Which of the following is natural acid-base indicator?

I. Litmus

II. Turmeric

The correct answer is Both I and II.

Key Points

• Indicators are the substances that are used to identify whether the given substance is an acid or a base.
• There are two types of indicators:
  • Natural: These indicators are obtained from natural sources like plants. 
  • Turmeric, litmus, china rose petals, etc., are some of the naturally occurring indicators. Hence, option 4 is correct.
  • The most commonly used natural indicator is litmus.
  • It is extracted from lichens.
  • It is available in the form of a solution, or in the form of strips of paper, known as litmus paper.
  • Generally, it is available as red and blue litmus paper.
  • Man-made: Phenolphthalein, Methyl orange are some of the man-made indicators.

Important Points

• The indicators change their color when added to a solution containing an acidic or a basic substance.
• Turmeric is a natural indicator. It contains the substance curcumin which changes color with pH.
• When a substance is basic, the color is red, whereas when the substance is acidic, the color of turmeric turns yellow to brown.
• Films containing aqueous hibiscus extract were developed as a natural pH indicator.
• Natural polymers (chitosan, gelatine, and starch) were used for the film production.
• Great color changes from pink (pH = 2.0) to yellow (pH = 13.0) were observed.
• Acids turn blue litmus red. Bases do not change the color of blue litmus.
• Acids do not change the color of red litmus. Bases turn red litmus blue.

Additional Information 

Some indicators and their color change are given in the table below: