SOP for ESTIMATION OF SULPHITE

PRINCIPLE:

An acidified sample containing Sulphite (SO­­­­­­­­­₃^2-.) is titrated with standardized potassium 

iodide-iodate titrant. Free iodine, liberated by the iodide-iodate reagent, reacts with SO­­­­­­­­­₃^2-. 

The titration end point is signaled by the blue colour resulting from the first excess of 

iodine reacting with starch indicator.

REAGENTS:

1.    Sulfuric Acid: H₂SO₄, 1+1.

2.    Standard Potassium Iodide-iodate Titrant 0.002083 M: Dissolve 0.4458 g primary 

grade anhydrous KIO₃ (dried for 4 hr at 120^OC), 4.35 g KI, and 310 mg sodium 

bicarbonate (NaHCO3) distilled water and dilute to 1000 ml. (1 ml = 500 mg SO₃^2-)

3.    Sulfamic Acid, NH₂SO₃ crystalline

4.    EDTA Reagent: Dissolve 2.5 g di-sodium EDTA in 100 ml distilled water.

5.    Starch Indicator: To 5 g starch in a mortar, add a little cold distilled water and grind to a 

paste. Add mixture to 1 lit. boiling distilled water, stir, and let settle overnight. Use 

clear supernatant. Preserve by adding 1.3 g salicylic acid, 4 g ZnCl₂, or a combination of 

4 g sodium propionate and 2 g sodium azide to 1000 ml starch solution.

PROCEDURE:

1.    Sample Collection: Collect a fresh sample, taking care to minimize contact with air. Fix 

cooled samples (< 50^OC) immediately by adding 1 ml EDTA solution / 100 ml sample. Cool 

hot samples to 50^OC or below. Do not filter.

2.    Titration: Add 1 ml H₂SO₄ and 0.1 g NH₂SO₃ H crystals to a 250 ml Erlenmeyer flask or 

other suitable titration vessel. Accurately measure 50 to 100 ml EDTA-stabilized sample into

 flask, keeping pipet tip below liquid surface.

3.    Add 1 ml starch indicator solution. Titrate immediately with standard KI-KIO₃ titrant, while swirling flask, until a faint permanent blue colour develops. Analyze reagent blank using distilled water instead of sample.

CALCULATION:

                                                          (A-B) x M x 6 x 40000

Mg SO₃^2-/ lit        =     ----------------------------------------                    

                                                      ml. Sample

Where,

            A = ml titrant for sample.

            B = ml titrant for blank.

            M = molarity of KI-KIO3 titrant.

REFERENCE:  APHA 4500 SO­­­­­­­­­_{_3}^2-. B (Pg. No. 4 –173). (Standard Methods for water and wastewater analysis)