Acid-Base Indicators Color Chart Cheat Sheet

Key Facts

• The pH of a solution is calculated with $\mathrm{pH} = -\log[\mathrm{H}^+]$, where $[\mathrm{H}^+]$ is measured in $\mathrm{mol/L}$.
• At $25^\circ\mathrm{C}$, acidic solutions have $\mathrm{pH} < 7$, neutral solutions have $\mathrm{pH} = 7$, and basic solutions have $\mathrm{pH} > 7$.
• The relationship between pH and pOH is $\mathrm{pH} + \mathrm{pOH} = 14$ at $25^\circ\mathrm{C}$.
• Blue litmus turns red in acidic solution, while red litmus turns blue in basic solution.
• Phenolphthalein is colorless below about $\mathrm{pH} = 8.2$ and pink to magenta above about $\mathrm{pH} = 10.0$.
• Methyl orange is red below about $\mathrm{pH} = 3.1$, orange in its transition range, and yellow above about $\mathrm{pH} = 4.4$.
• Bromothymol blue is yellow below about $\mathrm{pH} = 6.0$, green near neutral, and blue above about $\mathrm{pH} = 7.6$.
• Universal indicator shows a broad pH pattern, usually red or orange for acids, green near $\mathrm{pH} = 7$, and blue or purple for bases.

Vocabulary

Acid-base indicator

An acid-base indicator is a dye that changes color over a specific pH range.

pH

pH is a logarithmic measure of acidity defined by $\mathrm{pH} = -\log[\mathrm{H}^+]$.

Transition range

The transition range is the pH interval over which an indicator changes from its acid color to its base color.

Equivalence point

The equivalence point is the point in a titration where stoichiometric amounts of acid and base have reacted.

Endpoint

The endpoint is the observed color change of an indicator during a titration.

Universal indicator

Universal indicator is a mixture of indicators that produces different colors across a wide pH scale.