Proper Storage and Usage of pH Buffer Solutions
After preparation, the buffer solution shall be stored in glass bottles or polyethylene bottles. Alkaline pH buffer solutions such as pH9.18, pH10.01 and pH12.46 must be kept in polyethylene bottles. Tighten the bottle caps and store at low temperature (5～10℃) in a refrigerator. Generally, the solution can be used for about two months. Discard the solution if turbidity, mold or precipitation is observed.
For use, prepare several 50 mL small polyethylene bottles. Pour the buffer solution from the large bottle into the small ones and leave them at ambient temperature for 1 to 2 hours until temperature equilibrium is reached 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 to 3 days at temperatures above 10℃. Solutions of pH7.00, pH6.86 and pH4.00 usually have a longer service life. The pH values of pH9.18 and pH10.01 solutions tend to change easily because they absorb CO₂ from the air.
What are the applications of pH buffer solutions?
① Calibrate the pH meter before pH measurement. ② Verify the accuracy of the pH meter. For example, after calibrating the pH meter with pH6.86 and pH4.00 solutions, immerse the pH electrode into the pH9.18 solution to check whether the reading displayed by the instrument matches the pHs value of the standard solution. ③ Check whether recalibration is required for general-precision measurements. Drift may occur after the pH meter is calibrated and put into service. Before testing, immerse the electrode into a standard buffer solution with a pH close to that of the sample solution, and determine whether recalibration is needed according to the magnitude of error. ④ Test the performance of the pH electrode.
What is a pH indicator electrode?
An electrode that responds to hydrogen ion activity in the solution and changes its electrode potential accordingly is called a pH indicator electrode or pH measuring electrode. There are several types of pH indicator electrodes including hydrogen electrodes, antimony electrodes and glass electrodes, among which glass electrodes are most commonly used. A glass electrode consists of a glass stem and a hydrogen-ion-sensitive glass membrane made of special components. The glass membrane is usually in the form of a bulb. The bulb is filled with internal reference solution, into which an internal reference electrode (usually silver/silver chloride electrode) is inserted. It is sealed with an electrode cap to lead out the wire and fitted with a connector to form a pH indicator electrode. The commonly sold pH indicator electrode is Model 231 glass pH electrode. A single pH indicator electrode cannot perform measurement independently; it must be used together with a reference electrode.
What is a reference electrode?
A reference electrode is an electrode that does not respond to hydrogen ion activity in the solution and has a known and constant electrode potential. Types of reference electrodes include mercurous sulfate electrodes, calomel electrodes and silver/silver chloride electrodes. Calomel electrodes and silver/silver chloride electrodes are widely used. The function of the reference electrode in the measuring cell is to provide and maintain a fixed reference potential. Therefore, reference electrodes are required to have stable and reproducible potential, low temperature coefficient and small polarization potential when current passes through.
What is a pH combination electrode?
A pH combination electrode integrates the pH glass electrode and reference electrode. Those with plastic housings are plastic-body pH combination electrodes, while those with glass housings are glass-body pH combination electrodes. The biggest advantage of combination electrodes is the integrated design for easy operation. The structure of a pH combination electrode mainly consists of electrode bulb, glass stem, internal reference electrode, internal reference solution, housing, external reference electrode, external reference solution, liquid junction, electrode cap, electrode wire and connector.
