Standard curve for determining Kun content by silver salt method - Database & Sql Blog Articles

Determination of the standard curve of Kun content by silver salt method

Silver salt method to determine the standard curve of Kun content analysis instrument Keywords: food analysis; Kun; US instrument ; UV-1300 type one, silver salt method 1. Principle sample after digestion, with potassium iodide, chlorination Stannous reduces the high-priced arsenic to trivalent arsenic, and then forms new ecological hydrogen with zinc particles and acid. After absorption by silver salt solution, it forms a red colloid, which is colorimetric at 510 nm, which is quantitative compared with the standard series. The minimum detectable amount is 0.2 mg/kg. 2. Scope of application Standard method (GB/T5009.11-1996), suitable for the determination of total arsenic in various foods. 3. Reagents Unless otherwise specified, the reagents used are analytically pure reagents, and the water is distilled water or water of equivalent purity. (1) Nitric acid. (2) Sulfuric acid. (3) Hydrochloric acid. (4) Nitric acid + perchloric acid mixture (4+1): Measure 80 ml of nitric acid, add 20 ml of perchloric acid, and mix. (5) Magnesium nitrate solution (150g/L): Weigh 15g of magnesium nitrate〗 〖Mg(NO3)2·6H2O is dissolved in water and diluted to 100ml. (6) Magnesium oxide. (7) Potassium iodide solution (150 g/L): 15 g of potassium iodide was weighed and dissolved in water, and diluted to 100 ml, and stored in a brown bottle. (8) Acidic stannous chloride solution: Weigh 40.0 g of stannous chloride (SnCl 2 · 2H 2 O), dissolve it with hydrochloric acid and dilute to 100.0 ml, and add several metal tin particles. Stannous chloride (SnCl2), also known as tin dichloride, white or translucent crystals, with two molecules of crystal water (SnCl2 · 2H2O) is a colorless needle or flaky crystal, soluble in water, ethanol and ether. The stannous chloride reagent is unstable, oxidized to insoluble oxychloride in the air, and loses the reducing effect. In order to keep the reagent stable, the hydrochloric acid is dissolved into an acidic stannous chloride solution during the preparation, and the number is added. The metal tin particles are continuously reacted to form stannous chloride and new ecological hydrogen, so that the solution is reducible. The role of stannous chloride in this experiment is to reduce As5+ to As3+; depositing a tin layer on the surface of the zinc particles to suppress excessive hydrogen production. (9) Hydrochloric acid solution (1+1): Measure 50 ml of hydrochloric acid, carefully pour into 50 ml of water, and mix. (10) Lead acetate solution (100 g/L). (11) Lead acetate cotton: After impregnating the absorbent cotton with a 100 g/L lead acetate solution, the excess solution is removed, loosened, dried at 100 ° C or lower, and stored in a glass bottle. Lead acetate cotton is inserted into the airway tube to absorb the possible hydrogen sulfide, so that it forms lead sulfide and stays on the cotton, so as not to interfere with the absorption of the absorption liquid. Sulfide and silver ions produce gray-black silver sulfide, but acetic acid. It is not appropriate to have a lead cotton fortress that is not loose. (12) arsenic-free zinc particles. The arsenic-free zinc granules of different shapes and specifications have different reaction speeds with acids due to their different surface areas. The resulting hydrogen gas flow rate will directly affect the absorption efficiency and the measurement results. It is generally believed that good results can be obtained with 3 g of honeycomb zinc particles or 5 g of large particles of zinc. It is also believed that the mixed use of zinc particles of large and small particles is satisfactory. It is generally preferred to use zinc particles of the same specification for both the standard curve and the sample. (13) Sodium hydroxide solution (200 g/L). (14) Sulfuric acid solution (6+94): Measure 6.0 ml of sulfuric acid, carefully pour into 94 ml of water, and mix. (15) Silver diethyldithiocarbamate-triethanolamine-trichloromethane solution: Weigh 0.25 g of silver diethyldithiocarbamate (C2H5)2NCS2Ag in a mortar, add a small amount of chloroform Transfer to a 100 ml graduated cylinder, add 1.8 ml of triethanolamine, and then wash the chyle with chloroform in several portions. The