Real-time precision concentration measurement for flowing liquid solutions

The precise, real-time measurement of liquid concentration is important in fundamental research, chemical analysis, mixing processes, and manufacturing, e.g., in the food and semiconductor industries. This work presents a laser-based, noninvasive technique to measure concentration changes of flowing liquids in real time. The essential components in the system include a 5 mW laser diode coupled to a single-mode optical fiber, a triangular optical cell, and a high-resolution beam position sensor. The instrument provides a large range of concentration measurement, typically 0%-100% for binary liquid mixtures, while providing a resolution on the order of 0.05% concentration or better. The experimental configuration is small, reliable, and inexpensive. Results are presented for NaCl and MgCl₂ aqueous solutions with concentrations ranging from 0% to 25%, with very good agreement found between measured and true concentrations.