The synthesis of some water insoluble dyes for the measurement of pH in water immiscible solvents

Abstract

A series of water insoluble dyes for the measurement of pH in an inaccessible water phase in water immiscible solvents have been designed and synthesised based upon the fluorescein system. The solubilities of the anionic forms of the dyes depend strongly upon the nature of the ester substituent and the degree of substitution of the aromatic rings. For dye molecules with long chain alkyl esters and with bromine substituents in the rings, the sodium or potassium salts are soluble in organic solvents such as diethyl ketone or ethyl acetate and the anions do not partition measurably into aqueous solution. Other less highly substituted dyes can be solubilised as their anions by the addition of quaternary ammonium salts or crown ethers. Titration curves for the dyes show that they are able to respond reproducibly to the pH of a water phase in contact with the organic phase although the detailed physical chemistry of the ion exchange is complex. Certain of the dyes are suitable for monitoring the pH in reaction systems using enzymes as catalysts in water immiscible solvents.