accurate measurements. The universal indicator consists of multiple indicators, so there is a continuous color change from about pH 2 to pH 10. The universal indicator paper is made of an absorbent paper impregnated with a universal indicator. Another way to measure pH is to use an electronic pH meter.
Figure 1. Relation between p[OH] and p[H] (red = acidic region, blue = basic region).
BOC Sciences provides a series of probes that can be used to detect different PH, which can measure pH under acidic, alkaline and neutral conditions.
- Covington, A. K.; et al. Definitions of pH scales, standard reference values, measurement of pH, and related terminology. Pure Appl. Chem. 1985, 57 (3): 531–542.