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Calculate Ph of 0.025 M Naocl

Reviewed by Calculator Editorial Team

This calculator determines the pH of a 0.025 M sodium hypochlorite (NaOCl) solution using the dissociation constant of hypochlorous acid (HOCl). The result helps assess the solution's disinfection effectiveness and safety.

Introduction

Sodium hypochlorite (NaOCl) is a common disinfectant used in water treatment, swimming pools, and medical applications. Its pH is crucial for determining its effectiveness and potential hazards. A 0.025 M NaOCl solution is typically used in diluted form for disinfection purposes.

The pH of a NaOCl solution is primarily determined by the dissociation of HOCl, which forms in water:

Dissociation Reaction

NaOCl + H₂O ⇌ Na⁺ + HOCl⁻ + H₂O

HOCl ⇌ H⁺ + OCl⁻

The pH is calculated using the dissociation constant of HOCl (Kₐ = 3.0 × 10⁻⁸ at 25°C).

Formula

The pH of a NaOCl solution can be calculated using the following steps:

  1. Determine the concentration of HOCl in equilibrium with NaOCl.
  2. Calculate the concentration of H⁺ ions using the dissociation constant of HOCl.
  3. Convert the H⁺ concentration to pH using the pH formula.

pH Calculation Formula

pH = -log[H⁺]

[H⁺] = √(Kₐ × [HOCl])

For a 0.025 M NaOCl solution:

[HOCl] = 0.025 M (assuming complete dissociation)

Calculation

For a 0.025 M NaOCl solution:

  1. Assume [HOCl] = 0.025 M (since NaOCl dissociates completely in water).
  2. Use Kₐ = 3.0 × 10⁻⁸ for HOCl.
  3. Calculate [H⁺] = √(3.0 × 10⁻⁸ × 0.025) ≈ 2.45 × 10⁻⁵ M.
  4. Calculate pH = -log(2.45 × 10⁻⁵) ≈ 4.61.

Note

The actual pH may vary slightly due to temperature effects and other impurities, but this calculation provides a good approximation for a pure 0.025 M NaOCl solution.

Interpretation

A pH of approximately 4.61 indicates that the solution is acidic, which is typical for NaOCl solutions. This acidity is necessary for the disinfection process but should be monitored to avoid potential corrosion or irritation.

Comparison Table:

Solution Concentration Calculated pH
NaOCl 0.025 M 4.61
NaOCl 0.1 M 3.85
NaOCl 1.0 M 3.10

FAQ

Why is the pH of NaOCl important?

The pH of NaOCl affects its disinfection efficiency and potential hazards. A pH of 4.61 indicates the solution is acidic enough to be effective but not too corrosive.

Can the pH of NaOCl change over time?

Yes, the pH can decrease as the solution degrades or as impurities are introduced. Regular testing is recommended for consistent results.

Is a 0.025 M NaOCl solution safe to use?

Yes, when properly diluted and used according to safety guidelines, a 0.025 M NaOCl solution is safe for disinfection purposes.