Indicators Class 11 Notes | EduRev

Chemistry for JEE

Class 11 : Indicators Class 11 Notes | EduRev

The document Indicators Class 11 Notes | EduRev is a part of the Class 11 Course Chemistry for JEE.
All you need of Class 11 at this link: Class 11

Indicators Indicators Class 11 Notes | EduRev

Broadly defined, as indicator is substance which is used for the visual detection and determination of a specific constituent present in any sample. The visual observation used is primarily that of colour, but observations of fluorescence and turbidity are also used.Indicators under this general definition include all reagents which can be used in colorimetry flurimetry and turbidimetry. It is convenient, therefore, to define on indicator in a more limited way, as substance which is used for the visual detection of the completion of a particular reaction that is for the end point of a titration.

Acid-Base Indicators :- Acid- base indicator are organic substances which have one colour in acid solution while an altogether different colour in alkaline solution.

Theories of Acid-Base Indicators :-

Two important theories have been put forwarded to explain their behaviour:

(I) Ostwald's theory (1891) :  According to this theory:

(i) Acid- base indicators are weak organic acids or bases.

(ii) They have different colours in ionised and non-ionised states i.e.

         Hln       Indicators Class 11 Notes | EduRev     H+  + ln-

    (one colour)          (different colour)

(iii) The colour of the indicator depends on the relative proportions of the unionised indicator molecules and its ions.

On the basis of above postulates. Ostwald explained the action of phenophthalein, methyl orange, methyl red and other acid-base indicators.

 

(a) Action of Phenolphthalein:- Phenolphthalein is a weak acid (HPh) and is almost unionised. Its unionised molecules are colourless whilst on ionisation give colourless H  ions and pink coloured Ph- ions.

        HPh   Indicators Class 11 Notes | EduRev             H+         +     Ph-

   (colourless)            (colourless)     (pink)

In the presence of acid due to increase in the concentration of common H  ions, the dissociation of HPh is suppressed and thus the solution becomes colourless.

On the other hand, the addition of strong bases (like NaOH, KOH), however, the OH- ions produced from them combine with the H  ions from the phenonphthalein to form feebly ionised water. The equilbrium (i) is thus disturbed and more of the phenolphthalein ionises to produce Ph- ions. The latter combine with Na  ions to form the strongly ionised sodium salt NaPh and hence remains in the ionic state giving pink coloured Ph- ions.

Indicators Class 11 Notes | EduRev

(b) Methyl orange :- It is a weak base and can be represented as MeOH. Its undissociated molecule is yellow while gives red coloured Me  ions on dissociation,

MeOH     Indicators Class 11 Notes | EduRev      Me+         +          OH-

(yellow)                (Red)                 (colourless)

If a base (i.e., OH- ions) is added to the indicator, the OH- ions will suppress the ionisation of the indicator. Hence, the indicator will remain yellow in an alkali. However, if a small excess of acid (say, HCl) is added, the latter will force the equilibrium to the right by removing OH- ions to form H2O. This will result in the formation of red coloured Me  ions in the solution.

MeOH           Indicators Class 11 Notes | EduRev  Me+             +   OH-

(yellow)                 (Red)           (colourless)

 

HCl Indicators Class 11 Notes | EduRev Cl-              +          H+

                 Indicators Class 11 Notes | EduRev                           Indicators Class 11 Notes | EduRev

             MeCl                        H2O

           (Highly ionized)      (Unionised)

(II) Modern Quinoid Theory :- Main postulates of this theory are:

(i) The indicators used in acid-alkali titrations are aromatic organic compounds which are equilibrium mixtures of at least two tautomeric forms, ordinarily one form is benzenoid while the other is quinoid.

(ii) The two forms have different colours. The quinoid form is usually deeper in colour than the benzenoid form. Out of these one form exists in acidic solution while other in alkaline solution.

Indicators Class 11 Notes | EduRev

(iii) Change in pH causes the transition of benzenoid form to quinoid form and vice versa and consequently a change in colour.

This theory explains the action of phenolphthalein, methyl orange and other acid-base indicators.

(a) Action of Phenolphthalein:- Phenolphthalein is an acidic indicator undergoing the following transformation:

Indicators Class 11 Notes | EduRev

In alkaline medium, OH- ions combine with H  ions produced by the indicator, the equilibrium shifts to the right producing pink colour. In acid medium the dissociation of the organic acid is suppressed, the equilibrium shifts to the left and the solution becomes colourless.

(b) Action of Methyl Orange

Methyl orange is a basic indicator. More correctly it is an amphoteric compound containing both acidic group -SO3H and basic group -N(CH3)2. The quinoid form (red) combines with the OH-ions in alkaline medium favouring the formation of yellow.

Indicators Class 11 Notes | EduRev  

(c) Action of Methyl Red

Methyl red is a basic indicator and the colour change takes place according to the following scheme:

Indicators Class 11 Notes | EduRev

Acid-Base Indicators with pH range of colour change

Indicator

Colour in
 acid

Colour in alkali

pH range of colour change

Methyl orange

Red

Yellow

3.2 - 4.4

Bromo-cresol

green

Yellow

Blue

3.8 - 5.4

Methyl red

Yellow

Red

4.8 - 6.0

Bromothymol

blue

Yellow

Blue

6 - 7.5

Phenol red

Yellow

Red

6.3 - 8.4

Phenolphthalein

Colourless

Pink

8.2 - 10

Thymol blue

Yellow

Blue

8 - 9.6

Thymolphthalein

Colourless

Blue

9. 4 - 10.6

 

Choice of Indicators:-

At the equivalence point of acid- base titration there occurs a sudden jump in pH of the solution. An indicator, the pH range of colour change of which falls within this limit, is suitable and is used in that titration. No sudden pH jump of the solution in the titration of weak acid with weak base occurs and so not indicator is suitable for this titration.

Offer running on EduRev: Apply code STAYHOME200 to get INR 200 off on our premium plan EduRev Infinity!

Related Searches

Sample Paper

,

Extra Questions

,

past year papers

,

Summary

,

Objective type Questions

,

shortcuts and tricks

,

Indicators Class 11 Notes | EduRev

,

ppt

,

MCQs

,

Important questions

,

Indicators Class 11 Notes | EduRev

,

practice quizzes

,

Indicators Class 11 Notes | EduRev

,

pdf

,

Semester Notes

,

mock tests for examination

,

Exam

,

video lectures

,

Free

,

study material

,

Previous Year Questions with Solutions

,

Viva Questions

;