Adjusting pH in Your Vitamin C Serum

It's understandable that you're trying to correct the pH of your serum after accidentally adding too much Triethanolamine (TEA). However, without a pH meter, it is impossible to calculate the exact amount of citric acid needed to bring the pH back to the desired level.

Here's why:

• pH is Logarithmic: pH is not a linear scale. The amount of acid or base needed to change the pH depends on the starting pH and the buffering capacity of the solution (how well it resists pH changes). Adding a fixed amount of acid won't always result in the same pH change.
• L-Ascorbic Acid Stability: L-Ascorbic Acid (Vitamin C) is most stable and effective in a narrow pH range, typically between 2.5 and 3.5. Adding excess TEA would raise the pH significantly, causing the L-Ascorbic Acid to degrade rapidly. Guessing the amount of citric acid could result in a pH that is still too high (leading to degradation) or too low (potentially causing skin irritation).

Therefore, the only reliable way to adjust the pH is by using a pH meter or accurate pH strips and adding a dilute solution of citric acid gradually while measuring the pH until the target range (ideally 2.5-3.0 for L-Ascorbic Acid stability) is reached.

Since you don't have a pH meter, adding citric acid would be a risky guess. The best approach in this situation is to follow the advice given previously:

1. Store the serum in the refrigerator: This will help slow down the degradation of the L-Ascorbic Acid caused by the higher pH.
2. Use the serum quickly: Due to the compromised stability, the serum will likely have a shorter shelf life.

The tingling sensation you experienced is normal for serums containing L-Ascorbic Acid at a low, effective pH, similar to products like SkinCeuticals CE Ferulic. This sensation is due to the acidic nature of the Vitamin C itself.

For future formulations, investing in a pH meter is highly recommended for accurate pH adjustment, especially when working with sensitive ingredients like L-Ascorbic Acid, Vitamin E, and Ferulic Acid, and bases like Triethanolamine.