Explanation of Yellowing in Hada Labo + SAP + Alpha Arbutin Mixture

Based on the information from the conversation, the yellowing observed in your mixture of Hada Labo lotion, Sodium Ascorbyl Phosphate (SAP) 10%, and Alpha Arbutin 2% after about 15 days is most likely due to the degradation of the Sodium Ascorbyl Phosphate (SAP).

Here's why:

• Vitamin C Degradation: Sodium Ascorbyl Phosphate is a derivative of Vitamin C. While generally more stable than L-Ascorbic Acid, it still degrades over time, and yellowing is a common indicator of this process.
• pH Sensitivity: SAP is most stable within a specific pH range, typically around 6-7. If the pH of the base product (the Hada Labo lotion) is significantly outside this optimal range, particularly if it is more acidic, it can accelerate the degradation of the SAP.
• Likely Reason for Fast Degradation: The relatively quick yellowing (within 15 days) suggests that the pH environment provided by the Hada Labo lotion base was not ideal for SAP stability, likely being too acidic for optimal shelf life of the SAP in the mixture.

In summary, the yellowing is primarily caused by the degradation of the SAP, likely triggered or accelerated by the pH of the Hada Labo lotion base being outside the optimal stability range for SAP.