Trace Moisture in Ammonia: Gas Chromatography Using Calcium Carbide

Abstract

CaC2 is known to convert water to acetylene by the following reaction. CaC2 + 2H2O → Ca(OH)2 + C2H2 The conversion rate of water to acetylene is reported to depend on reaction temperature, cell size, flow rate, particle size, and the type of gas studied. Data will be presented quantifying the amount of C2H2 evolved from the reaction of CaC2 with 3 to 10 ppm H2O in both helium and ammonia gas under a defined set of experimental conditions. A calibration curve is presented to illustrate linearity for acetylene detection by gas chromatography/discharge ionization detection (GC/DID) under these experimental conditions down to 15 ppb ±1.5 ppb. Taking into account the stoichiometry of the above reaction, the method could potentially have a sensitivity for water in the range of 30 to 50 ppb. However, moisture challenges from 3 to 10 ppm in helium and ammonia were detected with an accuracy of only ±1 ppm. the precision of the measurement is a sum of the variance in the moisture input system plus variance in the reaction rate of water with calcium carbide. The reaction in NH3 requires a 2-hr residence time in the calcium carbide reaction cell for the water in the ammonia sample to fully react with the solid CaC2 at 40°C (104°F).