GB 5009.256-2025 National food safety standard Determination of phosphates in food
1 Scope
This standard specifies the methods for the determination of phosphates [represented by phosphate (PO43-)] in food.
Method I of this standard is applicable to the determination of phosphates, pyrophosphates, trimetaphosphates, and tripolyphosphates in food, but not applicable to the determination of sodium hexametaphosphate added to food.
Method II of this standard is applicable to the determination of phosphates [represented by phosphate (PO43-)] in food.
Method I Determination of four phosphates in food
2 Principle
Specimens are extracted with sodium hydroxide solution, purified by solid-phase extraction columns, eluted with potassium hydroxide solution, separated by an anion-exchange column, and detected by a conductivity detector. Qualitative analysis is performed based on retention time, and quantitative analysis is performed using the external standard method.
3 Reagents and materials
Unless otherwise specified, guaranteed reagent and Class-I water (defined in GB/T 6682) are adopted for the purpose of this method.
3.1 Reagents
3.1.1 Sodium hydroxide (NaOH).
3.1.2 Potassium hydroxide (KOH).
3.1.3 Methanol (CH3OH): Chromatographically pure.
3.2 Preparation of reagents
3.2.1 Sodium hydroxide solution (10 mmol/L): Weigh 0.4 g of sodium hydroxide, dissolve it in water, and dilute to 1,000 mL.
3.2.2 Sodium hydroxide solution (50 mmol/L): Weigh 2.0 g of sodium hydroxide, dissolve it in water, and dilute to 1,000 mL.
3.2.3 Potassium hydroxide solution (2 mol/L): Weigh 112.0 g of potassium hydroxide, dissolve it in water, and dilute to 1,000 mL.
3.3 Standards
3.3.1 Sodium phosphate (Na3PO4, CAS No.: 7601-54-9): With a purity ≥98%, or a reference material certified by the state and granted a reference material certificate.
3.3.2 Sodium pyrophosphate (Na4P2O7, CAS No.: 7722-88-5): With a purity ≥98%, or a reference material certified by the state and granted a reference material certificate.
3.3.3 Sodium trimetaphosphate ((NaPO3)3, CAS No.: 7785-84-4): With a purity ≥98%, or a reference material certified by the state and granted a reference material certificate.
3.3.4 Sodium tripolyphosphate (Na5P3O10, CAS No.: 7758-29-4): With a purity ≥98%, or a reference material certified by the state and granted a reference material certificate.
3.4 Preparation of standard solutions
3.4.1 Phosphate standard stock solution (1,000 mg/L): Accurately weigh 0.173 g (to the nearest of 0.000 1 g) of sodium phosphate standard, dissolve in 10 mmol/L sodium hydroxide solution, and dilute to 100 mL. This solution contains 1.0 g of phosphate per 1 L, and has a shelf life of 6 months if stored in a sealed container at room temperature.
3.4.2 Pyrophosphate standard stock solution (1,000 mg/L): Accurately weigh 0.153 g of sodium pyrophosphate (to the nearest of 0.000 1 g), dissolve in 10 mmol/L sodium hydroxide solution, and dilute to 100 mL. This solution contains 1.0 g of pyrophosphate per 1 L, and has a shelf life of 6 months if stored in a sealed container at room temperature.
3.4.3 Trimetaphosphate standard stock solution (1,000 mg/L): Dry sodium trimetaphosphate in an oven at 103±2°C for 3 h, and cool it to room temperature in a desiccator; accurately weigh 0.132 g of sodium trimetaphosphate standard (to the nearest of 0.000 1 g), dissolve in 10 mmol/L sodium hydroxide solution, and dilute to 100 mL. This solution contains 1.0 g of trimetaphosphate per 1 L, and has a shelf life of 6 months if stored in a sealed container at room temperature.
3.4.4 Tripolyphosphate standard stock solution (1,000 mg/L): Dry sodium tripolyphosphate in an oven at 103±2°C for 3 h, and cool it to room temperature in a desiccator; accurately weigh 0.148 g of sodium tripolyphosphate standard (to the nearest of 0.000 1 g), dissolve in 10 mmol/L sodium hydroxide solution, and dilute to 100 mL. This solution contains 1.0 g of tripolyphosphate per 1 L, and has a shelf life of 6 months if stored in a sealed container at room temperature.
3.4.5 Mixed standard intermediate solution of phosphate, pyrophosphate, trimetaphosphate and tripolyphosphate (100 mg/L): Pipette 10.0 mL of each of the standard stock solutions of phosphate, pyrophosphate, trimetaphosphate and tripolyphosphate, and dilute to 100 mL with 10 mmol/L sodium hydroxide solution.
3.4.6 Mixed standard series working solutions: Pipette 0 mL, 0.03 mL, 0.05 mL, 0.10 mL, 0.50 mL, 1.00 mL, and 2.00 mL of mixed standard intermediate solutions respectively, and dilute to 10 mL with 10 mmol/L sodium hydroxide solution to obtain mixed standard series working solutions with mass concentrations of 0 mg/L, 0.30 mg/L, 0.50 mg/L, 1.00 mg/L, 5.00 mg/L, 10.0 mg/L, and 20.0 mg/L, respectively. Prepare it immediately before use.
3.5 Materials
3.5.1 0.45 μm hydrophilic membrane with syringe filter.
3.5.2 Purification column: Polydivinylbenzene polymer reversed phase column, Ag column and Na column, or equivalents.
3.5.3 Syringes: 1.0 mL and 5.0 mL.
Note: All glassware needs to be soaked in 2 mol/L potassium hydroxide solution and water successively for 4 h before use, rinsed with water 3 to 5 times, and dried for later use.
4 Instruments and apparatus
Contents Foreword I 1 Scope 2 Principle 3 Reagents and materials 4 Instruments and apparatus 5 Analytical procedures 6 Expression of analysis results 7 Precision 8 Limit of detection and limit of quantification 9 Principle 10 Reagents and materials 11 Instruments and apparatus 12 Analytical procedures 13 Expression of analysis results 14 Precision 15 Limit of detection and limit of quantification Annex A Ion chromatogram for standard solutions of four phosphates Annex B Conversion factors for polyphosphate Annex C Ion chromatogram of standard phosphate solution
