Calibration of pH Meters and Preparation of Standard Buffer Solutions
1. What is the standard buffer solution for pH meters?
A pH buffer solution is a solution capable of maintaining a stable pH value. When a small amount of acid or base is added to it, or a small amount of acid or base is generated by chemical reactions inside the solution, or the solution is moderately diluted, its pH value remains basically unchanged. Such a solution that resists the effect of small amounts of acid, base or dilution and keeps its pH stable is defined as a pH buffer solution.
Standard buffer solutions for pH meters have the following characteristics: ① The pH value of the standard solution is known and meets specified accuracy requirements. ② The pH value of the standard solution features good reproducibility and stability, with high buffer capacity, small dilution value and low temperature coefficient. ③ The solution can be prepared easily.
2. What are the main functions of pH buffer solutions for pH meters?
① Calibrate pH meters before pH measurement. ② Verify the accuracy of pH meters. For example, after calibrating the pH meter with pH 6.86 and pH 4.00 buffer solutions, immerse the pH electrode into the pH 9.18 solution to check whether the reading on the instrument matches the standard pHs value of the solution. ③ Check whether recalibration is required for routine-accuracy measurements. Drift may occur after pH meter calibration and use. Before testing, immerse the electrode into a standard buffer solution whose pH is close to the sample solution, and judge whether recalibration is needed according to the magnitude of error. ④ Test the performance of pH electrodes.
3. How to prepare pH standard buffer solutions?
For general pH measurement, packaged pH buffer reagents (for preparing 250 mL solution) can be used. Deionized water shall be used for preparation, and the water shall be 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 the reagent with an appropriate amount of deionized water, rinse the reagent bag, transfer all rinsate into a 250 mL volumetric flask, dilute to the mark, and shake thoroughly.
4. How to properly store and use pH buffer solutions?
After preparation, buffer solutions shall be stored in glass or polyethylene bottles. Alkaline pH buffer solutions such as pH 9.18, pH 10.01 and pH 12.46 must be stored in polyethylene bottles. Tighten the cap and store at low temperature (5～10℃) in a refrigerator. The solution is generally usable for about two months. Discard it if turbidity, mold or precipitation appears.
For use, prepare several 50 mL small polyethylene bottles. Pour buffer solution from the large bottle into small bottles and leave them at ambient temperature for 1～2 hours to reach thermal equilibrium before use. Do not pour the leftover solution back into the large bottle after use to avoid contamination. The buffer solution in small bottles can be used for 2～3 days at temperatures above 10℃. Solutions of pH 7.00, pH 6.86 and pH 4.00 have a longer service life. The pH values of pH 9.18 and pH 10.01 solutions tend to change easily because they absorb CO₂ from the air.
