First, the principle of the pH meter
By measuring the potential difference measured by the working battery composed of the electrode and the reference electrode in the solution, and using the linear relationship between the pH value of the solution to be tested and the potential of the working battery, and then converting the value into a pH unit value by the ammeter. Achieve the measurement.
Second, the maintenance of the pH meter
1 pH meter glass electrode storage
When the pH meter is not used for a short period of time, it can be fully immersed in distilled water or 1×10-4 hydrochloric acid solution. However, if it is not used for a long time, it should be dried and should not be washed with washing liquid or other water-absorbing reagents.
2 pH glass electrode cleaning
Contamination of the glass electrode bulb may lengthen the electrode response time. Decontaminate can be removed with CCl4 or soap, then immersed in distilled water for a day and night. When the pollution is serious, immerse in 5% HF solution for 10-20 minutes, rinse immediately with water, then immerse in 0.1N HCl solution for a day and night and continue to use.
3 treatment of glass electrode aging
The aging of the glass electrode is related to the gradual change of the rubber layer structure. The old electrode has a slow response, high membrane resistance, and low slope. Etching the outer layer with hydrofluoric acid often improves electrode performance. If this method is used to periodically remove the inner and outer layers, the life of the electrode is almost unlimited.
4 Reference electrode storage
The best storage solution for the silver-silver chloride electrode is a saturated potassium chloride solution. The high concentration potassium chloride solution prevents the precipitation of silver chloride at the liquid junction and maintains the liquid junction at work. This method is also applicable to the storage of composite electrodes.
Third, common problems and solutions
1 For the same sample, the pH measured twice is different?
A change in temperature or a chemical reaction in the sample itself causes a change in pH. Therefore, try to keep the temperature consistent and avoid chemical reactions.
2 The same sample, measured on two pH meters at the same time, the readings are inconsistent?
Since the calibration conditions of the two pH meters are different (such as corrections made at different times), the measured values ​​are different. Therefore, the pH meter should be calibrated at the same time with the same buffer and then measured simultaneously.
3 Why is the buffer deteriorated within the validity period and cannot be used?
The expiration date of the buffer refers to the shelf life in the unopened use state. Once opened, the buffer is more susceptible to deterioration due to the action of various molds in the air.
Note: The used buffer must not be returned to the original bottle!
4 How often does the electrode need to be calibrated?
The calibration frequency of the electrode depends on the specific conditions of the electrode's use, maintenance, sample properties, and measurement accuracy. It is recommended to calibrate once a day, up to a maximum of one calibration per week. Replace the electrode and do not use it for a long time. It must be calibrated before use.
5 How to maintain the pH electrode?
After using the electrode for a period of time, if the slope is low and the response speed is slow, try the following methods:
1 If the sample contains protein, the electrode film can be washed with a pepsin/hydrochloric acid wash.
2 If the measurement sample is an oily/organic liquid, it can be rinsed with acetone or ethanol.
3 It is found that the electrode liquid portion becomes dirty and black, and the liquid mixture can be cleaned with a mercaptan cleaning solution.
4 Activate the electrode film. Activation method: The electrode regeneration solution was immersed for 30 seconds, and then immersed in a 3 mol/L KCl solution for 5 hours.
6 The sample temperature is 10 ° C. At this time, the meter shows the pH value at 10 ° C or 25 ° C?
The pH meter shows the pH of the solution at the current temperature. If measured at 10 ° C, the meter shows the value of the solution at 10 ° C. If you need to obtain a pH of 25 ° C, the solution temperature must be raised / lowered to 25 ° C, and then Make measurements. The temperature compensation of the acidity meter refers to the effect of the compensation temperature on the pH electrode, but the pH at any temperature cannot be compensated to 25 °C.
7 Why is the electrode displayed as 7.02 after being calibrated in a buffer of pH 7.00?
At this time, the buffer temperature is around 20 °C. Since the pH of the buffer will vary slightly with temperature, 7.00 is only the value of the buffer at 25 ° C, and the buffer should be 7.02 at 20 ° C. The pH meter automatically compensates for the effect of temperature on the buffer to ensure measurement accuracy.
8 How long is the pH electrode life?
The life of the pH electrode is related to the nature of the sample being measured, the temperature of the sample and the frequency of use, and maintenance. The pH electrode has a life of 1 to 2 years under normal use and proper maintenance.
9 Check pH meter is not allowed
The pH meter is not accurate. The only reliable and simplest method is to use a pH standard buffer solution for verification. Take three pH standard buffer solutions: pH 6.86, pH 4.00, pH 9.18 (preferably freshly prepared and at the same temperature), position calibration at pH 6.86, slope calibration at pH 4.00, and then test pH 9. 18 See if the pH meter is accurate, and whether it is qualified or not. If the accuracy is not acceptable, it can be further judged whether there is a problem with the pH meter or a problem with the pH electrode.
10 pH meter digital instability phenomenon summary
1 Check if the electrode is damaged.
2 should be the electrode used for too long, first check to see if it is effective.
3 Try to soak the probe with a 2.5mmoL/L KCL solution.
4 Washing the glass ball is not a long time, some organic matter is attached to it, which makes the reaction insensitive.
5 There is a chemical flat CO2 + H2O→H+ + HCO3- in the water. Since the general pure water or surface water is weakly alkaline, the equilibrium will move in the positive reaction direction, so the pH will rise all the time.
6 Add a neutral salt (such as KCL) to the water sample to be used as an ionic strength modifier to change the total ion intensity in the solution and increase the conductivity to make the measurement fast and stable. The national standard GB/T6P04.3-93 of this method stipulates: “In order to reduce the influence of the liquid junction potential and quickly achieve stability when measuring the water sample, a drop of neutral 0.1 mol/LKCL solution is added to each 50 ml water sample.†Although this method The ionic strength in the water sample was changed, and the pH value was changed to some extent. However, it was experimentally proved that the change only changed by 0.01 pH, which is completely acceptable. However, when using this method, it must be noted that the added KCL solution should not contain any alkaline or acidic impurities. Therefore, the KCL reagent should be of high purity, and the water quality of the solution should be high purity neutral water quality.
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