Padronização da solução de NaOH

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EXPERIMENT 10

Volumetric Analysis I

Standardization of NaOH Solution

Outcomes

After completing this experiment, the student should be able to:

1. Demonstrate the concept of quantitative analysis.

2. Make solution and standardize it.

3. Explain the difference between primary and secondary standard solutions.

4. Quantitatively determine the concentration of a base.

Introduction

Titration is a common method of quantitative analysis used to determine the concentration of

an unknown substance in a solution. The method is easy to use if the quantitative relationship

between two reacting solutions is known. It is particularly well-suited to acid-base and

oxidation-reduction reactions. In this course, we will analyze commercial products such as

household vinegar and bleach.

In the case of acid-base titrations, these permit the determination of concentrations of unknown

acids or bases with a high degree of accuracy. In order to analyze unknown acids or bases, we

must have a “standard solution” to react with the unknowns. A standard solution is one in

which the concentration is accurately known. We will first prepare a standard solution of

NaOH. One way to prepare a standard solution is to dissolve an accurately massed amount of

the substance and dilute it to a measured volume. In this way, the concentration can be

calculated exactly. However, it is usually impossible to obtain NaOH of sufficient purity to use

it as a primary standard. Moreover, NaOH is highly hygroscopic, therefore absorbs water

during the weighing process. An indirect method is more practical for obtaining a standard

solution of NaOH. We will prepare a solution of NaOH of approximate molarity and

standardize it against a primary standard of known purity. A primary standard is a substance

that should:

* be of high purity

* remain unchanged in air during massing and remain stable during storage

* have a high molar mass to reduce massing errors

* react with the solution to be standardized in a direct, well-defined reaction.

Potassium hydrogen phthalate is used as a primary standard in this experiment. This is a large

molecule (KHC8H4O4) with a molar mass of 204.2 g/mol. Instead of writing the whole formula,

we abbreviate it as KHP, where "P" stands for the phthalate ion, (C8H4O4)2−, not for

phosphorus. KHP is an acidic substance, with the ionizing hydrogen placed placed separately in

the formula for emphasis. Therefore, KHP is monoprotic (one ionizable proton, H+) and will

react with NaOH in a simple 1 to 1 relationship according to the following reaction:

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In this experiment, phenolphthalein will be used as an indicator which helps in determining endpoint and sodium hydroxide will be standardized.

Safety Precautions

Wear safety goggles while working in the laboratory. NaOH is corrosive so avoid skin contact. Follow all safety rules described in experiment 1.

Materials and Equipment