Complexometric titration
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Complexometric titration is a type of titration based on complex formation between the analyte and titrant.
EDTA Complexometric Titration
EDTA or (ethylenediamminetetraacetic acid), has four carboxyl groups and two nitrogen atoms that can act as electron pair donors, therefore it is also a bidentate ligand or Lewis base. This changes in an alkali solution where the EDTA molecule is more stable. It can be written as H4Y.
By definition, at high (basic) pH the solution surrounding a molecule will actively be trying to remove H+ protons attached to the molecule. For example at a certain pH a COOH group will have an H+ removed, making it a COO-. SO, in high pH EDTA forms a Y4- as the alkali pH removes the H+ ions from the carboxyl (COOH) groups. This is now a hexadentate ligand and can form an octahedral complex with most metal 2+ ions (M2+), hence the name complexometeric titration. The complex formed with the ion MY2- will be in a 1:1 ratio and is a reason why EDTA is so widely used to determine amounts of metal ions such as manganese and calcium in water or to help treat lead poisoning in humans as the EDTA binds with the heavy, toxic metal.
In the titration the use of a complexometric indicator (a suitable organic dye, eg: Fast Sulphon Black, Eriochrome Black T or Eriochrome Red B) needs to be used so that it will change color when bound to the metal ion and will be displaced by EDTA, as its bonded to the metal weaker than the EDTA would be. Changing back to original color once the EDTA fully reacted to form octahedral complexes with all the metal ions. This pH is best at 10 as any higher or lower and the EDTA becomes less stable, and is usually achieved and kept by addition of ammonia buffer solution.