Introduction to pH

1. BCH 201: Cell biology, pH and Buffer (Part 2)
Water and pH
Dr. MD Hassan

2. Water:
• Water is the most abundant substance in living systems
• Its makes up 70 % or more of the weight of most organisms
• Water is the solvent of life
• All chemical reactions in organisms occur in liquid-water
• Being polar, water has unique properties.
These include its role as;
- a solvent,
- a chemical reactant,
- a factor to maintain a fairly constant temperature.

3. Quality of water
1. Chemical water: pure water, contain H2O
2. Portable water: contain no harmful substances, fit for drinking
3. Palatable water: aesthetically good
4. Wholesome water: chemically not pure but does contain harmful
substances
5. Polluted water: contain harmful substance but not pathogens
6. Contaminated water: contain pathogens
7. Mineral water: contain minerals like calcium, iron, Mn etc.

4. Assessing Quality of water
Physical Parameters
1. Turbidity
2. Color
3. Odor/taste
4. Temperature
5. Conductivity
Chemical/biological parameters
1. Total solid
2. pH
3. Hardness
4. Presence of chemicals like; Cl,
Nitrate, sulphate etc.
5. Biological parameter e.g.
Microorganisms like
bacterial, virus, protozoa etc.

5. Properties of Water
Water Has Several Unique Properties
1. Surface Tension- cell membranes moist, protection.
2. Cohesion- water molecules stick to each other held together by H-bonds.
3. Adhesion- water molecules stick to unlike surfaces such as glass or plastic due to H-bonds
between water and other polar compounds.
4. High Specific Heat- water can absorb or release large amounts of heat energy with little temp-
erature change. E.g. Water stabilizes temp. in cells and ecosystems.
5. High Heat of Vaporization- many H-bonds must be broken before water can evaporate.
6. Lower Density of Ice- water molecules in an ice crystal are further apart because of H-
bonding.
7. Water is Polar- a good solvent; surrounds ions and other polar molecules. E.g. Transport of
nutrients in organisms- plasma, cytoplasm, and xylem.
• Medium for Chemical Reactions- hydration/dehydration reactions
• Hydration shells

6. Solvent Properties of Water
Water is a polar solvent. It readily dissolves most biomolecules, which are generally
charged or polar compounds. Compounds that dissolve easily in water are
hydrophilic (Greek, “water-loving”). In contrast, nonpolar solvents such as
chloroform and benzene are poor solvents for polar biomolecules but easily
dissolve those that are hydrophobic—nonpolar molecules such as lipids and waxes.
• Hydrophobic interactions: Water molecules in the immediate vicinity of a
nonpolar solute are constrained in their possible orientations as they form a
highly ordered cage-like (clathrate) structure around each solute molecule.
• Hydration shells formation: Water dissolves salts such as NaCl by hydrating and
stabilizing the Na+ and Cl- ions, weakening the electrostatic interactions between
them and thus counteracting their tendency to associate in a crystalline lattice.
• Hydrogen bonding with solutes; water form hydrogen bonds with non-ionic but
polar compounds such as sugar via their polar functional groups like hydroxyl,
amines, carbonyls etc. Water readily dissolves such compounds by replacing
solute-solute hydrogen bonds with solute-water hydrogen bonds

7. WATER AS A REACTANT
Water is a direct participant in many biochemical reactions. When a biomolecule is split
apart by water, the reaction is called a hydrolysis reaction. When a biomolecule is formed
from two components with the elimination of water, the reaction is called a condensation
reaction. The condensation reaction in which ATP is formed from ADP and inorganic
phosphate (HPO4
2-) is illustrated in Fig. below. Biopolymers such as proteins,
polysaccharides, and nucleic acids are formed and broken down by condensation and
hydrolysis reactions, respectively.

8. Biomedical importance of water
1. Water is essential constituent of all forms of life
2. Water serve as a medium for all cellular events.
3. It is required for the transport of solutes in the body
4. Water aids the folding of macromolecules like protein, nucleic acids etc.
5. Semi-fluid nature of body is due to water
6. Water regulates body temperature.
7. water accelerates biochemical reactions by providing ions.

9. Maintenance Of Water Balance
In a normal healthy individual water intake equals water out put. Water intake per day is about
2500ml. Drinking water contributes to about 1200ml, food water is about 1000ml and water of
metabolisms is about 300ml.
Water out put per day is about 2500 ml. Urine is major route of water elimination. About 1200ml
of water is removed in urine. Other routes of water removal are through lungs, skin and feces.
The body water homeostasis is achieved by combined action of hormonal and other factors. ADH
is hormone involved in water homeostasis.
Water intake and water out put are maintenance by two factors
Decrease in water content in the body leads to stimulation of thirst centre and thirst in caused.
At the same time another centre in the hypothalamus is stimulated anti-diuretic hormone is
released. Water is consumed due to thirst. Through circulation ADH reaches kidney and act on
renal tubular cells. As a result more absorption of water takes place in kidney. So by the
combined action of ADH and thirst centre water balance is attained.
Exactly reverse occurs when there is excess water in body. Due to excess water thirst centre
and other centre in hypothalamus are inhibited. Water in take is stopped. ADH secretion is
inhibited. Absence of ADH leads to elimination of water through kidney. These mechanisms bring
back water content to normal state. Thus the body water content of healthy adult is maintained.

10. DISEASES OF WATER BALANCE
1. Edema: Excess water in body leads to edema. It is also known as over hydration. It occurs
in excess secretion of ADH, water intoxication, cancer, drugs and overdose of
intravenous fluids.
2. Dehydration: In this condition water content of body is decreased. It occurs in vomiting,
diarrhea, hypothalamus lesions, diabetes insipidus etc.

