Aoac 936.16 Hidroxido De Sodio 1n6v6u

A.1.12 AOAC Official Method 936.16 Standard Solution of Sodium Hydroxide

Table 936.16. Volumes of NaOH (1 + 1) 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

A. Apparatus

0.50

270.0

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.

1.0

540.0

B. Reagents

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), con tain ing 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.

Standard Potassium Hydrogen Phthalate Method First Action 1936 Final Action

(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 ing 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 No. 100 sieve. Dry 2 h at 120°C. Cool in desiccator containing H2SO4.

Molarity (mol/L) = g KHC8H4O4 ´ 1000/mL NaOH ´ 204.229 References: JAOAC 19, 107, 194(1936). NIST Certificate for Standard Reference Material 84.

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

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 ti trate with solution be ing standardized. Molarity (mol/L) = g HCl ´ 1000/mL titer ´ G where G has value given in 936.15C (see A.1.06). References: JAOAC 25, 653(1942); 36, 96, 354(1953); 37, 122, 462(1954).

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