Distilled water normally contains dissolved CO2. When preparing NaOH standard solutions, it is important to use CO2 free distilled water. How does dissolved CO2 in distilled water affect the accuracy of the determination of a NaOH solution’s concentration?
Dissolved CO2 in distilled water can affect the accuracy of the determination of a NaOH solution's concentration because it can react with NaOH, leading to the formation of sodium carbonate (Na2CO3) and water. This reaction consumes NaOH, reducing the actual concentration of the NaOH solution.
To understand why this happens, we need to look at the chemical equation for the reaction between CO2 and NaOH:
2 NaOH + CO2 → Na2CO3 + H2O
In this reaction, two moles of NaOH react with one mole of CO2 to produce one mole of sodium carbonate and one mole of water.
During the preparation of NaOH standard solutions, the concentration of the NaOH solution is determined by titration with a known concentration of an acid, usually hydrochloric acid (HCl). The reaction between NaOH and HCl is used to determine the concentration of the NaOH solution.
However, if there is dissolved CO2 in the distilled water used to prepare the NaOH solution, it can react with NaOH during the titration process. This means that not all of the NaOH will react with the HCl, resulting in a lower measured concentration of the NaOH solution. In other words, the dissolved CO2 introduces a source of error in the determination of the concentration of the NaOH solution.
To avoid this, it is important to use CO2-free distilled water when preparing NaOH standard solutions. This can be achieved by degassing the water, which involves removing any dissolved gases, including CO2. One common method is to boil the distilled water for a short period of time and then allowing it to cool without exposing it to the air. Another method is to sparge the water with an inert gas, such as nitrogen, to remove the dissolved gases.
Using CO2-free distilled water ensures that there is no interference from dissolved CO2 during the titration process, leading to more accurate determination of the concentration of the NaOH solution.