【资料】标准溶液和参考物质

A.1.02 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 942.26
Standard Solutions
of Ammonium and Potassium Thiocyanates

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 5 mL 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 105°C and fuse at 220–250°C (mp 208°C), 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 105°C, and store in brown glass-stoppered bottle in dark over good desiccant.
(b) Reference solution.—To mixture of 5 mL HNO3 (1 + 1), 2 mL ferric alum solution, 941.18D(a) (see A.1.11), and 115 mL H2O, 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 40 mL (ca 0.7 g for 0.1M solution) and transfer with H2O through glass funnel to 250 mL glass-stoppered Erlenmeyer. Dissolve in ca 75 mL H2O (halogen-free), and add 5 mL HNO3 (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

A.1.03 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 939.12
Standard Solution of Arsenious Oxide

A. Reagent
Arsenious oxide.—Use NIST SRM 83. Dry 1 h at 105°C immediately before using.
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

A.1.04 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 964.24
Buffer Solutions
for Calibration of pH Equipment

Use H2O with pH of 6.5 but 7.5, obtained by boiling H2O 15 min and cooling under CO2-free conditions. Store standard buffer solutions except Ca(OH)2 in bottles of chemically resistant glass. Protect phosphate, borax, and Ca(OH)2 buffers from CO2. pH values as function of temperature are given in Table 964.24.
(a) Potassium tetroxalate buffer solution.—0.0496M; 0.05m. Transfer 12.61 g KHC2O4·H2C2O4·2H2O (air weight) (NIST SRM 189a) to 1 L volumetric flask, dilute to volume with H2O, and mix thoroughly. (It is not necessary to remove dissolved CO2 from the H2O or to dry salt before weighing.) Prepare fresh every 2 months.
(b) Potassium hydrogen tartrate buffer solution.—Saturated solution at 25°C, 0.034M. Add excess (ca 100%) of KHC4H4O6 (NIST SRM 188) to H2O in glass-stoppered bottle or flask, and shake vigorously; few minutes shaking is enough for saturation (100 mL H2O at 25°C dissolves ca 0.7 g KHC4H4O6). Adjust to 25°C, let solid settle, and decant clear solution, or filter if necessary. Discard when mold appears. Few crystals of thymol added during preparation will retard mold growth, and will alter pH by unit. For accuracy of ±0.01 pH unit, temperature of solution at saturation must be between 20 and 30°C.
(c) Acid potassium phthalate buffer solution.—0.0496M; 0.05m. Dissolve 10.12 g dried (2 h at 110°C) KHC8H4O4 (NIST SRM 185g) in H2O and dilute to 1 L. (Elaborate precautions for exclusion of atmospheric CO2 are unnecessary, although solution should be protected against evaporation and contamination with molds. Replace solution if mold appears.)
(d) Phosphate buffer solution.—0.0249M; 0.025m. Dissolve 3.387 g KH2PO4 and 3.533 g Na2HPO4 (NIST SRM 186e-I and II) in H2O and dilute to 1 L. (Dry salts 2 h at 110–130°C before use.)
(e) Phosphate buffer solution.—0.008663M, 0.008695m KH2PO4 and 0.03030M, 0.03043m Na2HPO4. Dissolve 1.179 g KH2PO4 and 4.303 g Na2HPO4 (NIST SRM 186e-I and II) in H2O and dilute to 1 L. (Dry salts 2 h at 110–130°C before use.)
(f) Borax buffer solution.—0.00996M; 0.01m. Dissolve 3.80 g Na2B4O7·10H2O (NIST SRM 187c) in H2O and dilute to 1 L. (Salt must not be dried in oven before use.) To avoid contamination with CO2, stopper bottle except when in use or protect with soda–lime tube. Use buffer solution within 10 min after removal from bottle.
(g) Sodium bicarbonate–carbonate buffer solution.—0.0249M; 0.025m (each). Transfer 2.092 g NaHCO3 (NIST SRM 191a; do not heat) and 2.640 g Na2CO3 (NIST SRM 192a; dry 2 h at 275°C) to 1 L volumetric flask. Dissolve and dilute to volume with CO2-free H2O.
(h) Calcium hydroxide buffer solution.—Saturated solution at 25°C, 0.02025M. Slowly heat finely granular CaCO3, low in alkalies, to 1000°C in Pt dish and maintain at this temperature 45–60 min. Cool in desiccator, and add to H2O with stirring. Heat to bp with continuous stirring. Cool, and filter on medium fritted glass filter. Dry at 110°C, cool, and crush to fine, granular powder.
Place crushed CaO in polyethylene bottle, add H2O, shake vigorously, let settle, and record temperature. [Keep large excess of Ca(OH)2 in bottle.] For use, filter solution through medium fritted glass filter. Use at same temperature at which saturation took place, and discard filtered solution if it becomes turbid. When more buffer solution is needed, add additional H2O to suspension, resaturate, and filter as above.
Table 964.24: pH Values for standard buffer solutions as function of temperature
Temp., °C    0.05mPotassiumtetroxalate, pH    Satd. Potassiumhydrogentartrate, pH    0.05mPotassiumphthalate, pH    0.025mPhosphate, pH    0.008695mand 0.03043mPhosphate, pH    0.01mBorax,pH    0.025m NaHCO3and 0.025m Na2CO3, pH    Satd. calciumhydroxide, pH
0    1.666    —    4.003    6.982    7.534    9.460    10.321    13.423
5    1.668    —    3.998    6.949    7.501    9.392    10.248    13.207
10    1.670    —    3.996    6.921    7.472    9.331    10.181    13.003
15    1.672    —    3.996    6.898    7.449    9.276    10.120    12.810
20    1.675    —    3.999    6.878    7.430    9.227    10.064    12.627
25    1.679    3.557    4.004    6.863    7.415    9.183    10.014    12.454
30    1.683    3.552    4.011    6.851    7.403    9.143    9.968    12.289
35    1.688    3.549    4.020    6.842    7.394    9.107    9.928    12.133
37    1.691    3.548    4.024    6.839    7.392    9.093    —    12.043
40    1.694    3.547    4.030    6.836    7.388    9.074    9.891    11.984
45    1.700    3.547    4.042    6.832    7.385    9.044    9.859    11.841
50    1.707    3.549    4.055    6.831    7.384    9.017    9.831    11.705
55    1.715    3.554    4.070    —    —    —    —    11.574
60    1.723    3.560    4.085    —    —    —    —    11.449

