I use tetrasodium EDTA in my batches (I have really hard water, too). I use it at .5% of the entire batch weight. By entire batch weight, I mean everything- oil, lye, water, FO, colorants, and any other additives I might use. And if I have some things that are too light to weigh (i.e., the weight of them is too light to register on the scale), I'll just count them as being 1 gram.
I make my own 39% solution out of the dry form (if you use less water than that, it doesn't dissolve properly), and then I do a couple of math calculations to make sure to add enough of the solution to equate to having .5% of the EDTA in my batch. It sounds complicated, but once you've done it a few times, it's as easy as buttoning your shirt. This is what I do:
First, I make up a bottle of 39% EDTA solution to store on hand.
Then I tally up the weight of everything in my batch
in grams. It's very important that you use grams to tally your batch weight for this and not ounces
, because the following calculations are based on using gram weights.
Next, I take that total gram weight and multiply it by .5% and jot the sum down on paper.
Then I take that sum and multiply it by 2.56 (I can give you the reasoning behind this specific figure later if you want, but for now, all that is necessary is to multiply the gram sum by 2.56). The resulting sum you get from that will be how much of the 39% solution
in grams you will need to weigh out for your batch.
Besides helping soap to lather in hard water, tetrasodium EDTA also helps to prevent DOS, and it acts as a preservative booster in lotions and liquid soaps. It's not a preservative per se, but its ability to starve bacteria of their food helps your chosen preservative to do it's job better.
IrishLass
