National standard of People s Republic of China GB5413.24-2010 Determination of chlorine in foods for infants and young children, raw milk and dairy products Issued at 2010-03-2 Implemented at:2010-06-01 Issued by the Ministry of Health, People s Republic of China
GB 5413.24-2010 Preface This standard is replaces GB/T5413.24-1997, Measurement of chlorine in formulated foods and milk powder for infants and young children Versions of previous standards replaced by this standard are: --GB 5413-1985 and GB/T 5413.24-1997.
1 Scope This standard regulate the determination method for chlorine in foods for infants and young children and dairy products. This standard applies to the determination of chlorine in foods and dairy products for infants and young children. 2 Normative cited documents The reference cited in this standard is necessary. For the cited documents which are labeled with date, all their subsequent modification sheets or modified versions are not applicable for this standard. For the cited documents which are not labeled with date, their latest versions are applicable for this standard. Method 1 Electrometric titration 3 Principles Acetone is added after the sample is subjected to acidification, the glass electrode is used as the reference electrode and the silver electrode is used as the indicator electrode, titrate with the standard solution of silver nitrate titration and the end point of titration is determined according to the sudden change in the electric potential. The content of chlorine in foods and dairy products for infants and children is calculated according to the consumed volumn of the standard silver nitrate titration solution. 4 Reagents and materials Unless specified, the reagents used by this method are all analytical pure reagents, and the water is the grade two water specified by GB/T6682. 4.1 K 4 Fe(CN) 6 3H 2 O 4.2 Zinc Acetate 4.3 Acetic Acid 4.4 Nitric Acid 4.5 Acetone 4.6 Sodium Chloride Standard 4.7 Silver Nitrate Standard 4.8 Protein precipitant 4.8.1 Precipitant I: dissolve 106 g potassium ferrocyanide (4.1) in water, transfer to a 1000 ml volumetric flask and make up to the mark with water. 4.8.2 Precipitant II: dissolve 220 g zinc acetate (4.2) in water, add 30 ml acetic acid (4.3), transfer to a 1000 ml volumetric flask and make up to the mark with water 4.9 Nitric acid solution (1+3): mix one volume of concentrated nitric with three volumes of water. 4.10 Sodium chloride standard solution (0.01 mol/l): weigh 0.5844 g standard and dry it in the oven with temperature 500 50 to constant weight, then dissolve in small
amount of water in 100ml flask, transfer to a 1000 ml volumetric flask, mixed well, make up to the mark 4.11 Silver nitrate standard solution (0.02 mol/l) 4.11.1 The preparation: accurately weigh 3.40 g silver nitrate standard (accurate to 0.01 g), dissolve in 100 ml flask with small amount of water, transfer to a 1000 ml volumetric flask, mix well, make up to the mark. stored in a dark place or transfer to a brown volumetric flask. 4.11.2 The calibration (second derivative method): pipette 10.00 ml sodium chloride standard solution (4.10) to a 50 ml beaker, add 0.2 ml nitric acid solution (4.4) and 25 ml acetone (4.5). immerse the glass electrode and the silver electrode and switch on the stirrer. Drip V ml of silver nitrate standard solution (90% of the estimated volume) measure the electric potential (E) of the solution. Continue titrate with silver nitrate solution. Every 1 ml added, electric potential (E) should be immediately measured The electric potential (E) should be immediately measured for 0.1 ml of standard silver nitrate titration solution when close or passed the end point. The standard silver nitrate titration solution is continued to be dripped in until the electric potential does not significantly change. The volume and the electric potential for each dripping of the standard silver nitrate titration solution should be recorded as demonstrated in the table 1 4.11.3 The determination of the end point for the titration: according to the titration records (4.11.2), Based on measured E and V calculate E, V, first derivative and second derivative, then fill into the table. Table 1 The records for the titration of the sodium chloride standard solution by using the standard silver nitrate titration solution V E E a V b First derivative ( E/ V) c Second derivative d 0.00 400 70 4.00 18 22 4.00 470 20 0.50 40 60 4.50 490 10 0.10 100 50 4.60 500 15 0.10 150 50 4.70 515 20 0.10 200 650 4.80 535 85 0.10 850-350 4.90 620 50 0.10 500-300 5.00 670 20 0.10 200-100 5.10 690 10 0.10 100 --
