Adjusting pH of Lactic Acid Solution

You are correct that the pH of your Lactic Acid solution (10% Lactic Acid 88% in Water) is very low at 1.9. However, adding more water will only dilute the solution and will not effectively raise the pH to the target of 3.5.

To increase the pH of an acidic solution like this, you need to add a base (alkaline substance) to neutralize some of the acid. As the staff reply suggested, you can use either Triethanolamine 99% or Sodium Hydroxide.

Recommendation:

It is generally recommended to use Triethanolamine 99% for adjusting the pH of cosmetic formulations containing AHA acids like Lactic Acid. This is because Triethanolamine is a milder base compared to Sodium Hydroxide, which is very strong and requires more caution during handling.

How to Adjust pH:

1. Add the base (Triethanolamine 99% or Sodium Hydroxide solution) to your Lactic Acid mixture very slowly, drop by drop.
2. After adding a few drops, stir the mixture thoroughly.
3. Measure the pH using a reliable pH meter or pH strips.
4. Repeat steps 1-3, adding the base gradually and checking the pH, until you reach your target pH of 3.5.

It is crucial to add the base slowly and check the pH frequently to avoid over-neutralizing the solution, which would make it too alkaline.

According to regulations for AHA products intended for consumers, the concentration of Lactic Acid should not exceed 10% and the final product pH should not be lower than 3.5. Your initial concentration is within the limit, but the pH needs to be adjusted upwards for safe use.