pH Indicators
Chemistry
pH Indicators
Experiment 1
1. Calculate the concentrations of H3O+ and the pH of each acidic solution. Since HCl is a monoprotic acid
(an acid that donates only one proton or hydrogen atom permolecule to an aqueous solution), the H3O+
concentration is equivalent to the HCl molarity. Summarize your results for each pH indicator, recording both
the pH values and the color of the solution at each pH.
.
Tube
#
1
2
3
[H3O+]
pH
bromothymol
blue
yellow
methyl
yellow
red
bromocresol
green
red
cabbage
extract
yellow
red
yellow
red
yellow
yellow
orange
yellow
yellow
green
4
5
7
pink/red
pink/purple
yellow
6
red
purple
yellow
yellow
blue
green
yellow
blue
teal
yellow
blue
purple/blue
blue
2. Estimate the useful range for each indicator, from a minimum to maximum pH. The range usually spans 1.5
to 2 pH units. For example, bromphenol blue is considered to be useful in the range from 3.0 to 4.6.
pH
bromothymol blue
3.0-4.6
methyl yellow
bromocresol green
red cabbage extract
3. Which indicator is the best to use for observing pH changes around pH = 2, pH = 4 and pH = 6?
2=methyl yellow
4=bromecresol green
6=bromothymol blue
Experiment 2
1. Calculate the concentrations of OH- and the pH of each basic solution. Since NaOH donates one OH- ion for
each NaOH molecule, the OH- concentration is equivalent to the NaOH molarity. The pOH is calculated as log[OH-] and the pH is calculated as 14 - pOH. Summarize your results for each pH indicator, recording both
the pH values and the color of the solution at each pH.
Tub
e#
[OH
-]
pO
H
p
H
bromothymo
l blue
red
cabbage
extract
alizari
n
yellow
yellow
red
1
purple
blue
2
3
4
5
blue
yellow
blue
blue
blue
teal
e
teal/blu
light blue
light
red
orange
yellow
yellow
6
7
phenolphtalei
n
purple
purple
purple
purple
gray
blue
medium
blue
teal
blue
yellow
yellow
gray
2. Estimate the useful range for each indicator, from a minimum to maximum pH. The range usually spans 1.5
to 2 pH units. For example, metacresol purple is considered to be useful in the range from 7.4 to 9.0.
pH
bromothymol blue
red cabbage extract
alizarin yellow
phenolphtalein
3. Which indicator is the best to use for observing pH changes around pH = 7, pH = 8, pH = 10 and pH = 12?