Composite electrode-knowledge
1 What is a pH combination electrode?
The electrode combining the pH glass electrode and the reference electrode is the pH composite electrode. According to the different shell materials, it is divided into plastic shell and glass. Compared with the two electrodes, the biggest advantage of the composite electrode is easy to use. The pH composite electrode is mainly composed of an electrode bulb, a glass support rod, an internal reference electrode, an internal reference solution, a housing, an external reference electrode, an external reference solution, a liquid junction, an electrode cap, an electrode lead, a socket, etc.
(1) Electrode bulb: It is made by melt blowing of lithium glass with hydrogen function, which is spherical, with a film thickness of about 0.1 ~ 0.2mm, and a resistance value of less than 250 megohms (25 ° C).
(2) The glass support tube is a glass tube body that supports the electrode bulb, and is made of lead glass with excellent electrical insulation, and its expansion coefficient should be consistent with that of the electrode bulb glass.
(3) Internal reference electrode: It is a silver / silver chloride electrode. Its main function is to extract the electrode potential, which requires stable potential and low temperature coefficient.
(4) Internal reference solution: the internal reference solution with a zero potential of 7pH is a mixed solution of neutral phosphate and potassium chloride. The glass electrode and the reference electrode constitute a battery to establish a zero-potential pH value, which mainly depends on the internal reference solution PH and chloride ion concentration.
(5) Electrode shell: The electrode shell is a shell that supports the glass electrode and the liquid junction, and contains the external reference solution. It is usually made of polycarbonate (PC) plastic compression molding or glass. PC plastic will dissolve in some solvents, such as carbon tetrachloride, trichloroethylene, tetrahydrofuran, etc. If the above solvents are included in the test, the electrode casing will be damaged, and the pH composite electrode with glass casing should be used instead.
(6) External reference electrode: It is a silver / silver chloride electrode. Its function is to provide and maintain a fixed reference potential, which requires stable potential, good reproducibility, and low temperature coefficient.
(7) External reference solution: potassium chloride solution or KCl gel electrolyte.
(8) Liquid junction: The liquid junction is the connecting part of the external reference solution and the measured solution, which requires stable penetration, usually with sand core.
(9) Electrode wire: low-noise metal shielding wire, the inner core is connected to the inner reference electrode, and the shielding layer is connected to the outer reference electrode.
2 About rechargeable and non-rechargeable pH composite electrodes
The rechargeable pH composite electrode has a liquid addition hole in the electrode shell. When the external reference solution of the electrode is lost, the liquid addition hole can be opened to replenish the KCl solution. The non-rechargeable pH composite electrode contains gel-like KCl, which is not easy to be lost and has no liquid hole.
The characteristic of the rechargeable pH composite electrode is that the reference solution has a higher penetration rate, the liquid junction potential is reproduced steadily, and the measurement accuracy is higher. And when the reference electrode is reduced or contaminated, the KCl solution can be replenished or replaced, but the disadvantage is that it is more troublesome to use. When the rechargeable pH composite electrode is used, the liquid addition hole should be opened to increase the liquid pressure and accelerate the electrode response. When the reference liquid level is 2 cm below the liquid addition hole, a new reference liquid should be added in time.
The characteristic of the non-rechargeable pH composite electrode is that it is easy to maintain and easy to use, so it is also widely used. However, when used as a laboratory pH electrode, under long-term and continuous use conditions, the KCl concentration at the liquid junction will decrease, affecting the test accuracy. Therefore, when the non-rechargeable pH composite electrode is not used, it should be immersed in the electrode immersion liquid, so that the electrode performance will be very good during the next test, and most laboratory pH electrodes are not long-term and continuous tests. The effect of accuracy is relatively small. Industrial pH composite electrodes have low requirements for test accuracy, so ease of use has become the main choice.
3 How to properly soak the pH composite electrode?
The pH electrode must be soaked before use, because the pH bulb is a special glass membrane with a thin hydrated gel layer on the surface of the glass membrane. It can only have a good relationship with the H + ions in the solution under a sufficiently wet condition. response. At the same time, after the glass electrode is soaked, the asymmetric potential can be greatly reduced and tend to be stable. The pH glass electrode can generally be soaked with distilled water or pH4 buffer solution. Generally, it is better to use pH4 buffer, and the immersion time is 8 hours to 24 hours or longer, depending on the thickness of the bulb glass film and the degree of electrode aging. At the same time, the liquid junction of the reference electrode also needs to be soaked. Because if the liquid junction dries up, the potential of the liquid junction will increase or become unstable. The immersion liquid of the reference electrode must be the same as the external reference solution of the reference electrode. The immersion time is usually a few hours.
Therefore, for the pH composite electrode, it must be immersed in the pH4 buffer solution containing KCl, so as to act on the glass bulb and the liquid junction at the same time. Special attention should be paid here, because in the past people used a single pH glass electrode has been used to soak in deionized water or pH4 buffer solution. Later, when using the pH composite electrode, this method of soaking is still used, even at some incorrect pH composite The wrong instruction is also given in the electrode instruction manual. The direct consequence of this wrong immersion method is to make a pH composite electrode with good performance into an electrode with slow response and poor accuracy, and the longer the immersion time, the worse the performance, because after a long immersion, the liquid junction The KCl concentration inside the boundary (such as inside the sand core) has been greatly reduced, making the liquid junction potential increase and unstable. Of course, as long as it is immersed in the correct immersion solution for several hours, the electrode will still recover.
