Determination of NaOH Concentration using Methyl Orange Indicator: A Step-by-Step Guide

NaOH, also known as sodium hydroxide, is a common base used in various applications, from industrial processes to laboratory experiments. Determining its concentration accurately is crucial for precise measurements. In this article, we will walkthrough the process of calculating the concentration of NaOH in mol/dm3 and g/dm3 using methyl orange as a pH indicator. This method is widely used in laboratory settings for titration experiments.

Understanding the Methyl Orange Indicator

Methyl orange is a popular pH indicator due to its precise color change range. It changes from red to yellow as the pH value shifts from 3.1 to 4.4. This wide range of color change makes methyl orange an excellent choice for determining the endpoint in many titration experiments.

Experiment Setup

25 ml of NaOH solution is pipetted into an Erlenmeyer flask. Good laboratory practice dictates that all equipment and solutions should be prepared and checked beforehand to ensure accuracy and safety.

Three drops of prepared methyl orange indicator solution are added to the NaOH solution. The solution should appear yellow, indicating an initial basic environment.

The titration is performed with a standardized HCl solution of similar strength as the NaOH solution. The endpoint is identified when the solution turns red, marking the equivalence point where the moles of HCl added equal the moles of NaOH present.

Calculating the pH and Endpoint

The pH at the midpoint of the color change (from yellow to red) is calculated by averaging the pH values at the color change range:

pH at midpoint (3.1 4.4) / 2 3.75

The Henderson-Hasselbalch equation can be used to calculate the hydrogen ion concentration [H ]:

pH -log[H ] 3.75

Thus, [H ] 10-3.75 0.001 M or 0.574 moles/liter.

This indicates the excess hydrogen ions from the HCl titrated to neutralize the NaOH.

Titration Equation and Calculation

The titration equation is as follows:

MHCl in mole/l x VHCl in l MNaOH in mole/l x VNaOH in l

Solving for MNaOH gives:

MNaOH in mole/l MHCl in mole/l x VHCl in l / VNaOH in l

After the titration, the MNaOH in mole/l is calculated based on the known MHCl, volume of HCl, and volume of NaOH:

MNaOH in mole/l MHCl in mole/l x VHCl in l / 0.025 l

Subtracting the initial concentration from the calculated concentration provides the precise concentration of NaOH in mole/dm3.

Finally, to convert the concentration from mole/dm3 to g/dm3, we use the molar mass of NaOH, which is 40 g/mole:

MNaOH in g/dm3 40 g/mole x MNaOH in mole/dm3

Key Takeaways and Conclusion

Understanding the use of methyl orange as a pH indicator and accurately titrating with HCl enables the determination of NaOH concentration in both mol/dm3 and g/dm3. This method is crucial for ensuring precise measurements in various applications, from industrial manufacturing to scientific research.

By following these steps and accurately interpreting the results, you can confidently determine the concentration of NaOH in your solution, providing valuable insights for further experiments and applications.