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How to Measure the pH Value of Culture Media More Accurately?

Most of the formulas for culture media are based on standards, such as national standards, pharmacopoeias, or other foreign standards. These standards have strict requirements on the pH of the culture medium, which are generally controlled within a very small range. Therefore, pH is important for quality control of culture medium manufacturers and the acceptance of culture medium by customers. Details about culture media introduction and types of media in microbiology.

Calibration of Culture Media in Microbiology

Firstly, when measuring the pH value, we always carry out calibration of the pH meter, and its calibration accuracy has a great influence on the subsequent measurement. The pH electrode needs to be calibrated regularly. Calibrating at least once a day is the most appropriate.

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

Meanwhile, it is also very important that the measured pH value of the sample falls within the selected range of standard solutions. When calibrating the electrode, it is best to wait for the buffer temperature to calibrate at the marked temperature so that the pH value measured is the one indicated by the buffer solution. For example, pH6.86 and pH9.12 buffer solutions indicate the pH value at 25 ℃. If you calibrate at 20 ℃, the displayed pH value will not be 6.86 or 9.12. However, most medium in microbiology manufacturers will attach a chart of the pH value measured at different temperatures.

The buffer solution used for calibration is a very accurate solution with fixed values and precision. After the buffer solution bottle is opened, to ensure the accuracy of calibration, pay attention to the following:

  • When using the buffer solution for the first time, mark the date on the package to ensure the sealing of the buffer solution package.

  • Do not pour the used buffer solution back into the original bottle or mix buffer solutions from different manufacturers together.

  • Store buffer solution at room temperature, and avoid direct sunlight.

  • Clean the electrode before calibration, and do not calibrate directly in the original bottle.

  • Absolutely do not use expired or contaminated buffer solutions.

  • Replace the buffer solution promptly when it expires.

Measurement of the pH Value of Culture Media in Microbiology

Culture media provided by culture media supplier can be divided into solid culture media and liquid culture media according to their forms. The common liquid pH electrode can be used for liquid culture media, and the corresponding solid pH electrode, such as the flat head electrode for gel surface, should be used for solid culture media. The results obtained by measuring solid culture media using a liquid pH electrode are not meaningful.

Firstly, during the process of preparing culture media in microbiology, it is necessary to pay attention to the use of distilled water or deionized water for preparation. Otherwise, it may affect the pH value afterwards.

Secondly, follow the instructions of the medium in microbiology manufacturer to prepare sterilization. The pH mark on the outer packaging of the culture medium generally indicates the pH value measured after sterilization at 25℃. Therefore, for liquid culture media, the pH value of the culture medium should be measured when the solution temperature reaches 25℃, and for solid culture media, the pH value should be measured after it has solidified using a special solid pH electrode, and the temperature for measurement should also be at 25℃.

After each measurement, rinse the electrode with deionized water and do not wipe the electrode with paper. The rough surface of paper will scratch and damage the gel layer on the pH-sensitive glass, and produce electrostatic charges on the electrode. Electrostatic charges will cause the measurement signal to be unstable.

If the electrode is not used for a long time or for a short period of time, it needs to be stored in a container containing special electrode storage solution, electrolyte, or buffer solution pH4 or pH7. Ensure that the liquid level of the electrolyte in the container is lower than that of the electrode filling liquid.

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