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This standard supersedes "National Food Safety Standard Determination of Chlorine in Foods for Infants and Young Children, Milk and Milk Products" (GB 5413.24-2010), "Determination of Sodium Chloride in Foods" (GB/T12457-2008), "Fruits, Vegetables and Their Products - Determination of Chloride Content" (GB/T 15667-1995), "Meat and Meat Products - Determination of Chloride Content" (GB/T 9695.8-2008), "Starches and Derived Products - Determination of Chloride Content" (GB/T 22427.12-2008) and determination of "14.2 Salt" in "Method for Analysis of Hygienic Standard of Meat and Meat Products" (GB/T 5009.44-2003).
This standard will integrate the above standards, mainly covering the following modifications:
- The standard name is revised as "National Food Safety Standard - Determination of Chloride in Foods";
- Three methods are integrated according to the determination principle of chloride: potentiometric titration method, Volhard method (indirect precipitation titration method) and argentometric method (Mohr method or direct titrimetric method);
- The original determination methods according to food category are deleted;
- The ultrasonic processing procedures are added.
National Food Safety Standard
Determination of Chloride in Foods
食品安全国家标准
食品中氯化物的测定
1 Scope
This standard specifies the methods for determination of chloride content in foods, such as potentiometric titration method, Volhard method (indirect volumetric precipitation method) and argentometric method (Mohr method or direct titrimetric method).
The potentiometric titration method in this standard is applicable to the determination of chloride in various foods.
The Volhard method (indirect volumetric precipitation method) and argentometric method (Mohr method or direct titrimetric method) in this standard are not applicable to the determination of chloride in foods with dark color.
Method I: Potentiometric Titration Method
2 Principle
After the specimen is acidized, acetone is added and silver nitrate standard volumetric solution is used to titrate the chloride in test solution with glass electrode as reference electrode and silver electrode as indicator electrode. The titration end point is determined according to potential "abrupt change". The chloride content in foods is calculated according to the consumption of silver nitrate standard volumetric solution.
3 Reagents and Materials
Unless otherwise specified, analytically-pure reagents and Class-III water (defined in GB/T 6682) are adopted for the purpose of this method.
3.1 Reagents
3.1.1 Potassium ferricyanide [K4Fe(CN)6•3H2O].
3.1.2 Zinc acetate [Zn(CH3CO2)2].
3.1.3 Silver nitrate (AgNO3).
3.1.4 Glacial acetic acid (CH3COOH).
3.1.5 Nitric acid (HNO3).
3.1.6 Acetone (CH3COCH3).
3.2 Standard product
Reference sodium chloride (NaCl), purity≥99.8%.
3.3 Reagent preparation
3.3.1 Precipitator I: weigh 106g potassium ferricyanide, dissolve it with water, scale the volume to 1L and mix it uniformly.
3.3.2 Precipitator II: weigh 220g zinc acetate, dissolve it into a small amount of water, add 30mL glacial acetic acid, scale the volume to 1L and mix it uniformly.
3.3.3 Nitric acid solution (1+3): uniformly mix nitric acid and water in a ratio of 1:3.
3.4 Preparation and calibration of standard solution
3.4.1 Sodium chloride reference solution (0.01000mol/L): weigh 0.5844g (accurate to 0.1mg) sodium chloride reference reagent which is burnt to constant weight at 500℃~600℃, dissolve it with a small amount of water in small beaker, then transfer it to 1000mL-volumetric flask, dilute to the scale and shake well.
3.4.2 Silver nitrate standard volumetric solution (0.02mol/L): weigh 3.40g silver nitrate (accurate to 0.01g) and place it in small beaker, dissolve it with small amount of nitric acid, transfer it into 1000mL brown volumetric flask, dilute it with water to the scale and shake well, the solution shall be kept in dark place or transferred to brown bottle. Or silver nitrate standard volumetric solution approved and awarded with reference material certificate by the nation shall be purchased.
