·       Sulphur Dioxide Is A Widely Accepted Preservative For Many Food Products Such As Beverages, Squashes, Grape Resins, Dehydrated Food Products, Caramel Etc. It Is Also Used For Bleaching Of Sugars And Often Occurs As A Residual Component In Sugar Samples.

(1)Qualitative Test:

·       Add Small Amount Of Sulphur Free Zinc And Several Ml Hydrochloric Acid To Approximately 25 Gm Sample (With Addition Of If Necessary) In 200 Ml Erlenmeyer Flask. Hydrogen Sulphide Generated May Be Detected With Lead Acetate Paper. Traces Of Metallic Sulphides Occasionally Present In Vegetables Give Same Reaction As Sulphites Under Conditions Of Above Test. Verify Positive Results Obtained By This Method By Monier-Williams Quantitative Method.

(2)Qualitative Test :

Reagents

·       Iodine – Barium Chloride – Dissolve 3 Gm Iodine In Water Containing 3 Gm Pot. Iodide. Add 2 Gm Barium Chloride Dissolved In Water And Dilute To 100 Ml

Apparatus

·       Conical Flask With A Small Bubbler In The Form Of A Small Thistle Funnel Bent Twice In The Stem So That Gases Evolved Pass Through The Reagent Placed In The Funnel

Procedure

·       Place 5 Gm Sample In The Flask, Add 0.1 Gm Copper Acetate, A Piece Of Marble And 10 Ml Of Conc Hydrochloric Acid And Fit On The Bubbler. Allow The Acid To Act On The Marble For 10 Minutes And Then Heat To Boiling. The Iodine Is Decolorized And In The Presence Of Sulphur Dioxide A Ppt Of Barium Sulphate Settles In The Tube. The Formation Of A Turbidity Is Inconclusive As It May Be Due To Other Substances Such As Volatile Oils

Quantitative Method:

Modified-Monier-Williams Method

·       Conical Flasks

·       Beakers & Pipette

·       Distillation Apparatus

·       (Note: In This Method, Back Pressure Inside Apparatus Is Limited To Unavoidable Pressure Due To Height Of 3% H₂O₂ Solution Above Tip Of Bubbler (F). Keep Back Pressure As Low As Possible To Avoid Loss Of SO₂ Through Leaks. Use Thin Film Of Stop Cock Grease On Sealing Surfaces Of All Joints Except Joint Between Separatory Funnel And Flask. Clamp Together Each Joint To Ensure Complete Seal Throughout Analysis.)

Assemble Apparatus Which Includes:

·       Inlet Adapter (A)

·       Separatory Funnel (B)

·       Round-Bottom Flask (C)

·       Gas In Let Tube (D)

·       Allihn Condenser (E)

·       Bubbler (F)

·       Vessel (G)

Reagents

·       Hydrogen Peroxide Solution (3%): Dilute 100 Ml Of 30% Hydrogen Peroxide To About 700 Ml In A 1000 Ml Graduated Cylinder. Take 100 Ml Portion Of Diluted Solution And Titrate In A 250 Ml Conical Flask With 0.01 N Naoh To Ph 4.1 Using Ph Calculate The Amount Of Naoh Required Neutralizing The Main Solution, Adding This Amount, Stirring And Checking The Ph. To Standardise The Hydrogen Peroxide, Pipette 10 Ml Of The Solution Into A 100 Ml Volumetric Flask And Make Up To Volume. Pipette 5 Ml Of This Diluted Solution Into A 500 Ml Flask, Add About 300 Ml Of Water And 10 Ml Of 6N H2SO4 And Titrate With 0.1 N Kmno4 To A Permanent Pink Colour.

·       Bromophenol Blue Solution (0.1%): Dissolve 1 Gm In 100 Ml Water.

·       Phenolphthalein Indicator (0.1%): Dissolve 0.1 Gm In 50 Ml Ethy1 Alcohol And Dilute To 100 Ml With

·       Sodium Carbonate- Saturated Solution: Dissolve Enough Sodium Carbonate To Prepare A Saturated Solution. Add Several Drops Of Phenolphthalein. Discard This Solution When It Becomes Decolourised.