washing solution was transferred to a graduated cylinder, diluted with chloroform to 100.0 ml, and left overnight. Filter into a brown bottle and store. Silver diethyldithiocarbamate, or diethyldithiocarbamic acid (Agsalt), (C2H5)2NC(S)SAg, molecular weight 256.15, yellow powder, insoluble in water and soluble in three Methyl chloride is extremely unstable in nature. When it is exposed to light or heat, it is easy to form silver oxide and is gray. Therefore, the concentration is not easy to control. If the commercial product is not applicable, the laboratory can also prepare it by itself. Preparation method of silver diethylaminodithiocarbamate: separately dissolve 1.7g of silver nitrate, 2.3g of sodium diethylaminodithioformate (DDCNa, copper reagent) in 100ml of distilled water, cool to below 20 ° C, slowly stir and mix, filter The resulting lemon yellow silver salt (AgDDC) was precipitated, and the precipitate was washed several times with cold distilled water, dried in a desiccator, and stored in the dark. The concentration of AgDDC in the absorption liquid is preferably 0.2% to 0.25%. If the concentration is too low, the sensitivity and reproducibility of the measurement will be affected. Therefore, when disposing the reagent, it should be left overnight or slightly heated in a water bath to help dissolve. Slight turbidity can be removed by filtration. If the solubility of the reagent is not good, it should be reconstituted and the absorbent must be clarified. (16) arsenic standard stock solution: accurately weigh 0.1320g arsenic trioxide dried in a sulfuric acid drier or dried at 100 ° C for 2h, add 5ml 200g / L sodium hydroxide solution, add 25ml sulfuric acid (6 + 94) solution after dissolution, and move in In a 1000 ml volumetric flask, dilute to the mark with freshly boiled water and store in a brown glass stopper. This solution is equivalent to 0.10 mg of arsenic per ml. (17) Arsenic standard use solution: Pipette 1.0 ml of arsenic standard solution, place it in a 100 ml volumetric flask, add 1 ml of sulfuric acid (6+94) solution, and dilute to the mark with water. This solution is equivalent to 1.0 μg of arsenic per ml. 4. Instrument (1) US-analyzed UV-1300 spectrophotometer. (2) Arsenic detection device 1100 ~ 150ml conical flask: No. 19 standard port. 2 air duct: the standard port of No. 19 of the nozzle or the rubber plug washed by alkali treatment should not leak when it is in close contact with the conical flask. The other end of the tube has a diameter of 1.0 mm. 3 absorption tube: 10 ml graduated centrifuge tube for absorption tube. 5. Method of operation 5.1 Sample digestion (1) Nitric acid-perchloric acid-sulfuric acid method A. Grain, vermicelli, vermicelli, dried bean products, cakes, tea, etc. and other solid foods with less water content: weigh 5.00g or 10.00g The pulverized sample is placed in a 250-500 ml nitrogen-fixing bottle. First, add a little water to make it moist. Add a few glass beads, 10-15 ml of a nitric acid-perchloric acid mixture, leave it for a while, and slowly heat it for a small time. Let it cool. Add 5ml or 10ml of sulfuric acid along the wall of the bottle and heat it. When the liquid in the bottle begins to turn brown, the nitric acid-perchloric acid mixture is continuously added along the wall of the bottle until the organic matter is completely decomposed. Increase the firepower to produce white smoke, the solution should be clear colorless or slightly yellowish, let cool. Care should be taken to prevent explosions during operation. Add 20 ml of water to boil, remove residual nitric acid until white fumes are produced, treat twice, and let cool. Transfer the cold solution into a 50ml or 100ml volumetric flask, wash the nitrogen fixed bottle with water, add the washing solution to the volumetric flask, let cool, add water to the mark, and mix. The solution after the constant volume is equivalent to 1 g of sample per 10 ml, and the amount of sulfuric acid is relatively added to 1 ml. The residual nitric acid in the sample digestive solution needs to be exhausted as usual. The presence of nitric acid affects the reaction and color development, which may result in low results. If necessary, increase the amount of sulfuric acid added for the measurement. Take the same amount of nitric acid-perchloric acid mixture and sulfuric acid as the digested