3.4.3 Calibration (second derivative method): pipet 10.00mL 0.01000mol/L sodium chloride reference solution into 50mL-beaker and add 0.2mL nitric acid solution and 25mL acetone into it. Immerge the glass electrode and silver electrode into the solution, then start the electromagnetic stirrer. Instill V'mL silver nitrate standard volumetric solution (90% of required quantity) with acid burette and measure the potential value (E) of solution. Continue to instill silver nitrate standard volumetric solution and measure the potential value (E) of solution immediately after each instilling of 1mL. When and after getting close to the end point, measure the potential value (E) of solution after each instilling of 0.1mL. Continue to instill silver nitrate standard volumetric solution till the potential value of solution not obviously changing any more. Record the distilling volume of silver nitrate standard volumetric solution and potential value for each time.
3.4.4 Determination of titration end point: calculate ΔE, ΔV, first and second derivatives according to titration record (3.4.3) and volume (V') of silver nitrate standard volumetric solution and electric potential values (E) based on the examples in Table A.1. Or automatically titrate with potentiometric titrator and record the volume of silver nitrate standard volumetric solution and potential value.
Titration end point is reached when the first derivative is largest and second derivative is zero; the volume (V1) of silver nitrate standard volumetric solution shall be calculated according to Formula (1) at the titration end point.
(1)
Where,
V1 - the volume of consumed silver nitrate standard volumetric solution when reaching the titration end point, mL;
Va - the volume of consumed silver nitrate standard volumetric solution at the time of a, mL;
a - the second derivative value before reaching zero;
b - the second derivative value after reaching zero;
ΔV - the volume difference between a and b, mL.
3.4.5 The concentration of silver nitrate standard volumetric solution is calculated according to Formula (2):
(2)
Where,
c - the concentration of silver nitrate standard volumetric solution, mol/L;
c1 - the concentration of sodium chloride reference solution, mol/L;
V1 - the volume of consumed silver nitrate standard volumetric solution at titration end point, mL.
4 Instruments and Apparatus
4.1 Tissue blender.
4.2 Grinder.
4.3 Mortar.
4.4 Vortex oscillator.
4.5 Ultrasonic cleaner.
4.6 Thermostat water bath.
4.7 Centrifuger: rotation speed ≥3000r/min.
4.8 pH meter: with precision of ±0.1.
4.9 Glass electrode.
4.10 Silver electrode or composite electrode.
4.11 Electromagnetic stirrer.
4.12 Potentiometric titrator.
4.13 Balance: with sensibility of 0.1mg and 1mg.
5 Analysis Steps
5.1 Specimen preparation
5.1.1 Powder, pasty or liquid sample
Take at least 200g representative sample, mix the sample sufficiently, then store the sample in an airtight glass container.
5.1.2 Blocky or granular solid sample
Take at least 200g representative sample, grind the sample with grinder or mortar, then store the sample in an airtight glass container.
5.1.3 Semi-solid or semi-liquid sample
Take at least 200g representative sample, pound with tissue blender, then store the sample in an airtight glass container.
5.2 Preparation of specimen solution
5.2.1 Foods and milk products for infants
Weigh 10g (accurate to 1mg) well-mixed specimen and place it into 100mL colorimetric cylinder with stopper, add 50mL hot water of about 70℃, shake the sample for dispersion, boil it for 15min in water bath and shake frequently, then take it out for 20min ultrasonic processing, cool to ambient temperature and successively add 2mL precipitator I and 2mL precipitator II, shake the solution well for each time of adding. Dilute it with water to the scale, shake well, keep it still for 30min at ambient temperature. Filter the solution with filter paper, discard the original filtrate and take partial filtrate for determination. Where necessary, the solution may also be centrifuged for 10min with centrifuger at 5000r/min, then take partial filtrate for determination.
Contents
Foreword I
1 Scope
2 Principle
3 Reagents and Materials
4 Instruments and Apparatus
5 Analysis Steps
6 Expression of Analysis Results
7 Precision
8 Others
9 Principle
10 Reagents and Materials
11 Instruments and Apparatus
12 Analysis Steps
13 Expression of Analysis Results
14 Precision
15 Others
16 Principle
17 Reagents and Materials
18 Instruments and Apparatus
19 Analysis Steps
20 Expression of Analysis Results
21 Precision
22 Others
Appendix A Table for Volume Calculation of Sodium Chloride Standard Solution Titrated by Silver Nitrate Standard Volumetric Solution
