chemistry of water ph acid and bases.

1. Ppt presentation on water
By
Asif Iqbal khattak
PhD Microbiology
Hazara University
Mansehra

2. Water
 A. ________ is the most abundant chemical in the body.
 B. Water has many characteristics that make it vital to our
bodies.
 1. _____—water is a very small molecule, so it moves fast
and can squeeze into tiny crevasses between other
molecules.
Water
Size

3. ©2002 Learning Zone Express3
Water
 Did you know?
 1/2 to 3/4 of the human body consists of water!
 Functions in the Body:
 Water carries nutrients to your cells and carries waste
from your body.
 Regulates body temperature.
 Dissolves vitamins, minerals, amino acids and other
nutrients.
 Lubricates joints.
It is recommended that teens drink 6-8 glasses (8 fl.oz each) of water each day.
This is in addition to around 4 cups of water you get from food each day.

4. Conti…
• water is essential for all living organisms.
• Water play a key role in the distribution of organisms in the biosphere, desert
contain less water and therefore have few species while rain forest have
numerous species and are full of life.
• Water is used in various metabolic reactions.
• Its amount is kept constant through water cycle.
• 75% earth's surface is water.
• Acts as a Solvent for many types of solutes.
• Creates a slightly negative Oxygen and a Slightly positive hydrogen.
• Allows formation of Hydrogen Bonds.
• The chemical formula of water is H2o.

5. Conti…
• Water can act as either an acid or a base, maintaining a stable pH in
our bodies.
• water absorbs and releases heat energy slowly, and can hold a great
deal of heat energy.
• This helps organisms maintain their body temperature in the safe
range.

6. Water is a Polar Molecule
• Polar: Molecule in which electrons are shared
unevenly between atoms, causing each end of the
molecule to have a slight charge
Negative end
Positive end

7. Water (H2O)
•Water is essential for life
•Two-thirds of our body is made up of water
•Water is the main component of blood, lymph
and digestive secretions, as well as all other
liquid parts of the body
•It is made up of hydrogen and oxygen molecules
in the ratio 2:1

8. Water
Properties
•Colourless, odourless and tasteless liquid
•Boils at 100°C and freezes at 0°C
•Neutral PH of 7
•Excellent solvent capable of dissolving a number of
substances
•Exists in three states: solid (ice), liquid (water), and
gas (steam)
•Able to absorb heat and maintain it
Sources
•Tap/ bottled water
•Beverages such as tea and coffee
•Fruit and vegetables
•All foods contain a certain amount of water

9. Water
Functions
•Transporting nutrients, oxygen, enzymes and
hormones around the body
•Removal of waste products from the body, e.g. from
the kidneys
•Quenches thirst
•Contains the minerals calcium and fluorine
•Controls body temperature through perspiration
•Significant in the hydrolysis of nutrients during
digestion
•Essential element of all body fluids and tissues
RDA
•Between 2 and 3 litres per day

10. Acid
An acid is a substance which, when dissolved in water, releases protons.
The extent of dissociation, that is, the amount of protons released compared to
the total amount of compound, is a measure of the strength of the acid.
For example, HCl (hydrochloric acid) is a strong acid, because it dissociates
completely in water, generating free [H+] and [Cl-].
Acidity can be measured on a scale called pH (more scarily, “the negative logarithm
of the hydrogen ion concentration”).
A substance that releases hydrogen ions (H+ or protons) when added to water.
Example
HCl ↔ H+ + Cl-
The hydrogen ion is called a proton (H+ ions)
Acids are called proton donors because they produce H+ ions.

11. Weak Acid
• Some substances, like acetic acid (vinegar!) dissociate poorly
in water.
• Thus, they release protons, but only a small fraction of their
molecules dissociate (ionize).
• Such compounds are considered to be weak acids.
• Thus, while 1 M HCl is pH = 0 (why?), 1 M acetic acid is only
pH = 2.4…
• Weak acids have only a modest tendency to shed their
protons (definition of an acid).

12. Water: A Very Weak Acid
12
But this hardly happens at all: In fact, at equilibrium,
[H+] = [OH-] = 0.0000001 M = 10-7 M = pH 7
Indeed, only two of every 109 (1 billion) molecules in pure water are ionized
at any instant - Can you confirm this?
+ +
hydronium ion hydroxide ion
2 H2O H3O+ + OH-
(an acid) (a base)

13. Bases
• A substance that releases OH-or hydroxyl ions when added to water OR an
ion that combines with H+ ions.
• Example
NaOH ↔ Na+ + OH-
• Bases produce negatively charged OH- or hydroxyl ions.
• Basic solutions are also called alkaline.
• Bases are called proton acceptors because they take up hydrogen ions.
• When this occurs water is formed.
OH- + H+ ↔ H2O

14. Salts
• A compound produced by a reaction between an acid and a base.
• Example
HCl + NaOH ↔ H2O+ NaCl
• Also, a salt is an electrolyte that dissociates into cations (+ ions) and
anions (- ions), neither of which is OH- + H+.
• Example
NaCl ↔ Na+ + Cl-

15. pH
Negative log of hydrogen ion concentration is called PH.
Most living cells have a very narrow range of tolerance for pH, i.e. [H+].
 The [H+] concentration will be important (either explicitly or implicitly)
for many other topics in biology.
 [H+] is controlled in all biological organisms, and in virtually all
biochemical experiments.
 Each pH unit represents a factor of 10 difference in [H+].

