An affordable ammonia temperature-programmed desorption equipment and its calibration using the thermal decomposition of ammonium dihydrogen phosphate

•New sensor for NH3-TPD.•Equipment for ammonia thermodesorption.•A way of calibrating NH3-TPD.•Ammonia temperature-programmed desorption apparatus.•Ionic conductivity sensor for ammonia. A temperature-programmed desorption (TPD) equipment was built to follow ammonia release from solids; ammonia is detected following the ionic conductivity changes of saturated boric acid dissolution of with an ionic conductivity detector. The conductivity probe response is fast; it is insensitive to water, which is an advantage in the analysis of acid solids like zeolites and can be used with low heating rates or small samples. Ammonium dihydrogen phosphate (ADP) decomposition was used to calibrate the equipment and to check for its repeatability. In that process, it was found that the thermal decomposition of ADP was not well reported before. The decomposition of ADP follows a first-order kinetics and takes place in two steps; first, an ammonia molecule is released, and ammonia pyrophosphate is formed, then this decompounds to metaphosphoric acid and ammonia. Activation energies of 153 (4.6) kJ mol−1, and 69 (2.8) kJ mol−1, respectively, were calculated. NH3-TPD equipment requires a blank experiment due to the ability of the detector of sensing impurities in the carrier gas. Calibration of the technique gave a factor of conversion from conductivity to moles of ammonia of 347 (2.2) S m−1 mol−1 at 95.45% of confidence, a factor ten orders of magnitude higher than the limit of detection of the method calculated as 8.6 × 10-8 mole of ammonia. The limit of quantification was established as 2.9 × 10-7 mole of ammonia, a value quite low and useful when working with solid acids.