Titration Analysis Method-Coordination Titration Method (2)

3.

In coordination titration, the indicator is used to indicate the change in the concentration of metal ions during the titration process, so it is called a metal indicator

(1) The working principle of the metal indicator

Figure 2.

Before the start of the titration to the stoichiometric point, since the product MgY ^2- produced by the reaction of EDTA and Mg ²⁺ is colorless, the solution is always the red of MgIn ^- .

The chemical formula of the titration reaction is

At the stoichiometric point, all the metal ions have reacted completely.

When pH=10, chrome black T is pure blue, so at the end of the reaction, the color of the solution changes from red to blue, and this color change can be used to indicate the end of the titration

(2) The conditions that the metal indicator possesses

1) Under the pH conditions required by the titration, the indicator and the metal-indicator integration have obvious color changes

2) The metal-indicator chelate should have proper stability, and its stability must be less than the stability of EDTA and metal ions, but the stability of the metal-indicator conjugate should not be too low, otherwise the conjugate should be chemically stable.

3) The reaction between the indicator and the metal ion should be rapid, and In and Mln should be easily soluble in water

4) The indicator should have good selectivity, and only show color with the measured ion under the measurement conditions

5) The indicator itself should be stable in the air and easy to store