EDTA, ethylenediaminetetraacetic acid, has four carboxyl groups and two amine groups that can act as electron pair donors, or Lewis bases.  The ability of EDTA to potentially donate its six lone pairs of electrons for the formation of coordinate covalent bonds to metal cations makes EDTA a hexadentate ligand. However, in practice EDTA is usually only partially ionized, and thus forms fewer than six coordinate covalent bonds with metal cations.
Disodium EDTA is commonly used to standardize aqueous solutions of transition metal cations.  Disodium EDTA (often written as NaHY) only forms four coordinate covalent bonds to metal cations at pH values ≤ 12.  In this pH range, the amine groups remain protonated and thus unable to donate electrons to the formation of coordinate covalent bonds.  Note that the shorthand form NaHY can be used to represent any species of EDTA, with x designating the number of acidic protons bonded to the EDTA molecule.
EDTA forms an octahedral complex with most 2+ metal cations, M, in aqueous solution. The main reason that EDTA is used so extensively in the standardization of metal cation solutions is that the formation constant for most metal cation-EDTA complexes is very high, meaning that the equilibrium for the reaction:
:M + HY → MHY + 2H
lies far to the right. Carrying out the reaction in a basic buffer solution removes H as it is formed, which also favors the formation of the EDTA-metal cation complex reaction product. For most purposes it can be considered that the formation of the metal cation-EDTA complex goes to completion, and this is chiefly why EDTA is used in titrations and standardizations of this type.