Preparation of NaOH
Solutions of NaOH can be prepared by either dissolving solid NaOH pellets in water or by diluting a concentrated solution of NaOH. However, the exact concentration of the solution prepared by these methods cannot be calculated from the weighed mass or using the dilution equation for two reasons:
1. Solid sodium hydroxide is hygroscopic (“water-loving”). Pellets of NaOH exposed to air will increase in mass as they become hydrated so the actual mass of pure NaOH is not accurately known.
2. Sodium hydroxide in solution reacts with carbonic acid and its concentration decreases over time. The acid is formed when small amounts of CO2 gas (which is always present in air) dissolves in solution.
H2CO3(aq) + NaOH(aq) —– H2O + Na+(aq) + HCO3 –(aq)
The water used to make the NaOH solution can be boiled to expel the dissolved CO2 gas but this time-consuming procedure is often not possible in a short laboratory period. A stock solution of NaOH can be made in advance with boiled water but will re-absorb CO2 over a period of time unless stored in airtight containers. Therefore, if we want to know the exact concentration of a freshly made NaOH solution, we need to “standardize” it. That is, determine its exact concentration by titrating it with a known mass of a primary standard acid.
A “primary standard” is a substance that is used to determine the concentration of a solution. A primary standard should have the following properties: It should be available in very pure form at reasonable cost and should have a high equivalent weight to minimize weighing errors. It should be stable at room temperature, easy to dry, and should not easily absorb water when exposed to air (hygrophobic).
Potassium hydrogen phthalate (“KHP”) is the primary standard reagent commonly used to standardize NaOH. It is a monoprotic acid whose formula is KHC8H4O4 and molecular weight is 204.22 g/mol.
KHC8H4O4(aq) + NaOH(aq) ⎯→ H2O + Na+(aq) + K+(aq) + C8H4O4(aq)
The white powdery acid is normally heated at 110°C for one hour to remove any loosely bound waters of hydration and then cooled in a desiccator before use.
The exact mass (and number of moles of acid) is determined by weighing the dried acid on an analytical balance. The acid is then dissolved in water and NaOH is added until an endpoint (the point at which an indicator changes color) is reached. The phenophthalein indicator used in this experiment is colorless in acid and pink in base. Therefore, the solution containing KHP will remain colorless as long as some KHP is still present. Once the last of the KHP has reacted, the solution will turn pink with one excess drop of base.
The exact concentration of NaOH is calculated by using the stoichiometry from reaction to convert the number of moles of KHP used to moles of NaOH and then dividing by the volume of NaOH used to reach the endpoint of the reaction.