pH
Potential Hydrogen
pH
A measure of
hydronium ion
concentration
pH
A measure of
+
[ H 3O ]
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pH of some solutions
HCl - 0
Stomach acid - 2
Cola drinks - 3
Tomatoes - 4
Coffee - 5
Milk - 6.5
pH of some solutions
HCl - 0
Pure water - 7
Stomach acid - 2
Blood - 7.5
Cola drinks - 3
Sea water - 8
Tomatoes - 4
Detergent - 10
Coffee - 5
Household cleaners - 11
Milk - 6.5
Oven cleaners - 14
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pH Scale - 17 min
Measuring pH
Meters
Indicators
strips
liquids
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Measuring pH
Most professional
laboratories use a
digital pH meter to
read the pH of
solutions directly.
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Measuring pH
These instruments
provide a very quick
and precise reading.
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Measuring pH
Bench-type pH meters have an
electrode on an adjustable
arm and are capable of
large-scale use,
with results to five
significant figures.
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Measuring pH
Portable pH meters are
battery-powered for use
in the field.
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Measuring pH
They are obviously less
expensive, but still
provide readings with
two or three
significant figures.
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Measuring pH
These are common in
high school chem labs.
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Measuring pH
Before pH meters, chemical
indicators were used
to determine pH.
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Measuring pH
Indicators are weak organic
acids and bases whose colors
differ from the colors of
their conjugate
acids or bases.
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Measuring pH
Indicators have two
major weaknesses:
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Measuring pH
1. They only provide a
"ballpark" determination
of a solution's pH.
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Measuring pH
2. They only work well with
solutions that begin
colorless.
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pH Indicators
Litmus paper is the
simplest test strip,
indicating only if
a solution is an
acid or base.
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pH Indicators
Red litmus
turns blue
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pH > 8.5
pH Indicators
Blue litmus
turns red
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pH < 5.5
pH Indicators
DO NOT dip test papers
into solutions.
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pH Indicators
This MAY contaminate
your sample.
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pH Indicators
Touch a clean stirring rod
to the sample, then touch
the rod to the
test paper.
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Measuring pH
Hydrion paper (pH paper)
is similar to litmus paper,
but provides a range
of values.
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Measuring pH
The color changes are
shown on the container.
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Measuring pH
pH paper is available in
several ranges, for use
in different types
of solutions.
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pH Indicators
Phenolphthalein solution
Colorless in acid
Faintly pink at neutral
Dark pink in base
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pH Indicators
Universal Indicator solution
A solution that
changes to
these colors
Ionization of Water
2 H2O
+
H3O (aq)
+
OH
(aq)
The Ion Product
Constant of Water
Kw =
+
[H3O ]
[OH ]
=1X
-14
10
Calculating pH
pH = - log [ H3O+ ]
[H3O+] is expressed
in powers of 10 from
10-14 to 100
Sample Problem #1
What is the pH of a solution with
-4
+
a [H3O ] of 1 x 10 M?
Problem Solution
p H = - log [H3O+]
p H = - log (1 x 10-4)
Problem Solution
p H = - log [H3O+]
p H = - log (1 x 10-4)
Do the following on your calculator:
Enter 1x10-4
Press "LOG"
PRESS "+/-"
PH=?
Problem Solution
p H = - log [H3O+]
p H = - log (1 x 10-4)
Do the following on your calculator:
Enter 1x10-4
Press "LOG"
PRESS "+/-"
PH=4
Sample Problem #2
0.01 moles of HCl is added to
water to make 1 dm3 of solution.
Assuming the HCl is completely
ionized, what is the pH of the
solution?
Problem Solution
p H = - log [H3O+]
What is the [H3O+]?
Problem Solution
p H = - log [H3O+]
p H = - log (1 x 10-2)
Problem Solution
p H = - log [H3O+]
p H = - log (1 x 10-2)
Do the following on your calculator:
Enter 1x10-2
Press "LOG"
PRESS "+/-"
PH=?
Problem Solution
p H = - log [H3O+]
p H = - log (1 x 10-2)
Do the following on your calculator:
Enter 1x10-2
Press "LOG"
PRESS "+/-"
PH=
2
Have you noticed anything
about these two problems?
If this # is 1
1X
-7
10
If this # is 1
1X
-7
10
The pH is this
as a positive #
Find the pH of a solution whose
[H3O+] is 9.5 x 10-8 M. Sample Problem #3
pH = - log [H3O+]
Find the pH of a solution whose
[H3O+] is 9.5 x 10-8 M. Sample Problem #3
pH = - log [H3O+]
pH = - log (9.5 x 10-8)
Find the pH of a solution whose
[H3O+] is 9.5 x 10-8 M. Sample Problem #3
pH = - log [H3O+]
pH = - log (9.5 x 10-8)
pH = - (log 9.5 + log 10-8)
Find the pH of a solution whose
[H3O+] is 9.5 x 10-8 M. Sample Problem #3
pH = - log [H3O+]
pH = - log (9.5 x 10-8)
pH = - (log 9.5 + log 10-8)
pH = - (.98 + (-8))
Find the pH of a solution whose
[H3O+] is 9.5 x 10-8 M. Sample Problem #3
pH = - log [H3O+]
pH = - log (9.5 x 10-8)
pH = - (log 9.5 + log 10-8)
pH = - (.98 + (-8))
pH = - (-7.02 )
Find the pH of a solution whose
[H3O+] is 9.5 x 10-8 M. Sample Problem #3
pH = - log [H3O+]
pH = - log (9.5 x 10-8)
pH = - (log 9.5 + log 10-8)
pH = - (.98 + (-8))
pH = - (-7.02 )
pH = 7.02
If this # is not 1
1.5 X
-7
10
If this # is not 1
1.5 X
-7
10
The pH is a positive decimal
number one less than this #
Sample Problem #4
What is the [H3O+] of a solution
with a pH of 12?
