Understanding the Reaction
Phosphoric acid is a triprotic acid, meaning it can donate three protons (H⁺) in a stepwise manner. However, in a typical titration with NaOH, the reaction proceeds in stages, and the endpoint we aim for is the first equivalence point, corresponding to the reaction:
H₃PO₄(aq) + NaOH(aq) → NaH₂PO₄(aq) + H₂O(l)
Procedure
1. Prepare Solutions:
- Make a standard solution of NaOH with a known concentration.
- Prepare the phosphoric acid solution of unknown concentration.
2. Titration:
- Pipette a known volume of the phosphoric acid solution into a flask.
- Add a few drops of a suitable indicator (e.g., phenolphthalein) to the flask.
- Titrate the phosphoric acid solution with the standard NaOH solution slowly, swirling the flask constantly.
- The indicator will change color at the equivalence point, indicating complete neutralization of the first proton.
- Record the volume of NaOH solution used.
Calculations
1. Moles of NaOH:
- Moles (NaOH) = Concentration (NaOH) × Volume (NaOH)
- Note: Convert the volume of NaOH used to liters.
2. Moles of H₃PO₄:
- From the balanced chemical equation, 1 mole of NaOH reacts with 1 mole of H₃PO₄. Therefore, the moles of NaOH at the equivalence point are equal to the moles of H₃PO₄ in the solution.
- Moles (H₃PO₄) = Moles (NaOH)
3. Concentration of H₃PO₄:
- Concentration (H₃PO₄) = Moles (H₃PO₄) / Volume (H₃PO₄)
- Note: Convert the volume of the phosphoric acid solution to liters.
Example
Let's say you titrate 25.0 mL of phosphoric acid solution with 15.5 mL of 0.100 M NaOH solution:
1. Moles of NaOH:
- 0.100 mol/L × 0.0155 L = 0.00155 mol NaOH
2. Moles of H₃PO₄:
- 0.00155 mol NaOH = 0.00155 mol H₃PO₄
3. Concentration of H₃PO₄:
- 0.00155 mol / 0.0250 L = 0.0620 M H₃PO₄
Important Points:
* The indicator choice is crucial. Phenolphthalein is suitable for the first equivalence point as the pH at this point is around 4.6.
* Ensure accurate measurements of volumes and concentrations.
* Repeat the titration multiple times to get reliable results.
Let me know if you have any specific questions or would like to work through a more complex example!
