Why do we use the disodium salt of EDTA but not its tetrasodium salt in complexometric titration?

It's probably a matter of convenience and .

EDTA is ##"E"##thylene##"D"##iamine ##"T"##etra##"A"##cetic acid.

EDTA is a tetrabasic acid. It is essentially insoluble in water.

The solubility increases as you add NaOH and convert the acid groups to their salts.

The disodium salt is soluble in water at room temperature up to a concentration of 0.26 mol/L.

The pH of the solution is in the range of 4 to 6.

At pH 8.5 a 0.5 mol/L solution at 4 °C is stable for months.

The disodium salt is available in standard purity as the dihydrate.

The tetrasodium salt is soluble in water at room temperature up to a concentration of 1.45 mol/L.

The solution is rather viscous and has a pH between 10 and 11.

You can use the tetrasodium salt at higher concentrations.

The drawback is that you must also work at a higher pH.

If pH is not a concern, you should be able to use either salt.

A pH of 11 may cause other, unwanted, reactions to occur in your system.

You should then switch to the disodium salt and its lower pH.