ANNEX IIU.K.METHODS OF ANALYSIS TO VERIFY THE COMPOSITION OF CERTAIN SUGARS INTENDED FOR HUMAN CONSUMPTION
METHOD 4U.K. MEASUREMENT OF REDUCING SUGARS EXPRESSED AS INVERT SUGARS U.K.(Berlin Institute method)U.K.
Scope and field of application U.K.
1.The method determines the reducing sugar content expressed as invert sugar in semi-white sugar.U.K.
2. Definitions U.K.
‘Reducing sugars expressed as invert sugar’: the content of reducing sugars as determined by the method specified.
3. Principle U.K.
The sample solution containing reducing sugars is used to reduce a solution of copper II complex. The copper I oxide formed is then oxidized with standard iodine solution, the excess of which is determined by back-titration with standardized sodium thiosulphate solution.
4. Reagents U.K.
4.1. Copper II solution (Muller's solution)U.K.
Dissolve 35 g of copper II sulphate, pentahydrate (CuSO4.5H2O) in 400 ml of boiling water. Allow to cool.
Dissolve 173 g of sodium potassium tartrate tetrahydrate (Rochelle salt or Seignette salt; KNaC4H4O64H2O) and 68 g of anhydrous sodium carbonate in 500 ml of boiling water. Allow to cool.
Transfer both solutions (4.1.1 and 4.1.2) to a one litre volumetric flask and make up to one litre with water. Add 2 g of activated carbon, shake, allow to stand for several hours and filter through thick filter paper or a membrane filter.
If small amounts of copper I oxide appear during storage, the solution should be re-filtered.
4.2. Acetic acid solution 5 mol/litre.U.K.
4.3. Iodine solution 0·01665 mol/litre (i.e. 0·0333 N, 4·2258 g/litre).U.K.
4.4. Sodium thiosulphate solution 0·0333 mol/litre.U.K.
4.5. Starch solution: to one litre of boiling water add a mixture of 5 g of soluble starch slurried in 30 ml of water. Boil for three minutes, allow to cool and add, if required, 10 mg of mercury II iodide as a preservative.U.K.
5. Apparatus U.K.
5.1. Conical flask, 300 ml; precision burettes and pipettes.U.K.
5.2. Water-bath, boiling.U.K.
6. Procedure U.K.
6.1.Weigh a portion of the sample (10 g or less) containing not more than 30 mg of invert sugar in a 300 ml conical flask and dissolve in about 100 ml of water.U.K.
Pipette 10 ml of the copper II solution (4.1), into the flask containing the sample solution. Mix the contents of the flask by swirling and place it in the boiling water-bath (5.2) for exactly 10 minutes.
The level of the solution in the conical flask should be at least 20 mm below the level of the water in the water-bath. Cool the flask rapidly in a stream of cold running water. During this operation the solution should not be stirred otherwise atmospheric oxygen will reoxidize some precipitated copper I oxide.
Add 5 ml of 5 mol/litre acetic acid (4.2) by pipette without shaking and immediately add an excess (between 20 and 40 ml) of the iodine solution 0.01665 mol/litre (4.3) from a burette.
Stir to dissolve the copper precipitate. Titrate the excess iodine against the sodium thiosulphate solution 0·0333 mol/litre (4.4) using the starch solution (4.5) as indicator. The indicator is added towards the end of the titration.
6.2.Carry out a blank test with water. This is to be carried out with each new copper II solution (4.4). The titration shall not exceed 0·1 ml.U.K.
6.3.Carry out a control test under cold conditions with the sugar solution. Allow to stand at room temperature for 10 minutes to permit any reducing agents such as sulphur dioxide which may by present to react.U.K.
7. Expression of results. U.K.
7.1. Formula and method of calculation U.K.
Volume of iodine consumed = ml 0·01665 mol/litre iodine added in excess minus ml 0·0333 mol/litre sodium thiosulphate used in titration.
The volume (in ml) of 0·01665 ml/litre iodine consumed is corrected by subtracting:
7.1.1.The number of ml consumed in the blank test carried out with water (6.2).U.K.
7.1.2.The number of ml consumed in the cold test with the sugar solution (6.3).U.K.
7.1.3.A value of 2·0 ml for every 10 g of sucrose present in the aliquot used, or a proportionate quantity where the sample contains less than 10 g sucrose (correction for sucrose).U.K.
After these corrections are made each ml of iodine solution (4.3) which has reacted corresponds to 1 mg of of invert sugar.
The invert sugar contents, as a percentage of the sample, is given by the formula:
where:
=
the number of ml of iodine solution (4.3) after correction,
=
the mass, in grams, of the sample used.
7.2. Repeatability U.K.
The difference between the results of two determinations when carried out simultaneously or in rapid succession on the same sample, by the same analyst, under the same conditions, shall not exceed 0·02 g per 100 g of sample.