What is a pH meter and how it works in the laboratory?


A pH meter takes advantage of this and works like a voltmeter: it measures the voltage (electrical potential) produced by the solution whose acidity we’re interested in, compares it with the voltage of a known solution, and uses the difference in voltage (the “potential difference”) between them to deduce the difference in pH.

The pH meter is a commonly used instrument and equipment, which is mainly used to accurately measure the pH value of the liquid medium. It can also measure the ion electrode potential MV value with the corresponding ion selective electrode.

The pH meter is widely used in environmental protection, sewage treatment, and scientific research. , Pharmaceutical, fermentation, chemical, aquaculture, tap water and other fields. The instrument is also an essential inspection device for certification of SC and HACCP in food factories and drinking water factories.

Table of Contents

What is pH standard buffer solution? What are its characteristics?

The pH buffer solution is a solution that can keep the pH value stable. If a small amount of acid or alkali is added to this solution, or the chemical reaction in the solution produces a small amount of acid or alkali, and the solution is appropriately diluted, the pH of this solution is basically stable and unchanged, which can resist a small amount of acid A solution whose alkali is more or less diluted without changing the pH is called a buffer solution.

The pH standard buffer 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, with 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 standard buffer solution?

For general pH measurement, a complete set of pH group reagents (250 mL) can be used. When preparing the solution, deionized water should be used and pre-boiled for 15 to 30 minutes to remove dissolved carbon dioxide. Cut the plastic bag and pour the reagent into the beaker, dissolve it with an appropriate amount of deionized water, rinse the bag, and 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 glass bottles or polyethylene bottles (alkaline pH buffer solution, such as pH=9.18, pH=10.01, pH=12.46, etc., which should be packed in polyethylene bottles). It can be stored in a refrigerator at a low temperature (5~10℃) and can be used for about six months.

If it is found that the mixer is turbid, moldy or precipitated, it cannot be used continuously. When using, several 50mL polyethylene vials should be prepared, and the group solution in the large bottle should be poured into the vial, and placed at ambient temperature for 1 to 2 hours. Wait until the temperature is balanced before use. Do not pour it into a large bottle after use to avoid contamination.

The buffer solution in the bottle can be used for 2 to 3 days under an environmental condition of >10℃. Generally, the three solutions of pH=7.00, pH=6.86, and pH=14.00 can be used for a long time. Longer, pH = 9.18 and pH = 10.01 solution because of the absorption of carbon dioxide in the air, its pH value is relatively easy to change.

What is the use of pH buffer solution?

(1) Calibrate and calibrate pH meter before pH measurement.

(2) Used to verify the accuracy of the pH meter. For example, use pH=6.86 and pH=14.00. After calibrating the pH meter, insert the pH electrode into the pH=9.18 solution and check whether the displayed value of the instrument is consistent with the pH value of the standard solution. .

(3) Check whether the pH meter needs to be re-calibrated during general accuracy measurement. After the pH meter is calibrated and used, there may be drift or change, so before testing, insert the electrode into a standard buffer solution that is relatively close to the measured solution, and determine whether re-calibration is required according to the size of the error.

(4) Detect the performance of the pH electrode.

Why should the pH electrode be soaked? 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 gel layer on the surface of the glass membrane. It can only have a good relationship with the hydrogen ions in the solution under a sufficiently wet condition. influences.

At the same time, after soaking the glass electrode, the asymmetric potential can be greatly reduced and tend to be stable. The pH glass electrode can generally be soaked with distilled water or pH=4 buffer solution.

Generally, it is better to soak in pH=4 buffer solution, the soaking time is up to 24 hours or longer, depending on the thickness of the bulb glass film and the aging degree of the electrode.

At the same time, the liquid junction of the reference electrode also needs to be soaked. Because, if the liquid junction is dry, the potential of the liquid junction will increase or become unstable. The soaking solution of the reference electrode must be the same as the external reference solution of the reference electrode, that is, 3.3mol/L KCl solution or saturated KCl solution The soaking time is usually a few hours.

Therefore, for the pH composite electrode, it must be immersed in pH=4 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 pH=4 group of flushing liquid. Later, when using the pH composite electrode, this method of immersion is still used, even in some incorrect The wrong instruction will also be carried out in the instruction manual of the pH composite electrode.

