Calculation of the chromium reducible sulfur content

Oxidation of iodine (to tetrathionate) involving one mole of thiosulfate corresponds to one equivalent of iodine (i.e. V212). The relevant chemical reaction in the absence of sodium carbonate or equivalent is  

A = The volume of iodine (in ml) used to titrate the zinc acetate trapping solution following the soil digestion, 

C=The molarity of the iodine solution as determined by titration with standard 0.025 M Na2S203 solution, noting that C = [(0.025 xD)/E], where D = titre (ml) of standard 0.025 M sodium thiosulfate and E=volume (ml) of iodine solution titrated. 

---