Monday, January 9, 2012

NEUTRALIZATION TITRATIONS- Neutralization indicators

Indicators for acid base titrations are substances which change color according to the hydrogen ion concentration of the solution. The chief characteristic of these indicators is that the change from a predominantly acid color to a predominantly alkaline color is not sudden and abrupt, but takes place within a small interval of pH (usually about two pH units), termed the color change interval of the indicator.
The equilibrium between the acidic form InA and the Basic form InB may be expressed as

The observed color of an indicator in solution is determined by the ratio of the concentrations of the acidic and basic forms
[InA] / [InB]

The color change equilibrium at any particular ionic strength can be expressed by,

Where pKIn = apparent indicator constant.

When the ratio [InA] / [InB] > 10 the acidic color will be visible and the corresponding limit of pH is,
pH = pKIn -1

The alkaline color will be visible when ratio [InB] / [InA] > 10, and the corresponding limit of pH is,
pH = pKIn + 1

The color change interval is accordingly,
pH = pKIn ± 1 (over approximately two pH units)


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