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Hydrolysis of ionsBuffer solutions
Protolytic reactions II
Medical Chemistry
Lecture 2 2007 (J.S.)
Liquid colloid dispersionsTensides
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Hydrolysis of ionsis the reaction of anions of weak acids and
cations of weak bases with water.
Hydrolysis of ions occurs in solutions of salts that contain an anion of a weak acid or a cation of a weak base.
All types of soluble salts are strong electrolytes – theydissociate completely in aqueous solution.
The salts of strong acids and strong hydroxides dissociate into"strong" (spectator) ions, which do not take part in protolyticreactions being only hydrated. Those solutions are neutral.
When a salt of a weak acid or a weak base is dissolves, high amounts of anions of a weak acid or cations of a weak base are passing into the solution. Those ion can exist only in equilibrium concentrations that comply with the ionization constants of particular acids or bases.
The equilibria are reached by means of hydrolysis – the reaction of those ions with water.
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Sodium acetate CH3COONa can serve as an example.
CH3COO–Na+(s) CH3COO– + Na+
Hydrolysis of the salt of a weak acid (and a strong hydroxide)
Complete dissociation during dissolving:
(a spectator cation)
Hydrolysis of acetate anion is running parallel to dissolution:
CH3COO– + H2O CH3COOH + OH–
The solution is not neutralbut slightly alkaline.
Acetate (a strong conjugate base) tears offprotons from water till the concentrationsof CH3COO–, H+, and CH3COOH molecules willreach those that comply with KA of the weak acetic acid.
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NH4+Cl–(s) NH4
+ + Cl–
Hydrolysis of the salt of a weak base (and a strong acid)
Ammonium chloride NH4Cl could be an example:
Complete dissociation during dissolving:
(a spectator cation)
Hydrolysis of ammonium cation is running parallel to dissolution:
NH4+ + H2O NH3 + H3O+
Ammonium (a strong conjugate acid) releasesprotons till the concentrations of NH4
+, OH–, andunionized molecules NH3 will reach those that comply with KB of the weak base ammonia.
The solution is not neutralbut slightly acidic.
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Hydrolysis of the salt derived from a weak hydroxide of a metal(and a strong acid)
For example: copper(II) chloride CuCl2
Cu2+Cl–2(s) Cu2+ + 2 Cl–Complete dissociation during dissolving:
Cu2+ + 4 H2O [Cu(H2O)4]2+
Cation of the metal is hydrated by forming a defined aquacomplex
The solution is not neutralbut slightly acidic.
and takes part in hydrolysis:
[Cu(H2O)4]2+ + H2O [Cu(H2O)3OH]+ + H3O+
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NH4+NO2
–(s) NH4+ + NO2
–
Hydrolysis of the salt derived from both a weak base and a weak acid
Example: Ammonium nitrite NH4NO2
Complete dissociation during dissolving:
Independent hydrolysis of both ions:
NH4+ + H2O NH3 + H3O+
NO2– + H2O HNO2 + OH–
Both ions H+ and OH– occur as the products of hydrolysis, butthey give water. The resulting pH value of the solution dependson pKB of the weak base and pKA of the weak acid.Mostly is the pH close to 7.0 .
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Buffer solutionsA buffer solution (a buffer) – resists a change in pH on addition of small amounts
of an acid or a base, it absorbs the change in acidity, – serves to maintain a fairly constant pH value.
A small amount of an acid added to water results in large drop in pH.If a buffer is present, the decrease in pH will be much smaller.
Simple buffer solutions are mixtures of a weak acid and the conjugate base of that ora weak base and its conjugate acid
Both components should be present at approximately equal (at least at comparable) concentrations.