A.1.05 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 941.17
Standard Buffers and Indicators
for Colorimetric pH Comparisons

A. Preparation of Sulfonphthalein Indicators
X = mL 0.01M NaOH/0.1 g indicator required to form monosodium salt. Dilute to 250 mL for 0.04% reagent. (See Table 941.17A.)
B. Preparation of Stock Solutions
Use recently boiled and cooled H2O.
(a) Acid potassium phthalate solution.—0.2M. Dry to constant weight at 110–1 15°C. Dissolve 40.836 g in H2O and dilute to 1 L.
(b) Monopotassium phosphate solution.—0.2M. Dry KH2PO4 to constant weight at 110–115°C. Dissolve 27.232 g in H2O and dilute to 1 L. Solution should be distinctly red with methyl red, and distinctly blue with bromophenol blue.
(c) Boric acid–potassium chloride solution.—0.2M. Dry H3BO3 to constant weight in desiccator over CaCl2. Dry KCl2 days in oven at 115–120°C. Dissolve 12.405 g H3BO3 and 14.912 g KCl in H2O, and dilute to 1 L.
(d) Sodium hydroxide standard solution.—0.2M. Prepare and standardize as in 936.16 (see A.1.12); 0.04084 g KHC8H4O4 = 1 mL 0.2M NaOH. It is preferable to use factor with solution rather than try to adjust to exactly 0.2M.
C. Preparation of Buffer Solutions
Prepare standard buffer solutions from designated amounts stock solutions, 941.17, and dilute to 200 mL. For use as calorimetric standard, mix 20 mL buffer solution with 0.5 mL indicator solution, A. (See Table 941.17B.)
Table 941.17A: Amounts of specific indicators required to form monosodium salts
    X    pH
Bromocresol green    14.3    3.8–5.4
Chlorophenol red    23.6    4.8–6.4
Bromothymol blue    16.0    6.0–7.6
Phenol red    28.2    6.8–8.4


