EDTA equations

EDTA Titration for the Hardness of Water
EQUATIONS

The STANDARD
CaCO₃+ 2 HCl -------> CaCl₂ + CO₂ + H₂O

Pure Calcium Carbonate is dissolved by adding a slight excess of HCl. This solution is then quantitatively transferred to a 100 mL volumetric flask, diluted to volume and mixed thoroughly.

This standard calcium solution, of known concentration, is used to determine the exact concentration of the EDTA solution.

TITRATION
Ca⁺²+ EDTA^{-4 -}-----> CaEDTA^-2

Adding the EDTA solution to the calcium, standard or unknown, is done by titration. The chelate, CaEDTA^-2, is very stable so this reaction proceeds quantitatively from left to right. It derives its stability in part to calcium being connected to the EDTA by six bonds.

On the left is a molecular model of CaEDTA^-2,
C₁₀H₁₂O₈N₂Ca

carbons are black

hydrogens are white

oxygens are red

nitrogens are blue

calcium ion is the green ball

In the titration of Ca⁺² with EDTA^-4 to produce CaEDTA^-2, all components are colorless so you can not tell when you have added the stoichiometric amount of EDTA to the reaction. To recognize the end point, you must add the indicator, eriochromschwartz-T.

END POINT
Mg In + EDTA^-4 ------> MgEDTA^-2 + In^-2

Eriochromschwartz-T is a dye. When chelated or acidified, it produces a wine red solution. Not chelated and under basic conditions (pH = 10 in this experiment) the dye is blue. This color change is used to determine the end point in this titration.


A	B	C

The three pictures depict the end point in this titration. There is a 1 drop difference of 0.01 M EDTA between A and B and between B and C. Two or three seconds were allowed for the colors in B and the color in C to develop after adding the additional drop. In each case the solution was thoroughly mixed.

The "slowness" of the end point is due in part to the compact nature of the complex.