The direct consequence of this wrong immersion method is to make a good pH composite electrode 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 (for example, 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 cannot 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 packet of pH=4.00 buffer (250mL), dissolve it in 250mL pure water, then add 56g of analytically pure KCl, heat it properly, and stir until it is completely dissolved.

What is the difference between rechargeable and non-rechargeable pH composite electrodes?

The pH composite electrode housing is distinguished by plastic and glass. The rechargeable pH composite electrode has a liquid addition hole in the electrode housing.

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 high penetration rate, the liquid junction potential is stable and reproducible, and the measurement accuracy is high.

When the reference electrode is reduced or contaminated, the KCl solution can be supplemented 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 dielectric liquid level is 2 cm below the liquid addition hole, a new dielectric 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, therefore, it is also widely used. However, when used as a laboratory pH electrode, under long-term and continuous use conditions, the concentration of KCl at the liquid junction will decrease, affecting the accuracy of the test.

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 some laboratory pH electrodes are not long-term and continuous tests.

Therefore, this structure The impact on accuracy is relatively small. Industrial pH composite electrodes have lower requirements for test accuracy, so ease of use has become the main choice.

How to use pH composite electrode correctly?

(1) There should be no bubbles at the front end of the bulb, if there are bubbles, they should be removed with force.

(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 membrane, 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-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. After the electrode is immersed in the solution, sometimes the gas in the cavity will be too late to remove the bubbles, which will cause bubbles The contact angle between the bulb or the liquid junction and the solution is poor, 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 away the sample with other reagents, 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 minimized and carefully cleaned after use.

(6) Avoid using 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, it will For electrode housings, pH composite electrodes with glass housings should be used instead.

How to clean the pH electrode?

After the bulb and liquid junction are contaminated, first clean with the following solvents, and then use deionized water to remove the solvent, and 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 blood cell sediment (Shimusheng tablets)
Pigment-like dilute bleach, hydrogen peroxide

How to repair 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, and the general customer can use appropriate methods to repair it:

(1) The electrode bulb and liquid junction are contaminated: you can use a fine brush, cotton or toothpick to carefully remove the dirt. The protective cover of the head of some plastic shell electrodes can be unscrewed, which is convenient for cleaning. If the pollution is serious, it can be cleaned with a detergent according to the method of Article 8.

(2) The external reference solution is contaminated; some electrode structures can be added with solution. At this time, the external reference solution of the electrode can be pumped out with a syringe to prepare a new 3.3mol/L or saturated KCl solution, and then add Go in, pay attention to the first and second time when you add it in, so as to clean the inner cavity of the electrode.

(3) Aging of glass sensitive film: Soak the electrode bulb with 0.1mol/L dilute hydrochloric acid (9mL hydrochloric acid diluted with pure water to 100mL) for 24 hours, wash with pure water, and soak in electrode soaking solution for 24 hours. If the passivation is serious, you can also immerse the lower end of the electrode in a 45-hydrofluoric acid solution for 3 to 5 seconds (solution preparation: 4mL of hydrofluoric acid diluted with pure water to 100mL), rinse with pure water, and then Soak the electrode for 24 hours to restore its performance.

What is the one-point calibration of the pH meter?

Any pH meter must be calibrated with a pH standard solution to measure the pH value of the sample. For samples with a measurement accuracy of less than 0.1 and pH, you can use a little calibration method to adjust the instrument, generally choose pH=6.86 or pH=7.00 Standard buffer solution. Some instruments are only 0.2pH or 0.1pH, so the instrument only has a positioning adjustment knob, the specific operation steps are as follows:

(1) Measure the temperature of the standard buffer, check the table to determine the pH value at this temperature, and adjust the temperature compensation knob to this temperature;

(2) Rinse the electrode with pure water and spin dry;

(3) Immerse the electrode in the buffer solution and shake it to stand still. After the reading is stable, adjust the positioning knob to make the instrument display the pH value of the standard solution;

(4) Take out the electrode to rinse and spin dry;

(5) Measure the temperature of the sample, and adjust the temperature compensation knob of the pH meter to the temperature value;

(6) Immerse the electrode in the sample solution, leave it still after shaking, and display the reading after stabilization.