Table 941.17B: Amounts of buffer solutions and base required to produce given pH solutions
Phthalate–NaOH mixtures
pH    0.2M Potassiumhydrogen phthalate (mL)    0.2M NaOH (mL)
5.0    50    23.65
5.2    50    29.75
5.4    50    35.25
5.6    50    39.70
5.8    50    43.10
6.0    50    45.40
6.2    50    47.00
KH2PO4–NaOH mixtures
pH    0.2M KH2PO4 (mL)    0.2M NaOH (mL)
5.8    50    3.66
6.0    50    5.64
6.2    50    8.55
6.4    50    12.60
6.6    50    17.74
6.8    50    23.60
7.0    50    29.54
7.2    50    34.90
7.4    50    39.34
7.6    50    42.74
7.8    50    45.17
8.0    50    46.85
H3BO3–KCl–NaOH mixtures
pH    0.2M H3BO3, KCl (mL)    0.2M NaOH (mL)
7.8    50    2.65
8.0    50    4.00
8.2    50    5.90
8.4    50    8.55
8.6    50    12.00

A.1.06 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 936.15
Standard Solution of Hydrochloric Acid

A. Preparation of Standard Solutions
Table 936.15 gives approximate volumes of 36.5–38% HCl required to make 10 L standard solutions.
B. Standard Sodium Hydroxide Method
Titrate 40 mL against standard alkali solution, 936.16C–E (see A.1.12), of ca same concentration as acid being standardized in 300 mL flask that has been swept free from CO2, using CO2-free H2O and 3 drops phenolphthalein.
Molarity =
If more concentrated than desired, dilute solution to required molarity value by following formula:
V1= V2 M2/M1
where M2 and V2 represent molarity and volume of stock solution, respectively, and V1 = volume to which stock solution should be diluted to obtain desired molarity, M1.
Check exact concentration of final solution by titration as above. Molarity will be exact only if same indicator is used in determination as in standardization. Restandardize if indicators other than phenolphthalein are used.

B. Constant Boiling Method
Dilute 822 mL HCl (36.5–38% HCl) with 750 mL H2O. Check specific gravity with spindle and adjust to 1.10. Place 1.5 L in 2 L flat-bottom distilling flask, add ca 10 SiC grains (ca "20 mesh"), and connect to long, straight inner-tube condenser. Heat on electric hot plate and distil at 5–10 mL/min, keeping end of condenser open to air. When 1125 mL has distilled, change receivers and catch next 225 mL, which is constant boiling HCl, in Erlenmeyer with end of condenser inserted into flask, but above surface of liquid. Read barometer to nearest mm at beginning and end of collection of 225 mL portion and note barometer temperature. Average readings.
Calculate air weight in g (G) of this constant boiling HCl required to give one equivalent weight of HCl from one of following equations:
For P0 = 540–669 mm Hg:
G = 162.255 + 0.02415 P0
For P0 = 670–780 mm Hg:
G = 164.673 + 0.02039 P0
where P0 = barometric pressure in mm Hg corrected to 0°C for expansion of Hg and of barometer scale. For brass scale barometer, following correction is accurate enough:
P0 = Pt (l – 0.000162t)
where t = barometer temperature in °C.
Weigh required amount of constant boiling HCl in tared, stoppered flask to at least 1 part in 10 000. Dilute immediately, and finally dilute to volume with CO2-free H2O at desired temperature.

