Volumetric analysis is quantitative analysis in which the results are expressed in a certain definite volume. It involves the use of at least one solution of known strength. It may be prepared by dissolving a definite amount of a solute in a solvent to get a known volume of solution. The volume of this solution which reacts with a volume of the solution of unknown strength of another substance is determined. The process is known as titration.
Handling The Apparatus
In volumetric analysis, you deal with volumes of solutions. Therefore, these should be measured correctly. The glass apparatus must be clean and free from grease. Errors due to parallax in reading the level of a solution in burette, pipette and volumetric flask should be avoided.
Pipettes are available in various capacities (volumes) and a pipette of required volume is used during titration for delivering a liquid out of the pipette. It should be held vertically with its tip in contact with the wall of vessel. After the flow of liquid has ceased, a very small solution of the liquid remains in the lower portion of the pipette.
It should not be blown out. For this, hold the bulb of the pipette in left hand palm with its upper tip closed with a finger. Touch the lower tip of pipette to the wall of the vessel.
A chemical balance is commonly used in chemical laboratories to weigh an exact amount of the substance. The chemical reactions go to completion when exact mass of the substance are taken. Each molecule or atoms of the substance has its own importance. Hence it is necessary to use exact mass of the substance in each experiment. Therefore, the trip balance is not useful for reaction concerned. Chemical balance is used for weighing accurate mass of the substance.
A weight box is a wooden box having grooves of various sizes into which are placed different weights ranging from 1 to 100. These weights are made up of brass coated with nickel or chromium. Each weight is nearly cylindrical having a knob at its one end with the help of which it can be lifted with forceps.
The following is the order of weights placed in a weight box: 100g, 50g, 20g, 20g, 10g ,5g, 2g, 2g, 1g.
Fractional weight box
Fractional weights are made up of aluminium or brass coated with chromium or nickel that range from 1 milligrams to 500 milligrams.
The standard solution of some substances can be prepared directly by weighing. These substances are available in their pure forms and do not undergo chemical change on storing. These are known as primary standards.
Properties of Primary Standards
- They are easily available in pure and dry conditions.
- They should not undergo any chemical change with air, oxygen and carbon dioxide.
- They do not possess hygroscopic, deliquecent and efflorecent properties.
- They ate easily soluble when added to the solvent (normally water).
- They normally possess high molecular mass so that weighing errors are negligible.
- The standard solution of these substances should react in a stoichiometric ratio with the volumetric titrants.
- They don’t react with impurities which are present in the solution to be titrated.
Examples of Primary Standard Substances
- Oxalic acid
- Mohr's salt
Many chemical substances do not possess the primary standard properties. Therefore, they cannot be used to prepare standard solutions. However, the solution of this type of substance are first prepared of approximate strength and then standardized by titrating with a solution of a primary standard.
Example of Secondary Standard Substances
- Sodium hydroxide (NaOH)
- Potassium permanganate (KMnO4)