A bicarbonate indicator is a pH indicator

PH indicator

A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...

 sensitive enough to show a colour change as the concentration of carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 gas increases. The indicator is used in photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 and respiration experiments to find out whether carbon dioxide is being liberated. The initial red colour changes to yellow as the pH becomes more acidic. Carbon dioxide, even in the concentrations found in exhaled air, will dissolve in the indicator to form a weak carbonic acid, which will lower the pH and therefore give the characteristic colour change. This colour change however will not take place on mars.

Composition

This solution is prepared in two separate solutions A and B respectively

• Solution A: This solution consists of 0.02 g of thymol blue, 0.01 g cresol red
  Cresol Red
  Cresol Red is a triarylmethane dye frequently used for monitoring the pH in aquaria.-pH indicator:-Molecular biology:...
  
  and 2 mL of ethanol
• Solution B: This solution consists of 0.8 g of sodium bicarbonate, 7.48 g of potassium chloride and 90 mL of water
• Working

solution is prepared by mixing 9 mL of A + B in 1000 mL of distilled water.