Standard Borax Method
C. Reagents
(a) Methyl red indicator.—Dissolve 100 mg methyl red in 60 mL alcohol and dilute with H2O to 100 mL.
(b) Reference solution.—Prepare reference solution of H3BO3, NaCl, and indicator corresponding to composition and volume of solution at equivalence point. For use in determination of end point of titration with 0.1M HCl, reference solution should be 0.1M in H3BO3 and 0.05M in NaCl.
(c) Standard borax.—Saturate 300 mL H2O at 55°C (not higher) with Na2B4O7·10H2O (ACS) (ca 45 g). Filter at this temperature through folded paper into 500 mL Erlenmeyer. Cool filtrate to ca 10°C, with continuous agitation during crystallization. Decant supernate, rinse precipitate once with 25 mL cold H2O, and dissolve crystals in just enough H2O at 55°C to ensure complete solution (ca 200 mL). Recrystallize by cooling to ca 10°C, agitating flask during crystallization.
Filter crystals onto small Buchner with suction, wash precipitate once with 25 mL ice-cold H2O, and dry crystals by washing with two 20 mL portions alcohol, drying after each washing with suction. Follow with two 20 mL portions ether. (Just before use, free alcohol and ether from any possible reacting acids by vigorously shaking each with 2–3 g of the pure, dry Na2B4O7·10H2O and then filtering.) Spread crystals on watch glass, immediately place dried Na2B4O7·10H2O in closed container over solution saturated with respect to both sucrose and NaCl, and let it remain 24 h before using. Then transfer the pure Na2B4O7·10H2O to glass-stoppered container and store in closed container over solution saturated with respect to both sucrose and NaCl (stable under these conditions 1 year).
D. Standardization
Accurately weigh enough standard Na2B4O7·10H2O to titrate ca 40 mL and transfer to 300 mL flask. Add 40 mL CO2-free H2O, 936.16B(a) (see A.1.12), and stopper flask. Swirl gently until sample dissolves. Add 4 drops methyl red and titrate with solution that is being standardized to equivalence point as indicated by reference solution.
Molarity (mol/L) = g Na2B4O7·10H2O 1000/mL acid 190.69

Standard Sodium Carbonate Method
E. Reagents
(a) Methyl orange indicator.—0.1% in H2O.
(b) Reference solution.—80 mL CO2-free H2O containing 3 or 4 drops methyl orange.
(c) Anhydrous sodium carbonate.—Heat 250 mL H2O to 80°C and add NaHCO3 (ACS), stirring until no more dissolves. Filter solution through folded paper (use of hot H2O funnel is desirable) into Erlenmeyer. Cool filtrate to ca 10°C, swirling constantly during crystallization. Fine crystals of trona that separate out have approximate composition: Na2CO3·NaHCO3·2H2O. Decant supernate, drain crystals by suction, and wash once with cold H2O.
Transfer precipitate, being careful not to include any paper fibers, to large flat-bottom Pt dish. Heat 1 h at 290°C in electric oven or furnace with pyrometer control. Stir contents occasionally with Pt wire. Cool in desiccator. Place the anhydrous Na2CO3 in glass-stoppered container and store in desiccator containing efficient desiccant. Dry at 120°C and cool just before weighing.

F. Standardization
Accurately weigh enough anhydrous Na2CO3, E(c), to titrate ca 40 mL, transfer to 300 mL Erlenmeyer, and dissolve in 40 mL H2O. Add 3 drops methyl orange and titrate until color begins to deviate from H2O tint (reference solution). (Equivalence point has not been reached.) Boil solution gently 2 min and cool. Titrate until color is barely different from H2O tint of indicator.
Molarity (mol/L) = g Na2CO3 1000/mL acid 105.988
Table 936.15: Volumes of concentrated HCl required to prepare solutions of different molarities
Approximate molarity    mL HCl to be diluted to 10 L
0.01    8.6
0.02    17.2
0.10    86.0
0.50    430.1
1.0    860.1

A.1.07 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 939.13
Standard Solution of Iodine

