Determination of formic acid vapour using piezoelectric crystals with 4-ethyl-3-thiosemicarbazide and 2,6-diacetylpyridine coatings

Abstract

Two sensors have been suggested for formic acid vapour. These sensors are based on piezoelectric crystals coated, by immersion, with films of 4-ethyl-3-thiosemicarbazide and 2,6-diacetylpyridine. The sensors have been perfected using a new static system comprising an oscillator mounted on a die-plate and placed inside a testing chamber which, by means of a series of auxiliary devices, can reproduce and control environmental conditions. The sensors show their sensitivity to formic acid for at least 60 d. Selectivity is adequate with both coatings. The sensors begin to respond to formic acid vapour immediately, reaching a stable frequency after 5 min; average recovery times are about 28 min. For the 4-ethyl-3-thiosemicarbazide sensor, the sensitivity is 29.7 Hz mg^–1 l (for an amount of coating corresponding to Δf₀= 475 Hz) with a correlation coefficient of 0.9997 and a detection limit of 0.17 mg l^–1. For the 2,6-diacetylpyridine sensor, the values are 10.9 Hz mg^–1 l^–1, 0.987 and 0.33 mg l^–1 respectively (Δf₀= 824 Hz). The sensors could be used to detect formic acid vapour in the atmosphere and in working environments if a suitable preconcentration device were connected. The advantages over existing techniques, i.e., gas chromatography and pulsed ultraviolet photoacoustic spectroscopy, should be evident from their simplicity, reduced cost and capacity for use in situ.