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• Angie Cerinza Acosta

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44.1.16 AOAC Official Surplus Method 906.03i i Invert Sugar in Sugars and Sirups

References:

J. Am. Chem. Soc. 28, 663(1906); 26, 541(1907). J. Res. Natl. Bur. Standards 24, 589(1940).

Munson-Walker General Method

Determination of Reduced Copper

Final Action Surplus 1989

By Direct Weighing of Cuprous Oxide 31.039

31.037 Reagents Asbestos.—Digest asbestos, amphibole variety (Caution: See Appendix B: Asbestos), with HC1 (1 + 3) 2–3 days. Wash acid-free, digest similar period with 10% NaOH soln, and then treat few hr with hot alk. tartrate soln, 923.09(A)(b)(alk. tartrate solns that have stood for some time may be used for this purpose). Wash asbestos alkali-free; digest several hr with HNO3 (1 + 3); and after washing acid-free, shake with H2O into fine pulp. In prepg gooch, make film of asbestos 6 mm thick and wash thoroly with H2O to remove fine particles. If pptd Cu2O is to be weighed as such, wash crucible with 10 mL alcohol and then with 10 mL ether; dry 30 min at 100E, cool in desiccator, and weigh. Other reagents and solns used are described in 923.09(A). Solns may be clarified by neut. Pb(OAc)2 soln, 925.46(B)(d)(never basic Pb(OAc)2). Remove excess Pb with dry Na oxalate. 31.038 Precipitation of Cuprous Oxide Transfer 25 mL each of CuSO4 and alk. tartrate solns to 400 mL beaker of alkali-resistant glass and add 50 mL reducing sugar soln; or if smaller vol. of sugar soln is used, add H2O to make final vol. 100 mL. Heat beaker on asbestos gauze over Bunsen burner, regulate flame so that boiling begins in 4 min, and continue boiling exactly 2 min. (It is important that these directions be strictly observed. To regulate burner for this purpose it is advisable to make preliminary tests, using 50 mL reagent and 50 mL H2O, before proceeding with actual detn. Elec. heater may be used instead of burner.) Keep beaker covered with watch glass during heating. Filter hot soln at once thru asbestos mat in porcelain gooch, using suction. Wash ppt of Cu2O thoroly with H2O at ca 60E and either weigh directly as CuO, 929.09i i (See 44.1.17), or det. amt of reduced Cu by one of methods described in 929.09i i (See 44.1.17). Conduct blank detn, using 50 mL reagent and 50 mL H2O, and if wt Cu 2O obtained is >0.5 mg, correct result of reducing sugar detn accordingly. Alk. tartrate soln deteriorates on standing, and wt Cu2O obtained in blank increases.

Determination

(Use only for detns in solns of reducing sugars of comparatively high purity. In products contg large amts of mineral or org. impurities, including sucrose, det. Cu in the Cu2O by one of methods described in 929.09i i, since the Cu2O is likely to be contaminated with foreign matter.) Prep. gooch as in 929.09i i. Collect pptd Cu2O on matt as in 929.09i; wash thoroly with hot H2O, then with 10 mL alcohol, and finally with 10 mL ether. Dry ppt 30 min in over at 100E, cool, and weigh. Obtain wt invert sugar equiv. to wt Cu2O from 940.39. Number of mg Cu2O reduced by given amt of reducing sugar varies, depending upon whether or not sucrose is present. In table, absence of sucrose is assumed except in entries under invert sugar and lactose, where, in addn to columns for these alone, columns are given for their mixts with sucrose in specified ratios in the case of lactose adn in specified amts of total sugar in 50 mL soln in the case of invert sugar. (Later Hammon table has replaced original MunsonWalker table, where applicable.) By Titration with Sodium Thiosulfate 31.040

Reagent

Thiosulfate std soln.—Prep. soln contg 39 g Na2S2O3.5H2O/L. Accurately weigh 0.2-0.4 g pure electrolytic Cu and transfer to 250 mL erlenmeyer roughly marked at 20 mL intervals. Dissolve Cu in 5 mL HNO3 (1+1), dil. to 20 or 30 mL, boil to expel red fumes, add slight excess ssatd Br-H2O, and boil until Br is completely removed. Cool, and add 10 mL NaOAc soln (574 g trihydrate/L). Prep. 42 g/100 mL KI soln made very slightly alk. to avoid formation and oxidn of HI. Add 10 mL of the KI soln and titr. with Na2S2O3 soln to light yellow. Add enough starch indicator, 922.03(f), to produce marked blue. As end point nears, add 2 g KSCN and stir until completely dissolved. Continue titrn until ppt is perfectly white. 1 mL Na2S2O2 soln - ca 10 mg Cu. It is essential for Na2S2O2 titrn that concn of KI in soln be carefully regulated. If soln contains