Food (1-10 lead) testing

1  Dithizone colorimetric method 2  Principle: After digestion of the sample, at a pH of 8.5-9.0, lead ions form a red complex with dithizone, which is soluble in trichloromethane. Add ammonium citrate, potassium cyanide, and hydroxylamine hydrochloride to prevent interference from iron, copper, and zinc plasma, and compare quantitatively with the standard series. 3  Reagents and Materials 3.1; Ammonia water (1+1). 3.2  Hydrochloric acid (1+1): Measure 100 mL of hydrochloric acid and add it to 100 mL of water. 3.3  Phenol red indicator solution (1 g/L): Weigh 0.10 g of phenol red; Dissolve in a small amount of ethanol multiple times and transfer to a 100 mL volumetric flask and bring to volume to the mark. 3.4  Hydroxylamine Hydrochloride Solution (200 g/L): Weigh 20.0 g of hydroxylamine hydrochloride, dissolve in water to 50 mL, add 2 drops of phenol red indicator solution, add ammonia water (1+1), adjust the pH to 8.5-9.0 (from yellow to red, add 2 more drops), extract with a dithizone trichloromethane solution until the green color of the trichloromethane layer remains unchanged, wash twice with trichloromethane, discard the trichloromethane layer, add hydrochloric acid (1+1) to the water layer until it becomes acidic, and add water to 100 mL. 3.5  Ammonium citrate solution (200 g/L): Weigh 50 g of ammonium citrate and dissolve it in 100 mL of water. Add 2 drops of phenol red indicator solution and ammonia water, adjust the pH to 8.5-9.0. Extract several times with a dithizone trichloromethane solution, 10 mL to 20 mL each time, until the trichloromethane layer remains green. Discard the trichloromethane layer and wash it twice with trichloromethane, 5 mL each time. Discard the trichloromethane layer and dilute it to 250 mL with water. 3.6  Potassium cyanide solution (100 g/L): Weigh 10.0 g of potassium cyanide, dissolve it in water, and dilute to 100 mL. 3.7  Trichloromethane: should not contain oxides. 3.7.1  Inspection method: Take 10 mL of chloroform, add 25 mL of freshly boiled water, shake for 3 minutes, let it stand for layering, take 10 mL of aqueous solution, add several drops of potassium iodide solution (150 g/L) and starch indicator solution, and shake until no blue color appears after shaking. 3.7.2  Treatment method: Add 1/10-1/20 volume of sodium thiosulfate solution (200 g/L) to trichloromethane for washing, then wash with water and add a small amount of anhydrous calcium chloride for dehydration before distillation. Discard the first and last tenths of the distillate, and collect the middle distillate for later use. 3.8  Starch indicator solution: Weigh 0.5 g of soluble starch, add 5 mL of water and stir well. Slowly pour into 100 mL of boiling water, stir while pouring, boil, let cool, and prepare when ready for use. 3.9  Nitric acid (1+99): Measure 1 mL of nitric acid and add it to 99 mL of water. 3.10 Dithizone trichloromethane solution (0.5 g/L): Store in a refrigerator and purify if necessary using the following method. Weigh 0.5 g of finely ground dithizone and dissolve it in 50 mL of chloroform. If it is not completely soluble, filter it with filter paper into a 250 mL separation funnel. Extract it three times with ammonia (1+99), with 100 mL each time. Filter the extract with cotton into a 500 mL separation funnel, adjust it to acidity with hydrochloric acid (1+1), and extract the precipitated dithizone 2-3 times with chloroform, with 20 mL each time. Combine the chloroform layer and wash it twice with equal amounts of water, Discard the washing solution and evaporate trichloromethane on a 50 ℃ water bath. The refined dithizone is placed in a sulfuric acid dryer and dried for later use. Alternatively, extract the precipitated dithizone three times with 200 mL, 200 mL, and 100 mL of chloroform, and merge the chloroform layer to form a dithizone solution. 3.11 Dithizone solution: Take 1.0 mL of dithizone solution, add chloroform to 10 mL, and mix well. Using a 1 cm colorimetric cup, adjust the zero point with chloroform, measure the absorbance (A) at a wavelength of 510 nm, and use equation (4) to calculate the number of milliliters (V) of dithizone solution required to prepare 100 mL of dithizone solution (70% transmittance). 3.12 Nitric acid sulfuric acid mixture (4+1). 313 lead standard solution (1.0 mg/mL): Accurately weigh 0.1598 g of lead nitrate, add 10 mL of nitric acid (1+99), dissolve all, transfer to a 100 mL volumetric flask, and dilute with water to the mark. 3.14 Lead standard solution (10.0 μ G/mL): Take 1.0 mL of lead standard solution and place it in a 100 mL volumetric flask. Dilute with water to the mark

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