Question

Why do we use disodium EDTA?

Answer 1

Because generally, we don't need all six teeth to bind to a given target in practice, and because EDTA by itself is poorly soluble in water. Disodium EDTA is water-soluble.

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EDTA, or ethylenediamminetetraaceticacid, a hexadentate, chelating ligand, looks like this:

So you can see that four "teeth" on EDTA belong to the acetate oxygens, and two of them belong to tertiary amine nitrogens. EDTA has a high complexation formation constant, which means the equilibrium is highly in favor of EDTA binding.

However, EDTA by itself is NOT very water-soluble.

In disodium EDTA, or `"Na"_2"H"_2"Y"` for short, the two nitrogens are protonated because their pKas (`~36` for ammonia) are higher than the pKa of acetic acid (`~4.76`), and thus, they are more easily protonated. In the meantime, the sodium is just an alkali metal counterion, which promotes solubility in water.

This makes `"Na"_2"H"_2"Y"` a water-soluble tetradentate (the two sodiums don't interfere with the binding capabilities of EDTA), not to mention, it works nicely under more typical pH's, i.e. `<= 12`.

In practice, it is generally more useful and convenient to have EDTA partially ionized as disodium EDTA since it is easier to dissolve in water for a complexometric experiment.