pH

SYNOPSIS-
INTRODUCTION
HISTORY
DEFINITION
MEANING OF ‘p’ IN pH
IONIZATION OF WATER
pH INDICATORS
MEASUREMENT OF pH- pH METER
pH SCALE
pH IN NATURE
pH IN LIVING SYSTEMS
pH VALUES OF SOME COMMON FOOD PRODUCTS
WATER QUALITY STANDARDS AND OTHER CRITERIA REGARDING pH
kamal.jaswani21@gmail.com

In chemistry, pH is a numeric scale used to specify the acidity
or alkalinity of an aqueous solution.
It is the negative of the logarithm to base 10 of the activity of
hydrogen ion.
Solutions with a pH less than 7 are acidic and solutions with a
pH greater than 7 are alkaline or basic.
Pure water has a pH of 7 and is neutral, being neither an acid
nor a base.
Contrary to popular believe, the pH value can be less than 0 or
greater than 14 for the strong acids or bases resp. However,
these extremes are difficult to measure precisely.

The concept of pH was first introduced by Danish chemist-
Sorensen in 1909.
In 1936 First Commercial pH meter was made by Arhold
Orville Beakman.
In 1970 Jenco Electronics (Taiwan) designed and
manufactured the 1st portable digital pH meter.

pH is a measure of the activity of hydrogen ions in a solution. The
activity of hydrogen ions can be expressed most conveniently in
logarithmic units. So pH is defined as the negative logarithm of the
activity of H+ ions:
pH = -log [H+]

IONIZATION OF WATER-
The degree of ionization of water at equilibrium is small; at 250C only about
two of every 109 molecules in pure water are ionized at any instant. The
equilibrium constant for the reversible ionization of water is
Keq =[H+] [OH-]/[H2O] -(equ 1)
In pure water at 25o C, the concentration of water is 55.5M –{grams of H2O in 1
L divided by its gram molecular weight: (1,000 g/L)/(18.015g /mol)}- and is
essentially constant in relation to the very low concentrations of H+ and OH-,
namely, 1 x 10-7M. Accordingly, we can substitute 55.5M in the equilibrium
constant expression (equ 1) to yield
Keq =[H+] [OH-]/[55.5 M]
On rearranging, this becomes-
(55.5 M)(Keq) = [H+][OH-] = Kw -(equ 2)
Where Kw designates the product (55.5 M) (Keq), the ion product of water at
250C.

The value for Keq, determined by electrical-conductivity measurements of pure
water, is 1.8 x 10-16 M at 250 C. Substituting this value for Keq in Equation 2
gives the value of the ion product of water:
Kw = [H+][OH-] = (55.5 M) (1.8 x 10-16 M )
=1.0 x 10-14 M2
Thus the product [H+][OH-] in aqueous solutions at 250 C always equals
1 x 10-14 M2. When there are exactly equal concentrations of H+ and OH-
, as in pure water, the solution is said to be at neutral pH . At this pH,
the concentration of H+ and OH- can be calculated from the ion product
of water as follows:
Kw = [H+] [OH-] = [H+]2 = [OH-]2
Solving for [H+] gives:
[H+] =√ Kw = √ 1 x 10-14 M2
[H+] = [OH-] = 10-7 M or 1.0 x 10-7

For such a neutral solution at 250 C in which the concentration of
hydrogen ions is 1.0 x 10-7 M, the pH can be calculated as follows-
pH = log 1/1.0 x 10-7 = 7.0

Indicators may be used to measure pH by making use of the fact that their
color changes with pH.
Visual comparison of the color of a test solution with a standard color chart
provides a means to measure pH accurate to the nearest whole number.
More precise measurements are possible if the color is measured using a
spectrophotometer.

Universal indicator consists of a
mixture of indicators such that
there is a continuous color
change from about pH 2 to pH 10.
Universal indicator paper is made
from absorbent paper that has
been impregnated with universal
indicator.

MEASUREMENT OF pH - pH METER
pH meter is an electronic device used for measuring the pH .
 The pH will indicate if the solution is
acidic or basic.
 pH meters work in liquids though special
probes are sometimes used to measure the
pH of semi-solid substances.
A typical pH meter consists of a special measuring probe (a glass
electrode) connected to an electronic meter that measures and displays
the pH reading.

