Water and Ph

Water, pH and dissociation of water

DR.SADIA HAROON

Lecture 3 Outline

• Homeostasis

• The structure and function of water

• Dissociation of weak acids and weak bases

• pH and the Henderson-Hasselbalch equation

• Buffers, biological/physiological examples

HOMEOSTASIS• The dynamic that defines the distribution of water and

the maintenance of pH and electrolyte concentrations• Water distribution maintained by the kidneys,

antidiuretic hormone, hypothalamic thirst response, respiration and perspiration

• Clinically, need to be aware of water depletion caused by decreased intake (coma, wandering the desert) or increased loss (diarrhea, renal malfunction, over-exercise), and excess body water due to increased intake (too much I.V.) or decreased excretion (renal failure)

Importance of water

• 70% of earths surface is water

• Cell contains

• Water is present intra cellular and extra cellularly

• Water is essential for life

• Chemical reaction takes place in aqueous environment

Water Chemistry

All living organisms are dependent on water.

The structure of water is the basis for its unique properties.

The most important property of water is the ability to form hydrogen bonds.

Water Chemistry

Within a water molecule, the bonds between oxygen and hydrogen are highly polar.

Partial electrical charges develop:

- oxygen is partially negative

- hydrogen is partially positive

Water Chemistry

Hydrogen bonds are weak attractions between the partially negative oxygen of one water molecule and the partially positive hydrogen of a different water molecule.

Hydrogen bonds can form between water molecules or between water and another charged molecule.

Hydrophobicity/Micelles

Fro

m L

ehni

nger

, 2nd

ed.

, Ch

4

Water Solubility / Hydrophilic

From Lehninger, 2nd ed., Ch 4

Water Chemistry

Structure of H20


Water Chemistry

The polarity of water causes it to be cohesive and adhesive.

cohesion: water molecules stick to other water molecules by hydrogen bonding

adhesion: water molecules stick to other polar molecules by hydrogen bonding

Water Chemistry

Structure of water

• Water has dipolar structure

• V –shaped structure ,bend geometry

• Two light atoms; hydrogen

• One heavy atom oxygen

• H-O-H

• Angle between them109.47

Why ice floats on water

• It expands on freezing

• It has density of 0.92g/ml

• Water at 0 degree has density of 1.0 g/ml

• Important in maintaining life and enviornment.

Structure of water

• Hydrogen bonding :non covalent bonding between hydrogen and oxygen of two water molecules

• Vander walls forces

• Electrostatic interaction

• Hydrophobic interactions; results in formation of droplet water water interaction)

Other properties of water

• Thermal properties ;liquid at room temp

• At 0 degree melts ,at 100 degree boils

• Ice formation; maximum hydrogen bonding

• Water vapour at 100 boiling

Properties of Water

1. Water has a high specific heat.

- A large amount of energy is required to change the temperature of water.

2. Water has a high heat of vaporization.

- The evaporation of water from a surface causes cooling of that surface.

Properties of Water

3. Solid water is less dense than liquid water.

- Bodies of water freeze from the top down.

4. Water is a good solvent.

- Water dissolves polar molecules and ions.

Other properties

• Solvent properties• Remarkable solvent• Ionic substances and polar substances due to

hydrogen bonding• Nonpolar lacks hydrogen bonding• Amphipathic rearrange to form micelles;non

polar aggregate at centre non polar out side

Properties of Water





Hydrophilic/HydrophobicF

rom

Leh

ning

er, 2

nd e

d., C

h 4

Water Chemistry

Properties of Water

Other properties

• Solvent properties• Remarkable solvent• Ionic substances and polar substances due to

hydrogen bonding• Nonpolar lacks hydrogen bonding• Amphipathic rearrange to form micelles;non

polar aggregate at centre non polar out side

Ionization of water

• Water very less ionize

• [H+]+[OH-]

• [H+]+[H2O]=H3O=hydronium ion

• Hydrated proton

• Ke=ionization constant of water

• =[H+]+[OH-]/[H2O]

• (1.8x10-16)(55.5)=1.0x10-14

Properties of Water5. Water organizes nonpolar molecules.

- hydrophilic: “water-loving”-hydrophobic: “water-fearing”- Water causes hydrophobic molecules to aggregate or assume specific shapes.

