ENGTECH 1CH3 – Chemistry

Experiment 2

Volumetric Analysis 2 - Oxalic Acid, Sodium Hydroxide and Vinegar

Summary

A sample of a commercial food vinegar unknown will be analyzed for its acetic acid content.

In the experiment you will:

Ø Weigh a sample of pure oxalic acid dihydrate and prepare a primary standard solution of oxalic acid of precisely known molarity;

Ø Titrate precisely measured portions of the oxalic acid solution with a supplied solution of sodium hydroxide, in order to determine the molarity of the sodium hydroxide solution;

Ø Precisely dilute a sample of a supplied commercial food vinegar unknown;

Ø Titrate precisely measured portions of the diluted vinegar solution with the solution of sodium hydroxide.

In the laboratory you will review these techniques. You will:

Ø Clean and use glassware;

Ø Use a laboratory top-load balance;

Ø Use a volumetric flask;

Ø Use a transfer pipet;

Ø Use a buret.

In reporting, you will:

Ø Write balanced chemical equations and do quantitative calculations using balanced equations;

Ø Answer questions relating to the practice and theory of what you have done.

References

Background Information and Worked Example Calculations

Vinegar

Ø Vinegar is a food product. It is a dilute solution of acetic acid in water. The major uses of vinegar are as a preservative or pickling agent; salad dressings; as an acidulant (adds a sharp taste to foods).

Vinegar is traditionally produced by bacterial oxidation of ethyl alcohol; the ethyl alcohol in turn

having been produced by yeast fermentation of sugars from fruits, germinated grains or plant starch

(Box 1).

Vinegar may also be produced from acetic acid produced from petrochemicals by any of several synthetic routes.

Vinegar for food use must meet a number of regulatory restrictions. The acetic acid content must be 4 % w/v at minimum. The product is usually pasteurized at about 77 – 78 °C before storage and sale.

As with any food product or other chemical product, quality control analysis is an important part of the production process.

Analysis of the Acetic Acid Content of a Vinegar Solution

The acetic acid content of vinegar must be determined for quality control, regulatory and commercial reasons. Vinegar solutions may be analyzed for their acetic acid content by a number of methods.

One such method is the basis of this experiment: titration using a secondary standard solution of sodium hydroxide.

The equation of the acid-base reaction is given below. The molecular equation is used for quantitative calculations, titration calculations, solution preparation, and molarity calculations. A net-ionic equation (see Post-Laboratory Question 3) is needed for equilibrium expressions, K_a and pH calculations.

CH₃COOH (aq)  +  NaOH (aq) ® NaCH₃COO (aq)  +  H₂O (l)   (molecular equation)

Acetic acid is a weak organic acid. Successful titration requires the use of a strong base such as sodium hydroxide solution with a suitable indicator if the end-point is to be observed visually. If the titration is performed with the weak acid in the flask and the strong base is added from the buret, then phenolphthalein (pK_In = 9) is a suitable choice of an acid-base indicator substance.

Sodium Hydroxide

Ø This is an important industrial chemical, produced as an aqueous solution along with hydrogen and chlorine from the electrolysis of aqueous sodium chloride as part of the chlor-alkalai industry. Sodium hydroxide is also called caustic soda. The production of this chemical is many Mtonne per year worldwide (More than 10 Mtonne annually in the U. S. alone). The major uses of sodium hydroxide are: organic and inorganic chemical production; paper production; natural soap and synthetic anionic detersive production; aluminum production; industrial acid neutralization.

Sodium hydroxide is usually produced and shipped as solid pellets of NaOH. Solid NaOH is very hygroscopic (absorbs water) and also absorbs carbon dioxide. The solid cannot be produced as a primary standard substance.

The heat of solution of NaOH is very large; dissolving the pellets creates a large amount of heat energy that may boil the solution. As a solid and as a concentrated solution it is an extremely corrosive and hazardous chemical. The solutions are able to attack human tissue and most kinds of glass and plastic. A solution of sodium hydroxide should never be left in a buret other than when it is being used in titration.

Solutions of sodium hydroxide are hard to maintain at a constant molarity because they absorb carbon dioxide from the air, but they may be standardized as secondary standard solutions for analytical use and stored for a short period of time.

Oxalic Acid Dihydrate

There are a number of primary standard solid acids that are suitable for the standardization of sodium hydroxide solutions. One such substance is oxalic acid dihydrate. This is a crystalline form. of oxalic acid (Box 2) that may be obtained from water solution in the form. of a solid lattice containing two water molecules per one molecule of the acid.

Oxalic acid is soluble in water and is a relatively strong weak acid (a diprotic acid) with two ionizable hydrogens. It reacts readily with sodium hydroxide in aqueous solution. The reaction of oxalic acid with sodium hydroxide actually occurs in two steps, but these are combined into one step above (Box 2) and into a single equation below:

H₂C₂O₄ (aq)  +  2 NaOH (aq) ® Na₂C₂O₄ (aq)  +  2 H₂O (l)   (molecular equation)

The net-ionic equations (see Post-Laboratory Question 3) for both ionization steps are needed for equilibrium expressions, K_a and pH calculations. Oxalic acid is a stronger weak acid than most other carboxylic acids (e. g. acetic acid or citric acid). It is considered to be a hazardous and toxic substance, largely due to its corrosively acidic nature. It is found in nature in many plant substances, in the form. of potassium and/or sodium oxalate in the leaves and roots of the plants. These plant parts are poisonous when eaten.

Oxalic acid has numerous industrial and analytical uses: metal treatment (oxalate coatings, anodizing, metal cleaning); textiles (dyeing, permanent press, flame-proofing); many other miscellaneous uses (see Encyclopedia). Some uses depend on its ability to act as a Bronsted acid. Some uses depend on its ability to act as a reducing agent. Still other uses depend on the ability of the oxalate ion to bind to metal ions as a chelating agent.

The solid dihydrate substance is available at a purity level that is adequate for the analysis of this experiment and can be used without drying as it does not absorb water from the air.

The molecular formula of oxalic acid is H₂C₂O₄ (molar mass = 90.04 g/mol). The molecular formula of oxalic acid dihydrate is H₂C₂O₄∙2H₂O (molar mass = 126.07 g/mol). Since the substance to be weighed in the experiment is the dihydrate, the latter molar mass must be used in the molarity calculation.