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