6. Water can form ions.

H2O OH-1 + H+1

hydroxide ion hydrogen ion

Ionization of water


• [H+]+[OH-]


• Hydrated proton


• =[H+]+[OH-]/[H2O]

• (1.8x10-16)(55.5)=1.0x10-14

pH =pH+pOH

• The product of both ion s as gram mole in one liter of water is the ionic product of water.

• P= -log (logarithm) to base 10

• Log means ratio number

• Log of a number is the index by which 10 is to be raised to get that number

Ionization of water


• [H+]+[OH-]


• Hydrated proton


• =[H+]+[OH-]/[H2O]

• (1.8x10-16)(55.5)=1.0x10-14

Ph scale

• Kw=[H+][OH-]=10-14

• [H+]=[OH-]=10-7

• pH =-log [H+]

• PH= -log[10-7]

• P H =-[ -7]

• Ph =7

Function of Water: Most of cellular activities are performed in

water solutions.

16% TBW

40% TBW4% TBW

- makes up ~60% of total body weight (TBW)

- distributed in three fluid compartments.

Body Fluid

16% TBW

40% TBW4% TBW

Fluid is continually exchanged between the three compartments.

16% TBW

40% TBW4% TBWExchange between Blood & Tissue Fluid

- determined by four factors:

capillary blood pressure

plasma colloid osmotic pressure

interstitium Hydrostatic Pressure

Interstitium colloid osmotic pressure

16% TBW

40% TBW4% TBW

- not affected by electrolyte concentrations

- Edema = water accumulation in tissue fluid

Exchange between Blood & Tissue Fluid

Water Gain

Water is gained from three sources.

1) food (~700 ml/day)

2) drink – voluntarily controlled

3) metabolic water (200 ml/day) --- produced as a byproduct of aerobic respiration

Routes of water loss

1) Urine – obligatory (unavoidable) and physiologically regulated, minimum 400 ml/day

2) Feces -- obligatory water loss, ~200 ml/day

3) Breath – obligatory water loss, ~300 ml/day

4) Cutaneous evaporation -- obligatory water loss, ~400 ml/day

5) Sweat – for releasing heat, varies significantly

ACID-BASE BALANCE

Acid An acid is any chemical that releases H+ in

solution.

Base A base is any chemical that accepts H+.

Acids and Bases

Most biological buffers consist of a pair of molecules, one an acid and one a base.

Acids and Bases

Hydrogen ion (H+1) is the basis of the pH scale.

Greater H+1 concentration --- lower pH (acidic)

Lower H+1 concentration --- higher pH (basic)

pH

is the negative logarithm of H+ concentration, and an indicator of acidity.

pH = - log [H+ ]

Example: [H+ ] = 0.1 M = 10 –7 M

pH


pH = - log [10 –7 ]

Example: [H+ ] = 0.1 M = 10 –7 M

= 7 log 10 = 7

pH


pH = - log [10 –8 ]

Example: [H+ ] = 0.01 M = 10 –8 M

= 8 log 10 = 8

[ H+ ] = pH

[ H+ ] = pH 0.01 M [ H+ ] = pH 8

0.1 M [ H+ ] = pH 7

Normal functions of proteins (especially

enzymes) heavily depend on an optimal pH.

pH7.35-pH7.45

Acids and Bases

Acid: a chemical that releases H+1 ions.

Base: a chemical that accepts H+1 ions.

Buffer: a chemical that accepts/releases H+1 as necessary to keep pH constant

Regulation of acid-base balance

1) Chemical Buffers

2) Respiratory Control of pH

3) Renal Control of pH

Properties of Water





Properties of Water

3. Solid water is less dense than liquid water.

- Bodies of water freeze from the top down.

4. Water is a good solvent.

- Water dissolves polar molecules and ions.

Acids and Bases

Hydrogen ion (H+1) is the basis of the pH scale.

Greater H+1 concentration --- lower pH (acidic)

Lower H+1 concentration --- higher pH (basic)

Acids and Bases

Acids and Bases

Acid: a chemical that releases H+1 ions.

Base: a chemical that accepts H+1 ions.