sample, and perform the reagent blank test in the same way. B. Vegetables and fruits: Weigh 25.00g or 50.00g of washed and homogenized samples, place them in 250-500ml nitrogen fixed bottle, add several glass beads, 10~15ml nitric acid-perchloric acid mixture, below According to the grain and other samples from the "place a moment" from the law, but the volume of the solution after the volume is equivalent to 5g sample per 10ml, a considerable amount of sulfuric acid 1ml. C. Sauce, soy sauce, vinegar, cold drink, tofu, fermented bean curd, pickled vegetables, etc.: Weigh 10.00g or 20.00g sample (or take 10.00ml or 20.00ml liquid sample), put it in 250~500ml nitrogen fixed bottle, add A number of glass beads, 5 ~ 15ml nitric acid - perchloric acid mixture, the following according to the food and other samples from the "place a moment" from the law, but the volume of the solution after 10ml is equivalent to 2g sample or 2ml sample. D. Alcoholic beverages or carbonated beverages: Pipette 10.00ml or 20.00ml sample, place in 250~500ml nitrogen fixed bottle, add several glass beads, first remove heat with small heat to remove ethanol or carbon dioxide, then add 5~10ml After the nitric acid-perchloric acid mixture is mixed, the following operations are carried out according to the sample such as grain from "placement moment", but the solution after constant volume is equivalent to 2 ml sample per 10 ml. Pipette 5 to 10 ml of water instead of the sample, and add the same amount of nitric acid-perchloric acid mixture and sulfuric acid as the digestive solution. The reagent blank test was performed in the same manner. E. Foods with high sugar content: Weigh 5.00g or 10.00g of pulverized sample, place it in 250~500ml nitrogen fixed bottle, add a little water to make it moist, add several glass beads, 10~15ml nitric acid-perchloric acid After mixing, shake well. Slowly add 5ml or 10ml of sulfuric acid, wait for the effect to slow down and then foam, then increase the firepower, until the organic matter is completely decomposed, white smoke occurs, the solution should be clear colorless or slightly yellowish, let cool. The following procedures are carried out according to the law, such as grain and boiled by adding 20 ml of water. F. Aquatic products: Take a portion of the edible portion and knead it into a homogenate, weigh 5.00g or 10.00g (the seaweed algae, shellfish can be appropriately reduced), put it in a 250-500ml nitrogen fixed bottle, add a few glass beads After mixing 10 to 15 ml of nitric acid-perchloric acid, the following procedures are carried out according to the law, such as adding 5 ml or 10 ml of sulfuric acid along the wall of the bottle. (2) Nitric acid-sulfuric acid method: operation is carried out by replacing nitric acid-perchloric acid mixed solution with nitric acid. (3) Ashing method A. Food, tea and other foods with less water content: Weigh 5.00g of ground sample, put it in the pot, add 1g of magnesium oxide, 1ml of nickel chloride and 10ml of magnesium nitrate solution, mix, Soak for 4h. Evaporate to dryness in a low temperature or water bath. It is charcoalized to a smokeless state, then transferred to a muffle furnace and heated to 550 ° C, burned for 3 to 4 hours, and taken out after cooling. After adding 5 ml of water to wet the ash, stir with a fine glass rod, and then wash the ash attached to the glass rod with a small amount of water to the crucible. After being placed in a water bath, it was evaporated to dryness, then transferred to a high temperature furnace at 550 ° C for 2 hours, and then taken out after cooling. Add 5 ml of water to wet the ash, then slowly add 10 ml of hydrochloric acid solution (1+1), and then transfer the solution into a 50 ml volumetric flask.