One important area of chemistry is the analysis of substances to determine their composition, either qualitatively or quantitatively. In qualitative analysis, we identify the elements and compounds that are present in a sample of a given substance. In quantitative analysis, we calculate the amount or quantity of an element or compound present in a given sample using chemical
processes called titrations. There are two approaches to quantitative analysis, namely volumetric analysis and gravimetric analysis. Volumetric analysis s based on volume measurements of solutions while gravimetric analysis is based on direct mass measurements of the substances. Volumetric analysis is commonly used as it is faster and more convenient, although less accurate, than the gravimetric method.
What is Titration?
Titration is the method employed in volumetric analysis. In this method, a solution from a graduated vessel is added to a known volume of a second solution until the chemical reaction between the two is just completed. This is shown by a colour change in the resulting solution or in an added indicator.
In any titration, a standard solution, i.e. one with a concentration which is accurately known, must be used to react with a solution of unknown concentration. The reacting volumes of solutions are then used to calculate the unknown concentration of the solution.
Important Notes on Titrations and Volumetric Analysis
1. There is no suitable indicator for titration of weak acid and weak base. Any indicator could be used for strong acid and base.
2. Volumetric analysis usually includes titrations of acid against base or trioxocarbonate (IV); oxidizing agent against reducing agent; one substance against another substance, giving a precipitate.
3. Acid-base or acid-troxocarbonate(IV) titration are actually neutralization reactions. We use indicators to determine the end-point of this type of titrations.
4. In titrations, the concentrations of the reacting solutions give an accurate picture of the quantitative behaviour of the reacting particles.
The following equations are useful in calculations involving concentration.
A Standard Solution is a solution of known concentration. the amount of solute in a given volume of solvent must be known. For example, 1.06g of Na2Co3* dissolved in 1dm3 of water gives a standard solution.
Some materials used during acid-base titrations and other precautions employed while using these materials:
- Weighing bottle
- Chemical balance
- Retort stand
- Filter paper
- White title
- Standard volumetric flask
- Conical flask
Precautions Employed while using the Pipette, Burette and Conical Flasks
- Rinse the pipette with the solution it should be used to measure, i.e base.
- Avoid air bubbles in the pipette.
- Make sure that the mark to be read is at the level with your eye.
- Do not blow the last drop on the pipette.
- Rinse the burette with acid or allow it to drain after rinsing with distilled water.
- make sure that the burette jet is filled.
- Avoid air bubbles in the burette.
- Make sure that the burette is not leaking.
- Take your burette readings with your eyes at the same level as the meniscus to avoid error due to parallax.
- Remove the funnel before taking your readings.
- Avoid inconsistent burette reading.
- Do not rinse it with any of the solutions used in the titration but with distilled water.
- Wash down with distilled water any drop of the solution that stick by the sides of the conical flask during titration.
The molar masses of sodium and potassium hydroxides are 40g and 56g respectively. Therefore, a molar solution of sodium hydroxide contains 1 mole or 40g of the hydroxide in 1 dm3 of the solution, while a molar solution of potassium hydroxide contains 1 mole or 56g of the hydroxide in 1 dm3 of the solution.
From table 10.2, we get following relationship which are helpful in calculations involving molarity.
- By definition, molar concentration C = mol dm-3
- By definition, number of specified entitles per dm3 = mo1 dm-3 x 6.02 1023 x 6.02 x1023
- Concentration (g dm3)
= mo1 dm3 x molar mass
= molarity (M) molar mass.