Flow Injection Analysis of Ammonia and Sulfur Dioxide with Piezoelectric Detection

Abstract

As crystal-coating materials used for ammonia sensors, hydrochlorides of pyridoxine, glutamic acid, histidine, methionine, alanine and cysteine were examined. By using the first three of them a satisfactory detection of ammonia in the wide range from 1ppb to 10³ppm could be obtained. Under optimized conditions the lifetime of a histidine hydrochloride-based sensor can be extended to 1 month, whereas for a pyridoxine hydrochloride-based detector, due to the possibility of regeneration, even up to 4 months is possible. Among several compounds examined as coating substances for sulfur dioxide detection, tripropylamine and tridodecylamine can be utilized only for the determination of a large concentration of SO₂, above 0.1% v/v. N, N, N′, N′-tetrakis(2-hydroxyethyl)ethylenediamine (EDTE) and N, N, N′, N′-tetrakis(2-hydroxypropyl)ethylenediamine (EDTP) can be employed for a much lower range of SO₂ concentrations, namely 1 to 1000 and 0.01 to 1000ppm for EDTE and EDTP, respectively. A lifetime study of SO₂ detectors has shown that although an EDTP-based SO₂ sensor can be used for 2 months, its detectability gradually deteriorates. Upon using an EDTP-based sensor some disadvantages of serial syringe dilution for the preparation of gas standard mixtures were noted. For a comparison, an alternative procedure employing gas-permeable tubing was proposed.