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What is a Universal Indicator?

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Acids are compounds that, when dissolved in water, produce free hydrogen ions (H+ ions). When bases are dissolved in water, they form hydroxyl ions (OH- ions). Hydrogen ions are abundant in acidic solutions and limited in basic solutions. The pH scale is used to determine the acidity or basicity of an acid or base. The pH scale ranges from 0 to 14 on the scale. A pH indicator made up of several chemical compounds is known as a universal indicator.

pH for Acids

The pH of acids or acidic solutions is less than 7. A solution with a pH of less than 7 is said to be acidic. For example, a pH of 4 indicates that the solution is acidic. It should be remembered that the lower the pH of a solution, the more acidic it is. A pH 1 solution, for example, is substantially more acidic than a pH 4 solution. To put it in another way, a pH 1 solution is a significantly stronger acid than a pH 4 solution. Strong acids are often defined as solutions with a pH of 0, 1, 2 or 3. Weak acid solutions have a pH of 4, 5, or 6. It is obvious that a substance’s acidity is proportional to its pH. The pH of strongly acidic chemicals is quite low. In fact, the stronger the acid, the lower the pH.

Since all solutions with a pH less than 7 are acidic in nature, they turn blue litmus red. In addition, they cause the methyl orange indicator to turn red.

pH for Bases

The pH of the bases or basic solutions is greater than 7. When a solution has a pH greater than 7, it is referred to as the basic solution. A solution with a pH of 12 will, for example, be basic in nature or a base. It should be remembered that the higher the pH of a solution, the more basic it is. A solution with pH 14, for example, is significantly more basic than a solution with pH 11.  In other words, a pH 14 solution will be a much stronger base than acid with a pH of 11. Strong bases are often defined as solutions with a pH of 11, 12, 13, or 14. Weak bases are defined as solutions with pH values of 8, 9, and 10. The pH of very basic compounds is extremely high. In fact, the stronger the base, the higher the pH.

Since all solutions with a pH greater than 7 are basic in nature, they turn red litmus blue. In addition, they make the phenolphthalein indicator pink.

pH for Neutral substances

The pH of neutral substances is exactly 7. Pure water is neither acidic nor basic; it is a neutral substance. As a result, pure water has a pH of 7. A sugar solution and a chloride solution are both neutral, with a pH of 7. As a result, any solution with a pH of 7 is considered neutral. Litmus paper, as well as other common indicators like methyl orange and phenolphthalein, will have no impact on a material with a pH of 7.

Universal Indicator

Litmus is a common indicator for determining if a chemical is an acid or a base. They are unable of determining whether a chemical is a strong acid, a weak acid, a strong base, or a weak base. To put it another way, typical indicators can’t tell us how strong acids or bases are. Litmus, for example, can determine that sulphuric acid and vinegar are both acidic, but it cannot determine whether sulphuric acid is a stronger acid or more acidic than the vinegar solution. We may, however, use a specific form of the indicator called a ‘universal indicator,’ which works by detecting the pH of a solution, to determine the strength of an acid or basic solution.

A universal indicator is a common method of measuring the pH of a solution in a school laboratory. A universal indicator is a mixture of several different indicators or dyes that produce varied colours at various pH levels across the pH scale. 

We can also remark that the universal indicator shows different colours at different concentrations of hydrogen ions in the solution since the pH of a solution is dependent on the hydrogen ion concentration. The universal indicator takes on a new colour when an acid or basic solution is put to it. By comparing the colour of the universal indicator to the colours on the pH colour chart, the pH value of the acid or base solution can be determined. We can tell whether a solution is a strong acid, weak acid, stronger base, or weak base by looking at the pH value. The colours that the universal indicator produces at various pH levels are shown below.

pH Colour
0 Dark red
1 Red
2 Red
3 Orange-red
4 Orange
5 Orange-yellow
6 Greenish-yellow
7 Green
8 Greenish blue
9 Blue
10 Navy blue
11 Purple
12 Dark purple
13 Violet
14 Violet

Universal indicator manufacturers give a pH colour chart that can be used to compare the colour created on indicator paper or indicator solution after adding acid or base solution to determine the pH value. The universal indicator, like litmus, can be used in the form of a solution or in the form of universal indicator paper.

How a universal indicator paper is used to measure the pH value of a solution?

On a strip of universal indicator paper, a drop of the solution to be tested is put. The colour of the indicator paper will change where the solution has been applied. The colour on the indicator paper is then compared to a colour on the standard pH colour chart. The pH value corresponding to this colour may be found on the pH colour chart. We can tell whether a solution is a strong acid, a weak acid, a strong base, or a weak base based on its pH value.

If a drop of a solution is placed on universal indicator paper and the paper turns dark red, the pH of the solution is near zero, indicating that it is a strong acid. However, if an orange colour is generated, the pH will be about 4 and the acid will be weak. Different colours can also be created using a different base solution. Weakly basic solutions are blue, whereas extremely basic solutions are violet with a universal indicator. If a solution turns universal indicator green, it is a neutral solution that is neither acidic nor basic. As a result, water will take on a green colour when tested with a universal indicator.

Sample Questions

Question 1: The colour of blue litmus is changed to red by a solution. What will the solution’s pH be?


A solution that changes the colour of blue litmus to red is acidic. Acidic solutions are those that have a pH of less than 7. As a result, the solution’s pH will be less than 7.

Question 2: The pH of fresh milk is 6. What is the pH of milk as it turns into curd?


When milk turns into curd, it becomes acidic, and its pH drops below 6.

Question 3: If a solution shows a pH of 1 on tested with a universal indicator, then the solution is a strong or weak acid?


A pH of 1 denotes an extremely acidic solution. As a result, this solution is a strong acid.

Question 4: A solution changes the colour of red litmus to blue. What will the solution’s pH be?


In nature, a solution that turns red litmus blue is basic. All solutions with a pH greater than 7 are considered basic. As a result, the pH of the solution will exceed 7.

Question 5: Which solution has a larger hydrogen ion concentration? Solution A has a pH of 4 while solution B has a pH of 9.


The concentration of hydrogen ions in a solution is inversely proportional to its pH. This means that a solution with a lower pH will have a higher concentration of hydrogen ions. Since solution A has a lower pH than solution B, solution A will have a higher hydrogen ion concentration.

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Last Updated : 28 Nov, 2021
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