2. WHY WAS THE CONCEPT OF pH
DEVELOPED?
• By the Danish biochemist Sorensen in
1909 to test the acidity of the beer he
was making.
• PH - Pouvoir hydrogene (the power of
hydrogen).
3. • Water has a slight propensity to dissociate into hydroxide ions and protons.
• The acidity of the aqueous solution is usually reported using the logarithmic pH scale.
• Buffers usually maintain the pH of the extracellular fluid between 7.35-7.45.
PH values in the cell and in the extracellular fluid are kept constant within narrow limits.
• In the blood, the pH value normally ranges only between 7.35 and 7.45.
• The pH value of cytoplasm is slightly lower than that of blood at 7.0–7.3. In lysosomes pH 4.5–
5.5
• Extreme values are found in the stomach pH 2 and in the small bowel > 8.
• Since the kidney can excrete either acids or bases, depending on the state of the metabolism, the
pH of urine has a particularly wide range of variation 4.8–7.5.
4. • PH IS DEFINED AS THE NEGATIVE log OF THE HYDROGEN
ION CONCENTRATION.
PH= -log [H⁺]
5. • PH IS A MEASURE OF THE ACIDITY AND BASICITY OF A SOLUTION.
• THE PH SCALE RANGES FROM 0 TO 14.
• WHEN [H+] = [OH-] THE SOLUTION IS NEUTRAL AND PH IS 7
• WHEN [H+] > [OH-] THE SOLUTION IS ACIDIC AND PH IS LESS THAN 7
• WHEN [H+] < [OH-] THE SOLUTION IS BASIC AND PH IS MORE THAN 7
• A CHANGE IN 1 PH UNITS IS = A TEN FOLD CHANGE IN HYDROGEN ION
CONCENTRATION
6. THE EXTENT OF IONIZATION OF A WEAK ACID IS A
FUNCTION OF ITS ACID DISSOCIATION CONSTANT PKa
• BRONSTED AND LOWRY ACID AND BASES
• ACID DONATES PROTONS
• BASES ACCEPTS PROTONS
• STRONG ACIDS DISSOCIATE NEARLY FULLY
• [H+] = [ ACID] AND THUS PH = -log [ACID]
• WEAK ACIDS ONLY PARTIALLY DISSOCIATE
• ACIDS WITH Ka < 1 ARE CONSIDERED WEAK ACIDS
• Ka FOR ACETIC ACID IS 1.76 X 10-5 -> DIFFICULT TO WORK WITH SO INSTEAD USE log SCALE:
• PKa = -log Ka
• SO THE PKa OF ACETIC ACID IS = -LOG 1.76 X 10-5 = 4.75
• THE PH IS A MEASURE OF ACIDITY AND THE PKa IS A MEASURE OF ACID STRENGTH
HA + H 2O = H 3 O+ + HA
acid base conjugate acid conjugate base
7. • WHEN PKa = PH, THERE IS EQUAL CONCENTRATION OF ACID AND ITS
CONJUGATE BASE.
• PKa OF AN ACID GROUP IS DEFINED AS THE PH AT WHICH THE
PROTONATED AND UNPROTONATED SPECIES ARE PRESENT IN
EQUAL CONCENTRATIONS.
• THE EXACT PROBABILITY THAT A MOLECULE WILL BE PROTONATED OR
DEPROTONATED DEPENDS ON THE PKA OF THE MOLECULE AND THE PH OF
THE SOLUTION.
8. • DEPROTONATION IS THE REMOVAL OF A PROTON (H+) FROM A
MOLECULE, FORMING THE CONJUGATE BASE.
• PROTONATION IS THE ADDITION OF A PROTON (H+) TO AN
ATOM, MOLECULE, OR ION.
• THE RELATIVE ABILITY OF A MOLECULE TO GIVE UP A PROTON IS
MEASURED BY ITS PKA VALUE.
• A LOW Pka VALUE INDICATES THAT THE COMPOUND IS ACIDIC
AND WILL EASILY GIVE UP ITS PROTON TO A BASE.