Buffer: a chemical that accepts/releases H+1 as necessary to keep pH constant

Acids and Bases

WATER

• Comprises approx 70% of human mass (45-60% intracellular, 25% extracellular/blood plasma)

• dipolar: partial negative charge on oxygen,

partial positive charge on hydrogens

• dipolar nature leads to formation of many low energy hydrogen bonds

Water Solubility / Hydrophilic


Hydrophilic/HydrophobicF

rom

Leh

ning

er, 2

nd e

d., C

h 4

Hydrophobicity

Fro

m L

ehni

nger

, 2nd

ed.

, Ch

4

Hydrophobicity/Micelles

Fro

m L

ehni

nger

, 2nd

ed.

, Ch

4

Summary of water and pH relationship

• Very low dissociation of H2O to H+ or OH-• The ion product of H2O, Keq X 55.5 M, leads to

this: [H+] = [OH-] = 1 X 10-7 M for pure H2O which is a constant in biological systems

• Therefore, if [H+] > 10-7 M, then [OH-] must be less than 10-7 M, and vice versa.

• Thus, if the negative logarithm of [H+] is derived ( pH = -log [H+] ), pure water would be pH = 7, acids pH < 7, and bases pH > 7

Ionization properties of water

• Water disociate

1) Polarity

Covalent bonds (electron pair is shared) between oxygen and hydrogen atoms with a bond angle of 104.5o.

Oxygen atom is more electronegative that hydrogen atom --> electrons spend more time around oxygen atom than hydrogen atom --> result is a POLAR covalent bond.

Creates a permanent dipole in the molecule.

Can determine relative solubility of molecules “like dissolves like”.

pH scale is widely used in biological applications

^pH 7is neutral ,means no acidity or

alkalinity

^when excess of H+ ions are added the solution are acidic

^when OH are added they become basic

^biomolecules have acidic or basic properties

The Relationship Between pH and pOH

pH pOH [H+] mol/L [OH-] mol/L

0 14 1.010-14

2 12 0.01 10-12

4 10 0.0001 10-10

6 8 10-6 10-8

8 6 10-8 10-6

10 4 10-10 0.0001

12 2 10-12 0.01

14 0 10-14 1.0

pH measurement

• pH paper: 1.Broad range (1-14whole no) 2.Narrow range (0.3-0.5pH unit)

• Universal indicator solution: -It is a mixture of many indicators. -Shows change of colour peculiar to the composition of that mixture.

• Litmus paper: -Its neutral point is pH 7. -Blue colour on alkaline side and red colour on acid side.

pH measurement

• pH meter:

-Electrometric method with

automatic temp compensating

device.

-Precision potentiometer.

-Controlled dial is calibrated in pH

units & milli volts

pH measurement

• Calomel electrode:

-It is used as reference electrode

• Glass electrode: -glass separates two solutions of different H+ conc. A small potential arises across the membrane this is propotional to the difference in pH and can be measured by amplifier.

ACIDS AND BASES

• Acids are defined as hydrogen donor or proton donor.

• Bases are defined as hydrogen acceptor• Strong acids and bases ionize completely in water.• HCl=H+ Cl and Na OH=Na+ OH• Strong acids are=sulfuric acid, nitric

Acid, hydrochloric acid and strong bases are Na OH and KOH.

Weak Acids and weak bases

• Weak acids partially ionize to release a H+ ,and lower the pH.

• Weak bases accept a H+ and increase the pH+

ACID BASE CONGUGATE PAIR

pH of various common fluids

• Blood plasma 7.4• Liver 6.9• Muscle 6.1• Saliva 6.6• Urine 5-8• Orange juice 4.3• Vinegar 2.9• Gastric juice 1.7

Acids produce by the body

• Carbonic acid• Sulphuric acid• Phosphoric acid• Lactic acid• Citric acid• Ammonium ion• Ketone bodies -Acetoactic acid -Betahydroxy butyric acid

Classification of acids

• First is based on strength: 1.Strong acids: -That ionizes completely. -High concentration of H ions. -Free H+ show less tendency to combine with base. (weakest conjugate) 2.Weak acids: -Slowly dissociates. -Give less no of H+ ions. -Has the strongest conjugate base.

