Indicators and pH

To learn about pH and how indicators

show a substance to be an acid or a base.

Objective:

Research:

Acid: solutions – high concentration of H3O

+ ions. Hydronium Base: solutions – high concentration of

OH- ions. Hydroxyl pH: percent of hydrogen – how acidic it

is.

pH scale: series of numbers 0-14. 7= neutral, 0-6 acid, 8-14 base.

Indicator: substance that shows a definite color change when mixed with acid or base.

Effect on Acids Effect on Bases

Blue litmus paper: pink ___ Red litmus paper: ___ blue Phenolphthalein: stays clear shocking pink Bromthymol blue: orange ___ Congo red: blue ___

• As the pH of a solution changes, the indicator shows a specific color change.

Procedure I – AcidAfter each step, record your

observations.

Drop a small drop of HCl onto blue litmus paper. Drop a small drop of HCl onto red litmus paper. Place a small amount of HCl into test tube A.

Add 1 drop of phenolphthalein. Rinse TT. Place a small amount of HCl into test tube A.

Add 2 drops of bromthymol blue. Rinse TT. Place a small amount of HCl into test tube A.

Add 1 drop of congo red. Rinse TT.

Procedure II – BaseAfter each step, record your

observations.

Drop a small drop of NaOH onto blue litmus paper.

Drop a small drop of NaOH onto red litmus paper.

Place a small amount of NaOH into test tube B.Add 1 drop of phenolphthalein. Rinse TT.

Place a small amount of NaOH into test tube B.Add 1 drop of bromthymol blue. Rinse TT.

Place a small amount of NaOH into test tube B.Add 1 drop of congo red. Rinse TT.

Reaction with Acid Base

Blue litmus paper

Red litmus paper

Phenolphthalein

Bromthymol blue

Congo red