Chloromercuric method

The content of sulfur dioxide (SO2) in the atmosphere varies widely and may reach values as high as 1-5 mg m in heavily polluted air. Direct monitoring with an SO2 gas probe is often insufficiently sensitive and thus SO2 must be preconcentrated by absorption in a suitable solution. The most common procedure is based on absorption of SO2 in a tetra-chloromercurate(II) solution (TCM), in which Hg(S03)2 is formed. The pH of the absorption solution is maintained at a value of 6.9 that ensures virtually complete absorption of S02- Prior to analysis, amidosulfonic acid is added to remove nitrite and the pH is decreased to about unity, so that most of the sulfite present is converted into free sulfur dioxide, which is determined by an SO2 probe. With very low SO2 contents, a filter is soaked with a TCM solution and air is passed through the filter at a high flow rate and for a rather long time. These methods provide discontinuous values of the SO2 concentration in regular intervals. 

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