Procedure

·       Circulate Cold Water Through Condenser. Add From A Graduated Cylinder 20 Ml Of 3% Hydrogen Peroxide To The Conical Flask And 5 Ml To The Trap. Assemble And Connect To Condenser. Weigh 50 Gm Of Blended Or Well Mixed Portion Of The Sample, Transfer Into 500 Ml Flask Through Gas Inlet Tube, Using 300 Ml Of Water. Replace Gas Inlet Tube Immediately, Making Sure All Connections Are Well Greased And Tight. Remove The Gas Inlet Tube, And Slowly Add 20 Ml Of Conc. HCI. Replace The Tube And See If Bubbles Nitrogen Enters The Receiving Flasks. If Not, Check Joint For Leaks. Adjust Nitrogen (99.9% Pure) To Give A Flow Of 15-20 Bubbles Per Minute Through The Tube. Heat The Solution To Boil. Dried Fruits And Vegetables Require Boiling For One Hour. For All Other Products 30 Min. Boiling Is Enough. After The Solution Boils For The Required Time, Wash The Hydrogen Peroxide Solution From The Trap Into The Conical Flask. Rinse The Trap With Water. Add 3 Drops Of Bromophenol Blue Indicator And Titrate With 0.05N Naoh Solution To A Pale Sky-Blue End Point Using 5 Ml Micro Burette. Run A Blank Titration On 20 Ml, Of Hydrogen Peroxide And Correct Results Accordingly.

Modified-Monier-Williams Method

Principle

·       Sulphur Dioxide Bleaches P-Rosaniline And By Addition Of Formaldehyde, Sulphur Dioxide Forms An Additional Product Releasing Free Rosaniline Which Is Pink In Colour, The Amount Of Free Rosaniline Released Is Equivalent To The Sulphur Dioxide Present.

Reagents

·       Formaldehyde Solution (0.015%): Prepare By Diluting 40% Formaldehyde In 2 Steps 10 To 1000 And 75 To 2000.

·       Acid Bleached P-Rosaniline Hydrochloride: Place 100 Mgs P- Rosaniline-Hcl And 200 Ml Water In 1 Litre Volumetric Flask. Add 150 Ml Hcl (1+1) And Dilute To Let Stand 12 Hours Before Use.

·       Sodium Tetra-Chloro Mercurate: Place 23.4 Gm Sodium Chloride And 54.3 Gm Mercuric Chloride In A 2 Litre Volumetric Flask. Dissolve In Approximately 1000 Ml Water, Dilute To

·       Sulphur Dioxide Standard Solution: Dissolve Approximately 170 Gm Sodium Bisulphate In Water And Dilute To One Litre. Standardize With 0.01N Iodine Solution Before Use (Approximately 100 µg SO2/Ml.)

Preparation Of Standard Curve:

·       Add 5 Ml Mercurate Reagent To Series Of 100 Ml Volumetric Flasks. Then Add 0, 1, 2, 3 Ml Of Sulphur Dioxide Standard Solution. Dilute To Volume And Mix.

·       Transfer 5 Ml Portions To 200 Ml Test Tubes Containing 5 Ml Rosaniline Reagent.

·       Add 10 Ml 0.015% Formaldehyde Solution, Mix And Hold For 30 Minutes At 22ºc. Read The Absorbance Against Zero Standard (Blank) And Plot Standard Curve.

Determination

·       Weigh 10 ± 0.02 Gm Ground Dried Fruit And Transfer To Blender With 290 Ml Water. Cover And Blend For 2 Minutes. Withdraw 10 Gm Aliquot From Bottom Of Blender With 10 Ml Calibrated Free Running Pipette And Transfer To 100 Ml Volumetric Flask Containing 5 Ml 0.5N Sodium Hydroxide (Use 2 Ml For Apples And 1 Ml For Golden Raisins). Swirl And Mix Approximately For 15-30 Seconds. Add 4 Ml 0.5N Sulphuric Acid (2 Ml For Apples And 1 Ml For Golden Raisins) And 20 Ml Mercurate Reagent And Dilute To Volume. For Blank, Omit 10 Ml Fruit Extract.

·       Transfer 2 Ml Sample Solution To 200 Mm Test Tube Containing 5 Ml Rosaniline Reagent. Add 10 Ml Of 0.015% Formaldehyde Solution And Mix And Hold It For 30 Minutes At 22ºc. Read Absorbance At 550 Nm Against Blank. Refer To Standard Curve And Convert Results To Ppm SO2.