Analysis of Your Formulation and the Burning/Stinging Sensation

Based on the ingredients and process you described, the burning and stinging sensation you experienced is most likely due to issues related to the pH of your formulation, particularly the use of Triethanolamine and Lactic Acid for adjustment.

Here's a breakdown of the potential causes:

• pH Imbalance: Triethanolamine is a strong alkaline ingredient used to increase pH. If too much was added initially, as you mentioned ("มือหนักไปนิดปรับเกินเลย"), the pH would have become too high, which can cause significant skin irritation, burning, and stinging. While you used Lactic Acid to lower the pH afterward, the final pH might still have been outside the optimal range for skin compatibility (typically pH 4.0-6.0 for many cosmetic products). A pH that is too high or too low can compromise the skin barrier and lead to irritation.
• Triethanolamine Concentration: The description for Triethanolamine 99% (Product ID 139) recommends a usage rate of 0.1-2.5% and warns against using more than 2.5% and direct skin contact due to its irritating nature. If you used a large amount to raise the pH significantly before correcting with Lactic Acid, the concentration might have been too high at that stage, or the resulting salts from neutralization could contribute to irritation.
• Lactic Acid and Final pH: Lactic Acid (Product ID 170) is an Alpha Hydroxy Acid (AHA) that helps with exfoliation and skin renewal. While beneficial, AHAs can cause sensitivity, especially at higher concentrations or lower pH levels. For consumer products, Lactic Acid concentration should generally not exceed 10%, and the final product pH should be no lower than 3.5. Your formula contains multiple actives, and if the final pH ended up too low after adding Lactic Acid, it could contribute to the burning sensation.
• French Pine Bark Solubility: You mentioned that the French Pine Bark Extract (Product ID 717) did not dissolve well. Undissolved particles can cause physical irritation on the skin. The description indicates it is water-soluble and should be mixed in the water phase. Ensuring it is fully dissolved is important for even distribution and efficacy.
• Combination of Actives: Your formula contains several active ingredients, including ODA (Octadecenedioic Acid), Niacinamide (Vitamin B3), French Pine Bark Extract, and Lactic Acid. While these ingredients offer various skin benefits, combining multiple actives, especially at relatively high concentrations (your formula has 7% Niacinamide), can increase the overall potential for skin sensitivity and irritation for some individuals.

Recommendations to Avoid Irritation:

1. Precise pH Adjustment: When adjusting pH, add Triethanolamine or Lactic Acid very gradually, using a pH meter or pH strips to check the pH frequently until you reach the desired range (ideally between 4.0 and 6.0 for this type of serum, ensuring it is not below 3.5). Avoid drastic pH swings.
2. Check Usage Rates: Double-check the recommended maximum usage rates for all ingredients, especially Triethanolamine (max 2.5%) and Lactic Acid (max 10% for consumer products, with final pH > 3.5).
3. Ensure Solubility: Make sure all ingredients are fully dissolved or properly dispersed according to their instructions. For French Pine Bark Extract, ensure it is completely dissolved in the water phase. For ODA, confirm its solubility properties and the best method for incorporation into your specific base.
4. Consider Ingredient Concentrations: If you continue to experience irritation, you might consider slightly reducing the concentration of actives like Niacinamide or Lactic Acid to see if your skin tolerates the formula better.
5. Review Mixing Order: Follow the recommended mixing instructions for each ingredient, particularly for the gel maker (Pro Polymer) and ingredients added to the water phase.

The staff's suggestion that Triethanolamine was a likely cause is valid, as over-adjusting the pH with this alkaline ingredient can be very irritating to the skin.