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Indicators Indicators | Chemistry Class 11 - NEET

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 | Chemistry Class 11 - NEET     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 | Chemistry Class 11 - NEET             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 | Chemistry Class 11 - NEET

(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 | Chemistry Class 11 - NEET      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 | Chemistry Class 11 - NEET  Me+             +   OH-

(yellow)                 (Red)           (colourless)

 

HCl Indicators | Chemistry Class 11 - NEET Cl-              +          H+

                 Indicators | Chemistry Class 11 - NEET                           Indicators | Chemistry Class 11 - NEET

             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 | Chemistry Class 11 - NEET

(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 | Chemistry Class 11 - NEET

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 | Chemistry Class 11 - NEET  

(c) Action of Methyl Red

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

Indicators | Chemistry Class 11 - NEET

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.

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FAQs on Indicators - Chemistry Class 11 - NEET

1. What are economic indicators and why are they important for understanding the state of the economy?
Ans. Economic indicators are statistics used to measure and assess the performance of an economy. They provide valuable insights into the overall health and direction of economic activities. These indicators help policymakers, investors, and businesses make informed decisions by providing information about inflation, employment, GDP, consumer spending, and other key aspects of the economy.
2. How are economic indicators classified and what are some examples of each type?
Ans. Economic indicators are classified into three main categories: leading indicators, lagging indicators, and coincident indicators. Leading indicators provide early signals of future changes in the economy, such as stock market performance and building permits. Lagging indicators reflect changes that have already occurred, such as unemployment rates and corporate profits. Coincident indicators move in tandem with the overall economy, such as industrial production and retail sales.
3. What is the significance of GDP as an economic indicator?
Ans. Gross Domestic Product (GDP) is a widely used economic indicator that measures the total value of goods and services produced within a country during a specific period. It provides a snapshot of the overall economic activity and growth. GDP helps policymakers, investors, and businesses gauge the size and health of an economy, identify trends, and compare the performance of different countries or regions.
4. How does inflation impact the economy and what are the key indicators used to measure it?
Ans. Inflation is the general increase in prices of goods and services over time, reducing the purchasing power of money. It can have a significant impact on the economy, affecting consumer spending, business investment, and overall economic stability. Key indicators used to measure inflation include the Consumer Price Index (CPI) and the Producer Price Index (PPI). These indicators track the average price changes of goods and services consumed by households and produced by businesses, respectively.
5. How do employment indicators provide insights into the labor market and the overall economy?
Ans. Employment indicators provide valuable insights into the labor market and the overall health of the economy. The unemployment rate, for example, measures the percentage of the labor force that is actively seeking employment but unable to find it. Other employment indicators include job creation numbers, labor force participation rate, and average hourly earnings. These indicators help policymakers, businesses, and investors understand the level of job opportunities, wage trends, and the overall strength of the labor market.
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