Tools and Materials for Calibrating a pH Meter
Standard buffer solution (a standard buffer solution with a pH value close to the test solution should be selected). There are currently 7 kinds of standard solutions. In my country, the following 3 solutions are generally used: (when the pH value is at 25°C)
1. Potassium hydrogen phthalate (KHC8H4O4) pH 4.003; 0.05M
2. Mixed phosphate (Na2HPO4): mixed salt solution of potassium dihydrogen phosphate and disodium hydrogen phosphate pH6.864; 0.025M
3. Borax (Na2B4O7·l0H2O) pH 9.182; 0.01M
The preparation methods of these three standard buffers are as follows:
1. pH 4, potassium hydrogen phthalate standard buffer:
Precisely weigh 10.12g of potassium hydrogen phthalate [KHC8H4O4] dried at 115±5℃ for 2 to 3 hours, add water to dissolve and dilute to 1000ml.
2. pH7, phosphate standard buffer (pH7.4):
Precisely weigh 4.303g anhydrous disodium hydrogen phosphate and 1.179g potassium dihydrogen phosphate dried at 115±5°C for 2 to 3 hours, add water to dissolve and dilute to 1000ml.
Also added: Phosphate standard buffer (pH6.8): Precisely weigh 3.533g anhydrous disodium hydrogen phosphate and 3.387g potassium dihydrogen phosphate dried at 115±5℃ for 2 to 3 hours, add water to dissolve and dilute to 1000ml .
3. pH9, borax standard buffer:
Precisely weigh 3.80g of borax [Na2B4O7·10H2O] (note: avoid weathering), add water to dissolve and dilute to 1000ml, place in a polyethylene plastic bottle, seal tightly to avoid contact with carbon dioxide in the air.