Classification of acids

• Second is based on volatile/non-volatile: 1.Volatile acid: -Carbonic acid:20000 m eq/day 2.Non-volatile acid: -Lactic acid -Sulphuric acid -phosphoric acid

Fro

m D

evli

n , 3

r d e

d ., C

h 1

From Marks, Marks, Smith, Ch 4

Dissociation Constant and pH

Henderson-Hasselbalch Equation

From Devlin, 3rd ed., Ch 1

Fro

m D

evli

n , 3

r d e

d ., C

h 1

Sample pH problemsF

rom D

evlin, 3rd ed., Ch 1

Sample pH Problem (cont)F

rom D

evlin, 3rd ed., Ch 1

Buffers

• Definition: A weak acid plus its conjugate base that cause a solution to resist changes in pH when an acid or base are added

• Effectiveness of a buffer is determined by: 1) the pH of the solution, buffers work best within 1 pH unit of their pKa 2) the concentration of the buffer; the more present, the greater the buffering capacity

Buffers and weak acids

• Buffers are solutions that resist change in pH when acid or base is added to it.

• It consist of weak acid and its salt (acetic acid and sodium acetate )or a weak base and its salt (ammonium hydro oxide and ammonium chloride )

Mechanism of buffer action

• Added H+ ions = in the form of strong acid ,combine with anions A-( largely form the salt component of buffer), to form the weakely dissociable HA, so that pH does become as acid as it would be in the absence of the buffer.

Mechanism cont..

• Acetic acid freely ionizable

• Sodium acetate to a large extent

• CH3COOH=CH3COO+H+

• CH3COONa=CH3COO +Na+

• H Cl is added acetate will combine with H+ to form acetic acid and NaCl is formed

Mechanism contd..

• When NaOH is added the H+ of the buffer (acetic acid) combine with OH- to form water ,which is weakely dissociated .

• Thus pH change due to base addition is also prevented by buffer

• OH+H+ =H2O

BUFFERING CAPACITY

• The efficiency of a buffer in maintaining a constant pH on the addition of acid or base is referred to as buffering capacity.

• The capacity to combine with added acid remains so long as there is supply of the buffer salt in the medium.

• OH can be buffered as long as some of the acid HA remains to supply the H+.

H-H EQUATION

• The quantitative relationship between the concentrate of weak (HA) and its conjugate (A-) is Henderson equation.

• HA=weak acid =H+ +A-

• H+ =proton

H-H EQUATION

• Is important for under standing buffer action and acid –base balance in the blood and tissue.

• Restating the expression for dissociation constant of an acid

• The pH of a solution of a weak acid (or base) and its salt is given by

• pH=pKa _log [HA/A-]

H-H EQUATION

• Ke =equilibrium constant

• K e =ionization constant

• Or dissociation constant of an acid or Ka

• Stronger acids :phosphoric acid, carbonic acid ,acetic acid have larger dissociation constant

Cont..

• Weaker acids mono hydrogen PO4 have smaller dissociation constant

• Pka=log 1/Ka

• pKa =-logka

• The stronger the tendency to dissociate a proton ,the stronger is the acid lower the pKa

H-H eq…

• Ka=[H+][A-]/HA

• First solve for [H+]=Ka [HA]/[A-]

• Take neg log on both side -log [H+]=-log Ka –log [HA]/A-

• Putting values pH=pKa -log[HA]

• pH=pKa+log[A-]/[HA]

• pH=pKa

Determination of pH

• Take three test tubes

• To one add 1ml sodium acetate+acetic acid=(log 1/10)=-1

• 10 ml both(log 10/10)=0

• 10ml Na acetate +1ml acid (log10/1)=1

Buffers of the body fluids

• Intracellular buffers;phosphate buffers cossist of disodium hydrogen phosphate and sodium dihydrogen phosphate.

• This has pka close to physiological ph

• Pka=6.8

Contd.

• Protein buffers :depend on ionizable side chain

• Histine imidazole group pka=6.1

• 16 histidine in albumin

• 38 histidine in haemoglobin

Buffers …

• Extracellular ;comprise43%(intracellular 57%)

• 65%=bicarbonate buffers

• 30%=haemoglobin

• 4%=protein

• 1%=phosphate buffers

• Buffers are first line of defence against acid load

Bicarbonate buffer system

• Consist of HCO3 and H2CO3• Most significant• 65% of plasma buffering• 40% of buffering of body• CO2 andHCO3 can diffuse easily across

membranes• HCO3 (Metabolic component) regulated by

kidney and co2 by respiration.