pH = - log [H3O+]
pH = - log [H3O+]
12 = - log [H3O+]
pH = - log [H3O+]
12 = - log [H3O+]
- 12 = log [H3O+]
pH = - log [H3O+]
12 = - log [H3O+]
- 12 = log [H3O+]
[H3O+] = antilog - 12
pH = - log [H3O+]
12 = - log [H3O+]
- 12 = log [H3O+]
[H3O+] = antilog - 12
antilog is "INV" + "log" on most calculators
pH = - log [H3O+]
12 = - log [H3O+]
- 12 = log [H3O+]
[H3O+] = antilog - 12
antilog is "INV" + "log" on most calculators
[H3O+] = 1 x 10-12
Sample Problem #5
What is the [H3O+] of a solution
with a pH of 5.45?
pH = - log [H3O+]
pH = - log [H3O+]
5.45 = - log [H3O+]
pH = - log [H3O+]
5.45 = - log [H3O+]
- 5.45 = log [H3O+]
pH = - log [H3O+]
5.45 = - log [H3O+]
- 5.45 = log [H3O+]
[H3O+] = antilog - 5.45
pH = - log [H3O+]
5.45 = - log [H3O+]
- 5.45 = log [H3O+]
[H3O+] = antilog - 5.45
antilog is "INV" + "log" on most calculators
pH = - log [H3O+]
5.45 = - log [H3O+]
- 5.45 = log [H3O+]
[H3O+] = antilog - 5.45
antilog is "INV" + "log" on most calculators
[H3O+] = 3.5 x 10-6
pOH
•
pOH = - log
[OH ]
pOH
•
[OH ]
pOH = - log
• [OH-] is expressed in powers
of 10 from 10-14 to 100
pOH
•
[OH ]
pOH = - log
• [OH-] is expressed in powers
of 10 from 10-14 to 100
• [H3O+] [OH-] = 1 x 10-14
pOH
•
[OH ]
pOH = - log
• [OH-] is expressed in powers
of 10 from 10-14 to 100
• [H3O+] [OH-] = 1 x 10-14
• pH + pOH = 14
Sample Problem #6
What is the pH of a solution
with a pOH of 1.36?
Sample Problem #6
What is the pH of a solution
with a pOH of 1.36?
pH + pOH = 14
Sample Problem #6
What is the pH of a solution
with a pOH of 1.36?
pH + pOH = 14
pH = 14 - pOH
Sample Problem #6
What is the pH of a solution
with a pOH of 1.36?
pH + pOH = 14
pH = 14 - pOH
pH = 14 - 1.36
Sample Problem #6
What is the pH of a solution
with a pOH of 1.36?
pH + pOH = 14
pH = 14 - pOH
pH = 14 - 1.36
pH = 12.64
Sample Problem #7
What is the pOH of a solution
with a [OH -] of 2.97 x 10-10?
Sample Problem #7
pOH = - log
[OH ]
Sample Problem #7
[OH ]
pOH = - log
pOH = - log (2.97 x 10-10)
Sample Problem #7
[OH ]
pOH = - log
pOH = - log (2.97 x 10-10)
pOH = - (log 2.97 + log 10-10)
Sample Problem #7
[OH ]
pOH = - log
pOH = - log (2.97 x 10-10)
pOH = - (log 2.97 + log 10-10)
pHO = - (.47 + (-10))
Sample Problem #7
[OH ]
pOH = - log
pOH = - log (2.97 x 10-10)
pOH = - (log 2.97 + log 10-10)
pHO = - (.47 + (-10))
pOH = - (-9.53)
Sample Problem #7
[OH ]
pOH = - log
pOH = - log (2.97 x 10-10)
pOH = - (log 2.97 + log 10-10)
pHO = - (.47 + (-10))
pOH = - (-9.53)
pOH = 9.53
Solutions that can absorb
moderate amounts of
acid or base without a
significant change in pH.
Buffers provide ions that
+
react with H3O or OH
if they are introduced
into the solution.
As either ion is neutralized,
the pH of the system
remains nearly
constant.
Buffer solutions are prepared
using a weak acid or base
with one of its salts.
The weak acid or base reacts
with one of the added ions,
the salt reacts with
the other ion.
The combination keeps the pH
fairly constant, up to a point.
By choosing the correct weak
acid or base, a buffer solution
can be prepared that
will maintain
almost any pH.
This buffer pH range
is between 6.5 and 7.5
Measuring pH
Why does
Alka-Seltzer
make your
upset stomach
better?