A. Preparation of Standard Solution
Dissolve weighed amounts of I2 (12.7 g/L for 0.1M solution) and KI, in proportion of 20 g KI to 13 g I2, in 50 mL H2O. When I2 dissolves, transfer solution to glass-stoppered volumetric flask. Dilute to volume with H2O and mix thoroughly. Store in dark brown, glass-stoppered bottle away from light and restandardize as frequency as necessary.
B. Standardization
Transfer accurately measured portion of standard As2O3 solution, 939.12B (see A.1.03) (40–50 mL ca 0.025M solution for 0.1M solution), to Erlenmeyer. Acidify slightly with H2SO4 (1 + 10), neutralize with solid NaHCO3, and add ca 2 g excess. Titrate with I2 solution, using ca 0.2% starch solution (5 mL/100 mL) as indicator. Saturate solution with CO2 at end of titration by adding 1 mL H2SO4 (1 + 10) just before end point is reached.
Molarity (mol/L) = mL As2O3 molarity As2O3/mL I2

A.1.08 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 947.13
Standard Solution
of Potassium Bromide–Bromate

A. Preparation of Standard Solution
Dissolve ca 2.8 g KBrO3 and 12 g KBr in boiled H2O and dilute to 1 L with boiled H2O for ca 0.01667M solution.
B. Standardization
Measure 40 mL standard As2O3 solution, 939.12B (see A.1.03), from buret into 300 mL Erlenmeyer. Add 10 mL HCl and 3 drops methyl orange, 936.15F(a) (see A.1.06). Titrate with KBr–KBrO3 solution until 1 drop causes color of methyl orange to fade completely. Swirl solution constantly and add last mL dropwise, swirling between drops.
Molarity (mol/L) =
mL As2O3 molarity As2O3/mL KBr–KBrO3

A.1.09 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 949.13
Standard Solution
of Potassium Dichromate

A. Reagent
Starch solution.—Mix ca 1 g arrowroot starch with 10 mL H2O and pour slowly, with constant stirring, into 200 mL boiling H2O. Boil until thin, translucent fluid is obtained. Let settle and use clear supernate. Preserve with Hg.
B. Assay of Stock Potassium Dichromate
If K2Cr2O7 is in small crystals, mix by shaking thoroughly in large, clean jar; if it is in lumps, grind representative sample to pass No. 60 sieve, and then mix by shaking. Dry portion for weighings 2 h at 100°C.
Weigh, into each of 3 glass-stoppered Erlenmeyers, enough K2Cr2O7, (NIST SRM 136e) to give titer of 100.5–102.0 mL 0.1M Na2S2O3, 942.27A (see A.1.13) (4.90–5.00 g for 0.01667M solution). Completely dissolve in 100 mL H2O, add 4.0 g KI, and swirl mixture until dissolved. With buret, add 4.0 mL HCl, stopper flask, mix by swirling, and let stand in dark 10 min. Cool flask ca 1 min in ice-H2O. While swirling flask, pipet in 100 mL Na2S2O3 solution. Add 5 mL starch solution and complete titration with Na2S2O3 solution added from 10 mL microburet (graduated in 0.05 mL). End point is from bluish green to clear green; change takes place within 0.01 mL. Record volume to nearest 0.01 mL. Calculate apparent molarity of Na2S2O3 solution for each of the 3 titrations, and average. Designate this average as MNIST.
Similarly titrate 3 portions of stock K2Cr2O7 and calculate the 3 apparent molarities. Designate each of these results as Mstock. Calculate percent purity of stock K2Cr2O7 = (MNIST 100)/Mstock.
Take average of the 3 results as percent purity of stock K2Cr2O7.
C. Preparation of Standard Solution
Dissolve theoretical weight K2Cr2O7 (NIST SRM 136e) (4.9032 g for 0.01667M solution), or weight stock K2Cr2O7, B, found to have oxidimetric value 99.95–100.05% of NIST SRM, in enough H2O to make 1 L. (Dry K2Cr2O7 2 h at 100°C before using.)


