Colorimetric pH Measurement for Seawater Samples Using a Three Light-Emitting Diodes Detector and a Calibration Method for Temperature Dependence

A colorimetric pH measurement of seawater samples using a light source comprising a three light-emitting diodes (TLED) detector and meta-cresol purple (mCP) as an indicator was investigated. The molar absorption ratios (e(1), e(2), and e(3)/e(2)) for mCP using the TLED detector at 25 degrees C were determined to be 0, 1.9994, and 0.1010, respectively. Next, the pH values of 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) and 2-aminopyride (AMP) seawater buffers were determined. Notably, the raw pH of TRIS buffer (similar to 8.1) agreed with the reference value, while that for the AMP buffer (similar to 6.8) had an error of +0.004 due to the small absorption ratio (R) and being out of the lower adequate pH range (>7.2) for mCP. pHT measurements obtained for seawater samples using the present colorimetric method agreed with those obtained using a glass electrode. These results demonstrate that this low-cost TLED detection system with a short cell length, 5 cm, can be used for seawater pHT analysis.