http://en.wikipedia.org/wiki/Phenol_red

Phenol red

The molecular formula of phenol red is C19H14O5S.it is also known as

phenolsulfonphthalein (PSP) or Guardde3x solution. Phenol red has following chemical structure.

We see it is a zwitter ion where the negative charge resides on the sulfate group and the positive charge on the ketone group bearing additional H+.. The PSP or Phenol red is a pH indicator it changes it color from pink to red in pH range of 6.8 to 8.2. The change in the color is due to the reason as that the pH of the solution increases the ketone group loses electrons. If the pH continues to be increasing it changes its color to pink. Because of losing one more H+ (proton) from the hydroxyl group.

The reaction can be represented as

H2+PS− → HPS−. +H+ (when pH increases from 6.8)

Yellow Orange red

HPS→→ PS2-+ H+ (when pH changes beyond 8.2)

red ion pink

Phenol red is used extensively in the biological laboratories. It is also used in home swimming pool test kit as Guardde3x solution. The scientist has proven its similarity in inducing the epithelial cells of ovary like estrogen hormone, it is great biotechnological breakthrough.

the image below has shown the the color change as the pH of the solution containing the phenol red is increased, below pH 6.8 the solution is changed from yellow to orange red and when the pH is changed beyond 8.2 it s color changes from red to pink. Like most of the indicators it is also a weak acid there exist equilibrium between the molecular and undissociated form.

H2+PS− ═══ HPS−. +H+ (when pH increases from 6.8)

Yellow Orange red

Ka = [HPS-] [H+] /[H2PS-]

Adding the H+ will the push the equilibrium backward resulting in the more quantity of zwitter ion and hence the yellow color changes to red when the pH decrease from 6.8.