Buffers ..

• HCO3 22-26 mmol/lt (24mmol)• Pka=6.1• H2CO3=(CO2 in forms dissolved)• pCO2=40• Solubility coeffeciant=0.3• pH=Pka+logHCO3/H2CO3• 7.4=6.1+LOg24/1.2• 7.4=6.1+LOg20• 7.4=6.1+1.3

Body buffers

• Three mechanism: to regulate pH and acid base balance and maintain the blood pH (around 7.4)

• 1.blood buffers

• 2.respiratory mechanism

• 3.renal mechanism

EFFECTS OF ACID BASE DISTURBANCES

• H+increased= acidosis,depression of C.N.S, Disorientation, death in coma.

• H+decreased= alkalosis, overexcitability of C.N.S,convulsions.

First in peripheral Nerves than C.N.SSensory effects,Tingling(pins and needle sensationMotor effects, muscle twitches, spasmExtreme alkalosis- spasm of respiratory muscles,

death

EFFECTS…..

2. CHANGES IN ENZYMES ACTIVITY

Altering shape and activity of protein molecule.

Some reactions are accelerated and some are depressed.

EFFECTS…..

3. CHANGES IN CELLULAR pHReduced contractility of actin and myosin in

muscles.CHANGES IN POTASSIUM LEVELH+ ions enters the cells for sodium and

potassium.H+ ions are eliminated more than potassium

so hyperkalemia, cardic disfunction.

Blood buffers

• Bicarbonate buffer system: NaHCO/H2CO3

• H2CO3 >H+ +HCO3

• pH=pKa +log [salt]/[acid]

• 20:1 is the ratio

• Alkali reserve : responsible for effective buffering of H+,generated in the body.

Blood buffers.

• Bicarbonate buffers ;is index to understand the disturbances.

• Phosphate buffers; this is important buffer intracellular .

• Sodium di hydrogen phosphate and disodium hydrogen phosphate

• Ratio of base to acid for phophate buffer is 4:1

• Pk is 6.8

Body buffers

• Three mechanism: to regulate pH and acid base balance and maintain the blood pH (around 7.4)

• 1.blood buffers

• 2.respiratory mechanism

• 3.renal mechanism

Physiological Buffers

• Carbon Dioxide-Bicarbonate System; a major regulator of blood pH

• Phosphate System; major regulator of cytosolic pH

• [CO2] and [HCO3] are much higher than [PO4] in blood; the reverse is true in the cytosol, [PO4] >>> [HCO3]

Examples - Physiological Buffers


Fro

m L

ehni

nger

, 2nd

ed.

, Ch

4

pH T

itrat

ion

Cur

ves

The bicarbonate blood buffer in a normal adultmaintains the blood pH at about 7.40. If the bloodpH drops below 7.35, the condition is referred toas an ACIDOSIS. A prolonged blood pH below7.0 can lead to death. Clinically for an acidosis, the acid-base parameters (pH, [HCO3

- ], [CO2] )of the patients blood should be monitored. The normal values for these are pH = 7.40; [HCO3

- ] = 24 mM; [CO2] = 1.2 mM.

Blood Bicarbonate and Metabolic Acidosis

Sample Problem – Metabolic Acidosis

• The blood values of a patient were pH = 7.03 and [CO2] = 1.1 mM. What is the patient’s blood [HCO3

-] and how much of the normal [HCO3

-] has been used in buffering the acid causing the condition?

• The pK’ for [HCO3-]/[CO2] = 6.10

Solution• Substitute into Henderson-Hasselbalch equation:• 7.03 = 6.10 + log [HCO3

-]/1.1 mM, or • 0.93 = log [HCO3

-]/1.1 mM• The anti-log of 0.93 = 8.5, thus:

• 8.5 = [HCO3-]/1.1 mM, or [HCO3

-] = 9.4 mM

• Since normal [HCO3-] equals 24 mM, there was a decrease of 14.6

mmol of [HCO3- per liter of blood in this patient. This would be

approaching the point where, if left untreated, the HCO3- buffering

capacity would be no longer effective in this patient.

Water and Ph

Documents

Transcript of Water and Ph