A standard solution is a solution of known concentration. For example, a solution containing 15g of sulphuric acid in 1 dm3 of solution is a standard solution. It has now been approved that volumetric work should be based upon the molar (M) solution.
A 1 molar (1M) solution of a compound is a solution which contains one mole of that compound in 1 dm3 of the solution. For example, 58.5g of sodium chloride (NaCl) dissolved in 1 dm3 of the solution makes a molar solution of sodium chloride (1M NaCl). Likewise, 106g of sodium carbonate (Na2CO3) in 1 dm3 of the solution gives a molar solution of sodium carbonate. Therefore, a 1 molar sodium carbonate solution contains 106g of the salt in 1 dm3 of the solution.
Preparation of standard solutions.
A stanedard solution is required as a starting point for volumetric analysis. We learned early that in order to find the unknown concentration of a substance in volumetric analysis, the concentration of one of the solutions must be known.
A small range of substances are suitable for direct preparation of accurately standard solutions. Substances that cannot be used for direct preparation of standard solutions include sodium hydroxide, potassium hydroxide and concentrated sulphuric acid.
These substances absorb water vapour from the air and hence cannot be weighed out precisely without taking extra precautions. Apart from absorbing water vapour from the air, sodium and potassium hydroxides react with carbon dioxide of the air to form respective carbonates.
2NaOH(s) + CO2(g)→ Na2CO3(s) + H2O(l)
Some solutions are volatile in nature and so are likely to change slowly in concentration during ordinary use. These include concentrated hydrochloric acid and ammonia.
A compound commonly used for preparation of a precisely standard solution is anhydrous sodium carbonate. It is best prepared from highly pure sodium carbonate. This is achieved by heating sodium bicarbonate to constant mass to make sure the compound is fully decomposed.
2NaHCO3(s) →Na2CO3(s) + H2O(g) + CO2(g)
The sodium carbonate so formed is suitable for preparation of a standard solution and can be weighed without undergoing any appreciable change in composition.
Precautions to be observed while preparing a standard solution.
(a) Transference of the substance from the weighing bottle to the beaker or flasks should be done with outmost care so that not a single particle of the substance is lost.
(b) Undissolved substance should not be transferred to the measuring flask. Make sure all the solid dissolves into solution before transferring the solution to flask.
(c) During making up of the volume, the last drop of the water should be added carefully. Do not blow out the final drop.