11. Water molecules have a very slight tendency to ionize to a hydrogen ion and a
hydroxide ion
H2O  H+ + OH-.
Actually, free protons (H+) do not exist in solution, and instead, hydrogen ions
formed in water are immediately hydrated to form hydronium ions (H3O+).
IONIZATION OF WATER

12. The equilibrium constant for the reversible ionization of water is
Keq = [H+] [OH-] / [H2O].
In pure water at 25˚C, the concentration of H2O is 55.5 M. On rearranging, the
equation becomes
(55.5 M) (Keq) = [H+] [OH-] = KW
where KW designates the product (55.5 M) (Keq) and is the ion product of water.
Since the value of Keq, determined by electrical conductivity measurements is 1.8 x
10-16 M, the equation becomes
KW = [H+] [OH-] = (55.5 M) ( 1.8 x 10-16 M)
or
KW = [H+] [OH-] = 1.0 x 10-14 M2.

13. The above calculations indicate that the product [H+] [OH-] in aqueous solution at 25˚C
always equals 1 x 10-14 M2. Thus, when the concentrations of H+ and OH- in solution are
equal (a neutral solution at neutral pH, see below)
[H+] = [OH-] = √KW = √1 x 10-14 M2 = 1 x 10-7 M.
The concentrations of H+ and OH- in solution are determined using the ion product of
water equation
KW = [H+] [OH-] = 1.0 x 10-14 M2.
The ion product of water, KW, is the basis for the pH scale. The pH scale is a convenient
way of designating the concentrations of H+ and OH- in solution in the range between 1.0
M H+ and 1.0 M OH-.
The pH is defined by the equation: pH = -log [H+].

14. Worked Example 1. Calculation of [H+]

15. Worked Example 2. Calculation of [OH-]

16. pH
• PH it is the negative log of the hydrogen ion concentration.
pH = -log [H+]
1. pH is a unit of measurement which describes the degree of acidity or alkalinity
(basic) of a solution.
2. It is measured on a scale of 0 to 14.
Where, Low pH values correspond to high concentrations of H+ and high pH values
correspond to low concentrations of H+.
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 OH- the solution is acidic.
If the OH- concentration is higher than H+ the solution is basic.
If the H+ concentration is equal to conc. OH- the solution is neutral.

17. THE PH SCALE
The pH scale corresponds to the concentration of hydrogen ions. For example, pure water H+ ion
concentration is 1 x 10-7 M, therefore the pH would then be 7.
Acid: Any compound which forms H⁺ ions in solution (proton donors) e.g: Carbonic acid releases H⁺
ions.
Base: Any compound which combines with H⁺ ions in solution (proton acceptors) e.g:
Bicarbonate(HCO3⁻) accepts H+ ions
The pH can be measured by:
1. pH strips
2. pH meter
3. pH indicators e.g. Phenophthalein 9.3-10.5 , Methyl orange 3.1-4.6 , Bromophenol blue 3.0-
4.6, Methyl red 4.4-6.2, Phenol red 6.8 – 8.4, Litmus 4.5-8.3 etc.
pH METER: The pH meter is a laboratory equipment which used to measure acidity or alkalinity of a
solution  The pH meter measures the concentration of hydrogen ions [H+] using an ion-
sensitive electrode.  It is the most reliable and convenient method for measuring pH.

18. FUNCTIONS OF BODY NORMAL pH
1. For the action of enzymes appropriate pH is essential. If pH is altered then enzyme
action is affected. This disturbs body homeostasis.
2. For transport of solute molecules in the body proper pH is required.
3. For the maintenance of structure of folded state of proteins, enzymes and nucleic acids
proper pH is required. Alteration in pH causes structural changes in these biomolecules
which in turn affects functions of these macro molecules.
4. Proper pH is required for maintenance of structure of coenzymes and metabolites.

19. ACID–BASE BALANCE DISTURBANCES
They are two types of acid-base disturbances, they are acidosis and alkalosis.
a. Acidosis : It occurs due to low blood pH. It is due to accumulation of acids. It is further sub divided
into 1. Metabolic acidosis and 2. Respiratory acidosis
Metabolic acidosis : Most common acid base disturbance is metabolic acidosis. More acids are
produced by metabolism. This leads to decreased bicarbonate concentration. It occurs in
uncontrolled diabetes mellitus and starvation. Loss of bicarbonate due to diarrhea and vomiting
also cause metabolic acidosis. Increased elimination of bicarbonate by kidneys also leads to
metabolic acidosis. Ingestion of acids and decreased elimination of proton by kidneys also leads to
metabolic acidosis.
Respiratory acidosis : Plasma partial carbon dioxide pressure is more due to abnormal lung function.
Decreased respiration or hypoventilation occurs due to depression of respiratory centre. Sedatives
like morphine and barbiturates depress respiratory centre. Hypoventilation also occurs due to
obstruction to air passage. In pnumonia, emphysema, asthma air passage is narrowed. Therefore
respiratory acidosis occurs in diseases of lung and respiratory centre depression.

20. b. Alkalosis : It occurs due to high blood pH. It is due to accumulation of alkali. It is further
subdivided into 1. Metabolic alkalosis 2. Respiratory alkalosis.
Metabolic alkalosis : Bicarbonate concentration is more in blood. It occurs due to loss of HCl. More
HCl is lost in prolonged vomiting. Excessive excretion of ammonia by kidney also leads to metabolic
alkalosis. Ingestion of alkali cause metabolic alkalosis.
Respiratory Alkalosis : Partial pressure of carbon dioxide is less. It occurs due to hyperventilation.
When respiratory centre is stimulated hyperventilation occurs. Hyper ventilation occurs at high
altitudes, head injury, drug poisoning, fever and anxiety.