Sulphites determination

8) Sulphites testing procedure:
 
The sample is titrated against Std iodate-iodide sol using starch indicator.

Reagents required:

  1. Dil HCl acid:- Add 200ml of concentrated hydrochloride acid to 100ml of distilled water.
  2. Starch indicator solution:- Mix starch (5g) and mercuric iodide (0.01g) with distilled water (30ml). Make up to one liter using boiling distilled water. Boil for 3min allow cooling and decanting off supernatant clear liquid. 
  3. Std iodate-iodide solution:- Weigh accurately 0.713g of potassium iodate and dissolve in about 150ml of distilled water. Add 7g of potassium iodide and 0.5g of sodium bicarbonate, when dissolved dilute the solution to exactly 1lit.
Procedure:
  • Since sulphite tends to be oxidized in content with air, great Care shall be exercised in sampling. The fallowing procedure shall be adopted:
  • Collect the sample by running it to the bottom of 250ml bottle but allowing about eight to ten times then volume of the bottle to run to waste before taking the sample.

  • For this test, a filtered sample shall not be used unless suspended matter is present containing interfering substance ,for example ,iron oxide ,in which case insert a filter in the sample line.
  • Measure 2ml of dil HCl acid into a 350ml porcelain basin and add 100ml of the sample and 1ml starch indicator to it. Titrate immediately with std iodate-iodide solution, stirring continuously, until a faint permanent blue colour is obtained.
Calculation:
  • Sulphite (SO3), mg/l = 8 V
  • V (ml)=  Std iodate-iodide solution volume consumed.