The pH of Sodium Benzoate in Water

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The pH of sodium benzoate in water is dependent on the conversion of sodium benzoate to benzoic acid, its active form, at a low pH. Benzoic acid is very pH dependent and shows some activity up to pH 6, but is most active at pH 3. Sodium benzoate is often combined with potassium sorbate in low pH products to provide a synergistic preservative effect against yeast and mold.

Understanding the pH of Sodium Benzoate

To calculate the pH of a solution of sodium benzoate, the Ka of benzoic acid and the molarity of the sodium benzoate solution are needed. The pH of a 0.10 M solution of sodium benzoate, given that the Ka of benzoic acid is 6.50 x 10^-5, can be calculated as follows:

  1. Determine the concentration of benzoate ion (C6H5COO-) in the solution: [C6H5COO-] = 0.10 M
  2. Calculate the concentration of benzoic acid (C6H5COOH) in the solution using the equilibrium expression for weak acids: K_a = [C6H5COOH][C6H5COO-] / [H+]
  3. Solve for [C6H5COOH]: [C6H5COOH] = K_a * [C6H5COO-] / [H+]
  4. Calculate the pH using the expression pH = -log[H+]: pH = -log([C6H5COOH] / (K_a * [C6H5COO-]) + 1)

The pH of a 0.10 M solution of sodium benzoate, given that the Ka of benzoic acid is 6.50 x 10^-5, is calculated to be 8.70.

Factors Affecting the pH of Sodium Benzoate

PH of sodium benzoate in waterImage source: picryl

  1. Conversion to Benzoic Acid: The pH of sodium benzoate in water is dependent on the conversion of sodium benzoate to benzoic acid, its active form, at a low pH. Benzoic acid is very pH dependent and shows some activity up to pH 6, but is most active at pH 3.

  2. Combination with Potassium Sorbate: Sodium benzoate is often combined with potassium sorbate in low pH products to provide a synergistic preservative effect against yeast and mold.

  3. Benzene Formation: There is concern that sodium benzoate in combination with L-ascorbic acid in products can form benzene, a known human carcinogen. The presence of ascorbic acid and benzoates alone does not lead to the formation of benzene. Certain additional conditions are required for trace levels of benzene to form, including heat, ultraviolet light, and metallic ions in the mixture.

  4. Role of EDTA: The use of EDTA, which chelates metallic ions, can be of assistance in the inhibition of benzene formation.

  5. Interaction with Citric Acid: Citric acid is not thought to induce significant benzene production in combination with benzoic acid, but some evidence suggests that in the presence of ascorbic acid and benzoic acid, citric acid may accelerate the production of benzene.

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Conclusion

The pH of sodium benzoate in water is a crucial factor in its effectiveness as a preservative and the potential formation of harmful compounds. Understanding the factors that affect the pH of sodium benzoate, such as its conversion to benzoic acid, its combination with other preservatives, and its interaction with other ingredients, is essential for ensuring the safety and efficacy of products containing this preservative.

References

  1. Testbooktestbook.com › … › Food Preservation
  2. Wyzant.com: Calculate the pH of 0.58 M solution of sodium benzoate
  3. Lotioncrafter.com: Sodium Benzoate
  4. Wyzant.com: Calculate the pH of a 0.10 M solution of sodium benzoate
  5. PubChem: Sodium Benzoate
  6. Homework.study.com: What is the pH of 0.100 M sodium benzoate (Ka = 6.28 × 10^-5)?