Blue-yellow-red indicator

Safe­ty pre­cau­tions

At­ten­tion! All ex­per­i­ments are per­formed by pro­fes­sion­als. Do not at­tempt.

Process de­scrip­tion

Thy­mol blue is a pop­u­lar acid-base in­di­ca­tor. An in­di­ca­tor is a par­tic­u­lar type of sub­stance, the struc­ture and col­or of which strong­ly de­pend on the acid­i­ty (pH) of the medi­um it is in. A so­lu­tion is con­sid­ered acidic if its pH < 7, neu­tral when its pH = 7, and al­ka­line at pH > 7. In a high­ly acidic medi­um, when the pH is ap­prox­i­mate­ly 1, thy­mol blue turns red. When the sodi­um car­bon­ate is added, it re­acts with the sul­fu­ric acid and the medi­um be­comes neu­tral:

Na₂­CO₃ + H₂­SO₄ → Na₂­SO₄ + CO₂ + H₂O

The struc­ture of the thy­mol blue im­me­di­ate­ly changes, and it turns yel­low. The ex­cess sodi­um car­bon­ate hy­drolyzes, cre­at­ing an al­ka­line medi­um, and the pH be­comes greater than 9:

Na₂­CO₃ + H₂O ⇋ NaH­CO₃ + NaOH

The struc­ture of thy­mol blue changes again, and this time it turns blue. The so­lu­tion is mixed un­even­ly, and ar­eas of vary­ing acid­i­ty are present in it si­mul­ta­ne­ous­ly, so it be­comes mul­ti­col­ored!

A safer ver­sion of this ex­per­i­ment is in­clud­ed in the “Col­or­ful chem­istry” set from the MEL Chem­istry sub­scrip­tion.