Examples: acetic acid / sodium acetate ammonia / ammonium chloride
sodium dihydrogen phosphate / hydrogen phosphate
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The buffer solution contains: cations Na+ of the salt (sodium acetate)
that are not taken into account because they are spectator ions;
molecules CH3COOH at concentration equal to cacid
because dissociation of the dissolved acetic acid is suppressed in the
presence of acetate anions from the sodium acetate;
anions CH3COO– at concentration equal to csalt
because hydrolysis of the dissolved acetate is suppressed in the
presence of undissociated CH3COOH of the second buffer component;
ions H+ at concentration that must comply with both equilibrium constants of acetic acid KA and water Kw.
The equilibria in buffer solutions
Example: The buffer solution of acetic acid and sodium acetate was prepared to contain acetic acid at concentration cacid
and sodium acetate at concentration csalt (i.e. cconj. base).
The equilibrium concentrations of the components must complywith the dissociation constant of acetic acid
KA [H+] [CH3COO–]
[CH3COOH]
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For the equilibrium in an acetate buffer, KA [H+] [CH3COO–]
[CH3COOH]takes the form
KA = cacid
[H+] csalt , from which
csalt
[H+] = KA cacid
The concentration of H+ ions in the buffer and its pH value depends on the KA value (i.e. on the type of the weak acid or base used) as well as on the ratio of the acidic and basic component concentrations.
The logarithmic form of that relation is known as
Henderson-Hasselbalch equation:
cbasepH = pKA + log cacid
cbase in buffer solution with an weak acid is csalt (concentration of the conjugate base),cacid in buffers with a weak base means concentration of the conjugate acid (csalt).
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An addition of acid to a buffer Concentration of H+ increases that upsets the equilibrium.New equilibrium will settle, the buffer base binds most of the added H+
ions which results in increase of the acidic buffer component.
The result - [H+] increases proportionally to the increase of cacid / cbase,, pH decreases proportionally to the decrease of the log cbase / cacid .
An addition of a strong hydroxide to a bufferIncrease in OH– concentration withdraws H+ from the buffer acid that transforms into its conjugate base.
The result - [H+] decreases proportionally to the decrease of cacid / cbase, pH increases proportionally to the increase of the log cbase / cacid . Buffer capacity β express the effectiveness of buffers.
It is defined as the ratio of the amount of a strong acid or a strongbase that have to be added to one litre of the buffer solution to change its pH by 0.1 .
β = n (H+ or OH– added) / l
Δ pH
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Buffer capacity depends
– on the ratio of buffer components concentration
pH = pKA + logcbase
cacidThe highest buffer capacity (at a given totalconcentration cbase+ cacid) occurs, if cbase/cacid = 1 , i.e. at pH = pKA .
In sufficient buffer solutions, the ratio cbase/cacid should take valuesfrom 1:10 to 10:1, i.e. in the range pKA ± 1.
– on the total buffer concentration ( cacid + cbase )
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n(OH–)/n(acid)0 0.2 0.4 0.6 0.8 1.0
12
10
8
6
4
2
0
pH
pKA
Determination of the pKA value of a weak acid:
A solution of a weak acid is titrated with a solution of hydroxide and the pH measured in the course of titration. The titration curve is plotted.
Acetic acid (c = 0,1 mol/l) serves as an example:
It can be deduced from Henderson-Hasselbalch equation that pH of weakacid or base solution equals pKA justwhen the ratio cbase/cacid equals 1.
The pH value of the titrated solutionequals pKA at the point when just one half of the weak acid or base isneutralized.
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The dependence of buffer capacity on buffer components rationcan be viewed on titration curves Weak acid ( acetic acid, cHA = 0.1 mol/l)
Area in which the buffer is effectiveThe highest effectiveness at pH = pKA.
