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Buffers & pHBuffers & pH

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Introduction

Molecules are formed by interaction ofpositively and negatively charged ionsthrough the formation of ionic bonds.

When certain compounds are dissociatedin water, they contribute either with H+

which referred to as an acid or OH-

whichreferred to as a base in a solution, andthat compound which contribute neitherare called salts.

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Revision

Acids and bases change the colours of

certain indicators.

Acids and bases neutralize each other.

Acids and bases react to form salts.

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Revision (continued)

An acid is a substance that liberateshydrogen ions (H+) in a solution.

Abase is a substance that liberateshydroxide ions (OH-) in a solution.

ANeutral solutionNeutral solution is a solution thatis a solution thathavehave equalequal concentration ofconcentration of HH++ ions &ions &

OHOH-- ions.ions.

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According to the Bronsted-Lowry concept:

An Acid is a substance that can donate a

proton.

ABase is a substance that can accept a

proton.

Revision (continued)

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Consider the reaction of NH3 with H2O:

NH3 (aq) +

H2

O(aq)

NH4

+

(aq) +OH-

(aq)

Aconjugate acid-base pair consists of two

species in an acid-base reaction, one acidone base, that differ by the loss or gainof a proton.

Revision (continued)Revision (continued)

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Note:


NH3 and NH4+ are a conjugate acid-basepair.

H2O and OH-

are also a conjugate acid-base pair.

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Relative strengths of acids and bases:

The strongest acids have the weakest

conjugate bases and the strongest baseshave the weakest conjugate acids.


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ThepH Scale

It is typically defined as the negativeIt is typically defined as the negativelogarithm of the hydrogen ion concentration.logarithm of the hydrogen ion concentration.

pH = Power of Hydrogen (H+) or PotentialHydrogen

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ThepH Scale(continued)

The equation used to determine the pHThe equation used to determine the pHof a solution is stated as:of a solution is stated as:

pH =pH = -- log [Hlog [H++]]

where [Hwhere [H

++

] is the concentration of hydrogen in mol/l.] is the concentration of hydrogen in mol/l.

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ThepH Scale(continued)

The pH scale uses the no. 0The pH scale uses the no. 0 14 to14 todescribe the acidity or alkalinity of adescribe the acidity or alkalinity of a

solution.solution.

0Acid7alkaline140Acid7alkaline14

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ThepH Scale (continued)

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About 50 to 100 mmol of hydrogen ionsare released from cells into extra cellularfluid each day either from metabolism ofamino acids or incomplete metabolism oforganic compounds.

Acid-BaseHaemostasis

The body yield H+ ions more than OH- ions.

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In human blood, normal pH lies

between 7.35 to 7.45 which isequivalent to the hydrogen ion

concentration of approximately

4 x 10 -8 mol/l.

Acid-BaseHaemostasis (continued)

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It is essential that the pH stays constantin order for any processes to workproperly. e.g. If the pH of blood varies by0.5 it can lead to unconsciousness andcoma.

Acid-BaseHaemostasis (continued)

The most important way that the pH ofthe blood is kept relatively constant is bybuffers dissolved in the blood.

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Acid BaseHaemostasis & Blood

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Abuffer is a solution characterised bythe ability to resist changes in pH when

limited amounts of acid or base are addedto it.

Buffers contain either a weak acid and its

conjugate base or a weak base and itsconjugate acid.

Buffers

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Thus, a buffer solution contains both anacid species and a base species in

equilibrium.

i.e. A pair of substance is involved: one

substance yield H+

ion when pH isincreased the other binds with H+ ion whenpH is decreased.

Buffer (continued)

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Buffer (continued)

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Henderson-Hasselbalch Equation

expresses the relation between pH andbuffer pair

pH = pKa + log [base][ acid]

Where, pK is the disassociation constant

Given concentrations of the acid and the base and the Ka,the pH of a solution can be calculated

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Acid-Base balance involves chemicalchemical andphysiologicphysiologic processes responsible for themaintenance of the acidity of body fluidsat levels that allow optimal function of thebody.

Buffer System

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Buffer System (continued)

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The chemical buffer systems are:


1. The carbonic acid-bicarbonate buffer

system (most important-in the ECF).2.The phosphate buffer system (in theICF).

3.The protein buffer systems (in theECF and ICF).

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The chemical buffer systems

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The Physiological buffer system includes:

1. Respiratory Mechanism(CO2excretion).

2.R

enal Mechanism (H+

excretion).

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Acid-base Disturbance

A condition in which blood pH drops below7.35 includes:

1.Respiratory acidosis primarydisorder is an increased carbonic acid(H2CO3) concentration.

I.Acidosis (Acidemia):I.Acidosis (Acidemia):

2.Metabolic acidosis primary disorderis a decrease in bicarbonateconcentration.

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Acid-base Disturbance (continued)

A condition in which blood pH rises above7.45includes:

II.Alkalosis (Alkalemia):II.Alkalosis (Alkalemia):

1.Respiratory alkalosis primarydisorder is a decreased carbonic acidconcentration.

2.Metabolic alkalosis primary disorderis an increased in bicarbonateconcentration.

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Measurement of pH

1. Calculation from the equation.

2.pH indicator.

4.Blood gas analyzer.

3.pH meter.

pH = pKa + log [base]

[ acid]

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pHMeter or pH Paper

pH meter pHpaper

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Blood gas analyzer

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Blood gas analyzer (continued)

8. buffers & ph

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