A.1.10 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 940.35
Standard Solution
of Potassium Permanganate

A. Preparation of Standard Solution
Dissolve slightly more than desired equivalent weight (3.2 g for 0.02M) of KMnO4 in 1 L H2O. Boil solution 1 h. Protect from dust and let stand overnight. Thoroughly clean 15 cm glass funnel, perforated porcelain plate from Caldwell crucible, and glass-stoppered bottle (preferably of brown glass) with warm chromic acid cleaning solution. Digest asbestos for use in Gooches on steam bath 1 h with ca 0.02M KMnO4 that has been acidified with few drops H2SO4 (1 + 3). Let settle, decant, and replace with H2O. To prepare glass funnel, place porcelain plate in apex, make pad of asbestos ca 3 mm thick on plate, and wash acid-free. (Pad should not be too tightly packed and only moderate suction should be applied.) Insert stem of funnel into neck of bottle and filter KMnO4 solution directly into bottle without aid of suction.
B. Standardization
For 0.02M solution, transfer 0.3 g dried (1 h at 105°C) sodium oxalate (NIST SRM 40) to 600 mL beaker. Add 250 mL H2SO4 (5 + 95), previously boiled 10–15 min and then cooled to 27 ± 3°C.
Stir until Na2C2O4 dissolves. Add 39–40 mL KMnO4 solution at rate of 25–35 mL/min, stirring slowly. Let stand until pink disappears (ca 45 s). If pink persists because KMnO4 solution is too concentrated, discard and begin again, adding few mL less of KMnO4 solution. Heat to 55–60°C, and complete titration by adding KMnO4 solution until faint pink persists 30 s. Add last 0.5–1 mL dropwise, letting each drop decolorize before adding next.
Determine excess of KMnO4 solution required to turn solution pink by matching with color obtained by adding KMnO4 solution to same volume of boiled and cooled diluted H2SO4 at 55–60°C. This correction is usually 0.03–0.05 mL. From net volume KMnO4, calculate molarity:
Molarity (mol/L) = g Na2C2O4 1000/mL KMnO4 133.999

A.1.11 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 941.18
Standard Solution
of Silver Nitrate

A. Preparation of Standard Solution
Dissolve slightly more than theoretical weight of AgNO3 (equivalent weight, 169.87) in halogen-free H2O and dilute to volume. Thoroughly clean glassware, avoid contact with dust, and keep prepared solution in amber glass-stoppered bottles away from light.
Mohr Method
B. Reagents
(a) Potassium chloride.—Recrystallize KCl 3 times from H2O, dry at 110°C, and then heat at ca 500°C to constant weight. Equivalent weight KCl = 74.555. Or, preferably, use NIST SRM 999.
(b) Potassium chromate solution.—5% solution of K2CrO4 in H2O.
C. Standardization
Accurately weigh enough KCl to yield titration of ca 40 mL (ca 0.3 g for 0.1M solution), and transfer to 250 mL glass-stoppered Erlenmeyer with 40 mL H2O. Add 1 mL K2CrO4 solution and titrate with AgNO3 solution until first perceptible pale red-brown appears. From titration volume, subtract mL of the AgNO3 solution required to produce end point color in 75 mL H2O containing 1 mL K2CrO4 solution. From net volume AgNO3, calculate molarity:
Molarity (mol/L) = g KCl 1000/mL AgNO3 74.555
Volhard Method
D. Reagents
(a) Ferric alum indicator solution.—Saturated solution of FeNH4(SO4)2·12H2O in H2O.
(b) Potassium or ammonium thiocyanate standard solution.—Prepare ca 0.lM solution, 942.26B (see A.1.02). Determine working titer by accurately measuring 40–50 mL standard AgNO3 solution, adding 2 mL ferric alum solution and 5 mL HNO3 (1 + 1), and titrating with the thiocyanate solution until solution appears pale rose after vigorous shaking.
E. Standardization
Accurately weigh enough KC1, B(a), to yield titration of ca 40 mL (ca 0.3 g for 0.1M solution) and transfer to 250 mL glass-stoppered Erlenmeyer with 40 mL H2O. Add 5 mL HNO3 (1 + 1) and excess AgNO3 solution. Mix, and let stand few minutes protected from light. Filter through Gooch prepared with medium pad of asbestos previously rinsed with 2% HNO3. Wash flask and precipitate with several small portions of 2% HNO3, passing washings through crucible until filtrate and washings measure ca 150 mL. Add 2 mL ferric alum solution and titrate residual AgNO3 with thiocyanate solution. From titration, together with ratio of the 2 solutions, calculate net volume AgNO3 solution. (Errors of blank are compensating and may be disregarded.) From net volume AgNO3, calculate molarity as in C.
.

