Preparation and Preservation of Standard Buffer Solutions
① pH reference materials shall be stored in a dry place. For example, mixed phosphate pH reference materials will deliquesce under high atmospheric humidity. Once deliquesced, the pH reference material cannot be used. ② Double-distilled water or deionized water shall be used for preparing pH standard solutions. Ordinary distilled water is acceptable for measurements with 0.1-grade pH meters. ③ Use a small beaker for dilution when preparing pH standard solutions to reduce the amount of standard solution adhering to the beaker wall. The plastic bag or other container holding the pH reference material shall be completely emptied and rinsed multiple times with distilled water. All rinsate shall be transferred into the prepared pH standard solution to guarantee accuracy. ④ The prepared standard buffer solution can generally be stored for 2–3 months. Discard the solution if turbidity, mold or precipitation is observed. ⑤ Alkaline standard solutions shall be hermetically stored in polyethylene bottles to prevent carbon dioxide from entering and forming carbonic acid, which will lower the pH value.
Proper Calibration of pH Meters
There are many types of pH meters with different electronic designs, and their operating procedures vary. Therefore, the pH meter must be operated strictly in accordance with its user manual. Calibration is an important step in pH meter operation. The data in Table 1 shows the measured values of a 0.01-grade, verified pH meter before and after calibration, which demonstrates the importance of calibration.
