Standardization of a NaOH Solution and Titration of Vinegar

I. Discussion: Locate and read the titration section in your textbook. A titration is a method for determining the concentration of a solution. The concentration of an acid could be determined. The concentration of a base could be determined. The concentration of a sodium hydroxide solution will be determined (it will be standardized). The standardized sodium hydroxide solution will be used to titrate vinegar for determination of the concentration of acetic acid.

Purposes:

A. Standardization of a Base: Potassium hydrogen phthalate is a weak monoprotic acid. It reacts with NaOH as shown below.

Neutralization of Potassium Hydrogen Phthlate with Sodium Hydroxide

The stoichiometry of the reaction is one mole of potassium hydrogen phthalate reacts with one mole of NaOH. The solution with the phthalate is acidic. The indicator is colorless when the solution is acidic. When enough NaOH has been added to react stoichiometrically with the acid, the solution will be neutral. This is called the endpoint of the titration. If just one more drop of NaOH (or a portion of a drop) is added, the solution is basic. The indicator will turn pink in a basic solution. The moles of acid will equal the moles of base (practically if not exactly). The molarity (M) of the NaOH can then be determined using the moles of NaOH and the volume of NaOH. This is the standardization of a base.

Molarity of NaOH = [(moles of NaOH) / (liters of NaOH)]

B. Determination of Acetic Acid Molarity in Vinegar: Acetic acid can be reacted with sodium hydroxide as shown in Equation 2.

CH3COOH (aq) + NaOH (aq) --> CH3COONa (aq) + HOH (l) (Equation 2)

The mole ratio of acetic acid to sodium hydroxide is one-to-one. The standardized base will be added to a measured volume of vinegar. The moles of base can be determined using the volume of NaOH delivered to reach the endpoint and the molarity of the NaOH. The moles of acetic acid can be determined from the moles of NaOH and the balanced chemical reaction. The molarity of acetic acid can be determined from the moles of acid and the volume of vinegar.

vol and M of NaOH -> moles NaOH -> moles acetic acid -> M acetic acid

Procedure: A. Standardization of a Base: Begin by making about 30 milliliters of approximately 0.5 M NaOH. This solution will be used to titrate a measured mass of potassium phthlate (about 0.2 grams). Rinse your microburet with two one-mL portions of your NaOH solution. Load the microburet with about 2 mL of the NaOH solution. Add about five milliliters of distilled water to the potassium hydrogen phthlate and one drop of phenolphthalein (an indicator). The phenolphthalein will indicate, with a solution color change from colorless to pink, when enough base has been added. Obtain the volume of NaOH added to obtain the color change. This process is called a titration. Repeat the titration.

B. Titration of Vinegar: Titrate a one milliliter sample of the vinegar with the sodium hydroxide solution. Measure the vinegar with a pipet into an Erlenmeyer flask. Add about five milliliters of distilled water and one drop of indicator to the vinegar. Fill the microburet with NaOH solution and titrate the vinegar sample. Repeat the titration with a fresh sample of vinegar.

Bring about ten mL of your favorite vinegar to class. Leave the bottle at home so that is does not become contaminated.

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