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3.1.2 Standardization of Sodium Hydroxide Standard Solution

Reference:
Standard Solution of Sodium Hydroxide (936.16) Official Methods of Analysis. 1990. Association of Official Analytical Chemists. 15th Edition.

Scope:
This method is applicable for the preparation and standardization of standard sodium hydroxide solution.

Basic Principle:
A basic solution is titrated with a standardized acidic solution to determine normality.

Equipment:
Buret, 50 mL, graduated to 0.1 mL Illuminated magnetic stirrer Volumetric pipet, 40 mL, class A Analytical balance, sensitive to 0.1 mg Container of alkali resistant glass pH meter with glass electrode (alternate to using phenolphthalein indicator)

Reagents:
Distilled water, carbon dioxide (CO2) free prepared either 1) by boiling for 20 min and cooling with soda-lime protection or 2) by bubbling air, freed from CO2 by passing through tower of soda lime, through water for 12 hr. Sodium hydroxide solution, to 1 part reagent grade NaOH add 1 part distilled, carbon dioxide-free water by weight. Acid potassium phthalate, NIST SRM for Acidimetry 84, dry for 2 hr at 120oC and cool in desiccator. Buffer solution, pH 8.6, 12.00 mL 0.2 N NaOH added to 50 mL 0.2M boric acid/potassium chloride solution made as follows: Boric acid-potassium chloride solution - dry boric acid (H3BO3) to constant weight in desiccator over CaCl2. Dry potassium chloride (KCl) 2 days in oven at 115 to 120oC. Dissolve 12.405 g H3BO3 and 14.912 g KCl in water and dilute to 1 L. Phenolphthalein indicator, 1%, dissolve 1 g phenolphthalein in 100 mL 95% ethanol.

Safety Precautions:

• Alkalis can burn skin, eyes and respiratory tract severely. Wear heavy rubber gloves and face shield to protect against concentrated alkali; if spilled on skin wash with copius amounts of water. Use effective fume removal device to protect against alkali dusts or vapors. Always add sodium hydroxide pellets to water, not vice versa.

Procedure: Preparation

• Add appropriate volume of NaOH solution (1 to 1) to CO2-free distilled water necessary to make 10 L of solution:

Standardize

1. Accurately weigh enough dried acid potassium phthalate (ca. 0.4 g) to titrate about 40 mL of NaOH solution and transfer to 300 mL flask.
2. Add 50 mL CO2-free water, stopper flask and swirl until sample dissolves.
3. Titrate to pH 8.6 with solution being standardized, taking precautions to exclude CO2 and using as indicator either glass-electrode or phenolphthalein. If using indicator, add 3 drops phenolphthalein to a flask containing 50 mL of pH 8.6 buffer and stopper. This flask is used as the reference endpoint for a pH 8.6 titration.
4. Determine volume NaOH required to produce endpoint of blank by matching color in another flask containing 3 drops phenolphthalein and same volume (50 mL) CO2-free water.
5. Subtract volume required to titrate blank from that used to titrate the potassium acid phthalate and calculate normality. Normality should be slightly high.
6. Adjust to desired concentration, mix well, and recheck standardization.
7. Record final standardization in logbook.

Calculations:

Normality = g KHC8H4O4 X 1000/mL NaOH X 204.229

Adjust to desired concentration by following formula: V1 = V2 X N2 / N1
Where N2 and V2 represent normality and volume of stock solution and V1 equals volume to which stock solution should be diluted to obtain desired normality, N1.

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