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How to Measure the pH Value of the Medium More Accurately?

Most media formulations are based on standards. These standards have strict requirements on the pH value of the medium, and are generally controlled within a small range. Therefore, the pH value is very important for the quality control of the medium manufacturer and the acceptance of the medium by the customer.


1. pH calibration of culture medium


First of all, when measuring the pH value, we all perform a calibration work on the pH meter, and its calibration accuracy has a great influence on the subsequent measurement. pH electrodes need to be calibrated regularly. It is most appropriate to calibrate at least once a day.


Since each electrode has its characteristic zero point and slope, a minimum two-point calibration is essential for reliable and accurate results. When measuring a wide range of pH values, a minimum 3-point calibration is required. Most pH meters can be calibrated at 3-5 points.


It is also very important that the pH value of the measured sample falls within the range of the selected standard solution. When the electrode is calibrated, it is best to wait for the buffer temperature to be calibrated at the marked temperature, so that the measured pH value is the value marked by the buffer. For example, the buffers of pH 6.86 and pH 9.12 indicate the pH value at 25°C , then you calibrate at 20°C, and the display is not 6.86 or 9.12. Buffers used for microbial media calibration are very precise solutions with fixed values and precision. After opening the buffer bottle, in order to ensure the accuracy of the calibration.


2. Measurement of the pH value of the culture medium


The medium can be divided into solid medium and liquid medium according to the form. The liquid medium can choose the pH electrode of ordinary liquid, and the solid medium can choose the corresponding solid pH electrode, such as the flat electrode for the surface of the gel. If the solid medium is measured with a liquid pH electrode, the results obtained are of no reference value.


(1) During the process of culture medium, be sure to pay attention to the preparation of water, generally distilled water or deionized water. Otherwise, it may have an effect on the pH value later.


(2) Configure sterilization according to the instructions of the medium manufacturer: Generally, the pH mark on the outer packaging label of the medium indicates the pH value measured at 25°C after sterilization, so the liquid medium should be measured when the solution temperature reaches 25°C. The pH value, solid medium, etc., are solidified and then measured with a dedicated solid pH electrode, and the measured temperature should also be at 25 degrees.


It should be noted here that the automatic temperature compensation function can only compensate the electrode itself, and cannot supplement the temperature of the solution to be tested. Therefore, the standard for microbial culture medium requires measurement at 25 °C, and the temperature of the solution to be tested should be controlled at about 25 °C. Test again.


3. Maintenance of pH value of culture medium


Rinse the electrode with deionized water after each measurement, never wipe the electrode with paper. The rough surface of the paper can scratch and damage the gel layer on the pH-sensitive glass and can create electrostatic charges on the electrodes. Electrostatic charges can cause very unstable measurement signals.


For short-term use of the measurement gap or electrode, the electrode needs to be stored in a container containing a dedicated electrode storage solution, or an electrolyte or buffer pH4 or pH7. Make sure that the electrolyte level in the container is lower than the fill level in the electrode.

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