ABSTRACT
Sulfur dioxide, in an air sample, is absorbed into a solution of potassium or sodium tetrachloromercurate, TCM, by aspirating a measured air sample through an absorber vessel. This procedure results in the formation of a monochlorosulfonato-mercurate (II) complex (3), which resists oxidation by the oxygen in the air (1),(2),(3). Ethylenediamine-tetraacetic acid disodium salt (EDTA) is added to this solution to complex heavy metals that catalyze the oxidation of the collected sulfur dioxide (4),(5). Once this monochlorosulfonatomercurate complex is formed, it is stable to strong oxidants (e.g. ozone, oxides of nitrogen and hydrogen peroxide) (2). After sampling is completed, any ozone in the solution is allowed to decay (5). The liquid is treated first with a solution of sulfamic acid to destroy the nitrite anion formed from the absorption of oxides of nitrogen present in the atmosphere (6). It is then treated with solutions of formaldehyde and specially purified, acid-bleached pararosaniline containing phosphoric acid to control pH. Pararosaniline, formaldehyde, and the bisulfite anion react to form the intensely colored pararosanilinemethylsulfonic acid, which behaves as a two-color pH indicator (λmax. = 548 nm at pH 1.6 ± 0.1, ϵ = 47.7 × 103). The ϵ value assumes quantitative production of the absorbing species. The pH of the final solution is adjusted to 1.6 ± 0.1 by the addition of a prescribed amount of 3M phosphoric acid to the pararosaniline reagent (5). This technique is the basis of the reference method for the determination of sulfur dioxide by the U.S. Environmental Protection Agency (7).