AOAC Official Method 936.16
Standard Solution
of Sodium Hydroxide
Standard Potassium Hydrogen Phthalate Method

A. Apparatus
Use buret and pipet calibrated by NIST or by analyst. Protect exits to air of automatic burets from CO2 contamination by suitable guard tubes containing soda–lime. Use containers of alkali-resistant glass.
B. Reagents
(a) Carbon dioxide-free water.—Prepare by one of following methods: (1) Boil H2O 20 min and cool with soda–lime protection; (2) bubble air, freed from CO2 by passing through tower of soda–lime, through H2O 12 h.
(b) Sodium hydroxide solution.—(1 + 1.) To 1 part NaOH (reagent quality containing <5% Na2CO3) in flask add 1 part H2O and swirl until solution is complete. Close with rubber stopper. Set aside until Na2CO3 has settled, leaving perfectly clear liquid (ca 10 days).
(c) Acid potassium phthalate.—NIST SRM for Acidimetry 84. Crush to pass No. 100 sieve. Dry 2 h at 120°C. Cool in desiccator containing H2SO4.
C. Preparation of Standard Solution
Table 936.16 gives approximate volumes of NaOH solution (1 + 1) necessary to make 10 L standard solutions.
Add required volume of NaOH solution (1 + 1) to 10 L CO2-free H2O. Check molarity, which should be slightly high, as in D, and adjust to desired concentration by following formula:
V1 = V2 M2/M1
where M2 and V2 represent molarity and volume stock solution, respectively, and V1, volume to which stock solution should be diluted to obtain desired molarity, M1. Standardize final solution as in D or E.
D. Standardization
Accurately weigh enough dried KHC8H4O4 to titrate ca 40 mL and transfer to 300 mL flask that has been swept free from CO2. Add 50 mL cool CO2-free H2O. Stopper flask and swirl gently until sample dissolves. Titrate to pH 8.6 with solution being standardized, taking precautions to exclude CO2 and using as indicator either glass-electrode pH meter or 3 drops phenolphthalein. In latter case, determine end point by comparison with pH 8.6 buffer solution, 941.17C (see A.1.05), containing 3 drops phenolphthalein. Determine volume NaOH required to produce end point of blank by matching color in another flask containing 3 drops phenolphthalein and same volume CO2-free H2O. Subtract volume required from that used in first titration and calculate molarity.
Molarity (mol/L) = g KHC8H4O4 1000/mL NaOH 204.229

Constant Boiling Hydrochloric Acid Method
E. Standardization
Accurately weigh from weighing buret enough constant boiling HCl, 936.15C (see A.1.06), to titrate ca 40 mL, into Erlenmeyer previously swept free from CO2. Add ca 40 mL CO2-free H2O, then 3–5 drops desired indicator, and titrate with solution being standardized.
Molarity (mol/L) = g HCl 1000/mL titer G
where G has value given in 936.15C (see A.1.06).
Table 936.16: Volumes of (1 + 1) NaOH solution required to prepare solutions of different molarities
Approximate molarity    mL NaOH to be diluted to 10 L
0.01    5.4
0.02    10.8
0.10    54.0
0.50    270.0
1.0    540.0

A.1.13 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 942.27
Standard Solutions
of Sodium Thiosulfate

