5c. pH Scale & Indicators

Question 1

pH Scale

Answer 1

pH measures the concentration of hydrogen ions in an aqueous solution

Neutral solution: conc. of H⁺ = conc. of OH^-
Acidic solution: conc. of H⁺ > conc. of OH^-
Alkaline solution: conc. of H⁺ < conc. of OH^-

The smaller the pH, the more acidic the solution, the higher the conc. of H⁺.
The larger the pH, the more alkaline the solution, the higher the conc. of OH^-.

Question 2

Litmus

Colour in acidic / alkaline solution & pH at which colour changes

Answer 2

Litmus:

Colour in acidic solution: red
Colour in alkaline solution: blue
pH at which colour changes: 7

Question 3

Phenolphtaelin

Colour in acidic / alkaline solution & pH at which colour changes

Answer 3

Phenolphtaelin:

Colour in acidic solution: colourless
Colour in alkaline solution: pink
pH at which colour changes: 9

Question 4

Methyl Orange

Colour in acidic / alkaline solution & pH at which colour changes

Answer 4

Methyl orange:

Colour in acidic solution: red
Colour in alkaline solution: yellow
pH at which colour changes: 4 (turns orange)

Question 5

Screened Methyl Orange

Colour in acidic / alkaline solution & pH at which colour changes

Answer 5

Screened methyl orange:

Colour in acidic solution: purple
Colour in alkaline solution: green
pH at which colour changes: 4 (turns grey)

Question 6

Bromothymol Blue

Colour in acidic / alkaline solution & pH at which colour changes

Answer 6

Bromothymol blue:

Colour in acidic solution: yellow
Colour in alkaline solution: blue
pH at which colour changes: 7 (turns green)

Question 7

Universal Indicator

Answer 7

Universal Indicator:

pH 0 (red)
pH 3 (orange)
pH 5 (yellow)
pH 7 (green)
pH 9 (blue)
pH 14 (purple)

Question 8

pH Meter

Answer 8

A pH meter consists of a pH electrode connected to a meter. The pH electrode is dipped into solution and pH value is shown on meter.

Advantages of using pH meter over indicators:

• More reliable and accurate (no human error: guessing shades)
• Can be used in data logging to record rapid changes in pH

Question 9

Problem: Soils tend to become acidic from acid rain and extensive use of chemical fertilisers (e.g. ammonium sulfate). What is the solution?

Answer 9

Solution: Adding of slaked lime (calcium hydroxide) to the soil to neutralise tha cids, which is known as ‘liming the soil’.