You want to use a dark bottle because it prevents the l-ascorbic powder from oxidizing. L-ascorbic powder is destabilized when it reacts with light and/or heat, so store your homemade C serum in a cool, dark place.
Mix well and use up the C serum as early as possible because l-ascorbic acid is highly unstable. Make a new batch every few days because as time goes on, the l-ascorbic acid grows less potent. Make sure the l-ascorbic acid you buy is 100 per cent with no additional ingredients. You can buy powder or crystals (from the vitamin section in your local health food store), but Christine likes to use powder because it dissolves faster. Sometimes vitamin C will just list "ascorbic acid" as its ingredient. If this is the case, call the company to see whether or not it's l-ascorbic acid. Also, make sure there are no unnecessary ingredients in the vitamin C capsules you buy (such as sweeteners, rose hip oils, etc).
If you find this percentage of vitamin C to be too irritating, you can always add less l-ascorbic acid. If it's not strong enough, you can add more, but don't go over 20 per cent.