Most sequestering agents are based on
phosphonic acid, as in 1-Hydroxyethane 1,1-diphosphonic acid (HEDP), not phosphoric acid, which is H
3PO
4. Phosphoric acid will not sequester metals.
What makes a chelating agent "good" for some metals and not others, is the structure of it's ligands, or the parts of the molecule that become negatively charged when they dissolve in water. In the case of HEDP, there are two phosphonate (H2PO3) ligands attached to a hydroxy ethane backbone, see this picture -
The -OH groups attached to the phosphorous will deprotonate (lose the hydrogen atoms) and form two, doubly negatively charged phosphonate groups, PO3
2- . So this is where the negative charge comes from to attract positively charged metal ions....however, it matters what metal ions are involved and you need to know something about the electronic structure of atoms to understand this. Calcium ions have a 2+ charge on them. These two electrons that it loses comes from the S-orbital of the atom which can be imagined as a spherical shell surrounding the atoms core. Because of this spherical symmetry, Ca
2+ ions can easily bind to the charged phosphonate ligands on either side of the HEDP molecule. There is sufficient distance across the molecule that the two very large calcium atoms will not repel each other too much. Think of it like giant round magnets (the calcium ions) attached to some toothpicks and marshmallows (the ethane backbone) - the calcium atoms will repel each other a bit but if they are sufficiently held apart, the repulsion will just cause the toothpicks and marshmallows to flex and bend a bit.
Iron, on the other hand, is a transition metal and, when it loses it's electrons, they come from the outermost electron orbitals. Because the S-shell (spherical) and P-shell electrons are very close in energy, different electrons can be involved in the bonding process. This is a picture of what the electron orbital shapes look like, s- and p- (there are other orbital shapes but they get very complicated) -
S and P orbitals can also mix during bonding causing very complicated bonding structures. Typically with iron, the p-orbital lobes will be involved in chelations and so the phosphonate groups will bond to two of the p-oribital axes (x & y, y & z or x & z). Since iron comes in both 2+ and 3+ oxidation states, the orbital bonding to the HEDP will become very complicated and mixed and will often involve two HEDP molecules binding to an iron atom. Sometimes the bonds involved don't allow for an easy structure to form and that will make certain metals less easy to chelate.
As different chelating agents (HEDP, PBTC, EDTA, CDTA, etc, etc.) have different ligand groups, there will be some that more favorably chelate a specific metal than others. HEDP and related phosphonate tend to bind to calcium and magnesium better than iron or copper while EDTA (and it's various derivatives) bind to transition metals (Fe, Cu, Ni, Mn, etc) a little better. Amines, carboxylates and phosphonate all have different bonding characteristics and so the choice of chelating agent really depends on what you want to achieve. HEDP tends to give good results in chlorinated pool water (although PBTC is better) while EDTA, and related compounds, will breakdown quickly and form CCs.
So, if you had a headache before this post, you have a migraine now....
