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
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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

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