A. Preparation of Standard Solution
Dissolve ca 25 g Na2S2O3·5H2O in 1 L H2O. Boil gently 5 min and transfer while hot to storage bottle previously cleaned with hot chromic acid cleaning solution and rinsed with warm boiled H2O. (Temper bottle, if not heat-resistant, before adding hot solution.) Store solution in dark, cool place; do not return unused portions to stock bottle. If solutions less concentrated than 0.1M are desired, prepare by dilution with boiled H2O. (More dilute solutions are less stable and should be prepared just before use.)
B. Standardization
Accurately weigh 0.20–0.23 g K2Cr2O7 (NIST SRM 136e dried 2 h at 100°C) and place in glass-stoppered iodine flask (or glass-stoppered flask). Dissolve in 80 mL chlorine-free H2O containing 2 g KI. Add, with swirling, 20 mL ca 1M HCl and immediately place in dark 10 min. Titrate with Na2S2O3 solution, A, adding starch solution after most of I2 has been consumed.
Molarity (mol/L) = g K2Cr2O7 1000/mL Na2S2O3 49.032


A.1.14 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 890.01
Standard Solutions
of Sulfuric Acid

A. Preparation of Standard Solution
Table 890.01 gives approximate volumes of 95–98% H2SO4 necessary to make 10 L standard solutions.
B. Standard Borax Method Standardization
See 936.15E (see A.1.06).
C. Specific Gravity Method
Dilute H2SO4 with enough H2O to make convenient volume of ca 70% H2SO4 by weight. Determine specific gravity in air at convenient temperature (0–40°C) as in 945.06C (see 26.1.06) (or specific gravity may be determined with Sprengel pycnometer), protecting solution from contact with air. Calculate exact percent H2SO4 by weight:
P = S(85.87 + 0.05T – 0.0004t2) – 69.82
where S = specific gravity (in air) at T°, compared with H2O at t°.
Weigh exactly W g prepared acid containing P% H2SO4 and dilute to n L to make required solution containing G g H2SO4/L. Calculate W from equation:
W = nG 100/P
Table 890.01: Volumes of concentrated H2SO4 required to prepare solutions of different molarities
Approximate molarity    mL H2SO4 to be diluted to 10 L
0.005    2.8
0.01    5.6
0.05    27.7
0.25    138.1
0.50    276.1

A.1.15 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 948.28
Standard Solutions
of Titanium Trichloride

A. Preparation of Standard Solution
To 200 mL commercial 15% TiCl3 solution add 150 mL HCl and dilute to 2 L. Make solution ca 0.1M, place in container with H2 atmosphere provision [e.g., JAOAC 5, 207(1921)], and let stand 2 days for absorption of residual O2.
B. Standardization
Weigh 3 g FeSO4(NH4)2SO4·6H2O and transfer to 500 mL flask. Introduce stream of CO2, and add 50 mL recently boiled H2O and 25 mL 40% (by weight) H2SO4. Then, without interrupting current of CO2, rapidly add 40 mL 0.01667M K2Cr2O7, 949.13C (see A.1.09). Add TiCl3 solution until near calculated end point. Then quickly add 5 g NH4SCN, and complete titration. Determine blank on 3 g FeSO4(NH4)2SO4·6H2O, using same volumes of H2O, H2SO4, and NH4SCN, and current of CO2. From net volume TiCl3, calculate molarity:
Molarity (mol/L) = mL K2Cr2O7 molarity K2Cr2O7/mL TiCl3


A.1.16 - Appendix A: Standard Solutions and Reference Materials
AOAC Official Method 982.35
Reference Materials
First Action 1982
A. Definitions
Reference material (RM).—Homogenous and stable material or substance one or more properties of which are sufficiently well established to be used for calibrating apparatus, assessing measurement method, or assigning values to materials.
Certified reference materials (CRM).—Reference material one or more of whose property values are certified by valid procedure, or accompanied by or traceable to certificate or other documentation which is issued by certifying body.
B. General Information
Table 982.35 lists selected issuing organizations and shows source codes together with names and addresses of these organizations.
Organizations listed are only representative of those supplying RMs. No claim is made as to completeness of information supplied. Catalogs or literature describing RMs are available from individual organizations. Because inventories of RMs are continually changing, inquiries should be made of the organizations concerning current availability of specific RMs.
Reference:
Technical Division on Reference Materials—Minutes of Executive Committee Meeting, January 31, 2000, AOAC INTERNATIONAL, Gaithersburg, MD, USA.

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