Chemistry of indicator

Definition: An acid-base indicator is either a weak acid or weak base that exhibits a color change as the concentration of hydrogen (H+) or hydroxide (OH-) ions changes in an aqueous solution.

Examples:

Thymol Blue, Phenol Red and Methyl Orange are all common acid-base indicators. Red cabbage can also be used as an acid-base indicator.

Principle working of indicators.

Indicators are complicate organic weak acids or bases with complicated structures. For simplicity, we represent a general indicator by the formula HIn, and its ionization in a solution by the equilibrium,

HIn = H+ + In-,and define the equilibrium constant as Kai, [H+][In-] Kai = ----------. [HIn]

Which can be rearranged to give

[In-] Kai

------- = ----- [HIn] [H+]

When [H+] is greater than 10 Kai, In- color dominates, whereas color due to HIn dominates if [H+] < Kai / 10. The above equation indicates that the color change is the most sensitive when [H+] = Kai in numerical value.

Taking the negative log of Kai gives,

[In-] -log Kai = -log[H+] - log------ [HIn]or [In-] pH = pKai + log----- [HIn]This is a very important formula, and its derivation is very simple.

Some Common Indicators

Name Acid color PH range Base color

Methyl violet yellow 0.0 - 1.6 Blue

Methyl orange red 3.2 - 4.4 yellow

Litmus red 5.0 - 8.0 Blue

Phenolphthalein colorless 8.2 - 10.0 Pink

Thymolphthalein colorless 9.4 - 10.6 Blue

Bromothymol blue yellow 6.0 - 7.6 Blue

types of indicators

there are 3 types of indicators:-

pH indicator, a chemical detector for protons in acid-base titrations Redox indicator, a chemical detector for redox titrations Complexometric indicator, a chemical detector for metal ions in complexometric titrations

A   pH indicator   and its function

A pH indicator is a halochromic chemical compound that is added in small amounts to asolution so that

the pH (acidity or basicity) of the solution can be determined visually. Hence a pH indicator is

a chemical detector for hydronium ions (H3O+) or hydrogen ions (H+) in theArrhenius model. Normally, the

indicator causes the colour of the solution to change depending on the pH.

Application

pH indicators are frequently employed in titrations in analytical chemistry and biology to determine the

extent of a chemical reaction. Because of the subjective choice (determination) of color, pH indicators are

susceptible to imprecise readings.

A   redox indicator   and its function

A redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a

definite color change at a specific electrode potential.

The requirement for fast and reversible color change means that the oxidation-reduction equilibrium for an

indicator redox system needs to be established very fast.

ApplicationThese indicators are frequently employed in titrations. these indictors are oxidizing reducing agents

Which are mainly use for the determination of the presence of the matalic ions like iron etc.

Determination of iron using potassium dichromate: Redox indicators 

As an oxidant, dichromate has some advantages over permanganate, but, as it is less powerful, its use is much more limited. It is obtainable in a state of high purity and can be used as a primary standard. Solutions of dichromate in water are stable indefinitely. Cr2O7

2- + 6 Fe2+ + 14H+ → 2Cr3+ + 6 Fe3+ + 7H2O

A   complexometric indicator and its function

A complexometric indicator is an ionochromic dye that undergoes a definite color change in presence

of specific metal ions.[1] It forms a weak complex with the ions present in the solution, which has a

significantly different color from the form existing outside the complex. Complexometric indicators are also

known as pM indicators.

Complexometric indicators are water-soluble organic molecules. Some examples are:

Calcein with EDTA for calcium Eriochrome Black T for calcium, magnesium and aluminium Fast Sulphon Black with EDTA for copper.

Application of complexometric indicator

any complexation reaction can be used as a volumetric technique provided that:

1. the reaction reaches equilibrium rapidly after each portion of titrant is added.2. interfering situations do not arise. For instance, the stepwise formation of several different

complexes of the metal ion with the titrant, resulting in the presence of more than one complex in solution during the titration process.

3. a complexometric indicator capable of locating equivalence point with fair accuracy is available.

In practice, the use of EDTA as a titrant is well established.