【Weekly Topic】(Forty) 2015.8.26 How should a sodium hydroxide standard solution be prepared? Why?
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How should a sodium hydroxide standard solution be prepared? Why? Answer: A saturated solution should first be prepared; after it settles and the foam is removed, the supernatant is taken to prepare a solution with a concentration close to the desired one. Since sodium hydroxide often contains a certain amount of sodium carbonate, it can affect the calibration results. Sodium carbonate has very low solubility in concentrated sodium hydroxide solutions; once complete precipitation occurs, the supernatant can be collected to prepare the sodium hydroxide standard solution.
Weigh 4.5 g of the reagent NaOH using a tray balance, add it to a beaker that already contains 300 mL of distilled water, stir to dissolve it, then add more distilled water to bring the volume to 1000 mL. Mix well, transfer the solution to a clean 1000 mL reagent bottle, and store it for calibration. When NaOH absorbs CO2, it forms Na2CO3. Since Na2CO3 is not a strong base, it can affect the determination of the titration endpoint; it may even cause the indicator to change color insensitively, thereby affecting the accuracy of the measurement. Therefore, for accurate determination, a NaOH standard solution free of Na2CO3 should be prepared. The method is as follows: Saturated NaOH method: Commercially available NaOH is prepared into a 50% saturated solution (50 g of NaOH dissolved in 50 mL of water); in this concentrated alkali solution, Na2CO3 hardly dissolves and settles at the bottom. After allowing it to stand for one day and night, the clear liquid on top is collected and diluted to the desired volume using distilled water free of CO2 (distilled water is boiled for a few minutes and can then be used after cooling). To prepare 1000 mL of a 0.1 mol•L-1 NaOH solution, 5.0 mL of the aforementioned saturated solution should be taken and diluted to 1000 mL.