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How to Tell if a Vitamin C Serum Oxidized

What is Vitamin C and Why is it Important?

 Vitamin C also known as L-Ascorbic Acid (LAA) is the most abundant water-soluble antioxidant found in your skin that is continuously depleted as environmental aggressors constantly attack it. Vitamin C plays an extremely important role in photoprotection, neocollagenesis, and inhibition of melanogenesis. Therefore, vitamin C must constantly be replenished and supplemented to protect skin from photodamage,and environmental aggressors known to expedite the premature signs of aging.

The Ideal Vitamin C Serum:

 The ideal vitamin C serum needs to be formulated correctly and packaged properly to prevent vitamin C from oxidizing. An effective vitamin C serum must meet three important criteria as established in clinical studies. The perfect vitamin C serum must be in the form of L-Ascorbic Acid, must have a pH range between 3.2-3.5, and must not exceed a concentration of 20% vitamin C. After a vitamin C serum has been opened and used as instructed, it begins to turn yellow over time which is the product beginning to oxidize.

How you can tell if a Vitamin C Serum Oxidized:

 It depends how oxidized the vitamin C serum is. Depending on the formulation, Vitamin C is typically clear or light yellow in color. Vitamin C (L-Ascorbic Acid) is typically unstable in water and oxidizes over time.

Color of Vitamin C:              

The color of the vitamin C serum will give you a good idea if it is still effective to use. If your vitamin c serum is clear or a light yellow in color, the serum was most likely manufactured less than a year ago and stored properly. If the vitamin C serum is yellow, it was most likely manufactured over 6 months ago with proper storage. A serum that is dark brown in color indicates the vitamin C serum has oxidized and will not be as effective and should be discarded. If the vitamin C serum hasn’t been opened, the serum could have been improperly stored or has been manufactured over a year ago.

Oxidized Vitamin C

Figure 1: Vitamin C Serums in Droppers 

Vitamin C Color:



Dark Brown

OK to Use




Manufactured Date


6-12 months old

> 1 Year


Factors that Influence Vitamin C Oxidation:

            The three factors that accelerate vitamin C oxidation are heat, light, and air.  Heat and light can be limited by storing your vitamin C serum in a cool, dark place such as the refrigerator. Air can be protected against by tightly capping the bottle immediately after using your serum until you’re ready to apply the serum again.

How to Properly Store a Vitamin C Serum:

 A vitamin C serum should be stored in a cool, dark place. Be sure to avoid storing your serums near sunlight, rooms with fluctuating temperature, and be sure to tightly cap the serum after each use. Additionally, as a precaution, vitamin C serums can be stored in a refrigerator until you are ready to use it.