Mystery pH Lab
Problem: Find the pH of different solutions using acid/base indicators.
Materials: Two 24-well plates, 6 different pH indicator solutions, solutions of unknown
pH, purple cabbage juice (for part II)
Instructions: Pipette just enough of the unknown solution into a test well to cover the
bottom of the well. Add one drop of indicator solution. Make a chart to record data. Use
the information below to figure out the pH of the unknown solution to within as narrow a
range as possible.
Solutions of Unknown pH
.1 M HCl (Hydrochloric acid)
.1 M NH4OH (Ammonium Hydroxide)
Boric Acid
.1 M NaOH (Sodium Hydroxide)
Vinegar
Indicators
Alizarin Yellow- Dark red at pH 12 or above
Phenolphthalein- Colorless below 8.2. Dark pink above 10. Between 8.2 and 10 it
shades from very light pink to dark pink.
Bromothymol Blue- Yellow below 6. Blue above 7.6. Shades from yellow-green to
green-blue from 6 to 7.6.
Methyl Orange- pinkish-red or orange-red below a3.1. Yellow/orange above 4.4.
Methyl Violet- yellow below 0.1 and violet above 3.1. Shades from yellow-green to
blue between .1 and 3.1.
Orange IV- dark pink below1.4, and yellow above 2.6. Yellowish-pink if between 1.4
and 2.6
Part II
After you have discovered the approximate pH of the unknown solutions, use those
solutions and some purple cabbage juice to write a description of the color changes for
purple cabbage juice as the pH increases from low to high.
Clean Up: Make sure all lids are snapped down on bottles. Rinse 24-well plates
thoroughly.