Determination of calcium in food

1. Purpose requirements

Master the principle and method of using EDTA titration to determine calcium content in food.

2. Principles of Practical Training

Calcium and amino carboxyl complexing agents can quantitatively form metal complexes, and their stability is stronger than the complexes formed by calcium and indicators. Within an appropriate pH range, titrate with ammonium carboxyl complexing agent EDTA. When the equivalence point is reached, EDTA extracts calcium ions from the indicator complex, resulting in the solution presenting the color of a free indicator (endpoint). The calcium content can be calculated based on the amount of EDTA complexing agent used.

3. Training supplies

(1) Commonly used laboratory glassware: high beaker (250mL), micro burette (1mL or 2mL), alkaline burette (50mL), graduated pipette (0.5-1mL), etc.

(2) Electric heating plate: 1000-3000W.

(3) 1.25mol/L KOH solution: Accurately weigh 70.13g of KOH and dilute with water to 1000mL.

(4) 10g/1 sodium cyanide solution: Weigh 1.0g of sodium cyanide and dilute with water to 100mL.

(5) 0.05mol/L sodium citrate solution: Weigh 14.7g of sodium citrate (Na3C6H5O4 · 2H2O) and dilute with water to 1000mL.

(6) Mixed acid digestion solution: nitric acid+perchloric acid=4+1.

(7) 20g/L lanthanum oxide solution: Weigh 23.45g of lanthanum oxide (purity greater than 99.99%), moisten with a small amount of water, add 75mL hydrochloric acid to a 1000mL volumetric flask, and dilute with deionized water to the mark.

(8) EDTA solution: Accurately weigh 4.50g of EDTA (disodium ethylenediaminetetraacetate), dilute with water to 1000mL, store in a polyethylene bottle, and store at 4 ℃. Dilute 10 times when using.

(9) Calcium standard solution: Accurately weigh 0.1248g of calcium carbonate (purity greater than 99.99%, dried at 105-110 ℃ for 2 hours), dissolve in 20mL of water and 3mL of 0.5mol/L hydrochloric acid, transfer to a 500ml volumetric flask, dilute with water to the mark, store in a polyethylene bottle, and store at 4 ℃. Every 1mL of this solution is equivalent to 100ug of calcium.

(10) Calcium Red Indicator: Weigh 0.1g of Calcium Red Indicator (C21O7N2SH14), dilute with water to 100mL, and dissolve before use. It can be stored in the refrigerator for more than a month and a half.

Note: All glass instruments are soaked in sulfuric acid potassium bicarbonate solution for several hours, then washed with laundry detergent, rinsed repeatedly with water, and finally rinsed with deionized water. They can only be used after natural drying or drying.