5.20 700 -- -- -- a. Corresponding change in electric potential; b. The increase in the volume of the standard silver nitrate titration solution that is continuously dripped in; c. The change in the electric potential that is induced by unit volume of the standard silver nitrate titration solution, and that is the ratio between E and V; d. It equals to the difference between the adjacent first derivative. When first derivative is the biggest and second derivative is zero, it is the end point for the titration, and the volume (V1) of the standard silver nitrate titration solution at the end point should be calculated according to formula (1). In this formula: V1 the volume of the standard silver nitrate titration solution at the end point of the titration, ml; Va the volume of the standard silver nitrate titration solution at time point a, ml; a the value of second derivative before it changes into zero; b the value of second derivative after it changes into zero; V the value between a and b, ml. Example: It is found in the table that the maximal first derivative is 850, then second derivative should be between 650 and -350 when second derivative is zero, therefore, a=650, b=-350, Va=4.8 ml, V=0.10 ml That is to say, the consumption of the standard silver nitrate titration solution at the end point of the titration is 4.87 ml. 4.11.4 The determination of the concentration of the standard silver nitrate solution: In this formula: C2- the accurate concentration of the standard silver nitrate solution, mol/l; C1 the accurate concentration of the standard sodium chloride solution, mol/l; V1 the volume of the standard silver nitrate titration solution that is consumed at the end point of the titration, ml. 5. Instruments and equipments 5.1 ph meter: the precision ±0.1; 5.2 Glass electrode 5.2 Silver electrode
5.4 Electromagnetic stirrer; 5.5 Burette 5.6 Balance: precision 0.1mg, 1mg 6. Operation procedures 6.1 The preparation of the sample solution Into 250 ml conical flask, Weigh about 20 g sample (accurate to 0.001 g) add 100 ml hot water of 70 and heat to boiling for 15 min, with stirred. Cool to room temperature, then add 4 ml precipitant I (4.8.1) and 4 ml precipitant II (4.8.2) in order. Each addition, mix well. Stand for 30 min. Transfer to a 200 ml volumetric flask and make up to the mark. Well mix and filter with filter paper, the first part of filter should be discarded. 6.2 Analytical procedures Pipette 10 ml sample solution (6.1) into 50 ml beaker, add 5 ml nitrate acid solution (1+3) (4.9) and 25 ml acetone (4.5). The analysis is carried out according the step 4.11.2, calculate volume of the standard silver nitrate titration solution consumed (V2) at the end point of the titration according to 4.11.4. 7. The calculation and expression of the analytical results 7.1 The calculation of the result The content of chlorine in the sample is counted by the mass fraction X1 and it is expressed by mg/100 g, it is calculated according to formula (3): In this formula: 35.5 the mass of chlorine that is equal to that in 1.00 ml standard silver nitrate titration solution [c(agno 3 )=1.000 mol/l], mg; V2 the volume of the standard silver nitrate titration solution that is consumed during the titration of the sample, ml; f the dilution factor; m1 the mass of the sample, g; c2 the accurate concentration of the standard silver nitrate titration solution, mol/l. 7.2 The expression of the result: The results are mean of two repeated results. The result is reported to three significant digits 8. The precision The difference between the two parallel measurements on the same sample should be no more than 0.2 g for every 100 g sample.