Different pH meters

Acidic and basic are two extremes
that describe a chemical property
chemicals. A substance that is
neither acidic nor basic is neutral.
The pH scale ranges from 0 to 14.
A pH of 7 is neutral. A pH less than
7 is acidic. A pH greater than 7 is
basic.

The pH scale is logarithmic and as a result, each whole pH value
below 7 is ten times more acidic than the next higher value, and each
whole pH value above 7 is ten times less basic than the next higher
value.

pH IN NATURE-
pH dependent plant pigments that can be used as pH indicators occur
in many plants, including hibiscus, red cabbage, red wine.
The juice of citrus fruits is acidic mainly because it contains citric
acid.
Other carboxylic acids occur in many living systems.
For ex- Lactic acid is produced by muscle activity.

Hydrangea macrophylla in
acidic soil.
Hydrangea macrophylla in
basic soil.

pH IN LIVING SYSTEMS-
The pH of different cellular compartments, body fluids and
organs, is usually tightly regulated in a process called acid-base
homeostasis.
The most common disorder in acid-base homeostasis is
Acidosis which means an acid overload in the body.
Alkalosis is the opposite condition, with blood pH being
extensively high.
The pH of the blood is usually slightly basic with a value of pH
7.36. This value is often referred to as physiological pH in
biology and medicine.
Plague can create a local acidic environment that can result in
tooth decay by demineralization.

Enzymes and other proteins have an optimum pH range and can
become inactivated or denatured outside this range.
The functioning of the oxygen- transporting molecule- haemoglobin
is also affected by pH.

pH of different components in human body-

pH VALUES OF SOME COMMON FOOD PRODUCTS-
PRODUCT APPROXIMATE pH VALUES
Apple 3.3 – 3.9
Aloe Vera 6.1
Banana 4.5 – 5.2
Beer 4.0 – 5.0
Bread(white) 5.0 – 6.2
Butter 6.1 – 6.4
Cabbage 5.2 – 5.4
Carrots 5.9 – 6.3
Cheese 4.8 – 6.4
Coconut 5.5 – 7.8
Cod liver 6.2
Eggs 7.6 – 8.0
Flour (wheat) 5.5 – 6.5
Grapes 3.5 – 4.5

Jams (fruit) 3.5 – 4.0
Ketchup 3.9
Lemons 2.2 – 2.4
Mangoes 5.8 – 6.0
Milk 6.4 – 6.8
Mustard 3.5 – 6.0
Oranges 3.0 – 4.0
Peas 5.8 – 6.4
Pickles 3.0 – 3.4
Soft drinks 2.0 – 4.0
Strawberries 3.0 – 3.9
Tea 7.2
Tomatoes 4.3 – 4.9
Water (drinking) 6.5 – 8.0
Water melon 5.2 – 5.6

WATER QUALITY STANDARDS AND OTHER CRITERIA
REGARDING pH-
The U.S. Environmental Protection Agency (U.S. EPA) sets a
secondary standard for pH levels in drinking water: the water should be
between pH 6.5 and 8.5 Secondary standards are unenforceable, but
recommended, guidelines.
Colorado Department of Public Health and Environment Water
Quality Control Division (CDPHE-WQCD) regulations state
that waters to be used for domestic water supply should have pH values
between 5.0 and 9.0

CDPHE-WQCD regulations state that waters used for primary
recreation (including such activities as swimming, rafting, and
kayaking) should have pH values between 6.5 and 9.0.
CDPHE-WQCD regulations state that waters classified as
"Class 1 Cold Water Aquatic Life" or "Class 1 Warm Water
Aquatic Life" should have pH values between 6.5 and 9.0.

REFERENCES-
Principles of Biochemistry-5th edition- Lehninger – Nelson and
Cox.
www.wikipedia.org
www.engineeringtoolbox.com
www.chemistry.elmhurst.edu

pH

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (13)

Similar to pH

Similar to pH (20)

Recently uploaded

Recently uploaded (20)

pH