pH electrodes will NOT give accurate pH values in distilled or deionized water because distilled and deionized water do not have enough ions present for the electrode to function properly. The readings will drift and be essentially meaningless. If you want to test the accuracy of your pH electrodes, use pH buffers.
General Cleaning: Soak the electrode in 1:10 dilution of household laundry bleach in a 0.1 to 0.5% liquid detergent solution in hot water with vigorous stirring for 15 minutes. Place junction under warm, running tap water for 15 seconds. Drain/refill the reference chamber.
To maintain a constant potential of the Ag/AgCl electrode an electrolyte solution is needed, which has sufficient chloride ions (Cl-) and which is pH-neutral. Commonly used are KCl solutions, which are 3 molar or saturated.
Remove the storage bottle and cap from the pH sensor and place the sensor in the beaker of liquid so it is immersed an inch or two in the liquid. Check the pH reading. If it is close to the known pH of the liquid, or anywhere below 7, then you can probably revive the sensor.
How do pH probes work? The glass pH probe is an example of an ion selective electrode (ISE). This measuring system basically consists of the ISE reacting on a special ion type, in this case the hydrogen ion, and a reference electrode that are jointly immersed in the sample to be measured.
Use distilled water to store the pH probe before it is used for testing. Rinse the probe and set the pH mode, and place it in a neutral buffer solution (with pH 7) for calibration. Similarly, place it in a buffer solution with pH 4. This allows the meter to calibrate for acidic solutions.
Accurate pH measurements cannot be accomplished with a pH meter unless the meter has been calibrated against standardized buffer. Without a proper calibration the meter has no way to determine the pH value of the solution you are testing.
The only reliable way to determine whether a pH meter is accurate or not is to test it in standard solutions. Just like the only way you can tell if a scale is accurate is to test the standard weights.
Rinse the probe with distilled or deionized water (which is ok for washing and rinsing – just not for submersion!) and then dry it quickly by blotting (NOT wiping!) before submerging it in your next solution or returning it to the storage solution.
Average pH meters have an accuracy of +/-0.01 to +/-0.2, though high-performing models can be accurate up to +/-0.001. The majority of pH meters measure within the pH range of 0 to +14, though certain meters have an extended range of -2 to +20.
To calibrate a pH meter you will need two types of buffer solutions: pH7 and pH4. These buffer solutions help you with displaying the right pH values, because when you use a pH meter you want to be sure that the pH meter displays the right measurement.
As solutions approach and exceed the pH level of 12, the high concentration of sodium ions interfere with the standard glass membrane and cause pH levels to be displayed lower than actual pH levels.
When a pH system is unstable, erratic, or the offset drifts, the most common problem is an electrical ground loop in the system, particularly if the tank and/or pipes are plastic. To verify this problem, remove the electrode and calibrate it in a known buffer in a beaker.
pH meter calibration is a necessary step of using a pH meter because of how the electrode changes over time. Regularly calibrating your pH meter will adjust your electrode based off any changes that may have occurred and ensures that your readings are accurate and repeatable.
To make 125 mL of 3M KCl
- Using digital scale, place 27.96 g KCl powder into weigh boat.
- In 150 mL flask, mix 27.96 g KCl with 100 mL H2O.
- Put flask on hotplate/stirrer with stirbar until dissolved.
- Add H2O to bring volume to 125 mL.
Weak AcidsIt releases fairly low concentrations of hydrogen ions in an aqueous solution, resulting in a pH range of about 5 to just below 7. A sufficiently concentrated weak acid can still produce a low pH readout. A 1.0 molar CH3COOH solution, for example, has a pH of 2.37.
The pH-responsive electrode is usually glass, and the reference is usually a silver–silver chloride electrode, although a mercury–mercurous chloride (calomel) electrode is sometimes used. When the two electrodes are immersed in a solution, they act as a battery.
