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.

Formula Calculation :::

mg/L SO₃  =    A x N x 40000 
                         mL sample

where:
VALUE of A = mL titrant 

N = Normality of KI-KIO titrant  (  0.0125 N )

To calculate as Na₂SO₃  

mg/L Na₂SO₃ = mg/L SO₃ x 1.57

Dosage

Interfering substance 

Interference level Metals Some metals, especially copper, catalyze the oxidation of sulfite to sulfate. Immediately add  Sulfite Indicator Powder: ::: RXSOL-62-5503-005  for each liter of sample during sample collection to prevent the interference. 

Nitrite Reacts with sulfite and causes low results. 

Organic compounds Oxidizable organic compounds can cause high results. 

Oxidizable compounds Cause high results. 

Sulfide Causes high results. 

Remarks

Samples must be analyzed immediately after collection and cannot be preserved for later analysis. 

Let the sample temperature decrease to 50 °C (122 °F) or less before analysis. 

Sulfite reacts quickly with oxygen in the air. Shaking or swirling the sample causes low results. Prevent agitation of the sample during the procedure. 

Review safety chemical handling procedure. Use the recommended personal protective equipment. Dispose of reacted solutions according to local, state and federal regulations. Follow the environmental safety, health safety staff for your facility and/or local regulatory agencies for further disposal information.

Supply Locations

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