PH meter related terms
pH buffer solution
A pH buffer solution is a solution that keeps the pH value stable. If a small amount of acid or base is added to this solution, or a small amount of acid or base is produced by the chemical reaction in the solution, and the solution is properly diluted, the pH value of the solution is basically stable. Or alkali or dilution, and the solution that makes the pH value not easy to change is called pH buffer solution.
pH standard buffer solution has the following characteristics:
(1) The pH value of the standard solution is known and reaches the specified accuracy.
(2) The pH value of the standard solution has good reproducibility and stability, and has a large buffer capacity, a small dilution value and a small temperature coefficient.
(3) The preparation method of the solution is simple.
How to prepare pH buffer solution?
For general pH measurement, a complete set of pH buffer reagents (250ml can be prepared) can be used. When preparing the solution, deionized water should be used and boiled for 15-30 minutes in advance to remove dissolved carbon dioxide. Cut the plastic bag and pour the reagent into a beaker, dissolve it with an appropriate amount of deionized water, rinse the packaging bag, then pour it into a 250ml volumetric flask, dilute to the mark, and shake well.
How to properly store and use pH buffer solution?
After the buffer solution is prepared, it should be packed in a glass bottle or a polyethylene bottle (alkaline pH buffer solutions such as pH9.18, pH10.01, pH12.46, etc. should be packed in a polyethylene bottle) and the cap should be tightly closed. Tight, store in the refrigerator at low temperature (5-10°C), generally it can be used for about two months, if it is found to be turbid, moldy or precipitated, it cannot be used any longer. When using, several 50ml polyethylene vials should be prepared, pour the buffer solution in the large bottle into the vials, and place it at ambient temperature for 1~2 hours, and then use it after the temperature is balanced. Do not pour it back into the large bottle after use to avoid contamination. The buffer solution in the small bottle can be used for 2~3 days under the environmental conditions of >10°C. Generally, the three solutions of pH7.00, pH6.86 and pH4.00 can be used for a long time. Longer, the pH value of the pH9.18 and pH10.01 solutions is easier to change due to the absorption of CO2 in the air.
What are pH buffer solutions used for?
(1) Calibrate and calibrate the pH meter before pH measurement.
(2) It is used to test the accuracy of the pH meter, for example, after calibrating the pH meter with pH6.86 and pH4.00, insert the pH electrode into the pH9.18 solution, and check whether the displayed value of the instrument is consistent with the pHs value of the standard solution.
(3) Check whether the pH meter needs to be reset during general accuracy measurement. After the pH meter is calibrated and used, it may drift or change. Therefore, before the test, insert the electrode into the standard buffer solution that is close to the solution to be tested, and determine whether it needs to be re-calibrated according to the size of the error.
⑷Check the performance of the pH electrode.
What are the advantages of using bottled pH buffer solutions?
There are generally pH standard buffer solutions in polyethylene plastic bottles sold at pH meter dealers. It is compared with the buffer solution prepared by itself with pH buffer reagent. Because it adds color developing agent and preservative, the buffer solution with different pH values has different colors. It is not easy to make mistakes when using it, and it will not be stored at room temperature Mildew and deterioration, the shelf life can be as long as one year, so it is very convenient to use. There are five kinds of pH4.00, pH6.86, pH7.00, pH9.18, pH10.01, and two specifications of 500ml big bottle and 50ml small bottle.
What is a pH indicating electrode?
The electrode that responds to the activity of hydrogen ions in the solution and the electrode potential changes accordingly is called a pH indicating electrode or a pH measuring electrode. There are several kinds of pH indicating electrodes, such as hydrogen electrode, antimony electrode and glass electrode, but the most commonly used one is glass electrode. The glass electrode is composed of a glass strut and a hydrogen ion-sensitive glass membrane composed of a special composition. The glass membrane is generally in the shape of a bulb, and the bulb is filled with an internal reference solution, inserted into an internal reference electrode (usually silver/silver chloride electrode), sealed with an electrode cap to lead out wires, and fitted with a socket to become a pH indicator. electrode. A single pH indicating electrode cannot be measured, it must be measured together with a reference electrode.
What is a reference electrode?
An electrode that does not respond to the activity of hydrogen ions in solution and has a known and constant electrode potential is called a reference electrode. There are several reference electrodes such as mercurous sulfate electrode, calomel electrode and silver/silver chloride electrode. The most commonly used are the calomel electrode and the silver/silver chloride electrode. The role of the reference electrode in the measurement battery is to provide and maintain a fixed reference potential. Therefore, the requirements for the reference electrode are stable and reproducible potential, small temperature coefficient, and small polarization potential when current passes.
What is a pH combination electrode?
An electrode that combines a pH glass electrode and a reference electrode is called a pH composite electrode. The case with plastic shell is called plastic shell pH composite electrode. The one with glass shell is called glass pH composite electrode. The biggest advantage of the composite electrode is that it combines two into one and is easy to use. The structure of pH compound electrode is mainly composed of electrode bulb, glass support rod, internal reference electrode, internal reference solution, shell, external reference electrode, external reference solution, liquid junction, electrode cap, electrode wire, socket and so on.
How to use the composite electrode of the pH meter correctly?
⑴ There should be no air bubbles at the front end of the bulb, if there are air bubbles, they should be thrown away with force.
(2) After the electrode is taken out of the soaking bottle, it should be shaken in deionized water and dried. Do not wipe the bulb with a paper towel, otherwise the time for potential stabilization will be prolonged due to the electrostatic induction of charge transfer to the glass membrane. A better method is to use a Rinse the electrodes with the measuring solution.
(3) After the pH composite electrode is inserted into the solution to be tested, it should be stirred and shaken for a few times before being placed statically, which will speed up the response of the electrode. Especially when using a plastic shell pH composite electrode, the stirring and shaking is more severe, because there will be a small cavity between the bulb and the plastic shell, and after the electrode is immersed in the solution, sometimes the gas in the cavity will produce bubbles if it is too late to get rid of it, making the ball Bubbles or liquid junctions are in poor contact with the solution, so vigorous stirring and shaking is necessary to remove air bubbles.
(4) After testing in viscous samples, the electrodes must be rinsed repeatedly with deionized water several times to remove the samples adhered to the glass membrane. Sometimes it is necessary to wash the sample with other solutions first, then wash with water to remove the solvent, and then immerse in the soaking solution for activation.
⑸Avoid contact with strong acid and alkali or corrosive solutions. If you test such solutions, you should minimize the immersion time and clean them carefully after use.
⑹ Avoid using in dehydrating media such as absolute ethanol and concentrated sulfuric acid, which will damage the hydrated gel layer on the surface of the bulb.
⑺The shell material of the plastic case pH composite electrode is polycarbonate plastic (PC). PC plastic will dissolve in some solvents, such as carbon tetrachloride, trichloroethylene, tetrahydrofuran, etc. If the above solvents are contained in the test, it will be damaged. Electrode casing, at this time should be changed to a pH compound electrode with a glass casing.
