Acids, Bases and Indicators: Universal indicator and pH scale

3.0 Acids, Bases and Indicators

3.2 Universal indicator and pH scale

What is a universal indicator?

Study the pictures of the universal indicator and its colour chart below.

Universal indicator is supplied in two alternative forms, namely:

1. Universal indicator solution (used with a dropper, like plant extract)
2. Roll of paper soaked in the solution and allowed to dry (like litmus paper)

Each of these is used with a colour chart printed on the bottle containing the solution, packet of the paper roll, or a separate rectangular card.

Questions 3.2(a)

1. What is the colour of the universal indicator solution?
2. What is the colour of the roll of universal indicator paper?
3. How many colours are there on the colour chart?---
4. What is the neutral colour of universal indicator solution?
5. What are the lowest and highest numbers used to label the colours?
6. What is common in the colours labelled (a) 1 to 6--- (b) 8 to 14?

Answers to Questions 3.2(a)

What is a pH scale?

You are probably aware that some acids cause serious burns. Others, such as citric acids in fruits are mild and safe for food. It means that there are varying degrees of acidity, from the weakest to strongest. The same is true with bases.

The degree of acidity or alkalinity of a substance is measured on a pH scale, which ranges from 0 to 14. Solution of a base is called an alkaline solution.

Questions 3.2(b)

1. Define the pH scale.
2. In what way or ways is universal indicator
   1. similar to litmus and leaf extract?
   2. different from litmus and leaf extract indicators?

Answers to Questions 3.2(b)

To measure the pH of a solution, add 2 to 3 drops of the universal indicator to it. Shake the mixture and match it with its closest colour on the chart. The number corresponding to this colour is the pH of the mixture. The strongest acid has a pH of 0; the strongest alkaline has a pH of 14.

If a pH paper is to be used, cut a small length, about 3 cm, and dip one end into the mixture. Match this end with its colour on the chart; then read the pH.

NB: Sometimes, there is lack of a perfectly matching colour on the chart, in which case you should take the closest colour.

What are the strengths of some of the acids and bases around us?

Observe the demonstration on measuring the pH

(courtesy YouTube - Introduction to Clinical Lab: pH paper measurement by Medical Lab Lady Girl) to learn how to measure the pH of a solution.

Questions 3.2(c)

1. From the colour of the pH paper and the colour chart shown in the last row of the Table, estimate the pH of each substance.
   


2. Based on the pH values, acids and alkalis can be classified as strong or weak.
   

Use this additional information, to identify two strong acids and two strong bases.

Answers to Questions 3.2(c)

What are the other acid-base indicators?

Other than litmus, there many commercial indicators available for use in the school laboratory. The most common ones are methyl orange and phenolphthalein. See their colours in the pictures that follow.

Questions 3.2(d)

Complete the table below to show the colours of the given indicators in the solutions.

Answers to Questions 3.2(d)

In life

The sour taste of ripe lemon, ripe orange, pineapple, and fermented milk and others is due to the edible acids in them. The bitter taste of raw lemons and bitter herbs is due to the bases in them.