Here's evidence of what we're talking about, Psfred.
I made 4 mixtures of NaOH in distilled water -- 50% w/w, 37.5%, 25%, and 12.5%. To each of these, I added 2 grams of freshly made tetrasodium EDTA solution, 50% w/w in distilled water. (edit: EDTA was purchased in powder form from Lotioncrafter.) Mixtures were swirled to mix.
50% NaOH + EDTA -> Mixture initially had a translucent curdled look, but quickly changed to a dense white precipitate. Slightly thickened.
37.5% -> Slight amount of white precipitate. No apparent thickening.
25% -> No apparent change. Solution remained water clear.
12.5% -> Ditto
Somewhere between a 25% NaOH concentration and a 37.5% NaOH concentration is the tipping point between the EDTA obviously reacting with the alkali vs. not apparently reacting.
I generally do what the others do -- mix the EDTA and other additives into the fats and then add the lye solution. I don't know whether that really accomplishes much besides hiding the formation of the precipitate, but it makes me feel better.
Edit: After thinking about it for a bit, most of us tend to soap with a final NaOH concentration of 33% or less, even if we use a 50% masterbatch, so I imagine adding the EDTA to the fats, etc. probably does do some good -- keeps the overall NaOH concentration lower.
Edit: I'll add distilled water to the two more concentrated solutions to bring them down to, say, 33% NaOH and see if the precipitate clears.
First photo: All four mixtures of NaOH, distilled water, EDTA. L to R: 50% NaOH, 37.5%, 25%, 12.5%
Second photo: 50% NaOH. Opaque white precipitate
Third photo: 37.5% NaOH. Slight white precipitate.
Fourth photo: 25% NaOH. Water clear. (The 12.5% mixture looks the same)