The structure of EDTA and its chemistry
Ethylenediaminetetraacetic acid (EDTA) is a weak acid with the chemical formula of C10H16N2O8 and a molecular mass of 292.24 g/mol. Its structure is seen in Fig 1 below. It is also a ligand, and can form complexes with metal ions, making it a lewis base (an electron pair donor).
Taking a closer look at EDTA, it has four carboxylic groups (COOH) and two amino groups (NH2). 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. These pairs of electrons can be donated to a metal ion, each forming a coordinate covalent bond. 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.
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).
With six coordinate covalent bonds, the resulting complex has an octahedral geometry which is very stable. 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.
Different forms of EDTA and how it works
There are various species of EDTA. The classic form, as seen in Fig 1.1, consists of four hydrogen. This can be written as H4Y. Apart from that, two more hydrogen atoms can be added to EDTA, each one binding to one of the nitrogen. This makes the fully protonated form of EDTA, which is H6Y2+. The charge is 2+ as two protons in the form of hydrogen atoms were added. EDTA is a polyprotic acid. This means that it can donate more than one proton or hydrogen atom per molecule to an aqueous solution. Thus, all the species of EDTA from fully protonated to fully deprotonated are: H6Y2+, H5Y+, H4Y, H3Y-, H2Y2-, HY3- and Y4-.
In an aqueous solution, the fractional composition, the fraction of each species of EDTA, is determined by the pH. At low pH, H6Y2+ is dominant, while at a high pH, Y4- is dominant. This is due to the pKa values of each hydrogen atom. pKa is the pH value at which a chemical species will accept or donate a proton. 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. At a high pH, the EDTA molecule will donate more protons in the form of hydrogen atoms, thus becoming more deprotonated.
From 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. 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. If hydrogen ions are present, the oxygen atoms are unable to form a coordinate covalent bond with the metal ion, and thus only Y4- is able to bond with metal ions.
Equilibrium constant and Complexometric Titration
Complexometric titration is based on forming complexes with metal ions. It is often used to determine the amount of metal ion, or metal ions in a solution. For complexometric titration involving EDTA, the EDTA bonds with the metal ion. The general equation is:
Mn+ + Y4- ⇌ MYn-4 , where the metal ion, M, has a charge of n+, and complexes with the fully deprotonated form of EDTA (Y4-).
For this equation, the formation constant (Kf), which is also the equilibrium constant, is Kf =[MYn-4] / [Mn+][Y4-]ー(2).
However, as mentioned before, the concentration of Y4- is dependent on pH. Therefore, equation (1) needs to be substituted into (2).
Going back to equation (1), αY4- = [Y4-]/[EDTA]
αY4- [EDTA] = [Y4-] ー(3)
Substituting (3) into (2),
Kf = [MYn-4] / [Mn+] αY4- [EDTA]
Since αY4- is constant at a constant pH, it can be multiplied out such that: K αY4- = [MYn-4] / [Mn+] [EDTA]
K αY4- can be written as Kf’. This is the conditional formation constant of the equation, conditional as it is dependent on the pH. Kf’ now describes the formation of [MYn-4] at any pH.
In order for a EDTA titration to be successful, the reaction has to be near completion, such that virtually all the metal ions are complexed with the EDTA. For this to happen, the equilibrium has to lie far to the right, meaning that the formation constant has to be very large. As a general value, it is assumed that Kf’ ≥ 10^8 in order for the titration to work.
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