Optical fibre titrations. Part 3. Construction and performance of a fluorimetric acid-base titrator with a blue LED as a light source

Abstract

An opto-electronic titration unit, which can be used for monitoring the course of acid-base titrations via an optical light-guide, is described. A blue light-emitting diode (LED) is used as a light source and an inexpensive photodiode as a detector to monitor changes in the fluorescence of an added indicator. The instrument is considered to have significant cost advantages over electrode-based titrators.

Various kinds of acids and bases have been titrated with excellent precision using the described technique. A voltage change of ca. 100 mV is measured at the end-point. 1-Hydroxypyrene 3,6,8-trisulphonate and fluorescein are used as indicators because of their water solubility and the close overlap of their absorptions with the emission of the blue LED. 7-Hydroxy-3-(4-pyridyl)coumarin is used as a two-step pH indicator for the titration of phosphoric acid, but this dye can only be excited by the UV light of, e.g., a xenon lamp.

The advantages of the method described include simplicity, broad applicability (as demonstrated by the titration of aggressive acids such as hydrofluoric acid), the ease of automation and the possibility of performing assays with sample volumes as small as 0.5 ml.