"...I thought it was like bread hydration where the EDTA was 39% of the water...."
You have a valid point, but your technical training encourages you to see the situation from a different perspective than most people. Non-chemistry folks almost always base the amount or percentage of an ingredient on the
total weight of the solution or mixture.
I'd say that's generally true here on SMF, although it never hurts to ask and make sure, as you wisely did. In soaping, we often see the "ppo" convention used, which is basing ingredients on the weight of fats, not on the total weight. So even in non-chemistry groups, there can be exceptions to this rule.*
There's only water and EDTA in this solution, so you'd use the calculation I gave previously to calculate % EDTA on the basis of
total solution weight --
% EDTA in the solution = (wt EDTA) / (wt EDTA + wt Water) X 100 = 39 / (39+61) X 100 = 39%
The other way to calculate concentration is to base the amount or percentage of an ingredient
on the main liquid (more correctly called the "solvent") in the solution or mixture. That would be the basis you were thinking of. It's not an everyday basis to use, but using a solvent basis is handy when making up a given solution or mixture -- there's less math to fiddle with.
For the EDTA solution, water is the solvent, so this is the calculation you'd use to calculate the % EDTA on a
solvent basis --
% EDTA based on solvent weight = (wt EDTA) / (wt Water) X 100 = 39 / 61 X 100 = 64%
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"...I'm still not sure why I would want to use 50% instead of some other number like 39%...."
I'd say the main reasons why people use a 50% solution rather than some other percentage are convenience and accuracy. This is the same reason one would use a solvent basis rather than a total solution basis -- the numbers are easier to calculate, so the math is easier and there's less chance for error.
Another reason is to increase the overall consistency of the soaping process. Many of us masterbatch NaOH as a 50% solution, so why not use a 50% concentration for EDTA and other solutions or mixtures? This common basis reduces the chance for mistakes in making these solutions and in using them.
Last -- probably least important for an EDTA solution -- is that a 50% solution requires a smaller container than a 39% solution.
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* There's another exception to the rule that I see in soaping circles. That's when people talk about making brine (solseife?) soap. You will see people get into arguments about how much table salt you can dissolve in plain water to make a saturated solution. Some say the answer is about 26% salt and others say it's about 35% salt.
They're actually both right except neither side is presenting the numbers correctly. The correct way to state this info is this way -- At room temperature, a brine of table salt and water is saturated at about 26% salt by weight on a
total solution basis or 35% by weight on a
water (solvent) basis.
I think this confusion comes when non-chemist people look for information about making a saturated salt brine and don't understand that chemistry info can be presented in a number of different ways. They just see a number -- 26% or 35% -- and don't realize there's more to the story.
