CREATING THE CONTEXT 2: 

DETERMINATION OF SO2 IN AIR: WEST-GAEKE METHOD

One of the methods to determine SO2 in the atmosphere is the procedure proposed by West and Gaeke (P. W. West and G. C.Gaeke, Analytical Chemistry 1956, 28, 1817-1819). This method is sensitive in the concentration range 0.005-0.2 ppm. The system obeys Beer’s law up to about 0.6 ml of SO2 per mililiter of absorbing solution. The air is bubbled through 0.1 M sodium tetrachloromercurate solution and the stable, nonvolatile dichlorosulfitomercurate is formed. The sulfur dioxide is then determined by spectrophotometric measurement of the red-purple pararosaniline methylsulfonic acid formed when acid-bleached pararosaniline and formaldehyde are added to the scrubbing solution.

Main reaction:

[HgCl4]2- + 2SO2 ---> [Hg(SO3)2]2- + 4Cl- + 4H+

REACTANTS AND GLASSWARE:

0.1 M solution of sodium tetrachloromercurate (II), hydrochloric acid-bleached p-rosaniline solution (0.04%), formaldehyde solution (0.2 %), 100 mg/mL standard solution of SO2 (sodium bisulfite).

scrubber (200 mL cylinder with appropriate tubing), crucible, 100 ml volumetric flasks, pipettes (10 ml), rubber pumps, air pump, plastic sample container, cuvettes, spectrophotometer.

 

SAFETY NOTES:

Sodium tetrachloromercurate is highly poisonous; if spilled on skin, flush off with water immediately.