Chemistry

UNIT 11: SOLUTIONS AND TITRATION

Key unit competency:

 Be able to prepare standard solutions and use them to determine concentration of

other solutions by titration.

Learning objectives:

•  Define the terms standard solution and primary standard solution.

•  Explain the properties of a standard primary solution.

•  Explain the titration process, emphasising the need for precise measurements.

•  Prepare solutions with different concentrations.

•  Properly use the burettes, pipettes during titration.

•  Interpret the experimental data obtained by titration and report.

•  Carry out acid-base, redox titrations and do calculations involved.

•  Develop a team approach and a sense of responsibility in performing the

experiments of titration.

•  Respect of procedure in practical experiment.

•  Develop a culture of orderliness in performing practical experiments.

•  Appreciate the use of appropriate measurements in daily life.

11.1. Definition of standard solution and primary standard solution.

In analytical chemistry, a standard solution is a solution containing a precisely known

concentration of an element or a substance and used to determine the unknown

concentration of other solutions. A known weight of solute is dissolved to make a

specific volume. It is prepared using a standard substance, such as a primary standard.

A primary standard is defined as a substance or compound used to prepare

standard solutions by actually weighing a known mass, dissolving it, and diluting to a

definite volume.

A good primary standard meets the following criteria:

•  High level of purity

•  High stability

•  Be readily soluble in water

•  High equivalent weight (to reduce error from mass measurements)

•  Not hygroscopic (to reduce changes in mass in humid versus dry environments)

•  Non-toxic

•  Inexpensive and readily available

•  React instantaneously, stoichiometrically and irreversibly with other substances

i.e. should not have interfering products during titration.

•  It should not get affected by carbon dioxide in air

Molar concentrations are the most useful in chemical reaction calculations because

they directly relate the moles of solute to the volume of solution.

The formula for molarity is:

The preparation of the solution requires a reagent that is so to say the quantity to be

weighed for mass if the reagent is in solid state ; The volume to be pipetted using

pipette if the solute is a liquid and then after dissolve it in water, So the solution can be

prepared by two methods such as dissolution method and dilution method.

In the preparation of solution, glasses, volumetric flask, pipette, glass rod, measuring

cylinder, analytical balance, spatula, beakers, magnetic stirrer and other laboratory

devices are used.

11.3.1. Preparation of standard solution by dissolution of solids

Scope: This method is applied for solute in solid state and you should be able to

determine the mass required from calculation to be weighed and provide distilled

water to dissolve the solute.

Examples:

1. Describe in details how you can prepare the following solution: 50 mL of

NaOH, 10%.

11.3.2. Preparation of standard solution by dilution

11.4. Simple acid-base titrations

Titration is the controlled addition and measurement of the amount of a solution

of known concentration required to react completely with a measured amount of a

solution of unknown concentration.

Acid-base titration

It is the determination of the concentration of an acid or base by exactly neutralizing

the acid or base of known concentration

Alkalimetry and acidimetry

•  Alkalimetry is the specialized analytic use of acid-base titration to determine

the concentration of a basic substance.

•  Acidimetry is the same concept of specialized analytic use of acid-base titration

to determine the concentration of an acidic substance.

Equivalence point

The point at which the two solutions used in a titration are present in chemically

equivalent amount is the equivalence point. At this point the moles of two solutions

will be equal.

Indicators and pH-meters can be used to determine the equivalence point. The

point in a titration at which an indicator changes color is called the end-point of the

titration.

Equipments and set up (Figure 11.1) of materials for Titration

The common equipment used in a titration are:

•  Burette

•  Pipette

•  pH-indicator/acid-base indicator

•  White tile: used to see a color change in the solution(a white paper can also be

used)

•  Conical flask (Erlenmeyer flask)

•  Titrant: a standard solution of known concentration

•  Analyte: a solution of unknown concentration

How to perform titrations

Knowing the use of pipette and burettes and how to handle them, the following

points are useful in order for a correct titration to be done:

1. The apparatus should be arranged as shown in the above Figure.

2. The burette tap is opened with the left hand and the right hand is used to

shake the conical flask.

3. The equivalence-point is reached when the indicator just changes

permanently the colour.

4. At the end-point, the level of the titrant is read on the burette

5. The titration is now repeated, three more times are recommended. Towards

the end-point, the titrant is added dropwise to avoid overshooting.

Notice: Before titration, check if the tip of the burette is filled with the titrant, and doesn’t

contain bulb of air. If there is a bulb of air, a quick opening and closing of the tap will expel

the air out of the burette.

Choice of indicators in acid-base titrations

When the technique of acid-base titration is extended to a wide variety of acidic and

alkaline solution, care needs to be taken about the choice of indicator for any given

reaction.

The choice of an inappropriate indicator would lead to incorrect results, and it is

therefore extremely important that the indicator is chosen carefully.

The principle on which a choice of indicator is made concerns the strength of the acid

or base involved in the reaction. Note that the strength of an acid or base is not to be

confused with the concentration of its solution. Example of strong and weak acids and

bases and choice of indicator are given in the Table below.

11.5. Titration involving redox reactions

11.5.1. Titrations with potassium manganate (VII); KMnO₄

76-104