16. pH Scale
• Acid and Base Concentrations are measured on the pH scale
• A measure of the hydrogen ion concentration of a solution.
• Ranges from 0 (most hydrogen ions) strong acid or very acidic to 14 (no
hydrogen ions) most alkaline or basic.
• The pH scale is a logarithmic scale.
• This means that every increment represents a 10-fold increase.
• Example
A pH of 4 represents a concentration of hydrogen ions that is 10
times greater than a pH of 5 and 100 times greater than a pH of 6.

17. pH Scale
•Measures concentration of hydrogen ions in a solution
•Ranges from 0 to 14
•7 is neutral
•0-7 have more hydrogen ions (H+) and are acidic
•7-14 have more hydroxide ions (OH-) and are basic

18. 10-2
10-3
10-5
10-4
10-8
10-7
10-6
[H+] M
10-10
10-9
10-11
10-12
10-13
10-14
10-1
100 A strong acid
A strong base
The pH scale goes from 0 to 14—because [H+][OH-] = 10-14

19. Determination the pH of a Solution
• Use a pH meter—read the number.
• Use pH paper (color patterns indicate pH).
• Titrate the solution with precise amounts of base or acid in
conjunction with a soluble dye, like phenolphthalein, whose color
changes when a specific pH is reached.

20. Ways to measure pH
 pH meter
 Electrode measures H+ concentration
 Must standardize (calibrate) before using.

21. Actually measuring a voltage – a
charge differential – between a
control solution and the external
fluid.

22. Ways to measure pH
 Indicator dyes and test strips
 Less precise
 Each indicator is only good for a small pH range (1-2 pH units)
 But may be good for field usage, or measuring small volumes, or dealing
with noxious samples.

23. pH
 pH is commonly expressed as –log[H+]
 Pure water has [H+]=10-7 and thus pH=7.
 Acids have a high [H+] and thus a low pH.
 Bases have a low [H+] and thus a high pH.
Bases contribute –OH ions when they dissociate. These bind to the H+ ions
produced when water dissociates. Thus, these OH ions “suck up” the H+ ions
in solution, reducing their concentration.
NaOH with a pH of 12.0 contributes so many –OH ions that almost all the H+
ions are bound into water molecules, reducing the free H+ (and hydronium) ion
concentration to 1 x 10-12 (1,000,000,000,000 = 1/trillion)

24. Buffer
• A buffer is a solution of a weak acid and its conjugate base that resists
changes in pH in both directions—either up or down.
• A buffer works best in the middle of its range, where the amount of
undissociated acid is about equal to the amount of the conjugate base.
• One can saturate up excess protons (acid), the other can saturate up
excess hydroxide (base).
• pH control is important, as many enzymes have a narrow range in which
they function optimally.
• Buffering capability is essential for the well-being of organisms, to
protect them from unwelcome changes in pH.

25. Conti…
• Substances that resist large and/or sudden changes in the pH of a
solution by reacting with a strong acid or base to form a weaker acid
or base.
• Deviations from the normal pH range are controlled by buffers.
• The function of a buffer is to convert strong acids, which are relatively
unstable and ionize easily (completely dissociate providing lots of H+),
into between acids which are relatively stable and do not ionize easily
(do not completely dissociate - provide only a few H+).

26. Conti…
• Most buffers in the human body consist of a weak acids and the salt
of that acid that functions as a weak base.
• Example
1. The carbonic acid-bicarbonate buffering system
2. Used to reduce the acidic effects of CO2 in body fluids.
3. Based on the bicarbonate ion (HCO3-), a weak base and carbonic
acid (H2CO3), a weak acid.

27. Factors in choosing a buffer
 Be sure it covers the pH range you need
 Generally: pKa of acid ± 1 pH unit
 Consult tables for ranges or pKa values
 Be sure it is not toxic to the cells or organisms you are
working with.
 Be sure it would not confound the experiment (e.g. avoid
phosphate buffers in experiments on plant mineral nutrition).

28. What to report when writing about a
buffer:
 The identity of the buffer (name or chemicals)
 The molarity of the buffer
 The pH of the buffer
 Examples:
 “We used a 0.5M Tris buffer, pH 8.0.”
 “The reaction was carried out in a 0.1M boric acid – sodium
hydroxide buffer adjusted to pH 9.2.”

29. Three basic strategies for making a buffer
1. Guesswork – mix acid and base at the pH meter until you get
the desired pH.
 Wasteful on its own, but should be used for final adjustments after
(2) or (3).
2. Calculation using the Henderson-Hasselbach equation.
3. Looking up recipe in a published table.

30. Acids, Bases, and pH
• Water molecules form ions
• H2O H+ + OH-
• Water hydrogen ion + hydroxide ion
• Very few ions are formed in pure water, but
there are equal numbers of hydrogen and
hydroxide ions.
• Water is neutral!

31. Thanks

32. Any Queries