Volumetric analyses-Kingsley Idiagbor

Volumetric Analyses (sometimes known as Quantitative Analyses) is an attempt to determine the quantity or volume of a specific substance in a sample.

A preparatory step to volumetric analyses is standardization, or calibration. The response and the sensitivity of the analytical method and apparatus used must be calibrated, or standardized, using a pure constituent or a sample containing a known amount of constituent.

This is achieved by way of titration. Titration is the chemical method of measuring how much of one solution is needed to react exactly with another solution with a known volume

A solution containing an unknown volume of a known substance is placed in a vessel, normally a conical flask. Another solution of accurately known concentration (called a standard solution) is then added to the first solutionoften drop by dropfrom a burette until the end point of the titration is reached.

The most encountered type of titration as recommended by WAEC and NECO syllabi is the acid-base titration. Other types of titration include gravimetric titration and redox titration.

It is not just any kind of solution that is used for the process of titration. Standard solutions are made use of in order to obtain accurate results.

Standard solution is defined as that which contains a known concentration of the solute in one litre of water. A special type of standard solution employed is the molar solution, which is the solution that contains the molar mass of the compound (that is the solute) in one litre of water. All molar solutions are standard solutions but the reverse is not true.

Mass concentration: this is the concentration of the solute in one litre of water. The unit is g dm^-3

Molar concentration: is the number of moles of the solute contained in one litre of water. Said differently, it is the concentration of the solute measured in moles per litre. The unit is mol dm^-3.

Indicator: This is weak organic acid or base that is used to determine the presence of alkali or acid by means of definite colour change. They are used to provide information about the degree of acidity or the pH of a substance

Acid-base indicators are often weak acids, in which the free weak acid has one colour, and its anion (negatively charged ion) has a different colour. Using HMe for the un-dissociated form of an indicator, its ionization in aqueous solution can be represented by the equation:

The extent of this dissociation depends on the hydrogen ion concentration, H⁺, in the solution. If the hydrogen ion concentration is high, that is, in an acidic solution, then most of the acid is present in its un-dissociated state, HMe. However, when the hydrogen ion concentration is low, that is, in an alkaline solution, it is mainly present in its ionized form. In the indicator phenolphthalein, for example, the un-dissociated acid is colourless, whereas its anion is pink; this is why phenolphthalein is colourless in acid solutions, but changes to pink in alkaline solutions. The precise pH at which the colour change occurs depends on the dissociation constant of the indicator.

It is difficult to see precisely when the end-point (or the equivalence point) is reached. However, indicators give an intense colour change at this point signifying that the reaction is complete.The table below shows the common acid-base indicators and their colour change in acid and base media:

End Point: This is also known as the equivalence point and is the point at which one solution (usually the acid) exactly neutralizes the other (the base). This is signified by a definite colour change by an appropriate acid-base indicator.

Strong Acid: Strong acid is that acid, which readily and completely dissociates into hydroxonium ion. Examples of strong acid include HCl, HNO₃, H₂SO₄ and H₃PO₄.

Weak Acid: Weak acid, on the other hand, is the acid that partially and incompletely dissociates into hydroxonium ion. Examples are ethanedioic acid and ethanoic acid.

Strong Acid/Strong Base titration: In this type of titration between strong acid, like HCl, and strong base like NaOH, the pH of the end-point falls at about 7.0. Since the pK_a of all the indicators range between 3.0 and 10.0, any indicator could be used for this type of titration.

Weak Acid/Strong Base: Weak acid like oxalic acid (or ethanedioic acid) could be titrated against NaOH which is a strong base. The pH of the end-point falls at about 9.0. Since the pK_a of the phenolphthalein is about 9.4, this is the most suitable indicator for the titration.

Strong Acid/Weak Base: Strong acid, such as HCl, is titrated against a weak base like ammonium hydroxide. The pH of the end-point of this titration is at about 4.0 and methyl orange whose pK_a is 4.6 is the most appropriate indicator.

Weak Acid/Weak Base: This type of titration between a weak acid like CH₃COOH and weak base such as NH₄OH is very difficult to carry out because there is no definite end-point or is extremely hard to accurately determine. No suitable indicator for this type of acid-base titration can be used.

Titration graph (or titration curve) is the graph of the pH plotted against the volume of the acid drop into the titration flask containing alkali and indicator.

The table below shows the titration graphs for the various types of the acid-base titrations.