There are two types of citric acid we can buy - anhydrous (water free) and monohydrate (contains water). I have found the anhydrous citric acid I've bought tends to be a powder and, for me, makes bath bomb creation a far easier process than using the monohydrate, which I've noticed tends to be more like a grain. The anhydrous is much less likely to fizz in high humidity, which means you can use it in bath bombs even when its damp outside! (I used to get mine at Voyageur, but they've stopped carrying the powder form, so I get it at Aquarius. Ask your supplier which one you have!) For other applications, it's irrelevant which one you choose.
It is a chelator like EDTA, which we can use to bind metal ions like calcium, magnesium, copper, zinc, nickel, and cobalt to keep our products from experiencing auto-oxidation. By binding these metals, it also keeps surfactant mixes nice and clear! It also helps to boost the efficacy of your preservative.
The difference in using citric acid as your chelating or sequestering agent is the pH changes you might find with it. Citric acid is a weak organic acid, which means it has a pH less than 7 (it ranges between 3.13 to 6.4 depending upon the concentration). When we add it to a lotion or surfactant mix, it can change the pH of our product. For something like a hair care product, we want a pH around the pH of our hair, which is 5.6 to 6.2. For a lotion, we want a pH around 5 to 6 - our skin is about 5.5.
Including too much citric acid in something like a shampoo can change your hair colour! Remember "Sun In"? (It was all the rage when I was a teenager - eek, I'm dating myself here!) It can lighten your hair if you're using too much of it! (You do have to use a decent amount - say 1% or more, so the 0.1% suggested below should be fine!)
Citric acid can be used as an AHA type acid in facial products, but as I've not used it in that context before, I'm not comfortable making any suggestions for use. It can also be used as a buffer (something that maintains the pH of a product) with sodium citrate if you're using things like AHA or Multifruit or Phytofruit or another acidic ingredient.
How much to add? You only want a titch when you're using it as a chelator and anti-oxidant - 0.1% or so - and this shouldn't be enough to change the pH dramatically. If you're using citric acid at higher levels, I'd suggest getting a pH meter (it's on my Christmas list) or using those pH strips.
If you are interested in learning more about acid-base reactions and pH, you can start here with the Wikipedia entry. If you're interested in learning more about buffers, again I offer you a Wikipedia entry!
Join me tomorrow for a discussion on how to determine the shelf life of your lotion, then a closer look at linoleic acid!