ISI 20-1e Determination of Sulphur Dioxide in Starch

1. Scope The method is applicable to native and modified starch.

 

 LT 13 Sep 1999
2. Principle Sulphur dioxide is released by boiling with acid. The gas is collected in hydrogen peroxide and oxidised to sulphuric acid and tirated with alkali.

 
3. Apparatus 3.1 Analytical balance weighing to the nearest 1 g.
3.2 Monier-Williams Sulphites Distillation Apparatus consisting of a three neck 1000ml boiling flask with a sloping Allihn reflux condenser, a right angle inlet adapter with a bubbler tube (scrubber), a 60-ml dropping funnel and 500 ml conical flask (collector / receiver)
3.3 Electrical heater

 
4. Reagents 4.1 Sodium carbonate solution: Dissolve approx. 15 g of Na2CO3 or 40 g of Na2CO3.10H2O in distilled water and dilute to 100 ml.
4.2 Hydrogen peroxide, 2%: Dilute 10 ml of neutral 30% hydrogen peroxide (H2O2) with distilled water to 100 ml.
4.3 Hydrochloric acid, concentrated (HCl)
4.4 NaOH 0.1N
4.5 Bromophenol blue 0.1% indicator (C19H10Br4O5S): Dissolve 0.1 g of bromophenol blue in 1.5 ml NaOH 0.1N (4.4) and 20 ml alcohol and dilute to 100 ml with distilled water.

 
5. Procedure Carbon dioxide is passed through the scrubber with sodium carbonate solution (4.1) and into the gas-inlet tube of the boiling flask. The scrubber removes chlorine
Place 20 ml hydrogen peroxide solution (4.2) in the receiving flask
Add through the dropping funnel 300 ml distilled water and 20 ml concentrated hydrochloric acid (4.3)
Assemble and start cooling. Boil for 10 min. in a current of carbon dioxide
Weigh 100 g of sample to the nearest g (w) and disperse in 300 ml of fresh boiled distilled water. Transfer the slurry to the boiling flask via the dropping funnel Adjust dropping rate and gas flow to prevent drawback of hydrogen peroxide or air
Boil gently for one hour with a slow current of carbon dioxide.

Titration

 Add immediately indicator (4.5) and titrate with 0.1N NaOH

Blank

 Carry out a blank test with water and reagents

 
6. Calculation Calculate sulphur dioxide content of sample by averaging results of two samples with two significant digits:
Sulphur dioxide % = ((s-b) * 0.0032 *100) / w where

s = ml 0.1N sodium hydroxide for sample
b= ml 0.1N sodium hydroxide for blank
w = weight of sample in g

Correct eventually to sulphur dioxide on sample dry matter.

 
7. Notes Additional gravimetric determination by precipitation with barium chloride after titration.

 
8. Reference JECFA

 
9. Monier-Williams Sulphites Distillation Apparatus


A slanting Allihn condenser is preferred in stead of the one shown. Connect the gas inlet tube to a scrubber and carbon dioxide source. Connect the outlet to a receiving flask.

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