AOAC Appendix A Appendix A_Standard Solutions and Reference Materials
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E4D2CD35912B422E818FBAD6D689FFCD |
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0.07 |
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8 |
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日期： |
2013-4-11 |
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Appendix A: Standard Solutions and Reference,Materials,A.1.01,AOAC Official Method 942.25,Standard Solutions and Materials,First Action 1942,Use accurately calibrated equipment, which meets NIST specifications.,Alkaline and other corrosive solutions dissolve glass. To,avoid volumetric errors, do not store such solutions in calibrated apparatus.,Burets used continuously with such solutions should be,recalibrated periodically.,Working temperature of standard solution should approximate,that of its temperature during standardization. If temperature corrections,are necessary, sufficient accuracy may be obtained by use of,Table 942.25.,Reference: JAOAC 25, 650(1942).,A.1.02,AOAC Official Method 942.26,Standard Solutions,of Ammonium and Potassium Thiocyanates,First Action 1942,Final Action,A. Reagents,(a) Purified silver nitrate.—Dissolve 50 g AgNO3 in 20 mL boiling,H2O containing ca 5 drops NHO3. Heat to dissolve, filter while,still hot through fritted glass filter, using suction, and collect filtrate,in clean borosilicate beaker. Wash beaker and filter with ca 5mL hot,H2O, adding washings to filtrate. Cool in ice bath, stirring to induce,crystallization, and place in refrigerator at ca 10°C until equilibrium,is reached. Decant liquid through fritted glass filter and transfer,crystals to filter. Cover filter with watch glass and draw air through,filter to remove adhering liquid. Transfer crystals to small, clean,borosilicate beaker. Cover beaker with watch glass and place inside,larger covered borosilicate beaker. Dry at 105EC and fuse at,220–250°C (mp 208EC), holding at this temperature ca 15 min after,crystals are melted. Protect from dust during preparation. Cool in,desiccator, remove product from beaker, powder in mortar, dry 0.5 h,at 105EC, and store in brown glass-stoppered bottle in dark over,good desiccant.,(b) Reference solution.—To mixture of 5mLHNO3 (1+1),2mL,ferric alum solution, 941.18D(a) (see A.1.11), and 115mLH2O, add,ca 0.02 mL 0.1M thiocyanate, B, noting exact volume used.,B. Preparation of Standard Solution,Prepare ca 0.1M solution from reagent that shows no chlorine using,7.612 g NH4SCN or 9.718 g KSCN/L.,C. Standardization,Accurately weigh, on tared watch glass, enough purified AgNO3 to,give titration of ca 40mL(ca 0.7 g for 0.1M solution) and transfer with,H2O through glass funnel to 250 mL glass-stoppered Erlenmeyer.,Dissolve in ca 75mLH2O(halogen-free), and add 5mLHNO3 (1 + 1),and 2 mL ferric alum solution, 941.18D(a) (see A.1.11). Titrate with,thiocyanate solution until titrated solution achieves a reddish brown,color, which remains after shaking vigorously 1 min. Record buret,reading and set flask aside 5 min, shaking occasionally and maintaining,end point color by addition of thiocyanate solution as required.,Then add additional thiocyanate solution, if necessary, to produce,permanent end point color, matching with color of reference solution,A(b). From total volume thiocyanate solution used in titration, subtract,volume contained in reference solution.,Molarity (mol/L) = g AgNO3 × 1000/mL titer × 169.87,References: JAOAC 25, 661(1942); 30, 105, 496(1947).,A.1.03,AOAC Official Method 939.12,Standard Solution of Arsenious Oxide,First Action 1939,Final Action,A. Reagent,Arsenious oxide.—Use NIST SRM 83. Dry 1 h at 105°C immediately,before using.,. 2000 AOAC INTERNATIONAL,Table 942.25 Temperature corrections for volumes of aqueous solutions,Volume,standard,solution, mL,Correction in mL at (°C),6 8 10 12 14 16 18 20 22 24 26 28 30,10 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 –0.01 –0.02 –0.02,20 0.03 0.03 0.03 0.02 0.02 0.01 0.01 0.00 –0.01 –0.02 –0.03 –0.03 –0.03,25 0.04 0.03 0.03 0.03 0.02 0.02 0.01 0.00 –0.01 –0.02 –0.03 –0.04 –0.05,30 0.04 0.04 0.04 0.03 0.03 0.02 0.01 0.00 –0.01 –0.02 –0.04 –0.05 –0.07,40 0.06 0.06 0.05 0.04 0.04 0.03 0.01 0.00 –0.02 –0.03 –0.05 –0.07 –0.09,50 0.07 0.07 0.06 0.06 0.05 0.03 0.02 0.00 –0.02 –0.04 –0.06 –0.09 –0.12,B. Preparation of Standard Solution,Accurately weigh As2O3 by difference from small,glass-stoppered weighing bottle (use ca 4.95 g/L for 0.025M). Dissolve,in 1M NaOH (50 mL/5 g As2O3) in flask or beaker by heating,on steam bath. Add ca same volume 0.5M H2SO4. Cool, quantitatively,transfer mixture to volumetric flask, and dilute to volume.,(Solution must be neutral to litmus, not alkaline.),Molarity (mol/L) = g As2O3 ′ 4000/mL final volume ′ 197.84,References: JAOAC 22, 568(1939); 24, 100, 639(1941).,A.1.04,AOAC Official Method 964.24,Buffer Solutions,for Calibration of pH Equipment,First Action 1964,Final Action 1965,Use H2O with pH of $6.5 but #7.5, obtained by boiling H2O,15 min and cooling underCO2-……
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