洗涤 Wash with hydrochloric acid solution (1+1) 3 times, each time 5ml, then wash 3 times with water, 5ml each time, the washing liquid is incorporated into the volumetric flask, add water to the mark and mix. The volume of the solution after constant volume is equivalent to 1 g of sample per 10 ml, which is equivalent to 1.5 ml of hydrochloric acid added (except for the neutralization requirement). When the full amount is measured by the silver salt method, it is not necessary to add hydrochloric acid. The same amount of magnesium oxide and magnesium nitrate solution were taken from the ashing sample, and the reagent blank test was performed according to the same operation method. B. Vegetable oil: Weigh 5.00g sample, put it in 50ml porcelain crucible, add 10g magnesium nitrate, cover 2g magnesium oxide on it, heat it on a small fire, just smoke, immediately remove it, to Prevent the contents from overflowing, and after the smoke is small, heat up until the carbonization is complete. Move the crucible to the muffle furnace, burn to ash at 550 ° C or less, and cool out. Add 5 ml of water to wet the ash, then slowly add 15 ml of hydrochloric acid solution (1+1), then transfer the solution into a 50 ml volumetric flask.洗涤 Wash 5 times with hydrochloric acid solution (1+1), 5 ml each time, wash the solution into a volumetric flask, add hydrochloric acid (1+1) to the mark, and mix. The volume of the solution after constant volume is equivalent to 1 g of sample per 10 ml, which is equivalent to 1.5 ml of hydrochloric acid added (except for the neutralization requirement). Take the same amount of magnesium oxide and magnesium nitrate from the digested sample and perform the reagent blank test according to the same operation method. C. Aquatic product: Take the edible part of the sample and homogenize it, weigh 5.00g into the mash, add 1g of magnesium oxide and 10ml of magnesium nitrate solution, mix and soak for 4h. In the following, according to the ashing method, the grain and other samples are operated according to the law from "drying on a low temperature or water bath". 5.2 Determination (1) Using a nitric acid-perchloric acid-sulfuric acid or nitric acid-sulfuric acid digestion solution, a certain amount of the digested constant volume solution (corresponding to 5 g sample) and the same amount of reagent blank solution were respectively placed in a 150 ml Erlenmeyer flask. In addition, sulfuric acid is added to a total amount of 5 ml, and water is added to 50 to 55 ml. Pipette 0.0, 2.0, 4.0, 6.0, 8.0, 10.0ml arsenic standard solution (equivalent to 0, 2, 4, 6, 8, 10μg arsenic) in 150ml Erlenmeyer flask, add water to 40ml, add 10ml sulfuric acid (1+1). Add 3ml of 150g/L potassium iodide solution, 0.5ml acid stannous chloride solution to the sample digestive solution, reagent blank and arsenic standard solution, mix and let stand for 15min. Add 3g of arsenic-free zinc particles, immediately plug the air tube containing lead acetate cotton, and insert the tip of the tube into the liquid surface of the centrifuge tube containing 4ml of silver salt solution. After reacting at room temperature for 45min, remove the centrifuge. Tube, add chloroform to make up 4ml. Using a 1 cm cuvette, adjust the zero point with a zero tube, measure the absorbance at a wavelength of 520 nm, and compare the standard curve. Occurrence and absorption should be prevented from being carried out under direct sunlight. At the same time, the temperature should be controlled at about 25 °C. If the temperature is too high, the reaction will be fast and the absorption will be incomplete. If the temperature is too low, the reaction time will be prolonged. The action time is preferably 1 h, and the summer time can be shortened to 45 min. The chloroform was partially volatilized at room temperature, and 4 ml was added with chloroform before colorimetry, which did not affect the results. When the absorption liquid contains moisture, when the temperature of the absorption and colorimetric environment changes, it will cause slight turbidity, and it can be slightly warmed to make it clear when colorimetric. (2) Using the ashing method, the ashing solution and the reagent blank were taken in a 150 ml Erlenmeyer flask. Pipette 0.0, 2.0, 4.0, 6.0, 8.0, 10.0ml arsenic standard solution (equivalent to 0, 2, 4, 6, 8, 10μg arsenic) in a 150ml Erlenmeyer flask, add water to 43.5ml, plus 6.5 Ml hydrochloric acid. The following is operated according to law according to 1 from "digested in sample". 6. Calculation (A1-A2)×1000X=--------------------------------------M ×V2/V1×1000 type: content of arsenic in X-sample, mg/kg or mg/L; content of arsenic in digestive solution of A1-measurement sample, μg; content of arsenic in blank of A2-reagent, μg M-sample mass (volume), g (ml); V1 - total volume of sample digestive juice, ml; V2 - volume of sample digestive solution for measurement, ml. Key words: food analysis; Kun; US instrumentation ; UV-1300