![]() This can be seen as there are no hydrogen atoms, allowing the O- ions to have a pair of unbonded electrons to form coordinate bonds with the metal ion. At a high pH, the EDTA molecule will donate more protons in the form of hydrogen atoms, thus becoming more deprotonated.įrom Fig 2, showing the EDTA bonding to a metal ion, it shows that only the Y4- species is able to bond with the metal ion. Acid-base equilibria is an important yet challenging topic in the syllabus that can be made simple with the help of our highly experienced chemistry tutor. pKa is the pH value at which a chemical species will accept or donate a proton. This is due to the pKa values of each hydrogen atom. At low pH, H6Y2+ is dominant, while at a high pH, Y4- is dominant. In an aqueous solution, the fractional composition, the fraction of each species of EDTA, is determined by the pH. Thus, all the species of EDTA from fully protonated to fully deprotonated are: H6Y2+, H5Y+, H4Y, H3Y-, H2Y2-, HY3- and Y4. This means that it can donate more than one proton or hydrogen atom per molecule to an aqueous solution. The charge is 2+ as two protons in the form of hydrogen atoms were added. This makes the fully protonated form of EDTA, which is H6Y2+. Apart from that, two more hydrogen atoms can be added to EDTA, each one binding to one of the nitrogen. The classic form, as seen in Fig 1.1, consists of four hydrogen. This is why all metal-EDTA complexes have a one to one stoichiometry, meaning that one molecule of EDTA bonds with one metal ion, as the metal ion bonding to an additional EDTA molecule would cause the complex to become less stable. With six coordinate covalent bonds, the resulting complex has an octahedral geometry which is very stable. The concepts of ligands and lewis bases are taught in our chemistry tuition classes under the topic “Introduction to Transitin Elements” (a common last topic which was removed for the past 2 years in the ‘A’ level examination due to COVID). EDTA with a metal ion forming a hexadentate complex Thus, this allows EDTA to form a maximum of six coordinate covalent bonds with a metal ion, as seen in Fig 2, making it a hexadentate ligand. These pairs of electrons can be donated to a metal ion, each forming a coordinate covalent bond. The nitrogen of the two amino groups and the four oxygen that form a single bond with carbon in the carboxylic groups each have a pair of unbonded electrons. Taking a closer look at EDTA, it has four carboxylic groups (COOH) and two amino groups (NH2). It is also a ligand, and can form complexes with metal ions, making it a lewis base (an electron pair donor). Ethylenediaminetetraacetic acid (EDTA) is a weak acid with the chemical formula of C10H16N2O8 and a molecular mass of 292.24 g/mol.
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