Preparation and Safety Precautions
Before starting the titration, make sure you have the necessary equipment and chemicals:
- Phosphoric acid (H3PO4) solution (0.1 M)
- NaOH solution (0.1 M)
- Buret
- Conical flask
- Calibration curve (optional)
Wear protective gloves and goggles to prevent skin and eye irritation. Work in a well-ventilated area, away from open flames or sparks.
Phosphoric acid is a weak acid, and NaOH is a strong base. The reaction between them is highly exothermic, so be prepared for a rapid temperature increase during the titration process.
Standardization of NaOH Solution
Before titrating the phosphoric acid, standardize the NaOH solution by reacting it with a known amount of potassium hydrogen phthalate (KH2C4H4O4). This step ensures the accuracy of the NaOH concentration.
Measure 25 mL of 0.1 M NaOH solution into a conical flask. Add 25 mL of 0.1 M KH2C4H4O4 solution. Use a pH meter to monitor the pH change during the reaction.
Continue adding NaOH solution from the burette until the pH reaches a stable value of around 7. This indicates the endpoint of the reaction. Record the volume of NaOH added and calculate the average concentration of the NaOH solution.
Titration Procedure
Measure 20 mL of 0.1 M phosphoric acid solution into a conical flask. Add a few drops of indicator (such as phenolphthalein or bromothymol blue) to the solution.
Titrate the phosphoric acid solution with the standardized NaOH solution from the burette while stirring the mixture. Record the initial pH of the solution before adding the first drop of NaOH.
Continue adding NaOH solution until the pH reaches the endpoint, which is indicated by a color change of the indicator. Record the final pH reading and the volume of NaOH added.
Calculations and Data Analysis
Calculate the number of moles of NaOH added using the formula:
moles NaOH = (volume NaOH x concentration NaOH) / 1000
Calculate the number of moles of phosphoric acid using the formula:
moles H3PO4 = (moles NaOH x (1 + 3x))
where x is the ratio of the number of moles of H+
Compare the calculated number of moles of phosphoric acid to the known amount of phosphoric acid in the initial solution. This will give you the accuracy of the titration.
Example Calculations and Comparison Table
| Titration Run | Volume NaOH (mL) | moles NaOH | moles H3PO4 | Accuracy |
|---|---|---|---|---|
| 1 | 25.4 | 0.00254 | 0.00249 | 98.0% |
| 2 | 24.9 | 0.00249 | 0.00251 | 100.4% |
| 3 | 25.2 | 0.00252 | 0.00248 | 98.4% |
Compare the accuracy of each titration run by calculating the percentage difference between the calculated and known amount of phosphoric acid.
Repeat the titration process several times to ensure consistent results and to obtain an accurate average value.
Common Issues and Troubleshooting
Some common issues that may occur during the titration of phosphoric acid with NaOH include:
- Difficulty in obtaining a sharp endpoint: Check the pH meter and indicator for proper function. Ensure the pH reading is accurate and consistent.
- Variability in the accuracy of the titration: Check the standardization of the NaOH solution and the concentration of the phosphoric acid solution.
- Insufficient or excessive pH change: Adjust the indicator used or the pH range of the titration.