Define Negative Correlation

 

Indicators :-

--> An indicator is a substance which is used to determine the end point in a titration. In. acid-base titrations, organic substances (weak acids or weak bases) are generally used. as indicators. They change their colour within a certain pH range.

--> An indicator is a chemical compound that changes its colour in presence of an acid or base. Indicators are generally derived from plant pigments and are mildly acidic or basic in nature.

Theory of indicators :-

In acid-base titrations, organic substances (weak acids or weak bases) are generally used as indicators.

Indicators change their colour within a certain pH range.The colour change and the pH range of some common indica­tors used are tabulated below:

Indicator	pH Range	Colour of Acidic Solution	Colour of Basic Solution
Methyl Orange	3.2-4.5	Orange	Yellow
Methyl Red	4.4 – 6.5	Red	Yellow
Bromothymol blue	6.0 -7.8	Yellow	Blue
Phenolphthalein	8.3- 10.0	Colourless	Pink
Alizarin Yellow	10.1 – 12.1	Yellow	Red
Litmus	5.5-7.5	Red	Blue
Phenol red	6.8-8.4	Yellow	Red

Theory of indicators :-

1) Ostwald's theory :-

According to this theory:-

(a) The colour change is due to ionisation of the acid- base indicator. The unionised form has different colour than the ionised form.

(b) The ionisation of the indicator is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionisation is very much low in acids due to common H+ ions while it is fairly ionised in alkalies. Similarly if the indicator is a weak base, its ionisation is large in acids and low in alkalies due to common OH- ions.

2) ​Quinonoid theory :-

According to quinonoid theory, an acid-base indicators exist in two tautomeric forms having different structures which are in equilibrium. • One form is termed benzenoid form and the other quinonoid form.

The two forms have different colors. The color change is due to the interconversation of one tautomeric form into other. One form mainly exists in acidic medium and the other in alkaline medium. Thus, during titration the medium changes from acidic to alkaline or vice-versa. The change in pH converts one tautomeric form into other and thus, the colour change occurs.

Phenolphthalein has benziod form in acidic medium and thus, it is colourless while it has quinonoid form in alkaline medium which has pink colour.

Methyl orange has quinonoid form in acidic solution and benzenoid form in alkaline solution. The color of benzenoid form is yellow while that of quinoniod form is red.