Solutions to Common Problems and Application Scope of pH Meter
One What is the pH value of water? What does it mean?
PH value is one of the important physicochemical parameters of aqueous solutions. All natural phenomena, chemical changes, and production processes involving aqueous solutions are related to pH value. Therefore, pH value measurement is necessary in the fields of industry, agriculture, medicine, environmental protection, and scientific research. The pH value of water is the negative logarithm of the activity of hydrogen ions in water, expressed as pH=- lgaH+pH value, sometimes also known as the hydrogen ion index. Due to the small value of hydrogen ion activity, it is inconvenient to apply it. Therefore, the concept of pH value is used as an indicator to judge the acidity and alkalinity of aqueous solutions. Moreover, the negative logarithm of hydrogen ion activity can indicate the magnitude of changes in acidity and alkalinity, which is very convenient for application. As a result, (1) neutral aqueous solution, pH=-1g aH+=-1g 10-7=7 (2) acidic aqueous solution, pH 7, the higher the pH value, the stronger the alkalinity.
2, What is pH scale?
PH measurement is a relative measurement that only indicates the pH difference between a standard solution and an unknown solution. In actual measurement, regular calibration with a standard buffer solution is required. Therefore, in order to achieve consistency in measurement values, a pH scale must be established. The pH scale range is set from 0 to 14 pH, and the magnitude of the pH scale is determined by the pH value of the reference buffer solution. Therefore, the meaning of pH scale can be expressed as: according to the definition of pH, several pH buffer solutions are selected as fixed points for pH scale within the range of 0-14 pH, and their pH values are measured using accurate methods that can be achieved by contemporary technology. There are two types of pH scales internationally, namely multiple benchmark pH scales and a single benchmark pH scale. China uses multiple benchmark pH scales. The pH values of six reference buffer solutions at 0-95 ℃ are shown in Table (1).
3, Do pH measurements need to be calibrated?
There are usually two methods for pH measurement: colorimetric method (pH test paper or colorimetric dish) and electrode method. Of course, colorimetric methods do not require calibration, while electrode methods must be calibrated because electrode method pH measurement involves comparing an unknown solution with a standard solution with a known pH value in a measuring cell. This is determined by the "operational definition" of electrode method pH measurement.
4, What is pH standard buffer solution? What are its characteristics?
A pH buffer solution is a solution that can maintain a stable pH value. If a small amount of acid or base is added to this solution, or if a chemical reaction in the solution produces a small amount of acid or base, and the solution is appropriately diluted, the pH value of this solution is basically stable and unchanged. This solution that can resist a small amount of acid or base or dilution and make the pH value less likely to change is called a pH buffer solution. The pH standard buffer solution has the following characteristics: (1) The pH value of the standard solution is known and achieves the specified accuracy. (2) The pH value of the standard solution has good reproducibility and stability, with a large buffer capacity, small dilution value, and small temperature coefficient. (3) The preparation method of the solution is simple.
5, How to prepare pH buffer solution?
For general pH measurement, a complete set of pH buffering reagents (250ml can be prepared) can be used. When preparing the solution, deionized water should be used and boiled for 15-30 minutes in advance to remove dissolved carbon dioxide. Cut open the plastic bag and pour the reagent into a beaker. Dissolve it in an appropriate amount of deionized water, rinse the packaging bag, and then pour it into a 250ml volumetric flask. Dilute to the mark and shake well.
