pH
By
Dr. Mohammed Golam Rasul
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Overview of Today’s lecture
1. Introduction to pH
2. pH scale
3. Importance of pH in Everyday Life
4. Testing the pH of Solutions
6.pH demonstration
Introduction
Living things have certain pH ranges for
normal biochemical processes to occur.
pH is a logarithmic scale for measuring
acids and bases.
Measured through indicators or pH meter.
What is pH?
The term "pH" is defined as the negative
logarithm of H+ ion concentration of a given
solution.
pH = -log10 [H+]
pH value
The pH value of a substance is directly
related to the ratio of the hydrogen ion and
hydroxyl ion concentrations.
If the H+ concentration is higher than OHthe material is acidic.
If the OH- concentration is higher than H+
the material is basic.
7 is neutral, < is acidic, >7 is basic
pH Scale
The pH of a solution
is used to indicate the acidity of a solution
has values that usually range from 0 to 14
is acidic when the values are less than 7
is neutral with a pH of 7
is basic when the values are greater than 7
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Strongest
acid
Weak
acid
Neutral
Weak Base
Strongest base
Importance of pH in Everyday Life
• pH plays a very significant role in biochemical
reactions. For example, the blood in our bodies is
maintained at a pH value of 7.35 - 7.45.
• The stomach produces hydrochloric acid that helps
in the digestion of food.
• The bacteria present in the mouth act on sugar and
food particles remaining in the mouth after eating to
produce acids. The best way to prevent this is to
clean the mouth after eating food or use
toothpastes , which are generally basic, for cleaning
the teeth so as to prevent tooth decay.
Learning Check
Identify each solution as A) acidic, B) basic,
or N) neutral
___ 1) HCl with a pH = 1.5
___ 2) pancreatic fluid [H3O+] = 1 x 10−8 M
___ 3) Sprite soft drink, pH = 3.0
___ 4) pH = 7.0
___ 5) [OH−] = 3 x 10−10 M
___ 6) [H3O+ ] = 5 x 10−12 M
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Solution
Identify each solution as A) acidic, B) basic, or
N) neutral
A 1) HCl with a pH = 1.5
B 2) Pancreatic fluid [H3O+] = 1 x 10−8 M
A 3) Sprite soft drink pH = 3.0
N 4) pH = 7.0
A 5) [OH-] = 3 x 10−10 M
B 6) [H3O+] = 5 x 10−12
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Testing the pH of Solutions
The pH of solutions can be determined using
a pH meter
pH paper
indicators that have specific colors at
different pH values
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pH meter
pH paper
Measuring pH of various solutions using a pH paper
The pH paper is orange in color (before use) and changes color depending on
the pH being tested. Look at the pH key to determine the pH of the solution
ranging from 1-12.
A pH strip has been dipped into the
solution and is displayed sitting
outside the beaker
pH paper
pH paper
Indicators
SAFETY
NO tasting, smelling, touching of liquids
NO tasting, smelling of papers
NO wandering – be aware while cleaning up.
First Aid – wash affected area with plenty of
water.
Calculating pH
Mathematically, pH is the negative log of the
hydronium ion concentration
pH = −log [H3O+]
For a solution with [H3O+] = 1 x 10−4,
pH = −log [1 x 10−4 ]
pH =[4.0]
pH = 4.0
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Example of Calculating pH
Find the pH of a solution with a [H3O+] of 1.0 x 10−3.
STEP 1 Enter the [H3O+] value:
STEP 2 Press log key and change the sign:
log (1 x 10−3) = [3]
STEP 3
Make the number of digits after the decimal
point (2) equal to the number of significant figures in
the coefficient (2):
[H3O+] = 1.0 x 10−3 pH is 3.00
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Learning Check
What is the pH of coffee if the [H3O+] is
1 x 10−5 M?
1) pH = 9.0
2) pH = 7.0
3) pH = 5.0
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Solution
What is the pH of coffee if the [H3O+] is 1 x 10−5 M?
STEP 1 Enter the [H3O+] value:
STEP 2 Press log key and change the sign:
log (1 x 10−5) = [5]
STEP 3 Make the number of digits after the decimal
point (1) equal to the number of significant
figures in the coefficient (1):
[H3O+]= 1 x 10−5, pH is 5.0
(3)
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