The scientific name for Vitamin C is Ascorbic Acid.
Ascorbic Acid has three primary functions:

Boost collagen production : It help keeping the skin firm and reduces wrinkles for anti aging
Antioxidant : It protects the skin from oxidative damage, caused by UV sunlights and Air Pollution. This is why it is so important to use Ascorbic Acid serums in the morning along with sunscreen of minimum SPF 25.
Brighten the skin: including fade hyperpigmentation from acne scars, sun damage and melasma.

How to Keep Ascorbic Acid from oxidize
Vitamin C is extremely unstable. Which means that it will oxidizes and making it less potent and effective when exposed to sunlight and air. This is why it is recommend to be storage in cool area and away from the expose to the sun.
It is for best result to use your vitamin C within 3 months once the it is open and being used. After 3 months, you might expect to see the formula begin to decrease and oxidize.

Vitamin C being very unstable means you have to consider the packaging and storage of keeping your serums. The Aarde packaging are airless tubes, pumps and in amber bottles which help keeping product from oxidize. But keep in mind that the recommended time for Vitamin C is about 3 months or less before they turn brown.
It is most effective between 5-15% except it’s a derivative. A higher concentration might cause irritaton which might cause hyerpigmentation. if you have sensitive skin, it also recommended to use Vitamin C serums under 15%.