In situ measurements of the solubilities of salt-water systems by a fiber sensor

A novel fiber sensor for measuring solid solubilities of salt-water systems in situ is described. The sensor consists of a diode laser as light source, three couplers, two sensing fiber ends with protective cladding, and two photodetectors. The measurement principle is based on relative Fresnel reflective intensity. This method enabled us to observe in situ transitional process of the equilibration of a solid-liquid system with a small quantity of specimens. By the relatively simple technique, fast determination of the solid solubility is possible. We applied this method to measure a temperature dependence of the solubility of potassium chloride in water in situ and compared with the previous data obtained by other techniques. The measured result has the long-term standard deviation of the concentration of 0.1%, and agrees with the data obtained by the classical method within the error of +/-1%. This method is precise and sample saving and is suitable to measure the solubilities of rare and expensive materials.