Standardizing a Sodium Hydroxide Solution with Potassium Hydrogen Phthalate (KHP)

Objective:
To accurately determine the concentration of a sodium hydroxide (NaOH) solution using potassium hydrogen phthalate (KHP) as a primary standard. This experiment demonstrates how to use a primary standard to find the concentration of an unknown solution by titration.

Materials:

• KHP (C₈H₅KO₄), a weak acid used as a primary standard
• NaOH solution of unknown concentration
• Phenolphthalein indicator (turns pink in basic solution)
• Burette
• Pipette
• Erlenmeyer flask (125 mL or 250 mL)
• Analytical balance
• Distilled water

Procedure:

1. Preparation:
   • Clean and rinse all glassware, especially the burette and Erlenmeyer flask, to avoid contamination.
   • Accurately weigh about 0.7-0.9 grams of KHP (the exact mass should be recorded) on an analytical balance.
2. Dissolving the KHP:
   • Transfer the weighed KHP to an Erlenmeyer flask and dissolve it in approximately 50 mL of distilled water. Swirl gently until the KHP fully dissolves.
3. Adding the Indicator:
   • Add 2-3 drops of phenolphthalein indicator to the KHP solution. The solution will remain colorless since it is acidic.
4. Titration:
   • Fill the burette with the NaOH solution, ensuring that there are no air bubbles, and record the initial volume.
   • Begin titrating by slowly adding NaOH to the KHP solution while continuously swirling the flask.
   • As you approach the endpoint, a faint pink color will appear momentarily in the solution. Continue adding NaOH dropwise until a stable, pale pink color persists for 30 seconds. This indicates that the endpoint has been reached.
5. Calculations:
   • Record the final volume of NaOH in the burette.
   • Calculate the volume of NaOH used by subtracting the initial volume from the final volume.
   • Use the molar mass of KHP (204.22 g/mol) and its known mass to find the moles of KHP, which is equivalent to the moles of NaOH at the endpoint due to the 1:1 stoichiometric ratio.

Calculations

1. Calculate the moles of KHP:

2. Determine the concentration of NaOH:

Since the reaction between KHP and NaOH is 1:1, the moles of KHP are equal to the moles of NaOH at the endpoint.

Results:
The concentration of the NaOH solution is approximately 0.163 M.

Key Concept

Using a primary standard, like KHP, allows for accurate determination of a solution’s concentration. KHP is ideal because it is pure, stable, and has a known molar mass, making it reliable for standardization.

FAQs

1. Why is KHP used as a primary standard?
   • KHP is pure, stable, and has a high molar mass, which minimizes weighing errors. Its acidic nature allows it to react completely with bases like NaOH, making it ideal for accurate titrations.
2. Why do we use phenolphthalein as an indicator?
   • Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions. It provides a clear endpoint when the NaOH neutralizes the KHP.
3. What if the pink color fades after 30 seconds?
   • A brief color change might indicate incomplete neutralization. Ideally, the endpoint is reached when the pink color persists for 30 seconds, indicating a stable pH level in the basic range.
4. Why should I titrate slowly as I approach the endpoint?
   • Near the endpoint, small amounts of NaOH cause a significant pH change. Adding NaOH too quickly could overshoot the endpoint, leading to inaccurate results.
5. Can I use a different acid instead of KHP?
   • While other acids can be used, KHP’s properties make it a preferred primary standard for NaOH titrations. It’s more accurate than many alternatives because it is highly pure and stable.
6. How should I calculate errors in my experiment?
   • To assess precision, perform multiple titrations and calculate the mean NaOH concentration. You can calculate the standard deviation to evaluate the consistency of your results.
7. What could cause errors in this titration experiment?
   • Errors can arise from inaccurate measurements, not reaching the true endpoint, or improper rinsing of equipment. Using distilled water and careful technique can minimize these errors.