• AT PH VALUES LESS(ACIDIC) THAN PKa, PROTONATED ACID FORM IS MORE ( IT WILL GIVE
OUT ITS H⁺ IONS )
• AT PH VALUES(BASIC) GREATER THAN THE PKa, DEPROTONATED BASE FORM IS MORE IN THE
SOLUTION ( IT WILL TAKE H⁺ IONS TO NEUTRALIZE THE PH AND BRING IT DOWN).
10. WHY IS PH IMPORTANT IN BIOLOGY?
• PH AFFECTS SOLUBILITY OF MANY SUBSTANCES.
• PH AFFECTS STRUCTURE AND FUNCTION OF MOST PROTEINS -
INCLUDING ENZYMES.
• MANY CELLS AND ORGANISMS (ESP. PLANTS AND AQUATIC
ANIMALS) CAN ONLY SURVIVE IN A SPECIFIC PH ENVIRONMENT.
• IMPORTANT POINT -
• PH IS DEPENDENT UPON TEMPERATURE
11. IONIZATION OF WATER:
• PURE WATER IS NEUTRAL. BUT WHEN CHEMICALS ARE MIXED WITH WATER, THE
MIXTURE CAN BECOME EITHER ACIDIC OR BASIC
• WATER MOLECULES EXIST IN EQUILIBRIUM WITH HYDROGEN IONS AND
HYDROXIDE IONS.
H2O <--> H+ + OH-
12. • THE PRODUCT [H+] [OH–]—THE IONIC PRODUCT OF WATER—IS CONSTANT EVEN
WHEN ADDITIONAL ACID–BASE PAIRS ARE DISSOLVED IN THE WATER.
• AT 25 °C, PURE WATER CONTAINS H+ AND OH– AT CONCENTRATIONS OF 1 × 10–7
mol L–1 EACH;
• IT IS NEUTRAL AND HAS A PH VALUE OF EXACTLY 7.
13. • THE WATER EQUILIBRIUM CONSTANT IS WRITTEN AS:
Kw = [H+] [OH-]
EXPERIMENTALLY, IT HAS BEEN FOUND THAT THE CONCENTRATION
OF:
H+ = OH- = 10-7
THEREFORE: Kw = [10-7][ 10-7] = [10-14]
14. TITRATION
• TITRATION IS USED TO DETERMINE THE AMOUNT OF AN ACID / BASE IN A GIVEN
SOLUTION.
• A MEASURED VOLUME OF THE ACID IS TITRATED WITH A SOLUTION OF A
STRONG BASE, USUALLY SODIUM HYDROXIDE (NaOH) , OF KNOWN
CONCENTRATION.
• THE NaOH IS ADDED IN SMALL INCREMENTS UNTIL THE ACID IS CONSUMED
(NEUTRALIZED), AS DETERMINED WITH AN INDICATOR DYE OR A PH METER. THE
CONCENTRATION OF THE ACID IN THE ORIGINAL SOLUTION CAN BE
CALCULATED FROM THE VOLUME AND CONCENTRATION OF NaOH ADDED.
15. HENDERSON - HASSELBALCH EQUATION
PH= PKa+ log (A-)
(HA)
THE QUANTITATIVE RELATIONSHIP BETWEEN PH AND PKa IS
DESCRIBED BY THE HENDERSON- HASSELBALCH EQUATION
16. CONSIDER THE IONIZATION OF A WEAK ACID HA WHICH HAS SOME
PKa.
By the law of mass action:
HA H+ + A-
Rearrange to isolate the hydrogen ion
concentration
17. • MULTIPLY EACH SIDE WITH - log
-log [H+] =- log Ka - log [HA]
[A-]
Ie, pH= pKa – log [HA]
[ A-]
pH=pKa + log
[A-]
[HA]
18. IMPORTANCE
• MEASUREMENT OF PH
IS ONE OF THE MOST IMPORTANT AND FREQUENTLY USED
PROCEDURES IN BIOCHEMISTRY. THE PH
AFFECTS THE STRUCTURE AND ACTIVITY
OF BIOLOGICAL MACROMOLECULES;
1. FOR EXAMPLE, THE CATALYTIC ACTIVITY OF ENZYMES IS STRONGLY
DEPENDENT ON PH
2. MEASUREMENTS OF THE PH OF BLOOD AND URINE ARE COMMONLY USED IN
MEDICAL DIAGNOSES.
