An apparatus for detecting a chemical of interest in a fluid or estimating a concentration of the chemical in the fluid includes: a carrier configured to be conveyed through a borehole penetrating an earth formation; a first temperature sensor disposed at the carrier and configured to sense a temperature of the fluid and provide a first temperature output; and a second temperature sensor disposed at the carrier and covered with an exothermic reaction material that experiences an exothermic reaction when exposed to the chemical of interest and configured to sense a temperature and provide a second temperature output. The apparatus further includes a processor coupled to the first temperature sensor and the second temperature sensor and configured to detect the chemical or estimate the concentration using the first temperature output and the second temperature output.