Determination of the pH of Various Samples of NaOH Solution using pH Papers and Universal Indicator Solution

In our daily life, we use many compounds like water, baking soda, lemon juice, etc. Do they all lie in the same category? They all fall into different categories. Water is neutral, lemon juice is acidic and baking soda is alkaline.

There are three types of chemicals used in laboratories: basic, acidic and neutral. Their characteristics are determined by the ions they release. A chemical that releases H+ ions in its aqueous solutions is said to be acidic, while a chemical that releases OH– ions in its aqueous solutions is said to be basic. The acidity or alkalinity of a solution can be measured using pH which is the hydrogen ion concentration in the solution.

Table of Content

• Aim

• Theory

• Apparatus Required

• Procedure

• Observation

• Result

• Precautions

• Lab Manual Questions

• Viva Questions

• Practical Based Questions

Aim

To determine the pH of different samples of NaOH solution using: (i) pH paper method and (ii) universal indicator solution method.

Theory

The alkalinity and acidity of a solution are determined by pH, which is a measurement of hydrogen ion concentration. In general, if a solution has a pH value of less than 7, it is an acidic solution, if it has a pH value greater than 7, it is a basic solution, and if it has a pH value equal to 7, it is a neutral solution.

Apparatus Required

• Test tubes

• Measuring cylinder

• Dropper

• pH paper

• Universal indicator solution

• Different concentrations of NaOH solution (0.1 M, 0.01 M, 0.001 M, 0.0001 M and 0.00001 M)

Procedure

i) pH paper Method

• Add 2-3 drops of NaOH solution on pH paper with the help of a glass rod.

• Observe the change of colour on the pH paper.

• Take a look at the 'pH indicator chart' and compare the shade of colour formed with different colours.

• Based on this information, record the pH of sodium hydroxide samples approximately in a table.

ii) Universal Indicator Solution

• Take five clean and dry test tubes.

• With the help of a measuring cylinder, pour 5ml of each solution into different test tubes.

• Add 2-3 drops of ‘universal indicator solution’ to each test tube with the help of a dropper.

• Observe the colour of each test tube and compare it with the different colour shades given in the "pH indicator chart".

• Record the pH of the solution in the table after comparing the colour in each tube.

pH scale

Observation

To find out the pH of the sample, first we need to find out H3O+ ion concentration,

\[[O{H^ - }][{H_3}{O^ - }] = 1 \times {10^{ - 14}}\]

\[Or\,[{H_3}{O^ + }] = \dfrac{{1 \times {{10}^{ - 14}}}}{{[O{H^ - }]}}\]

\[pH =  - \log [{H_3}{O^ + }]\]

Result

After finding the pH of the following samples, from the above experiment, we can conclude that the pH of the solution decreases with a decrease in concentration.

Precautions

• Prepare the samples freshly before the experiment.

• Wash droppers and glass rods before putting them into different samples.

Lab Manual Questions

1. What is the pH scale?

Ans: pH scale is a scale that determines whether a solution is neutral, basic or acidic by measuring its pH value.

2. Can you explain the difference between acids and bases?

Ans: 

3. How do you measure pH? 

Ans: pH of a solution can be determined by various methods. pH can be determined by the pH paper method, pH metre or by using a universal indicator solution.

4. What is a universal indicator solution?

Ans: Universal indicators are pH indicators made from solutions of several compounds that exhibit smooth colour changes over a wide range of pH values.

Viva Questions

1. When NaOH is added to water, what happens to its pH?

Ans: pH of water increases if NaOH is added.

2. What is the pH of 0.01 M NaOH?

Ans: pH of 0.01 M NaOH is 12.

3. What is an indicator solution?

Ans: An indicator is a weak acid or weak base that changes colour with a change in hydrogen ion concentration or pH.

4. What is the colour of the pH paper at neutral pH? 

Ans: pH paper colour is green at neutral pH.

5. Which method gives accurate results to find pH?

Ans: pH metre gives accurate results to find pH.

6. Which ions are released by acids in a solution?

Ans: Hydrogen ions are released by acids in a solution.

7. What is the disadvantage of pH paper?

Ans: Accuracy is not adequate in the case of pH paper as compared to pH metre.

8. What are the main components of a universal indicator solution? 

Ans: Thymol blue, methyl red, phenolphthalein, and bromothymol blue are the main components of a universal indicator solution.

9. What is the colour of pH paper if the solution has a pH of 14?

Ans: The colour of the pH paper is blue if the solution has a pH of 14.

10. What is the range of acidic pH?

Ans: Acidic pH ranges from 0 to 7.

Practical Based Questions

1. Which is not a component of the universal indicator solution?

A) Thymol blue

B) Methyl red

C) Phenolphthalein

D) safranin

Ans: D) safranin

2. What is the range of pH of the basic solution?

A) 0 to 7

B) 0 to 5

C) 7 to 14

D) 7

Ans: C) 7 to 14

3. Since the pH of NaOH solution is more than 7, it is ____.

A) acidic

B) Basic

C) Neutral

D) None of the above

Ans: B) Basic            

4. Can you tell me what the pH of 0.01M NaOH is?

A) 12

B) 11

C) 10

D) 14

Ans:  A) 12

5. What is the colour of the pH paper at pH zero?

A) Orange

B) Blue

C) Green

D) Red

Ans: D) Red

6. What happens to the pH of an acidic solution on dilution?

A) pH increases

B) pH decreases

C) pH does not change

D) None of the above

Ans: A) pH increases

7. Which among the following is more efficient?

A) pH paper

B) Universal indicator solution

C) pH metre

D) All the above

Ans: C) pH metre

8. What is the value of neutral pH?

A) 7

B) 9

C) 3

D) 4

Ans: A) 7

9. pH of 0.1M NaOH solution is ____.

A) 11

B) 12

C) 13

D) 14

Ans: C) 13

10. The negative logarithm of OH- ion concentration is known as ___.

A) pH

B) pOH

C) Both A and B

D) None of the above

Ans: B) pOH

Conclusion

There are three types of chemicals used in laboratories: basic, acidic and neutral. Their characteristics are determined by the ions they release. Add 2-3 drops of NaOH solution on pH paper with the help of a glass rod. Observe the change of colour on the pH paper. Take a look at the 'pH indicator chart' and compare the shade of colour formed with different colours. After finding the pH of the following samples, we can conclude that the pH of the solution decreases with a decrease in concentration.