Fiber optic and non-fiber optic double-beam pH sensors for use in a flow-through cell

Two modules are presented for optical measurement of the pH of solutions in a flow-through cell. One module is linked to fiber optic cables, while the other is non-fiber optic, with source and detector being located inside the module. In order to compensate for (a) temporal fluctuations of the light source, (b) sample turbidity and (c) adverse effects of sample coloration, a two-wavelength detection scheme is applied. The system measures pH from 5.5-9.0, and displays low temperature coefficients (which are in the range of 0.001 pH/°C for the pH-sensitive membrane, and 0.05%/°C for the LED light source). An algorithm is derived allows for the calculation of pH by ratioing light intensity data. The accuracy is ±0.02 over the pH 6-8.5 range, and ±0.1 over the pH 8.5-9.5 and 5.5-6.0 ranges. The respective merits of (a) the optical fiber approach, (b) the non-fiber optic module (both based on absorbance measurements) and (c) of a micro-electrochemical pH flow-through cell are compared.