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International Starch Institute
Science Park Aarhus, Denmark
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
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.|
|Add immediately indicator (4.5) and titrate with 0.1N NaOH|
|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
Correct eventually to sulphur dioxide on sample dry matter.
|7. Notes||Additional gravimetric determination by precipitation with barium
chloride after titration.
|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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Keywords: laboratory method determination sulphur sulfur dioxide starch