EDTA
/ ee-dee-tee-AY /
Picture a molecule built like a hand with six fingers, all reaching inward to grip a single metal ion from every side at once. That is EDTA: a man-made acid whose six binding points fold around almost any metal and hold it tightly in a cage.
Chemically, EDTA is ethylenediaminetetraacetic acid, a molecule with four acid groups and two nitrogen atoms — six sites in all that can bind a metal. Crucially, it grabs nearly every metal in a one-to-one ratio, no matter whether the metal carries a charge of two or three, which makes calculations clean.
EDTA matters because it is the workhorse titrant of complexometry and a common preservative and cleaning agent. Its one big catch: it is not picky — it will bind almost any metal present — so a titration must use pH control and masking agents to make sure only the metal you want is being measured.
A 0.0100 mol/L EDTA solution, standardised against pure calcium carbonate, is the everyday titrant for measuring water hardness; because it binds one-to-one, the moles of EDTA equal the total moles of calcium plus magnesium.
EDTA binds metals one-to-one, so moles of titrant equal moles of metal.
The disodium salt Na2H2Y is used in practice rather than the free acid, because it dissolves readily in water; the symbol Y4- stands for the fully deprotonated form that does the binding.