Chiang Mai Journal of Science

The determination of soils  pH  by observing the change in additive color (red (R), green (G), blue (B)) via a smartphone was succeeded by providing the definite values of pH with respect to the values obtained from a  pH meter.  The experiments were carried out under the control conditions, i.e.,  light intensity and focus range of a mobile phone camera, via a home-made detection box which is robust, convenience, and adjustable to support various sizes of  mobile phones with the maximum area of 630 cm².  Three indicators; bromothymol blue, methyl red, and phenol red, were buffered in the pH range of 1-13 before monitored the RGB values.  Characteristic calibration graph of each indicator was constructed by plotting between pH versus RGB value. Soil composition defines the pH of soil solution and the pH affects on the forms of the indicators, leading to variation in color of the solution. The pH of soil sample was obtained from calibrating the RGB values of the soil solutions mixed with indicators to the mentioned calibration graphs. The proposed method could overcome the problem from observing the color change of indicators when mixed to the colored sample solution of soil, and  also applied to neutral, acidic, and basic soil analysis. With economic and simple method, the definite pHs were found and agreed with the results obtained by using a  pH meter.