Development of hydrogen-chloride research gas mixtures required for SI-traceable measurements of stack gas emissions

摘要

Measurements of the HCl concentration determined using the two independent Cl~- and H+ concentration techniques agreed to within 1 % over the range 7 to 12 μmol/mol, suggesting systematic relative uncertainties (k=2) of this magnitude. An important potential source of measurement error has to do with non-equilibrium effects caused by interactions of the analyte with internal surfaces of the impingers and the flow manifold. To see evidence of this effect, HCl concentration measurements were repeated at regular time intervals. It was found that in the absence of sample preconditioning, apparent HCl concentrations varied substantially with time. However, with preconditioning steady state conditions were achieved quickly, thus reducing the amount of analyte consumed during the measurement process and reducing concentration bias. These results are illustrated in Figure 2 where the steady behavior under preconditioning contrasts with the relatively slow approach to equilibrium in the absence of preconditioning. The presented method provides the basis for accurate, Si-traceable determination of HCl concentrations in N_2. Here we have demonstrated accurate assignment in the range 1 to 10 μmol/mol, although the procedure can be readily extended up to concentrations as high as 1000 μmol/mol using quantitative dilution schemes. It is expected that this approach will yield relative combined standard uncertainties in HCl concentration of 1-2 % over this three-decade-wide range.