The application of phenol red has been widely applied in cell culture media to enable monitoring of the pH value of cell growth. However, sometimes it is difficult for a human observer to identify the colour transformation using only the naked eye. Furthermore, different people will give different interpretations of colour transformation. Thus the current study was performed to provide the spectrophotometry and colorimetry profiling of pure phenol red and cell culture media to pH variation. Pure phenol red and cell culture media were used as samples. The pH values of both types of sample were manipulated using citric acid. Spectrophotometry analysis results show that the pH value of the pure phenol red samples could be well profiled by using the wavelengths in the green light region (in the range of 500‐600 nm). The pH value of the cell culture medium samples could also be well profiled, by using wavelengths in either the green or blue light region. Colorimetry analysis results conducted using CIELab colour space indicate that the colour transformation of both samples is linear, with a high value for the coefficient of determination. This profiling proves that phenol red can be used to quantify the pH values of the samples and shows good potential to be applied in the development of spectroscopy‐based and colorimetry‐based pH sensors.

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The phenol red is commonly used in cell culture medium as visual pH indicator by changing its color from darker red color to the orange-yellowish color as the pH is changing from neutral to acidic level. The color change of phenol red is hardly to be determined by using naked eyes that will be interpreted differently by different person. Thus, this study is executed to develop a specialized colorimeter for phenol red pH measurement by using Raspberry Pi as a microcontroller unit, green LED as a light source and photodiode in tandem with capacitor as a light sensor. The Raspberry Pi calculated the capacitor charging time which is in relative to the amount of light sensed by the photodiode. Based on the results, the capacitor charging time can be related to the pH value by using non-linear relationship with high accuracy of R2 = 0.8946 and RMSE = 0.4447. This study successfully introduced the development of low-cost system for phenol red pH measurement by applying the capacitor charging time which is in correspond with the absorbance value.