When a Sulfur containing sample is combusted at 1000 ℃, Sulfur dioxide is formed SO2: R-S + O2 –> SO2 + CO2 + H2O

The combustion gas, carrying Sulfur dioxide (SO2), is led into a sulfuric acid scrubber for effective water and interference removal. The dried and clean gas is led into the temperature-controlled titration cell, where the Sulfur dioxide reacts with Tri-iodine, present in the titration cell. The amount of charge (the integral of the regeneration current over the measuring time) used to regenerate the lost Tri-iodine, is directly related to the Total Sulfur content of the sample.

The equations for this reaction are:
SO2 + I3- + H2O –> SO42- + 3 I- + 4 H3O+
2 I- –> I2 + 2 e
I2 + I- –> I3-