Determination of pH Values of Hydrochloric Acid at Different Strengths using pH Papers and Universal Indicator Solution

pH is the amount of hydronium ions present in a given solution. The pH of a solution can be found by the formula: pH =-log[H3O+]. An acidic solution contains a pH starting from 0, and it extends up to 6. A solution is considered acidic if it contains more H+ ions. A solution is called basic if it contains more OH- ions and the pH value comes to 8 or above 8. Neutral solutions have a pH of 7. The pH is an essential parameter in the functioning of various metabolic processes in humans, animals, plants and various microorganisms; hence, its study and maintenance of pH are very important. 

Table of Contents

• Aim 

• Apparatus Required

• Theory

• Procedure

• Using pH Paper

• Using a Universal Indicator Solution

• Observation

• Result

• Precautions

• Lab Manual Questions

• Viva Questions

• Practical Based Questions

Aim

To determine the pH values of hydrochloric acid at different strengths using (i) pH papers and (ii) universal indicator solution.

Requirements

1. Test Tubes

2. HCl of Varying Concentrations - 0.1M HCl

3. 0.01 M HCl

4. 0.001 M HCl

5. 0.0001 M HCl 

6. 0.00001 M HCl

7. pH Paper

8. Universal Indicator Solution

9. pH Indicator Chart 

10. Glass Rods

11. Measuring Cylinder

Theory

1. ‘p’ in pH stands for Potential, and ‘H’ indicates hydronium ion concentration. Hence, pH is the power of hydronium ions present in a given solution. 

2. The pH paper and universal indicator work on the principle of colour change. 

3. When a pH paper or universal indicator is dipped/poured into the solution, they indicate the pH of a solution by showing a change in colouration of the pH paper or the universal indicator solution. 

4. Acids generally show warm colours that range from Red to Yellow colours while bases show cool colours from blue to violet colours. 

5. On a pH scale, the colours on the extremities indicate strong acids and strong bases whereas the colours in the centre of the pH scale indicate weak acids or weak bases. 

6. The green colour on the pH scale indicates neutral solutions. 

Procedure

1. Using pH Paper:

1. Take 5 test tubes and label them with varying concentrations of HCl solutions.

2. Cut the pH paper strips into equal parts.

3. Using forceps, dip the pH paper in each of the test tubes containing different concentrations of HCl.

4. Observe the colour change of the pH paper and compare different shades of colour with the pH indicator chart and tabulate the results.

5. Determine the approximate pH of the sample solutions using this method.

2. Using a Universal Indicator Solution:

1. Take 5 test tubes and pour 5ml of each of the given solutions using a measuring cylinder in the test tubes. 

2. Label the test tubes carefully. 

3. Using a dropper, add 2-3 drops of universal indicator solution in each of the given solutions.

4. Observe the change in colour of the solution and compare the various shades obtained, with the pH indicator chart.

5. Record your observations and determine the approximate pH of the sample solutions using this method.

pH scale chart

Observations and Result

Result

1. The pH of acids is less than 7 on the pH indicator chart.

2. The pH of the acidic solution is increased or moved towards neutral, with a decrease in the concentration of an acidic substance.

3. Colour of HCl in universal indicator is colourless.

Precautions

1. Hydrochloric acid is a very strong acid and hence should be handled with care.

2. Universal indicators should be used in minimal quantities and should not be wasted.

3. The pH papers should be used judiciously.

Lab Manual Questions

1. How is a universal indicator made? 

Ans: Universal indicator is made by mixing different indicators; hence, it can detect the pH of acids, bases and neutral solutions of varying strengths.

2. What is the pH scale?

Ans: pH scale is used to measure the acidity or basicity of a solution. The pH scale ranges from 0 to 14. 0 indicates that the solution is a strong acid, whereas 14 indicates that the solution is a strong base.

3. What is an Arrhenius acid?

Ans: Arrhenius acids are substances which contain hydrogen and upon dissociation give H+ ions or protons. They can donate hydrogen ions. It converts blue litmus paper to red; on pH paper and universal indicator show red-yellow colouration.

