二氧化硫检测方法Word格式文档下载.docx

1、 as described in EN 1988-1 （1998） or AOAC 990.28 （2000） and with a dual bubble-through system （see Figure）. 2. 1000 mL round-bottom flask with three 29/32 （or 24/40） tapered joints （vertical arms） and appropriate heating mantle. 3. 250 mL dropping funnel with stopcock and tapered joint to fit round

2、bottom flask. 4. 25 mL buret. 5. Graduated cylinders of 25, 50, 100 and 500 mL. 6. 1 L volumetric flask. 7. 3 mL graduated pipet. . Cryostat set at - 5?C. 89. pH meter. B. Reagents 1. Deionized water. 2. 0.01 N NaOH solution. 3. Indicator: Dissolve 250 mg of methyl red in 100 mL of ethanol （analytic

3、al grade）. This indicator is red for pH 6.2. 4. 30% w/w or 3% w/w hydrogen peroxide HO solution, analytical grade. 225. 37% concentrated hydrochloric acid HCl for analysis. 6. 85% phosphoric acid HPO for analysis. 34Page 1 of 6 October 2010 7. 85% potassium hydroxide KOH in pellets for analysis. 8.

4、Potassium dihydrogen phosphate KHPO of purity 99% for analysis. 249. Toluene, analytical grade. 10. Cryostat circulating fluid: mixture of monopropylene glycol/water （2/1）. 11. Nitrogen flow with precision valve allowing a flow rate of about 200 mL/minute. 12. Buffer solution Solution A （acid part）:

5、 0.59 M HPO and 0.96 M HCl solution. Introduce approximately 34200 mL of water into a 1 L volumetric flask, carefully add 40 mL of 85% HPO 34followed by 80 mL of 37% HCl with, respectively, a 50 and 100 mL graduated cylinder and, finally, dilute to volume with water. Solution B base part: 0.25 M KHP

6、O and 0.258 M KOH solution. Introduce 34 g 24KHPO and 17 g KOH into a 1 L volumetric flask and dilute to volume with water. 24Check that the buffer components are properly made by mixing solutions A and solution B in a 1:4 ratio （e.g. 15 mL of solution A and 60 mL of solution B）. The pH must range f

7、rom 2.3 to 2.5. Deoxygenate these 2 solutions by bubbling nitrogen through them for 15 min, less than 1 h before use. 13. Hydrogen peroxide （3%, HO） solution. Into a 50-mL graduated cylinder, add 3 mL of 2230% HO solution with a graduated pipet and water to give a volume of 30 mL. 22Alternatively, o

8、ne may purchase and use 30 mL of 3% HO solution directly for this 22purpose. Add 3 drops of methyl red indicator using a Pasteur pipet. With stirring, add 1 to 5 drops of 0.01 N NaOH and until the color of the indicator just turns yellow. This solution should be prepared fresh daily. C. Preparation

9、of Sample Dried allium powders can be tested for evidence of added sulfite without preparation but quantitative precision will be improved if powders are finely ground to improve homogeneity （see Statistics section）. D. Procedure Part A. Collection of sulfite from the sample. 1. Start circulating th

10、e coolant in the condenser, set the cryostat to -5?C and wait for approximately 30 minutes until this temperature is reached. （Note 1） Page 2 of 6 October 2010 2. Weigh approximately 25 g of test sample, to the nearest 0.1 g, into a dry round-bottom flask. （Note 2） 3. Place the round-bottom flask in

11、 the heating mantle. Connect the dropping funnel （stopcock closed）, the condenser and the nitrogen inlet to the round-bottom flask and the chain of 2 bubblers to the upper condenser outlet as shown in the Figure. 4. Add 25 mL toluene into the first bubbler and place the 50 mL graduated cylinder with

12、 3% HO solution into position as the second bubbler （see Figure）. 225. Add 120 mL of solution A via the dropping funnel with a 250 mL graduated cylinder. If the system is sealed properly, gas should begin to flow through the bubblers as liquid enters the round bottom flask. 6. Lower the height of th

13、e second bubbler cylinder to improve the flow and to ensure that no HO solution goes up in the bubbler containing toluene. The tip of the bubbler tube 22should be submerged at least 1 cm below the surface of the HO solution. 227. Close the dropping funnel tap when a few milliliters of solution A remain in the funnel and raise the second bubbler cylinder such that the tip of the second bubbler