Hydrogen peroxide solution— Dilute 30 percent hydrogen peroxide with water to obtain a 3% solution. Just before use, add 3 drops of methyl red TS, and neutralize to a yellow endpoint with 0.01 N sodium hydroxide. Do not exceed the endpoint.

Nitrogen— Use high-purity nitrogen, with a flow regulator that will maintain a flow of 200 ± 10 mL per minute. Guard against the presence of oxygen by passing the nitrogen through a scrubber, such as alkaline pyrogallol, prepared as follows. Add 4.5 g of pyrogallol to a gas-washing bottle, purge the bottle with nitrogen for 3 minutes, and add a solution containing 85 mL of water and 65 g of potassium hydroxide, while maintaining an atmosphere of nitrogen in the bottle.

Apparatus— The apparatus (see Fig. 1) is designed to effect the selective transfer of sulfur dioxide from the specimen in boiling aqueous hydrochloric acid to the Hydrogen peroxide solution. The backpressure is limited to the unavoidable pressure due to the height of the Hydrogen peroxide solution above the tip of the bubbler, F. Keeping the backpressure as low as possible reduces the likelihood that sulfur dioxide will be lost through leaks. Preboil vinyl and silicone tubing. Apply a thin film of stopcock grease to the sealing surfaces of all of the joints except the joint between the separatory funnel and the flask, and clamp the joints to ensure tightness. The separatory funnel, B, has a capacity of 100 mL or greater. The inlet adapter, A, with a hose connector provides a means of applying headpressure over the solution. [NOTE—A pressure-equalizing dropping funnel is not recommended because condensate, which may contain sulfur dioxide, is deposited in the funnel and the side arm.]

Procedure— Position the Apparatus in a heating mantle controlled by a power-regulating device. Add 400 mL of water to the flask. Close the stopcock of the separatory funnel, and add 90 mL of 4 N hydrochloric acid to the separatory funnel. Begin the flow of nitrogen at a rate of 200 ± 10 mL per minute. Start the condenser coolant flow. Add 30 mL of Hydrogen peroxide solution to vessel G. After 15 minutes, remove the separatory funnel, and transfer a mixture of 50.0 g of Maltodextrin, accurately weighed, and 100 mL of alcohol solution (5 in 100). Apply stopcock grease to the outer joint of the separatory funnel, return the separatory funnel to the tapered joint flask, and concomitantly resume the nitrogen flow. Apply headpressure above the hydrochloric acid solution in the separatory funnel with a rubber bulb equipped with a valve. Open the stopcock of the separatory funnel to permit the hydrochloric acid solution to flow into the flask. Continue to maintain sufficient pressure above the hydrochloric acid solution to force it into the flask. [NOTE—The stopcock may be temporarily closed, if necessary, to pump up the pressure.] To guard against escape of sulfur dioxide into the separatory funnel, close the stopcock before the last few mL of hydrochloric acid drain out. Apply power to the heating mantle sufficient to cause about 85 drops of reflux per minute. After refluxing for 1.75 hours, remove vessel G, add 3 drops of methyl red TS, and titrate the contents with 0.01 N sodium hydroxide VS, using a 10-mL buret with an overflow tube and a hose connection to a carbon dioxide-absorbing tube, to a yellow endpoint that persists for at least 20 seconds. Perform a blank determination, and make any necessary correction (see Titrimetry 541). Calculate the quantity, in µg, of SO2 in each g of the Maltodextrin taken by the formula: in which 32.03 is the milliequivalent weight of sulfur dioxide; V is the volume, in mL, of titrant consumed; N is the normality of the titrant; and W is the weight, in g, of Maltodextrin taken. Not more than 40 µg of sulfur dioxide per g of Maltodextrin are found (.004%).

Standard solution— Dissolve an accurately weighed quantity of USP Dextrose RS in water, and dilute quantitatively with water to obtain a solution having a known concentration of about 10 mg per mL.

Test solution— Transfer about 5 g of Maltodextrin, accurately weighed, with the aid of hot water to a 100-mL volumetric flask, cool, add water to volume, and mix.

Procedure— Transfer 25.0-mL portions of alkaline cupric tartrate TS to each of two boiling flasks. Bring the contents of one flask to boiling within about 2 minutes while titrating with Standard solution to within 0.5 mL of the anticipated endpoint. Boil gently for 2 minutes. Continue to boil gently, add 2 drops of methylene blue solution (1 in 100), and complete the titration within 1 minute by adding the Standard solution dropwise or in small increments until the blue color disappears, determined by viewing against a white background in daylight or under equivalent illumination. If more than 0.5 mL of the titrant was required after the addition of the indicator, repeat the titration, adding the necessary volume of titrant before adding the indicator. Bring the contents of the second flask to boiling, and similarly titrate with Test solution. Calculate the Dextrose equivalent, on the dried basis, taken by the formula: in which A is the percentage Loss on drying of the Maltodextrin taken; CU is the concentration, in mg per mL, of Maltodextrin in the Test solution; CS is the concentration, in mg per mL, of USP Dextrose RS in the Standard solution; and VU and VS are the titrated volumes, in mL, of the Test solution and the Standard solution, respectively. The Dextrose equivalent is less than 20. [NOTE—This is a limit test. For Maltodextrins with lower reducing values, other procedures may give other results.]

(Official January 1, 2007)