4. What is the colour of dilute HCl on pH paper?

Ans: The dilute HCl colour on pH paper is  Red.

Viva Questions 

1. What is a Universal indicator colour in acids?

Ans: Universal indicator shows warm colours such as red, yellow, orange etc. in acidic solutions.

2. What colour would Hydrochloric give on treatment with pH paper?

Ans: Acids show a colour range from Red to Yellow on a pH paper; hence, dilute HCl will show red colour on a pH paper.

3. What is the pH of human blood?

Ans: Human blood is a neutral solution and its pH ranges between 7.35 and 7.45.

4. Name various methods to detect the pH of a solution.

Ans: pH of a solution can be detected by using the following:

1. pH paper

2. Universal indicator

3. pH metre

5. What colour does potassium hydroxide give to the universal indicator?

Ans: When the universal indicator is put into KOH, the colour changes to dark violet colour which indicates that KOH is a strong base.

6. What is pH level of Hydrochloric acid?

Ans: The pH of the hydrochloric acid is 2 according to the given data, which indicates that it is acidic in nature.

7. What is methyl orange?

Ans: Methyl orange is an indicator which is used to detect acidic and basic solutions. Methyl orange gives red colouration when put into an acidic solution and yellow colour when put into the basic solution.

8. What is the pH of KCl salt?

Ans: Salts such as KCl are made from a neutralisation reaction between an acid and base and hence are neither too acidic nor too basic but are neutral and hence have a pH of 7.

9. What are Arrhenius’s bases?

Ans: Arrhenius bases are those compounds which help in increasing the OH- ions concentration in a solution.

10. Define acids and bases based on Lewis’s definition.

Ans:  According to Lewis, acids are electron pair acceptors and bases are electron pair donors.

Practical Based Questions (MCQs)

1. What is the colour of hydrochloric acid?

1. Colourless

2. Pink colour

3. Red colour

4. Blue colour

Ans: Colourless

2. What is the pH level of hydrochloric acid?

1. 0-3

2. 6-7

3. 8-11

4. 0-14

Ans: 0-3

3. Acids and bases can be detected using:

1. pH paper

2. Litmus paper

3. Universal indicator

4. All of the above

Ans: All of the above

4. As you move towards the right on the pH scale, the acidity of a solution

1. Increases

2. Decreases

3. Remains constant

4. None of the above

Ans: Decreases

5. Basicity of the solution increases towards the _____of the pH scale.

1. Right 

2. Left

3. Center

4. Downwards

Ans: Right

6. The pH of a strong acid is _____on the _____.

1. 2, litmus paper scale

2. 3, universal indicator chart

3. 0, pH scale

4. 14, pH scale

Ans: 0, pH scale

7. pH 14 is_____, pH 6 is_____ and pH 8 is_____.

1. Strong acid, Weak base, Weak base

2. Strong base, Weak acid, Weak base

3. Weak base, Weak acid, Strong base

4. Strong base, Weak base, Weak acid

Ans: Strong base, Weak acid, Weak base

8. At pH 1.0, 7.0, 9.5 and 5.5, the colour of the solution is_____, ____, ___and_____,respectively.

1. Orange, Green, Blue, Red

2. Red, Blue, Green, Orange

3. Red, Green, Blue, Orange

4. Green, Red, Orange, Blue

Ans: Red, Green, Blue, Orange

9. Which of the following is the strongest acid?

1. HCl

2. H2SO4

3. HNO3

4. CH3COOH

Ans: H2SO4

10. _____is the acid obtained from tamarind.

1. Citric acid

2. Acetic acid

3. Vinegar

4. Tartaric acid 

Ans: Tartaric acid

Conclusion

Acids are compounds which can donate H+ ions when present in a solution. They turn blue litmus paper red. They are colourless and sour to taste. Furthermore, they are very harmful as they have corrosive properties. Acids when put on pH paper show red to yellow colouration depending on whether they are strong acids or weak acids. Some common strong acids are H2SO4, HNO3 and HCl and some weak acids are CH3COOH, Carbonic acid, etc. Hydrochloric acid colour is red.