In addition, the pH electrode can not be immersed in a neutral or alkaline buffer solution. Long-term immersion in such a solution will make the pH glass film slow to respond.
The preparation of the correct pH electrode soaking solution: Take a sachet of pH 4.00 buffer (250ml), dissolve it in 250ml of pure water, then add 56g of analytically pure KCl, heat it properly, stir until it is completely dissolved.
In order to make the use of the pH composite electrode more convenient, some imported pH composite electrodes and some domestic-made electrodes are equipped with a sealed plastic vial at the head of the pH composite electrode, which contains the electrode immersion liquid. Just wash it, very convenient. This preservation method is not only convenient, but also very beneficial to prolong the life of the electrode, but the soaking solution in the plastic vial should not be contaminated and should be replaced.
4 What is a liquid junction compound electrode-knowledge
In the pH measurement process, a reference electrode is used. In most cases, the reference electrode is provided with a salt bridge solution. The reference electrode is connected to the measured solution through the salt bridge solution, so that a liquid junction is formed between the measured solution and the reference salt bridge solution. The ions in the solution on both sides of the liquid junction continue to diffuse across the interface to the opposite side, because The diffusion speed of each ion is different, and the two sides of the liquid junction are charged with opposite signs, resulting in a potential difference, which is called the liquid junction potential, also known as the diffusion potential, which will affect the measurement accuracy of the electrode. In order to reduce the potential difference of the liquid junction or keep it stable and reproducible, a high concentration KCl solution (≥3mol / L) must be used as the external reference salt bridge solution of the electrode, which has a sufficiently high ionic strength The maximum possible ionic strength of the measured medium is still 5 to 10 times greater. Therefore, at the interface of the liquid junction, K + and Cl- always diffuse outward, and since the mobility of K + and Cl- is almost equal, the positive The negative charge distribution is basically uniform, so a small liquid junction potential is formed, which is why the external reference solution in the reference electrode must use a high concentration KCl solution.
5 How to use pH composite electrode correctly?
(1) There should be no air bubbles at the front end of the bulb.
(2) After the electrode is removed from the soaking bottle, it should be shaken in deionized water and dried. Do not wipe the bulb with paper towels. Otherwise, the electrostatic induction charge will be transferred to the glass film, which will prolong the time for the potential to stabilize. A better method is Rinse the electrode with the test solution.
(3) After the pH composite electrode is inserted into the solution to be tested, it should be stirred and shaken a few times and then placed still, which will speed up the response of the electrode. Especially when using a plastic pH composite electrode, the stirring and shaking is more severe, because there will be a small cavity between the bulb and the plastic shell. After the electrode is immersed in the solution, sometimes the gas in the cavity will be too late to remove the bubbles. The bubble or liquid junction has poor contact with the solution, so it must be stirred and shaken vigorously to eliminate bubbles.
(4) After testing in a viscous sample, the electrode must be repeatedly rinsed with deionized water multiple times to remove the sample adhering to the glass film. Sometimes it is necessary to wash off the sample with other solvents first, then wash off the solvent with water, and immerse in the soaking solution to activate.
(5) Avoid contact with strong acids, strong bases or corrosive solutions. If testing such solutions, the immersion time should be reduced as much as possible, and carefully cleaned after use.
(6) Avoid use in dehydrating media such as absolute ethanol and concentrated sulfuric acid, as they will damage the hydrated gel layer on the surface of the bulb.
(7) The shell material of the plastic 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 included in the test, then The electrode casing will be damaged, and the pH composite electrode with glass casing should be used instead.
6 How to clean the pH electrode?
When the pollution between the bulb and the liquid junction is serious, you can first use the following solvent to clean, then use deionized water to remove the solvent, and finally immerse the electrode in the immersion liquid to activate.
Contaminant cleaning agent
Inorganic metal oxide less than 1mol / L dilute acid
Dilute detergent for organic fats and oils (weakly acidic)
Resin polymer substance dilute alcohol, acetone, ether
Acidic enzyme solution of protein hematocrit
Pigment-like dilute bleach, hydrogen peroxide
7 How to "repair" the pH electrode?
The "damage" of the pH composite electrode is caused by a decrease in the sensitivity gradient, a slow response, and poor reading repeatability. It may be caused by the following three factors. Generally, customers can use appropriate methods to repair it.
(1) The electrode bulb and liquid junction are contaminated. You can use a fine brush, cotton ball or toothpick to remove the dirt carefully. The protective cover of the head of some plastic shell pH electrodes can be unscrewed, which is more convenient for cleaning. If the pollution is serious, you can use the cleaning method to clean it.
(2) The external reference solution is contaminated. For the rechargeable electrode, a new KCl solution can be prepared and added. Note that the first and second additions should be poured out again to clean the inner cavity of the electrode.
(3) Aging of glass sensitive film: Soak the electrode bulb with 0.1 mol / L dilute hydrochloric acid for 24 hours. Wash with pure water and soak in electrode soaking solution for 24 hours. If the passivation is serious, the lower end of the electrode can also be immersed in a 4% hydrofluoric acid solution for 3 to 5 seconds (solution preparation: 4ml of hydrofluoric acid diluted with pure water to 100m1), washed with pure water, and then on the electrode Soak in the soaking solution for 24 hours to restore performance.
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