Preparation and Application of Standard Buffer Solutions for pH Meters
The pH value of the standard solution is known and achieves the specified accuracy. The pH value of the standard solution has good reproducibility and stability, with a large buffer capacity, small dilution value, and small temperature coefficient. The preparation method of the solution is easy.
1. Potassium hydrogen phthalate at pH 4.003 and 25 degrees can be prepared into a 250ML solution
2. A 250ML solution can be prepared by mixing phosphate at pH 6.864 and 25 degrees Celsius
3. Boric acid at pH 9.182 and 25 degrees can be prepared into a 250ML solution
1) Principle of buffer solution action and pH value
When a certain amount of acid and alkali is added to a solution, it has a buffering effect that hinders the pH change of the solution. Such a solution is called a buffer solution. Mixed solutions of weak acids and their salts (such as HAc and NaAc), and mixed solutions of weak bases and their salts (such as NH3 · H2O and NH4Cl) are buffer solutions. The buffering effect of a buffer solution composed of weak acid HA and its salt NaA on acids is due to the presence of a sufficient amount of base A - in the solution. When a certain amount of strong acid is added to this solution, H ions are basically consumed by A ions: therefore, the pH value of the solution remains almost unchanged; When a certain amount of strong alkali is added, the weak acid HA present in the solution consumes OH - ions and hinders the change in pH.
2) The buffering capacity of a buffer solution
Adding a small amount of strong acid or strong base to the buffer solution does not cause significant changes in the pH value of the solution. However, if a large amount of acid or base is added, the buffer solution loses its buffering effect. This indicates that its buffering capacity has a certain limit. The buffering capacity of a buffer solution is related to the concentration of the components that make up the buffer solution. The buffer solution composed of 0.1mol · L-1 HAc and 0.1mol · L-1 NaAc has a greater buffering capacity than the buffer solutions of 0.01mol · L-1 HAc and 0.01mol · L-1 NaAc. This can be confirmed through calculation. But the concentration of the components in the buffer solution cannot be too high, otherwise the interaction between ions cannot be ignored. When the ratio of the two components that make up the buffer solution is not 1:1, the buffering effect decreases and the buffering capacity decreases. When the ratio of c (salt)/c (acid) is 1:1, the △ pH * is small and the buffering capacity is large. It has a significant buffering effect on both acids and bases.
