An analytical comparison of two versions (US EPA and BIS) of pararosaniline method used for monitoring of ambient SO₂

The two versions of the reference method of sulphur dioxide (SO₂) measurement in ambient air (pararosaniline method) available as the Environment Protection Agency (EPA) USA (CFR 40 Part 50, Appendix A) and Bureau of Indian Standards (BIS) India (IS 5182 (Part 2):2001) standard methods were analytically studied. For accuracy and precision of the data obtained, a certain set of specifications should be ascertained before using the method for sample analysis as per the guidelines. On comparing the two methods of operation, the stated set of specifications are fulfilled for the EPA method but not for the BIS method. A different set of specifications were observed for the BIS method (absorbance blank: 0.012 ± 0.001 abs units, slope of calibration curve: 0.014 ± 0.001 abs units/µg SO₂, intercept of calibration curve: 0.003 ± 0.002, calibration factor:72.0 ± 4.2). The absorption efficiency as well as the method efficiency of both methods were tested using three different concentrations (0.3 ppm, 0.5 ppm, and 0.8 ppm) of standard SO₂ gas. The absorption efficiency of both the methods was found to be 100%. The average method efficiency of the EPA method at 0.3 ppm, 0.5 ppm and 0.8 ppm were found to be 81 ± 8%, 81 ± 6% and 87 ± 1% while that of BIS method was observed to be 91 ± 5%, 93 ± 2%, 89 ± 4% at the respective concentrations. An uncertainty estimation study was also performed considering factors involved in sampling and analysis. A combined uncertainty < 9% and < 7% was observed for EPA and BIS method, respectively. This study presents a comprehensive examination of the operational aspects of two versions of pararosaniline method employed for measuring SO₂ in ambient air. The results indicate a need for redefining the specifications outlined in the BIS method. Notably, the BIS method displays greater sensitivity to low blank values compared to the EPA method. Additionally, the study introduces, for the first time, working factors and specifications associated with the BIS method for SO₂ measurement. These findings suggest their potential inclusion in the standard method as a means to enhance data quality and reliability in the assessment of SO₂ levels in ambient air.