Optical sensors and the salt effect: A dual-transducer approach to acidity determination in a salt-containing concentrated strong acid

A dual-transducer approach has been developed to decompose the optical signals of acid sensors in salt-containing concentrated acid solutions and to give acid and salt concentrations in concentrated LiCl-HCl, CaCl2-HCl, and AlCl3-HCl solutions, respectively. The optical acid sensors in this approach are films of porous sol-gel SiO2 or SiO2-Nafion composite doped with low-pKa indicators. A novel linear relationship (partial derivativeA/partial derivativeC(salt))c(acid) = beta. (dA(0)/dC(acid))c(salt=0) (A = absorbance of the sensor in a salt-containing HCl solution; A(0) = absorbance of the sensor in a salt-free acid solution) was found, and the current approach is based on a set of nonlinear equations derived from this relationship.