Food (1.2 Lead) Testing

The use of lead containing pesticides in the production of food raw materials, the use of lead containing tin plated pipes, utensils, and containers in food raw material processing, and the use of lead containing pigments in ceramic edible materials can directly or indirectly cause food pollution, resulting in a certain amount of lead in the food. Due to the accumulation of harmful elements such as lead, frequent consumption of lead containing foods can lead to chronic poisoning

1. Principle

After sample reduction, at a pH of 8.5-9.0, lead ions form a red complex with dithizone, which is soluble in trichloromethane. Ammonium citrate, potassium chloride, and hydroxylamine hydrochloride are added to prevent interference from copper, iron, and zinc plasma, and compared quantitatively with the standard series.

Secondly, reagents

(1) Ammonia water (1+1)

(2) Hydrochloric acid (1+1): Measure 100mL of hydrochloric acid and add it to 100mL of water

(3) 1g/L phenol red indicator solution: Weigh 0.10g phenol red, dissolve it in a small amount of ethanol multiple times, and transfer it to a 100mL volumetric flask to volume

(4) 100g/L potassium chloride solution: Weigh 10.0g of potassium cyanide, dissolve it in water, and dilute to a constant volume of 100mL

(5) Trichloromethane: should not contain chlorides

(6) Nitric acid (1+99): Take 1mL of nitric acid and add 99mL of water

(7) Nitric acid sulfuric acid mixture (4+1)

(8) 200g/L hydroxylamine hydrochloride solution: Weigh 20g of hydroxylamine hydrochloride, dissolve it in water to 50mL, 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 trichloromethane layer remains green, then wash twice with trichloromethane, discard the trichloromethane layer, add hydrochloric acid (1+1) to the water layer, and add water to 100mL

(9) 200g/L ammonium citrate solution: Weigh 50g of ammonium citrate and dissolve it in 100mL of water. Add 2 drops of phenol red indicator solution, add hydrogen water (1+1), adjust the pH to 8.5-9.0, extract several times with a dithizone trichloromethane solution, 10-20mL each time, until the green color of the trichloromethane layer remains unchanged. Discard the trichloromethane layer, wash twice with trichloromethane, 5mL each time, discard the trichloromethane layer, and dilute with water to 250mL

(10) Starch indicator solution: Weigh 0.5g of soluble starch, add 5mL of water and shake well. Slowly pour into 100mL of boiling water, stir while pouring, boil, and let cool for later use. Prepare during temporary use

(11) Dithizone trichloromethane solution 0.5g/L: Weigh 0.5g of refined dithizone and dissolve it in 50mL of trichloromethane. If it is not completely soluble, filter it with filter paper in a 250mL separation funnel. Extract three times with ammonia (1+1), each time 100mL. Filter the extraction solution with cotton into a 500mL separation funnel, adjust it to acidity with hydrochloric acid (1+1), extract the precipitated dithizone with trichloromethane, and finally dilute it to 1L with trichloromethane. Store in the refrigerator

(12) Dithizone solution: Take 1.0mL of dithizone solution, add chloroform to 10mL, and mix well. Using a 1cm colorimetric cup, adjust the zero point with chloroform, measure the absorbance (A) at a wavelength of 510nm, and use the following formula to calculate the volume (V) of the dithizone solution required to prepare 100mL of dithizone solution (70% transmittance):

(13) Lead standard solution: Accurately weigh 0.1598g of lead nitrate, add 10mL of nitric acid (1+99), dissolve all, transfer to a 100mL volumetric flask, dilute with water to the mark, and each milliliter of this solution is equivalent to 1.0mg of lead

(14) Lead standard solution: Take 1.0mL of lead standard solution and place it in a 100mL volumetric flask. Dilute with water to the mark. Each milliliter of this solution is equivalent to 10.0 μ G Lead

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