1 Scope
This standard specifies determination methods of iodine content in foods.
Oxidation-reduction titration (Method I) is applicable to the determination of iodine in algae like kelp, porphyra and wakame and their products.
As(III)-Ce4+ catalytic spectrophotometry (Method II) is applicable to the determination of iodine in foods such as grains, vegetables, fruits, beans and their products, milk and its product, meat, fish and eggs.
Gas chromatography (Method III) is applicable to the determination of iodine in foods for infants and young children, milk and milk products.
Method I Oxidation-reduction Titration
2 Principle
After being carbonized and incinerated, the organic iodine in sample is transformed to inorganic iodide ion, which is then oxidized to iodate radical ion by bromine water in acid medium; after that, iodine is separated from the generated iodate radical ion by reduction in acid solution of potassium iodide and titrated with sodium thiosulfate solution.
I-+3Br2+3H2O→IO3-+6H++6Br-
IO3-+5I-+6H+→3I2+3H2O
I2+2S2O32-→2I-+S4O62-
3 Reagents and Materials
Unless otherwise specified, analytically-pure reagents and Grade III water (defined in GB/T 6682) are adopted for the purposes of this method.
3.1 Reagents
3.1.1 Anhydrous sodium carbonate (Na2CO3).
3.1.2 Liquid bromine (Br2).
3.1.3 Sulfuric acid (H2SO4).
3.1.4 Sodium formate (CHNaO2).
3.1.5 Sodium thiosulfate (Na2S2O3).
3.1.6 Potassium iodide (KI).
3.1.7 Methyl orange (C14H14N3SO3Na).
3.1.8 Soluble starch.
3.2 Preparation of reagents
3.2.1 Sodium carbonate solution (50g/L): weigh 5g of anhydrous sodium carbonate and dissolve it into 100mL of water.
3.2.2 Saturated bromine water: measure 5mL of liquid bromine into a brown bottle which has a stopper coated with vaseline, add into 100mL of water, oscillate sufficiently and make it become saturated solution (a small amount of bromine liquid is remained at the solution bottom and the operation shall be carried out in fuming cupboard).
3.2.3 Sulfuric acid solution (3mol/L): measure 180mL of sulfuric acid and slowly inject it into a beaker containing 700mL of water while stirring, cool the solution to room temperature, dilute with water to 1 000mL and mix well.
3.2.4 Sulfuric acid solution (1mol/L): measure 57mL of sulfuric acid and prepare the solution in accordance with the method specified in 3.2.3.
3.2.5 Potassium iodide solution (150g/L): weigh 15.0g of potassium iodide, dissolve it and dilute the solution to 100mL with water, and then store the solution in a brown bottle; the solution is prepared immediately before use.
3.2.6 Sodium formate solution (200g/L): weigh 20.0g of sodium formate, dissolve it and dilute the solution to 100mL with water.
3.2.7 Sodium thiosulfate standard solution (0.01mol/L): prepare and calibrate the solution in accordance with those specified in GB/T 601.
3.2.8 Methyl orange solution (1g/L): weigh 0.1g of methyl orange powder and dissolve it into 100mL of water.
3.2.9 Starch solution (5g/L): weigh 0.5g of starch into a 200mL beaker, add into 5mL of water to make it into starch paste, pour into 100mL of boiling water and reboil for 0.5min after stirring, and then cool the solution for future use; the solution is prepared immediately before use.
4 Apparatuses
4.1 Tissue blender.
4.2 High speed disintegrator.
4.3 Analytical balance: with the sensitivity of 0.1mg.
4.4 Electric thermostatic drying oven.
4.5 Muffle furnace: ≥600℃.
4.6 Porcelain crucible: 50mL.
4.7 Adjustable electric furnace: 1 000W.
4.8 Iodine flask: 250mL.
4.9 Brown acid burette: 25mL, minimum scale of 0.1mL.
4.10 Micro acid burette: 1mL, minimum scale of 0.01mL.
5 Analysis Procedures
5.1 Specimen preparation
5.1.1 For dry sample, pulverize it with high speed disintegrator, sieve it with standard sieve of 425μm in mesh size, store it in an inclosed and dark place or at low temperature.
