Preservative Recommendations for Hypoallergenic Whitening Serum (pH 5.5)

For a hypoallergenic serum with a pH of 5.5 and a variety of water-soluble actives, I recommend using a "Preservative-Free" system. These systems use ingredients that provide broad-spectrum antimicrobial protection but are not classified as traditional preservatives, making them ideal for sensitive skin and hypoallergenic claims.

Recommended Preservative Systems

1. Mild Preserved HD™ (Preservative-Free)

   • Composition: 1,2-Hexanediol and Hydroxyacetophenone.
   • Why it fits: This is the most suitable choice for your formula. Hydroxyacetophenone is a powerful antioxidant that also acts as an anti-irritant, which directly supports your "hypoallergenic" goal.
   • Compatibility: It is 100% water-soluble, stable at pH 3-8, and will not cloud your serum.
   • Usage Rate: 0.8% - 1.2% (1.0% is recommended for water-based serums).

2. Mild Preserved Eco™ (Preservative-Free)

   • Composition: Ethylhexylglycerin and 1,3-Propanediol.
   • Why it fits: Very gentle and also acts as a humectant to help retain skin moisture.
   • Compatibility: Works well in pH 4-9.
   • Usage Rate: 1.0% - 1.5%.

Formulation Considerations for Your Actives

• Synergy with Glycols: Your formula already contains 1,3-Propanediol, Pentylene Glycol, and Butylene Glycol. These glycols act as "preservative boosters," significantly increasing the efficacy of the systems mentioned above.
• Active Stability: Your key whitening actives (Alpha Arbutin, 4MSK, Ethyl Ascorbic Acid, Tranexamic Acid) and peptides (Matrixyl 3000) are all stable and effective at pH 5.5, which perfectly matches these preservative systems.
• Zinc PCA & Sodium PCA: These electrolytes are compatible with the recommended systems, as they do not rely on ionic charge for efficacy.

Mixing Instructions

• These ingredients should be added at the final stage of mixing, once the serum base has been formed and cooled (if heat was used).
• Ensure thorough mixing for at least 5-10 minutes to ensure complete dissolution and distribution.

Summary Recommendation: Use Mild Preserved HD™ at 1.0% for the best balance of safety, efficacy, and skin-soothing properties.