Sulphite (Sulfite) Lab Test Method

Summary of Method The water sample is acidified and titrated with a potassium iodide-iodate standard solution. The acid releases free iodine, which is reduced to colorless iodide by the sulfite in the sample. When all of the sulfite is gone, the iodine reacts with the starch indicator to form a blue color. 

Reagents 
Sulfuric acid, H2 SO4 ::: RXSOL-60-6186-500
Starch indicator  ::: RXSOL-60-6221-100 
Sulfite Indicator Powder: ::: RXSOL-62-5503-005 a proprietary formulation containing sulfamic acid to destroy nitrite. 
Standard potassium iodide-iodate titrant ::: RXSOL-62-5503-006  This titrant  is equivalent to 500 mu (μ) g SO3 /1.00 mL.
Sulfamic Acid: Crystalline 
EDTA Reagent ::: RXSOL-60-6168-500 Dissolve 2.5 g EDTA in 100 mL distilled water. 

Procedure 

Sampling Contact with air must be minimized. If the sample temperature is greater than 50°C, it must be cooled in a air tight apparatus. Immediately add 1 mL of EDTA Solution (5.6) per 100 mL of sample. 

1st Method 
Place 1 mL H2SO4 in titration vessel.
Add 0.1 g sulfamic acid crystals. 
Add 50 mL sample. 
Add approximately 0.1 g starch indicator. 
Titrate with potassium iodide-iodate titrant (5.4) until a faint permanent blue color develops. Keep the pipet tip below the surface of the sample. View the color change against a white background. Record the mL titrant. —- VALUE of A
Run a reagent blank using distilled water instead of sample. 

2nd Method
Place 50 mL sample in a titration vessel. 
Add 3-4 drops phenolphthalein indicator. 
Add sufficient scoops (1 g) of indicator ( Sulfite Indicator Powder: ::: RXSOL-62-5503-005 )  to discharge the red color. 
Titrate with potassium iodide-iodate titrant until a faint permanent blue color develops. View the color change against a white background. Record the mL titrant. —- VALUE of A
Run a reagent blank using distilled water instead of sample.