In West-Gaeke colourimetric procedure, the intensity of the red purple colour is measured photometically and it is proportional to the concentration of sulphur dioxide. What is the red purple coloured compound?

• A. Sodium tetra chloromercurate
• B. Dicholorosulphitomercurate complex
• C. Ammonia molybdate
• D. Paraosaniline sulphonic acid