| Abstrakt: | SUMMARIES. A portable, inexpensive sensor for the quantification of formaldehyde vapour in air has been developed. The vapour is trapped by a solution containing 0.07 mM pararosaniline, 2 mM sulphite and 0.02 M hydrochloric acid (Schiff’s reagent). Reaction mechanisms were explored to explain the reduction of pararosaniline by sulphite and/or dilute hydrochloric acid, and to understand how the coloured alkyl sulphonic acid product forms. The optimized trapping reagent was doped into a transparent, robust, porous glass, (1 cm2, approximately 1 mm thick), prepared by the sol–gel method. The absorbance of the glass sensor was measured at 620 nm. When formaldehyde vapour (3–22 parts per million by volume (ppmv)) was passively sampled for 6 h in the laboratory, a linear correlation was achieved between absorbance and concentration. The sensors were also exposed to lower concentrations of formaldehyde vapour (sub-ppmv) for 1–6 days. Initial results indicate linear responses to increasing vapour phase concentrations, and exposure times. To allow on-site measurements, a hand-held spectrophotometer has been designed to measure the intensity of transmitted light passing through the sensor. It employs an eight-bit microcontroller to modulate two light emitting diodes, one that emits light at a reference wavelength ( λr) the other at the absorbing wavelength ( λmax) of the derivatized product. After the light has passed through the absorbing media it is detected by a photodiode. The net intensity of the transmitted light is measured as the difference between the intensity of light measured at λmax and the intensity of light measured at λr. The data obtained are processed by a microcontroller and the absorbance measurements are output to a liquid crystal display. The accuracy and precision of the hand-held device were assessed by comparing absorbance measurements with those obtained from a bench top ultraviolet spectrometer. |
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