The level of nitrate and nitrite in food is one of the most important pollutants that affect food quality and jeopardize human health. Its content in food materials is mainly affected by the nitrogen content in the environment. The purpose of this experiment is mainly through the experiment to let students understand the different agricultural products in the nitrate and nitrite content of the situation, to master the determination of its methods and steps.
Operation method
Determination of nitrite in agricultural products
Principle
(i) Nitrite determination: After the sample is precipitated for protein and fat removal, nitrite is diazotized with p-aminobenzene sulfonic acid under weak acidic conditions, and then coupled with N-1-naphthylethylenediamine to form a purplish-red dye, which is quantified by comparing with the standard. Move I. Experimental apparatus and reagents For more product details, please visit Aladdin Scientific website.
(A) Instruments: small pulverizer; spectrophotometer; analytical balance
(B) reagents: the experimental water is distilled water, reagents without description, are analytically pure reagents.
1 ammonium chloride buffer: 1L volumetric flask add 500mL water, accurately add 20.0mL hydrochloric acid, shaking and mixing, accurately add 50mL ammonium hydroxide, dilute with water to the scale. If necessary, use dilute hydrochloric acid and dilute ammonium hydroxide debugging to pH9.6 ~ 9.7.
2 Zinc sulfate solution (0.42mol/L): weigh 120g of zinc sulfate (ZnSO4-7H2O), dissolve with water and dilute to 1000mL.
3 Sodium hydroxide solution (20g/L): weigh 20g sodium hydroxide, dissolve with water and dilute to 1L.
4 p-aminobenzenesulfonic acid solution: weigh 10g p-aminobenzenesulfonic acid, dissolve in 700mL water and 300mL glacial acetic acid, put in a brown bottle and mix well, keep at room temperature.
5 N-1-naphthylethylenediamine solution (1g/L): weigh 0.1g N-1-naphthylethylenediamine, add 60% acetic acid to dissolve and dilute to 100mL, mix well, placed in a brown bottle, stored in the refrigerator, stable within one week.
6 Color Developer: Mix equal volume of N-1-naphthylethylenediamine solution (1g/L) and p-aminobenzenesulfonic acid solution before use.
7 Sodium nitrite standard solution: accurately weigh 250.0mg of sodium nitrite dried in a silica gel desiccator for 24h, dissolved in water and transferred to a 500mL volumetric flask, add 100mL of ammonium chloride buffer, dilute with water to the scale, mix, and store at 4℃ away from light. This solution is equivalent to 500μg of sodium nitrite per milliliter.
8 Sodium nitrite standard solution: before use, absorb 1.00mL of sodium nitrite standard solution, put it in 100mL volumetric flask, dilute with water to the scale, each milliliter of this solution is equivalent to 5.0μg of sodium nitrite.
Experimental Methods and Steps
1 Sample treatment
Weighing about 10.00g (grain take 5g) by grinding and mixing the sample, placed in a pulverizer, add 70mL water and 12mL sodium hydroxide solution (20g / L), mix well, with sodium hydroxide solution (20g / L) to adjust the sample pH = 8, quantitatively transferred to a 200mL volumetric flask with 10mL zinc sulfate solution, mix well, such as does not produce a white precipitate, and then add 2 to 5mL sodium hydroxide, mix well. If no white precipitate, add 2-5mL of sodium hydroxide, mix well. Place in 60℃ water bath and heat for 10min, remove and cool to room temperature, add water to the scale and mix well. Place 0.5h, filter with filter paper, discard the initial filtrate 20mL, collect the filtrate standby.
2 Determination
2.1 Preparation of nitrite standard curve: suck 0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0mL sodium nitrite standard use solution (equivalent to 0, 2.5, 5, 10, 15, 20, 25μg of sodium nitrite), respectively, placed in a 25mL stoppered colorimeter.
Add 4.5mL ammonium chloride buffer to the standard tube, add 2.5mL 60% acetic acid, then add 5.0mL colorant, add water to the scale, mix well, and let it stand in the dark for 25min, then use a 1cm colorimetric cup (2cm colorimetric cup can be used for the low sensitivity), adjust the zero point with zero tube, and then measure absorbance at the wavelength of 550nm to draw the standard curve.
Low-content samples are calculated by preparing a low content standard curve, the standard series is: absorb 0, 0.4, 0.8, 1.2, 1.6, 2.0mL sodium nitrite standard use solution (equivalent to 0, 2, 4, 6, 8, 10μg of sodium nitrite).
2.2 Sample Determination: Pipette 10.0mL of the above filtrate (1.1) into a 25mL stoppered cuvette, and then add 4.5mL of ammonium chloride buffer to the standard tube as described in 2.1 "Add 4.5mL of ammonium chloride buffer to the standard tube". Do the reagent blank at the same time.
Calculation 