Colorimetric determination of formaldehyde in air using a hanging drop of chromotropic acid

A simple and sensitive method to determine parts per billion (ppb) of atmos pheric formaldehyde in situ, using chromotropic acid, is described. A color imetric sensor, coupled to a droplet of 15.5 muL chromotropic acid, was con structed and used to sample and quantify formaldehyde. The sensor was set u p with two optical fibers, a right emitting diode (LED) and two photodiodes . The reference and transmitted light were measured by a photodetection arr angement that converts the signals into units of absorbance. Air was sample d around the chromotropic acid droplet. A purple product was formed and mea sured after the sampling terminated (typically 7 min). The response is prop ortional to the sampling period, analyte concentration and sample flow rate . The detection limit is similar to2 ppb and can be improved by using longe r sampling times and/or a sampling flow rate higher than that used in this work, 200 mL min(-1). The present technique affords a simple, inexpensive n ear real-time measurement with very little reagent consumption. The method is selective and highly sensitive. This sensor could be used either for out door or indoor atmospheres.