Some Useful Tips for pH Measurements |
- A pH meter may require a warm up time of several minutes. When a pH meter is routinely used in the laboratory, it is better to leave it "ON" with the function switch at "standby."
- Set the temperature control knob to the temperature of your buffer solution.
- Before you begin make sure the electrode is well rinsed with deionized water and wiped off with a clean absorbent paper.
- Always rinse and wipe the electrode when switching from one solution to another.
- Calibrate your pH meter by using at least two standard buffer solutions.
- Do not allow the electrode to touch the sides or bottom of your container. When using a magnetic bar to stir the solution, make sure the electrode tip is high enough to prevent any damage.
- Do not stir the solution while taking the reading.
- Inspect your electrode periodically. The liquid level should be maintained as per the specification provided with the instrument.
- Glass electrode should not be left immersed in solution any longer than necessary. This is important especially when using a solution containing proteins. After several pH measurements of solutions containing proteins, rinse the electrode in a mild alkali solution and then wash several times with deionized water.
- Water used for preparation of buffers should be of the highest possible purity. Water obtained by a method combining deionization and distillation is highly recommended.
- To avoid any contamination do not store water for longer than necessary. Store water in tightly sealed containers to minimize the amount of dissolved gases.
- One may sterile-filter the buffer solution to prevent any bacterial or fungal growth. This is important when large quantities of buffers are prepared and stored over a long period of time.
- Select a buffer that has a pKa value close to the middle of the range required. If you expect the pH to drop during the experiment, choose a buffer with a pKa slightly lower than the working pH. This will permit the buffering action to become more resistant to changes in hydrogen ion concentration as hydrogen ions are liberated. Conversely, if you expect the pH to rise during the experiment, choose a buffer with a pKa slightly higher than the working pH.
- Citrate and phosphate buffers are not suitable for systems that are highly calcium-dependent. Citric acid and its salts are chelators of calcium and calcium phosphates are insoluble and will precipitate out. Use of these buffers may lower the calcium levels required for optimum reaction. Tris (hydroxymethyl aminomethane) is known to weakly chelate calcium and other essential metals.