Method 2 Precipitation titration 9. Principles Organic acids are used to precipitate the protein in the sample and silver nitrate is used to titrate the chloride ion in the sample and produce silver chloride precipitation. Excessive silver nitrate will react with the potassium chromate indicator and changes the solution into orange, it is the end point for the titration. The chlorine content is calculated according to the consumption of the silver nitrate solution. 10. Reagents and materials Unless specified, the reagents used by this method are all analytical pure reagents, and the water is the grade three water specified by GB/T6682. 10.1 Sodium hydroxide 10.2 potassium chromate 10.3 Sodium Chloride standard, purity higher than 99% 10.4 Silver nitrate 10.5 Trichloroacetic acid solution (500 g/l): weigh 500g trichloroacetic acid into 1000 ml water 10.6 Sodium Hydroxide solution (50g/L): weigh 25g Sodium Hydroxide into 500 ml water 10.7 Silver nitrate solution (0.1mol/L): Pipette 6 ml nitric acid, dilute to 1000 ml with Water 10.8 potassium chromate solution (50g/L): dissolve 5 g potassium chromate into 100 ml water. 10.9 NaCl standard solution (0.1 mol/l): Into 1000 ml volumetric flask, to precision, weigh 5.8840 g NaCl which dry to constant under 500±50ºC, make up to the mark with deionized water 10.10 Silver nitrate solution (0.05 mol/l): into 100 ml flask, to the precision, weigh 8.50g (nearest to 0.01mg) silver nitrate (10.4), dissolve with small amount of water, transfer to 1000 ml volumetric flask, make up to the mark with water, mix well, store from light or in amber volumetric flask. Pipette10 ml sodium chloride standard solution (10.9) into 125 ml flask, add 10 drops of potassium chromate indicator, titrate with the above mentioned silver nitrate solution. The actual concentration of the silver nitrate solution is calculated according to the volume of the silver nitrate solution consumed. Calculate on formula 4 0.1 10 c = (4) V In which, c- the concentration of silver nitrate, mol/l V- the volume of silver nitrate, ml 0.1- the concentration of sodium chloride standard solution, mol/l 10- volume of the sodium chloride standard solution, ml
10.11 Phenolphthalein ethanol indicator: 10 g/l: dissolve 1g Phenolphthalein into 95% ethanol 11 Instruments and equipments 11.1 balance: nearest to 0.1mg 11.2 Volumetric flask, 100 ml 11.3 Conical flask, 125 ml 11.4 Brown burette, 10 ml 12 Operation procedures 12.1 into small flask, weigh 10 g homogenate sample (nearest to 0.1 mg), add 50 ml water to dissolve the sample, transfer to a 100 ml volumetric flask, add10 ml trichloracetic acid solution (10.5), mix well and make up to the mark, stand for about 1 min then filter. 12.2 Piptte10 ml filtrate into 125 ml conical flask, add three drops of Phenolphthalein indicator (10.11),adjusted to slightly red with sodium hydroxide solution (10.6) and then adjusted backward with nitric acid solution (10.7) until the red color has just disappeared. Add another 10 drops of potassium chromate solution (10.8) then the solution is titrated with silver nitrate solution (10.4) to orange and the color can not disappear within 1 minute, and thus it is the end point for the titration, a piece of yellow paper is placed at the bottom of the conical flask during the titration and thus the end point can be easily distinguished. The blank experiment should be carried out at the same time. 13. The calculation and expression of the resut The chlorine content in the sample is calculated according to formula (5): V 5 100 X 2 = ( V 3 V 4) C3 35.5 (5) V 6 m2 In this formula: X2- content of chlorine in the samples, mg/100g V3 the volume of the silver nitrate solution that is consumed for the titration of the sample solution, ml; V4 the volume of the silver nitrate solution that is consumed by the blank solution, ml; C3 the concentration of the silver nitrate solution, mol/l; V5 the volume of the sample solution, ml; V6 the volume of the filtrate that is taken, ml; m2 the mass of the sample, g. The result is the mean of two repeat measurement results. Report results to two digital. 8. The precision The absolute difference between the two results of repeat measurements should be less then 5% of the mean.