What is the two-point calibration of the pH meter?

For precision pH meters, in addition to the “positioning” and “temperature compensation” adjustments, there is also an electrode “slope” adjustment, which requires two standard buffers for calibration. Generally, pH=6.86 or pH=7.00 is used for “positioning” calibration, and then pH=4.00 (acidic) or pH=9.18 or pH=10.01 (alkaline) buffer solution is used for “slope correction” according to the acid and alkali conditions of the test solution , The specific operation steps are:

(1) The electrode is washed and dried, immersed in a standard solution with pH=6.86 or pH=7.00, and the temperature compensation knob is placed at the temperature of the solution. After the displayed value is stable, adjust the positioning knob to make the instrument display the pH value of the standard solution.

(2) Take out the electrode, wash and spin dry, immerse in the second standard solution. After the displayed value is stable, adjust the slope knob of the instrument so that the displayed value of the instrument is the pH value of the second standard solution.

(3) The electrode is washed and dried, and then immersed in pH6.86 or pH7.00 standard solution. If the error exceeds 0.02pH, repeat steps (1) and (2). Until the two standard solutions can display the correct PH value without adjusting the knob.

(4) Take out the electrode, wash it and spin it dry, adjust the pH temperature compensation knob to the sample temperature, immerse the electrode in the sample solution, shake it and place it still, display the reading after it stabilizes.

How much does temperature affect pH accuracy measurement?

For the pH electrode, the temperature affects each pH to 0.003pH/°C. For example, a 0.2-level pH meter is calibrated in a 30°C pH buffer, and then the solution at 60°C is tested (assuming the pH of the solution is in the range of 6-8 There is a difference between pH7.00 and pH7.00), then the maximum error of temperature effect is 30×0.003=0.09pH.

If it is 3 pH units (in the range of pH 4-10), the maximum error is 0.27pH, from which we can see that the temperature has a great influence on the pH. Of course, we can also draw conclusions from it, in order to reduce the temperature error of pH measurement, we should pay attention to the following three points:

(1) Try to select a buffer solution close to the pH value of the measured solution to calibrate the pH meter.

(2) Try to make the temperature of the calibration solution consistent with or close to the temperature of the measured solution.

(3) A pH meter with temperature compensation should be selected.

All pH meters with an accuracy higher than 0.1pH have temperature compensation adjustment, while a pH meter of 0.2 grade does not have temperature compensation. Some 0.2-level pH meters also claim to have 0.1-level accuracy. In fact, this is impossible. Some people confuse the concepts of resolution 0.1pH and precision 0.1pH. Even in a pH unit, the pH error between 60°C is 0.003×60=0.18pH, therefore, the pH meter without temperature compensation, the highest accuracy is only 0.2pH.

Can temperature compensation eliminate all errors caused by temperature?

It must be particularly pointed out that the temperature compensation set on the pH meter is only to compensate the slope term of the electrode (2.303RT/F). Affected by temperature is also the standard potential of the glass electrode, liquid junction potential, etc., which are not strictly linear with temperature. At the same time, it takes a certain amount of time for the pH electrode to reach equilibrium at the new temperature. Therefore, neither manual temperature compensation nor automatic temperature compensation is sufficient.

According to the operation definition of pH measurement, in order to obtain precise measurement results, the sample solution and the standard solution should be measured at the same and constant temperature. This is the principle of isothermal measurement.
For pH measurement with general accuracy requirements, temperature compensation can be used when the temperature of the sample solution and the standard solution are different.

How to judge whether your pH meter is accurate?

Many users are confused when using the pH timer. Is this pH meter accurate? Some people judge by work experience, some judge by pH test paper, others judge by pH meter used in the past, these are unreliable.

In fact, the only reliable and easiest way is to use pH standard buffer solution to carry out the verification. This is the only test standard. Take three standard buffer solutions: pH=6.86, pH=4.00, pH=9.18 (preferably freshly prepared and the same temperature), position calibration with pH=6.86F, slope calibration with pH=4.00, and then test pH= 9.18. It is clear whether the pH meter is accurate and qualified.

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