-
Call Now
1800-102-2727
Theory of Indicators- Definition, Calculation of pH, pH Range, Practice Problems and FAQs
Yashvi is very happy today as her mother is cooking her favorite dish Cheese and Garlic Noodles. Yashvi wants to follow and look up every step in cooking very closely. Her mother is making cheese at home from milk because she wants healthy noodles. Yashvi don't know earlier that cheese can be made at home and found this procedure very interesting. On adding vinegar to boiling milk, Yashvi’s mother told her that after a particular time cheese will get separated from milk. The indicates that when cheese is formed, it is a change in physical state from liquid to solid.
In Acid-Base reactions,the point of neutralization and change of acidity/ basicity of the mixture does not show any colour or observable physical change. Indicators are used as an indicating agent to exactly pin point the complete neutralization by changing or imparting new colour.
Table of Content
- Indicators
- Calculation of pH
- ph range and colour change
- Practice Problems
- Frequently asked questions-FAQ
Indicators
Acids and bases are mostly colourless. The salts generated as a result of acid and base reactions are also mostly colourless. As a result, determining the titration's equivalence point or endpoint or neutralization point is challenging. Indicators are utilised to determine the reaction's endpoint by their changing colour.
Indicators are compounds that exhibit different colours at different hydrogen ion concentration..
For Example Phenolphthalein, methyl orange etc.
Similarly, indicators are used as an indicating agent
Weak organic acids or bases that have distinctively different colours in ionized and unionized states are used as indicators.
HX may have different colour and 
may have a different colour.
Example:
Phenolphthalein: It is a weak acid represented by HPh. It is colourless and unionized at lower pH( in acid medium), whereas it is pink and ionized at higher pH(in basic medium).
The following is the equilibrium between the ionized and unionized forms of phenolphthalein (HPh), an acidic indicator:
Colourless Pink
Case-1: The concentration of 
rises when a lot of acids is added to the phenolphthalein. According to Le Chatelier's principle, if the product's concentration rises, the equilibrium moves in the opposite direction to counteract the effect. As a result, in the presence of an acid, the equilibrium backwards, undissociated form is present in significant quantities, and the indicator is colourless.
Case-2: The base, on the other hand, reacts with ions when it is introduced. As a result, the product's concentration falls. To counteract this impact, the equilibrium shifts forward, resulting in the production of ions, which changes the colour of the solution from colourless to pink.
Methyl Orange: MOH stands for methyl orange, which is a weak base. In basic solutions, it is yellow and unionized, but in acidic solutions, it is red and ionized.
The following is the equilibrium between the ionized and unionized forms of methyl orange (
), a basic indicator:
Yellow Red
Case-1: When a large amount of base is added to a methyl orange solution, the concentration of 
rises. If the product concentration rises, the equilibrium shifts backwards, according to Le Chatelier's principle. As a result, the solution has a yellow colour due to the presence of more undissociated forms.
Case-2: However, as the acid is added, it reacts with , lowering the product's concentration. As a result, the equilibrium shifts forward to neutralize this impact, resulting in the creation of ionized form and a change in the colour of the solution from yellow to red.
Calculation of pH
Using Law of Mass Action
By taking negative log on both side
range and colour change
The human eye is incapable of accurately detecting colour changes in indicators. There is always a balance between the ionized and unionized forms. As a result, at equilibrium, both forms or both the colours are there, and we are unable to see the precise colour.
We'll need to use one approximation for this. If the ratio of the ionized and unionized forms is 10: 1, one of the two colours is distinct or particularly perceived by the human eye.
Example: Methyl Orange
| Case-I | Case-II | Case-III |
 |
 |
 |
| Only red colour appears | Only yellow colour appears | Mixture of both yellow and red colour appear |
| Case-I | Case-II |
 |
 |
 |
 |
Therefore, pH range is defined as the range in between which the indicator changes its colour but we can't see that change. If 
, then only colour change can be observed. pH range is given as follows:
Important Note: In between this range, the indicator does not work because we get mixed colours from the indicator.
| Indicator | pH range | Colour change | pH range and corresponding colour change |
| Phenolphthalein | 8.3-10 | Colourless to Pink | |
| Methyl Orange | 3.1-4.4 | Red to yellow | |
| Phenol red | 6.4-8.2 | Yellow to reddish pink |
Practice Problems
Q 1. 
, an acid type indicator, is coloured differently from its corresponding base In-. Only at the ratio 
, does the human eye become sensitive to colour distinctions. What is the minimum pH adjustment in the solution required to see a complete colour change?(
)
Solution: 
Case - 1: 
Case - 2: 
Minimum pH change to observe a complete colour change = 7-5 =2
Q2. Bromothymol blue is an indicator with 
a value of 
. What % of this indicator is in its ionized form at a pH of 5?
Solution: 
At initial concentration C 0 0
Concentration at equilibrium C-x x x
Q 3. In acid solution, an acid-base indicator is red, and in basic solution, it is blue. At pH=5, the indicator is present in the solution in its blue form for 80% of the time. Calculate the pH at which the indicator seems to be 60% red.
Solution:
Red Blue
80% blue in basic form means 
60% red in acidic form means 
Q 4. The indicator is red in its acid form and blue in its basic version. How much does the pH have to alter for the indication to go from 60 % red to 60 % blue? (=
)
Solution: 
Red Blue
60% red in acidic form means
60% blue in basic form means 
Change in pH = 4.706-4.353=0.3531
Frequently asked questions-FAQ
Q 1.What is universal indicator?
Answer: A pH indicator formed from a solution of many compounds that exhibits several smooth colour changes across a wide range of pH values to indicate the acidity or alkalinity of solutions is known as a universal indicator.
Q 2. What are olfactory indicators?
Answer: An olfactory indication is a material that changes its smell depending on whether it's mixed with an acidic or basic solution. In the laboratory, olfactory indicators can be used to determine whether a solution is a basic or an acid, a procedure known as olfactory titration. For example Onion
Q 3.Is there any natural indicator exist which is used for acid-base titration?
Answer: Red cabbage, turmeric, grape juice, turnip skin, curry powder, cherries, beetroots, onion, and tomato are examples of Natural indicators that can be found in nature and can be used to identify whether a substance is acidic or basic.
Q 4.Can you predict whether litmus is a natural indicator or a synthetic one?
Answer: Litmus paper is a type of natural indicator.
It's a purple dye derived from lichen, which is a type of plant. Litmus paper is divided into two categories: red litmus paper and blue litmus paper. If the substance is acidic, the blue litmus paper becomes red. If the substance is basic or alkaline, the red litmus paper becomes blue.
Related Topics
| Buffer-Acidic Buffer | Buffer-Basic buffer and Buffer Capacity |
| Acid-Base Titrations | Acids, Bases, and Salts |
| Ionic Product of water |
Call Now
Let's Chat
Share