EDTA is Ethylene Diamine Tetra-acetic acid (Edetic Acid), and is rarely used
as it is very difficultly soluble in water (0.5 grams per liter) and is less
stable than its salts. It forms compounds with sodium and other metals,
which are the forms in which normally used. The most common is EDTA
Tetrasodium Salt, which I think you call "Tetra EDTA." There are also
monosodium, disodium, and trisodium salts, plus ferric EDTA and salts with
EDTA of almost any other metal (the ferric EDTA is the major ingredient in
Bleach-Fixer).
I think it's important that you know this stuff so you can order the right
materials. Common names can lead to wrong materials, and sometimes
disastrous results!
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net