5.1.2 As for fresh and frozen samples, take edible part of them and homogenize it, preserve it by inclosed and cold storage or freezing.
5.1.3 As for liquid samples like condensed juice of alga or alga beverage, carry out sampling after mixing well.
5.2 Specimen analysis
5.2.1 Weigh 2g~5g of specimen (accurate to 0.1mg), put it into a 50mL porcelain crucible, add into 5mL~10mL of sodium carbonate solution to infiltrate the specimen sufficiently; and then keep it still for 5min, place it into a 101℃~105℃ electric thermostatic drying oven for 3h, dry the sample and take it out.
5.2.2 Heat the specimen with electric furnace in fuming cupboard to make it carbonize sufficiently to smokeless, and then put it into 550℃±25℃ muffle furnace to burn for 40min, cool to about 200℃ and take it out. Add a small amount of water into the crucible and grind, transfer all the solution and residues to a 250mL beaker, wash the crucible with water for several times and combine the washing solution into the beaker to make the total volume of solution in beaker is about 150mL ~ 200mL, and then boil for 5min.
5.2.3 For sample with higher iodine content (kelp and its product etc.), filter the solution and residue obtained in 5.2.2 to a 250mL volumetric flask with filter paper while hot, repeatedly flush the residue in beaker and funnel with hot water, cool the washing solution and bring to volume. Then accurately transfer an appropriate amount of filtrate into a 250mL iodine flask for future use.
5.2.4 As for other samples, filter the solution and residue obtained in 5.2.2 with filter paper to a 250mL iodine flask while hot for future use.
5.2.5 Add 2 or 3 drops of methyl orange solution into the iodine flask, adjust the solution to red with 1mol/L sulfuric acid solution, add 5mL of saturated bromine water inside fuming cupboard, heat and boil until the yellow fades. After the solution cools slightly, add into 5mL of sodium formate solution, heat and boil for 2min on electric furnace, and then take it down, cool the solution to below 30℃ with water bath, add into 5mL of 3mol/L sulfuric acid solution and 5mL of potassium iodide solution, put on the bottle cap and leave it still for 10min; after that, titrate with sodium thiosulfate standard solution until the solution appears light yellow, add into 1mL of starch solution and continue to titrate until the blue disappears exactly. Carry out blank test simultaneously and respectively record the volume V and V0 of consumed sodium thiosulfate standard solution.
6 Expression of Analysis Results
The content of iodine in specimen is calculated according to Formula (1):
(1)
Where,
X1 - the content of iodine in the specimen, mg/kg;
V - the volume of sodium thiosulfate standard solution consumed for titrating sample solution, mL;
V0 - the volume of sodium thiosulfate standard solution consumed for titrating reagent blank, mL;
c - the concentration of sodium thiosulfate standard solution, mol/L;
21.15 - the mass of iodine equivalent to 1.00mL of standard titration solution of sodium thiosulfate [c (Na2S2O3)=1.000mol/L], mg;
V1 - the constant volume of sample solution with higher iodine content, mL;
V2 - the volume of pipetted filtrate with higher iodine content, mL;
m1 - the mass of sample, g;
1 000 - the unit conversion coefficient.
The result is accurate to one decimal place.
7 Accuracy
The absolute difference of the results from two independent determinations under repeatability condition shall not exceed 10% of the arithmetic mean value.
8 Others
The method detection limit is 1.4mg/kg.
Foreword i
1 Scope
2 Principle
3 Reagents and Materials
4 Apparatuses
5 Analysis Procedures
6 Expression of Analysis Results
7 Accuracy
8 Others
9 Princple
10 Reagents and Materials
11 Apparatuses
12 Analysis Procedures
13 Expression of Analysis Results
14 Accuracy
15 Others
16 Principle
17 Reagents and Materials
18 Apparatuses
19 Analysis Procedures
20 Expression of Analysis Results
21 Accuracy
22 Others
Appendix A Gas Chromatogram of Iodine Standard Derivative
