Understanding the Rust-Like Smell in L-Ascorbic Acid Solutions

The rust-like smell you observe when mixing L-ascorbic acid (standard type) at 15% concentration is a common indicator of oxidation. L-ascorbic acid is highly susceptible to degradation, especially when dissolved in water and exposed to air, light, or heat.

What Causes the Rust-Like Smell?

The rust-like smell is typically associated with the breakdown products of L-ascorbic acid as it oxidizes. This process is significantly accelerated by the presence of metal ions, such as iron, copper, or nickel, which can act as catalysts for the oxidation reaction. These metal ions can be present as impurities in the water used, other ingredients in the formulation, or even leached from mixing equipment.

How to Prevent the Rust-Like Smell?

To help prevent or minimize the rust-like smell caused by oxidation catalyzed by metal ions, you should incorporate a chelating agent into your formulation. A common and effective chelating agent for this purpose is Disodium EDTA. Disodium EDTA works by binding to the metal ions, preventing them from catalyzing the oxidation of L-ascorbic acid.

Additionally, consider the inherent instability of L-ascorbic acid in water-based formulas. For better stability, especially at higher concentrations like 15%, formulators often explore alternative approaches:

• Anhydrous Formulations: Creating formulations without water (e.g., using silicones or oils as the base) can significantly improve the stability of L-ascorbic acid.
• Stabilized Vitamin C Derivatives: Using more stable forms of Vitamin C, such as Sodium Ascorbyl Phosphate (SAP), Magnesium Ascorbyl Phosphate (MAP), Ascorbyl Glucoside, or Ascorbyl Tetraisopalmitate, which are less prone to oxidation in water-based systems.

When working with L-ascorbic acid in water, always ensure your water is deionized or distilled to minimize metal ion content, mix at temperatures below 30°C, maintain the recommended pH range (typically 2.0-4.0), and store the final product properly sealed away from light and heat.