pH meter is mainly used to accurately measure the pH value of liquid medium as a laboratory instrument with high popularity, easy to use and simple operation.But often we see many problems in use, such as reading fluctuation, calibration problem, buffer solution problem, etc. So what’s the reason behind it? Let’s discover…..

Q1: For the same sample, the two measured pH values are different?

A: Temperature change or chemical reaction of the sample itself will cause the change of pH value. Therefore, the temperature should be kept as consistent as possible and chemical reaction should be avoided.

Q2: The same sample was measured on two pH meters at the same time, and the readings were inconsistent?

A: Because the calibration conditions of the two pH meters are different (such as calibration at different times), the measured values are different. Therefore, it is necessary to calibrate the pH meter at the same time with the same buffer, and then measure at the same time.

Q3: Why the measurement is unstable and takes a long time?

A: Because the electrode is aging. The response time of the electrode in the buffer can be tested. If it is longer than 1 minute, the electrode needs to be activated or replaced with a new electrode. If the response time of the measurement buffer is very short, but the measurement sample is unstable, it means that the electrode is not suitable for measuring the sample under test. Please select a suitable electrode according to the electrode selection guide.

Q4: Why has the buffer deteriorated and can’t be used within the validity period?

A: The validity period of buffer refers to the storage period under unopened use. Once unsealed, the buffer is easy to deteriorate due to the action of various molds in the air. Note: the used buffer must not be poured back into the original bottle!

Q5: How often does the electrode need to be calibrated?

A: The frequency of electrode calibration depends on the specific conditions of electrode use, maintenance, sample properties, and measurement accuracy. It is recommended to calibrate once a day; the longest should not exceed once a week. To replace the electrode and not use it for a long time, it must be calibrated before use.

Q6: How to maintain the pH meter electrode?

A: After extreme use for a period of time, if you find that the slope becomes lower and the response speed becomes slower, you can try the following methods:

  • If the measurement sample contains protein, the electrode membrane can be cleaned with pepsin/hydrochloric acid washing solution.
  • If the measurement sample is oily/organic liquid, it can be washed with acetone or ethanol.
  • Activate the electrode membrane. Activation method: soak in the electrode regeneration solution for 30 seconds, and then soak in 3mol/L KCl solution for 5 hours.

Q7: The sample temperature is 10℃. Does the meter display the pH value at 10℃ or 25℃?

A: The pH meter shows the pH value of the solution at the current temperature; If it is measured at 10 ℃, the instrument displays the value of solution at 10 ℃; If a pH of 25 ℃ is required, the solution temperature must be raised / lowered to 25 ℃ before measurement. The temperature compensation of the pH meter refers to the compensation of the influence of temperature on the pH electrode, but the pH value at any temperature cannot be compensated to 25 ℃.

Q8: No matter what sample the electrode is in, the display remains the same?

A: Because the electrode is not really connected to the meter. The solution is to turn off the machine first, and then reconnect the electrode and the meter. Because the electrode is broken, it must be replaced with a new one timely.

Q9: Why is it displayed as 7.02 after the electrode is calibrated in a pH7.00 buffer?

A: At this time, the buffer temperature is about 20 ℃. Since the pH value of the buffer will change slightly with the change of temperature, 7.00 is only the value of the buffer at 25 ℃, while the value of the buffer at 20 ℃ should be 7.02. The pH meter can automatically compensate the influence of temperature on the buffer to ensure the measurement accuracy.

Q10: What should I do when the correction slope is greater than 105%?

A: Check whether the buffer has expired. If it expires, replace the buffer with a new one.

Q11: What is the purpose of pH buffer solution?

A: Calibrate the pH meter before pH measurement.

  • It is used to verify 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.
  • Check whether the pH meter needs to be recalibrated during general accuracy measurement. The pH meter may drift or change after calibration and use. Therefore, before the test, insert the electrode into the standard buffer close to the measured solution, and determine whether it needs to be recalibrated according to the error.
  • Test the performance of pH electrode.

Q12: How to properly store and use buffer solution?

A: After the buffer solution is prepared, it should be packed in a glass bottle or polyethylene bottle (alkaline pH buffer such as ph9.18, ph10.01, ph12.46, etc., should be packed in a polyethylene bottle). The bottle cap should be tightly closed and stored in the refrigerator at a low temperature (5 ~ 10 ° C). Generally, it can be used for about two months. If turbidity, mildew or precipitation are found, it can not be used any more. When using, prepare several 50ml polyethylene vials, pour the buffer solution into the vials, and place them at ambient temperature for 1 ~ 2 hours, and then use them after the temperature is balanced. The buffer solution in the vial can be used for 2 ~ 3 days under the environmental conditions of > 10 ° C. generally, the use time of ph7.00, ph6.86 and ph4.00 solutions can be longer. The pH values of ph9.18 and ph10.01 solutions are easy to change due to the absorption of CO2 in the air.

Q13: How should pH electrodes be stored?

A: If the electrode is not used, please keep it in 3mol / L potassium chloride solution (3M KCl); It can be stored in ph7.00 buffer for a short time; The service life of the pH electrode will be shortened if the pH electrode is dry for a long time or soaked in distilled water.

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