Determination of the pH of Solutions of Some Salts using pH Papers or Universal Indicator Solution

Soren Peder Lauritz Sorensen, a Danish chemist, was first introduced to pH. Based on the concentration of hydrogen ions, the pH value varies. The solution where hydrogen ions are rich is the acidic solution while basic solutions are poor in hydrogen ions. The substance that generates hydrogen ions when dissolved in water is known as acids. Bases generate hydroxyl ions when dissolved in water.

Table of Content

• Aim

• Apparatus Required

• Theory

• Procedure

• Observation

• Result

• Precautions

• Lab Manual Questions

• Viva Questions

• Practical Based Questions

Aim

To determine the pH of solutions of some salts using pH paper or a universal indicator.

Apparatus Required

• Test tubes

• Measuring Cylinder

• Glass rod

• pH paper

• Universal indicator solution

• 0.1 M and 0.01 M solutions of CH3COONa, Na2CO3 , FeCl3 , BaCl2, NH4Cl, NaCl, salts.

Theory

1. In the pH paper method, the pH paper is dipped in a specific solution, and the change of colour is observed. 

2. Then the change of colour is compared to the colour chart and determines the pH of the solution. 

3. A universal indicator is a mixture of various indicators and shows a range of colours depending on the pH of the solution. 

4. It comes in a liquid solution of ethanol. A universal indicator is constituted of a solution of several compounds. 

5. Thymol blue, methyl red, bromothymol blue, phenolphthalein etc. are the components of universal indicators.

pH scale

Procedure

Using pH Paper:

1. Take a strip of pH paper.

2. Next, put 1-2 drops of CH3COONa (0.1M) salt solution on it with the help of a glass rod.

3. Observe the colour of the pH paper.

4. Now compare the colour of the pH paper strip to the standard pH chart.

5. Note down the value of the pH of the tested samples.

6. Repeat the process with different salt solutions and record the observations.

Using a Universal pH Indicator

1. Take some clean and dry test tubes.

2. Make sure that 5 ml of each salt solution should be transferred to the clean test tubes with the help of a measuring cylinder.

3. Add 3-4 drops of a universal indicator in each solution with a dropper and mix it thoroughly.

4. Observe the colour of each solution and compare it with the chart that presents in the bottle.

5. Note down the value of the tested samples.

Observation

Results

• The solutions of CH3COONa, FeCl3 and NH4Cl have values less than 7 and hence, they are acidic in nature. 

• The solutions of Na2CO3 have pH value of 11.8, hence, it is basic in nature. 

• The pH of common salt and BaCl2 are 7.

Precautions

1. Test tubes should be properly cleaned and dry.

2. Avoid using the same dropper for the transfer of salt solutions.

3. The universal indicator should be equal in quantity for all the samples.

4. pH paper should be clean and away from fumes.

5. Observe the colour of the solution that appears with the standard pH chart.

Lab Manual Questions

1. What are strong acids?

Ans: The acids that dissociate completely and produce H+ ions are known as strong acids. For example, HCl, HNO3 and H2SO4.

2. How is pH paper made?

Ans: A blotting paper is dipped in a universal indicator. When the paper absorbs the indicators, it is allowed to dry.

3. What is the pH scale?

Ans: The pH scale ranges from 0 to 14 and is used to measure the strength of acids and bases.

4. How to measure pH?

Ans: The pH of a solution can be determined using an instrument like a pH metre or various indicators which change their colour accordingly depending upon the nature of the test solution.

Viva Questions

1. Name the scientist who has introduced pH.

Ans: Soren Peder Lauritz Sorensen, a Danish chemist, first introduced the concept of pH.

2. What do you understand the pH of a solution means?

Ans: pH of the solution means the amount of H+ ions present in a given solution.

3. What will be the colour of the pH paper if the pH value is 7?

Ans: The colour of the pH paper will be green if the pH value is 7.

4. What is an indicator?

Ans: It is a chemical that changes its colour when it comes in contact with an acid or base.

5. Which method of measuring pH is more accurate?

Ans: In comparison to others, the pH metre gives accurate results.

6. What will be the colour of the pH paper when a common salt solution is added to it?

Ans: When common salt dissolves in water, it gives a neutral solution and, as a result, the colour of the pH paper remains green.

7. What will be the colour of the pH paper when the NH4Cl solution is added to it?

Ans: As the NH4Cl solution is acidic, it turns the pH paper to orange colour.

8. What will be the colour of pH paper if the solution is weak alkaline?

Ans: The colour of the pH paper turns blue if the solution is weak alkaline.

9. What is the common name for sodium chloride?

Ans: Sodium chloride is commonly known as common salt or table salt.

10. Give examples of strong bases.

Ans: Strong bases are NaOH, KOH, CsOH etc.

Practical Based Questions (MCQs)

1. Which of the following is a component of a universal indicator?

1. Methyl red

2. Safranin

3. Thymol blue

4. Both A & C

Ans: Both A & C

2. Which of the following turns the pH paper green?

1. Milk of magnesia

2. Baking soda

3. Oxalic acid solution

4. NaCl solution

Ans: NaCl solution

3. Which one of the following is not required to find the pH of a solution?

1. pH paper

2. NaCl

3. Universal indicator

4. Standard pH value chart

Ans: NaCl

4. If the pH of the solution changes from 6 to 4, the solution becomes

1. Less basic

2. Less acidic

3. More acidic

4. Neutral

Ans: More acidic

5. If the pH paper colour is obtained orange on putting a drop of solution, the solution is

1. Acidic

2. Highly acidic

3. Basic

4. Highly basic

Ans: Acidic

6. The two colours seen on the extreme ends of the pH chart are

1. Red and blue

2. Red and green

3. Green and blue

4. Orange and green.

Ans: Red and blue

7. The colours obtained on a pH paper for highly acidic, basic and neutral solutions, respectively, are

1. Blue, orange, green

2. Yellow, blue, green

3. Red, blue, green

4. Red, green and blue

Ans: Red, blue, and green

8. Which one of the following solutions is used to test the pH of a sample?

1.  Blue litmus solution

2. Red litmus solution

3. Universal indicator solution

4. A mixture of red and blue litmus solution

Ans: Universal indicator solution

9. H2SO4 is a____acid and weak____

1. Strong, base

2. Alkaline, base

3. Weak, alkaline

4. Strong, acidic

Ans: Strong, base

10. HCl donates_____H+ ions and Sulphuric acids donates_____H+ ions.

1. 1, 2

2. 2,1

3. 1,1

4. 2,2

Ans: 1,2

Conclusion

From the above experiment, we conclude that pH helps us understand the acidity or basicity of a solution. Understanding and knowing the pH of any chemical is of paramount importance, as pH is an important parameter for various life processes occurring in organisms. When the pH of a solution is below 7, then such solutions are acidic and show colours such as red, yellow, orange etc. If the pH of a solution is above 7, then such solutions are basic and show blue, dark blue and violet colourations. If the pH is 7 and the colour of the solution/pH paper is green, then it is a neutral solution.