Determination of the pH of Vegetable and Fruit juices using pH Paper and a Universal Indicator

Soren Peder Lauritz Sorensen, a Danish chemist, was first introduced to pH. Based on the concentration of hydrogen ions, the pH value varies. Acids in water dissolve and donate H+ ions, whereas bases in water dissolve and give rise to OH- ions. Acids are proton donors and bases are proton acceptors. The acidity or basicity of a solution can be determined by knowing its pH. The pH of a solution is defined as the amount of hydrogen present in a given solution. The pH of a solution can be found out by the following formula: pH= -log[H+].

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 vegetable and fruit juices using pH paper and a universal indicator.

Apparatus Required

• Test Tubes

• Measuring Cylinder

• Glass Rod

• Watch Glass

• Vegetable and Fruit Juice

• Universal Indicator Solution

• pH Paper

Theory

There are many methods to determine the pH of a solution. The most common method is pH paper, a universal indicator and a pH metre. The pH paper and universal solution when in contact with a solution show a change in colouration depending upon pH. A universal indicator is made up of several compounds. Thymol blue, methyl red, bromothymol blue, phenolphthalein etc. are the indicators which help in detecting pH over a wide range. 

pH scale

Procedure

Using pH Paper

1. Take some clean and dry test tubes.

2. Next, put various vegetable and fruit juices into the test tubes.

3. Place the pH paper strip on a clean surface.

4. Using a clean dropper, put the sample of fruits and vegetables into the pH paper strips.

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

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

Using Universal Indicator

1. Take a given sample of juices like lemon juice, orange juice, tomato juice, pineapple juice, amla juice and mango juice in a separate test tube.

2. Make sure that 10 ml of juices should be transferred to the clean test tubes with the help of a measuring cylinder.

3. Add a universal indicator in each of the solutions 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 pH of the tested samples.

Observation

Results

From this above experiment, we can reveal that:

• The pH value of lemon juice is 2.

• The pH value of tomato juice is 4.30-4.90.

• The pH value of orange juice is 3.

• The pH value of pineapple juice is 3.20-4.00.

• The pH value of amla juice is 5.70-6.00.

• The pH value of mango juice is 6.

Precautions

1. Test tubes should be properly cleaned and dry.

2. Avoid using the same dropper for the transfer of juice samples.

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 do you mean by pH?

Ans: It is defined as the negative logarithm of hydrogen ions in a solution. It is used to determine the strength of an acid or base. The term pH refers to the “potential of hydrogen ions”.

2. What do you understand about the pH scale?

Ans: It is a scale containing various colours which indicate the strength of the acid or the base. pH is normally measured in a range of 0-14. 0 is a strong acid and 14 is a strong base.

3. How to analyse the pH of vegetables and fruit juices.

Ans: The pH of a solution can be determined using pH paper or a universal indicator. To analyse the pH of fruits or vegetable juices, either the pH paper is dipped into the solution or a universal indicator is put into the juices. Depending upon the pH, different coloured solutions are formed and this colour change can determine the pH of the solution which can be understood from the pH scale indicator.

4. What is a universal indicator?

Ans: A universal indicator is a mixed indicator (it contains several colour-changing substances) and shows a range of colours depending on the pH of the solution. Universal indicator is a liquid solution.

Viva Questions

1. What is the formula to find the pH of a solution?

Ans: Formula of pH is pH= -log[H+].

2. What does the pH of a solution mean?

Ans: pH is a scale used to determine a liquid solution's acidity and basicity.

3. Name some weak acids and strong acids.

Ans: Sulphuric acid, Hydrochloric acid and nitric acid are strong acids. Acetic acid, Carbonic acid etc. are weak acids.

4. What are weak acids?

Ans: Weak acids are acids that when dissolved in solutions do not completely dissociate into their respective ions, i.e., upon dissociation do not release all the hydrogen ions present in them. 

5. What are the components of universal indicators?

Ans: The components of the universal indicator are thymol blue, methyl red, bromothymol blue, phenolphthalein etc.

6. Give another name for NaOH.

Ans: NaOH is also known as caustic soda.

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

Ans: The colour of the pH paper becomes orange when lemon juice is added to it.

8. In a pH metre, which part of it is sensitive to hydrogen ions?

Ans: A particular electronic bulb in a pH metre is sensitive to hydrogen ions.

9. Give examples of weak bases.

Ans: Ammonium hydroxide, Lead hydroxide, zinc hydroxide etc. are examples of weak bases.

10. What are strong bases?

Ans: Strong bases are bases which when in a solution completely dissociate into respective components, giving rise to OH- ions.

Practical Based Questions (MCQs)

1. Finding the pH of a solution, which of the following is not essential?

1. pH paper

2. HCl

3. Universal indicator

4. Standard pH value Chart

Ans: HCl

2. Which of the following is not a component of a universal indicator?

1. Methyl red

2. Thymol blue 

3. Safranin

4. Phenolphthalein

Ans: Safranin

3. Find the odd one out.

1. Litmus paper

2. pH paper

3. pH metre

4. Universal indicator

Ans: Litmus paper

4. If a pH paper is dipped in an orange juice solution, what will be the possible colour of the pH  paper?

1. Red

2. Blue

3. Orange 

4. Violet

Ans: Orange

5. pH on the pH scale becomes ______as we move from left to right.

1. Basic

2. Acidic

3. Neutral

4. Remains unchanged

Ans: Basic

6. Find the odd man out.

1. Sodium hydroxide

2. Ammonium hydroxide

3. Aluminium hydroxide

4. Copper hydroxide

Ans: Sodium hydroxide

7. State which of the following is true.

1. pH helps in the digestion of food

2. pH helps in not important in soil.

3. HCl is the strongest acid

4. pH is not significant for various life processes.

Ans: pH helps in the digestion of food.

8. Fruit juice is tested for its pH value. What could be the possible pH if the colour is changed to yellow?

1. Less than 3.5

2. More than 7

3. 7

4. Between 5.5 and 6.5

Ans: Between 5.5 and 6.5

9. What are the end products of the neutralisation reaction?

1. Acid and base

2. Salt and water

3. H+ ions and OH- ions

4. Water and OH- ions

Ans: Salt and water

10. Which of the following is not a property of acid?

1. Converts blue litmus paper red

2. pH value lower than 7

3. Are highly corrosive

4. pH value higher than 7

Ans: pH value higher than 7

Conclusion

From this experiment, we can conclude that the given solutions of vegetable juice are acidic and basic. That helps us determine which liquid samples are safe to consume. The pH can be measured with the help of a pH metre, pH paper and universal indicator.