Chemistry: Sulphite And Sulfite

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Sulphite ion (SO32-) is a weak base, but undergoes some hydrolysis to produce basic solutions. In acidic solution, the equilibria are shifted to form sulfurous acid, resulting in the evolution of SO2 gas. Sulfur dioxide is a colorless gas with a characteristic choking odor. The sulfites of Na+, K+, and NH4+ are soluble in water. Most other sulfites are insoluble in water. However, due to the basic nature of SO32-, all sulfites dissolve in acidic solution. Sulphites may occur in natural waters or wastewaters as a result of industrial pollution and in treatment plant effluents dechlorinated with sulphur dioxide. Control of sulphite ion in wastewater treatment and discharge may be important environmentally, mainly because of its toxicity to fish …show more content…
The remaining iodine when titrated gives a blue color with starch indicator. The volume of titrant used is proportional to the sulfite concentration
In this protocol, iodide ion is generated by the following slow reaction between the iodate and bisulfite:
IO3− + 3 HSO3− → I− + 3 HSO4−
This is the rate determining step. The iodate in excess will oxidize the iodide generated above to form
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Interferences
• Sulphide, ferrous iron and organic matter interferes cause positive error in titration. The presence of other oxidizable materials, such as sulphide, thiosulphate, Fe2+ ions can apparently give higher readings of sulphites. Interferences due to the presence of sulphides may be removed by the addition of zinc acetate.
• Nitrate, if present oxidizes sulphite during acidification. Addition of sulfamic acid helps to eliminate interference due to nitrate.
• Copper ions rapidly accelerate the oxidation of sulphite ions. Certain heavy metals may also react in a similar manner as copper. Proper sampling and immediate fixing by acid addition minimize such difficulties. Apparatus
(i) Conical flask of 500 ml
(ii) Pipette of 5 ml and 10 ml

Reagents
(i) Sulphuric acid 1+1(distilled water). Dilute 100 ml concentrated sulphuric acid L.R grade to 200 ml with distilled water.
(ii) Standard potassium iodide - iodate titrant, 0.0125 M. Dissolve 0.4458 g primary grade anhydrous KIO3 (dried for 4 hours at 1200C) , 4.35 g Potassium iodide and 310 mg Sodium bicarbonate (NaHCO3) in distilled water and dilute to 1000

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