An easy way to make sure your Vitamin C is in the perfect form to get the most benefits you can use a pH test strip. Your serum should be right around 3.5.
Secondly, make a small batch and use it only for 3-5 days. It is some extra work, but trust me, the benefits are worth it.
Feel free to use this on the backs of your hands as well. As most of us already know, sun spots show up on the hands first. Vitamin C can cause your skin to be irritated at first. When you apply it, a small amount of tingling is normal. If it is burning, remove it immediately. Don't worry - you can just add more water and/or glycerin to your mixture. This will dilute the Vitamin C so it is more suited for your skin.