The slopes of tangents to the curve are indirectly proportional to thebuffer capacity.
n(OH–)/n(acid)0 0.2 0.4 0.6 0.8 1.0
12
10
8
6
4
2
0
pH
pKA
pKA + 1
pKA – 1
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Diprotic or triprotic weak acids (e.g. H2CO3, H3PO4) act as buffers in two or three pH ranges
CO2 + H2CO3 CO32–HCO3
–
Percentage of particlespresent
100 %
50 %
0
pH of the solution
0 2 4 6 8 10 12 14
pKA 1 pKA 2
Buffer(CO2 + H2CO3) / HCO3
–
BufferHCO3
– / CO3
2–
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Buffer systems in human body
The pH value of blood is 7.40 ± 0.04 .Most biological happenings occur in the pH range 6 to 8.
Blood buffer bases: Buffer:
Hydrogen carbonate HCO3– / (H2CO3+CO2)
Plasma proteins protein / protein-H+
Haemoglobin of red blood cells haemoglobin / haemoglobin-H+
Hydrogen phosphate HPO42– / H2PO4
–
All those buffer systems cooperate – a surplus of H+ is accepted byall buffer bases but distributed proportionally to their concentration in blood.
Each of those four buffer systems has its own pKA .
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( CO2+ H2O H2CO3 ) H+ + HCO3–
CO2 dissolves in water physically, only a minute part of in reacts withwater to give unstable H2CO3 .
When evaluating the acid-base status in blood by means of measuredpH values and pCO2 (partial pressure of CO2 in blood), the
Henderson-Hasselbalch equation for hydrogen carbonate bufferis modified from
Hydrogen carbonate buffer in blood
[CO2+ H2CO3]pH = pKA1 + log
[HCO3–]
pKA1 (25 °C, in water) = 6.37
to
6.10 + log[HCO3
–]
pCO2 0.22pH = (37 °C, higher ionic strength),
[HCO3–] in mmol/l,
( pCO2 0.22 ) transformes pCO2 in kilopascals into
[CO2+ H2CO3] in mmol/l
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Amino acid Ionizable group in the side chain pKA
Aspartate
Glutamate
Histidine
Cysteine
Tyrosine
Lysine
Arginine
-carboxyl (-COOH)
-carboxyl (-COOH)
imidazolium
sulfanyl (-SH)
phenolic hydroxyl
-ammonium (-NH3+)
guanidinium –NH-C(NH2)=NH2+
3,9
4,3
6,0
8,3
10,1
10,5
12,5
Plasma proteins and haemoglobin as buffersIn all proteins, only ionizable groups can take part in acid-base reactions.At physiological pH values, imidazole groups of histidine residues aloneact as effective buffer bases.
Hydrogen phosphate buffer is of second-rate significance inthe blood due to relatively low concentration. However, within thecells, phosphates with proteins are the major buffer bases.
Liquid colloid dispersionsSurfactants (tensides)
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Liquid colloid dispersions In this part, the aqueous colloid dispersions are discussed.
In general, lyophobic particles are those without any interactions between their surface and molecules of the solvent;
lyophilic particles are of similar polarity as the molecules of the solvent and are surrounded by a layer of solvent molecules attracted by weak intermolecular forces
If water is the solvent, then the colloid particles of solutes are either hydrophobic or hydrophilic.
The size of colloidal particles ranges from 1 nm to an upper limitof 500 (– 1000) nm.
The colloid particles are able to pass through filter paper (not throughcellophane or parchment paper), exhibit Brownian movement, Tyndalleffect and opalescence; they do not settle out on standing spontaneously.
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Aqueous colloid dispersions
Examples: colloid dispersion of sulfur, silver, gold.
Dispersed particles are
HYDROPHOBIC
COLLOID SOLS
The dispersion is stabilized by electric charge only, more diphasic than homogenous, less stable.
Flocculation occurs afteraddition of a small amountof electrolytes, unlessa protective colloid is added.
Dispersed particles are (at least partly)
HYDROPHILIC
COLLOID SOLUTIONS
stabilized by both electric chargeand solvation shell,
more stable than hydrophobic sols,aggregation caused by change in pH,.
alcohol, acetone, and by salting-out
MOLECULARcolloid solutions
of macromolecularpolymers (starch,proteins, nucleic acids, etc.)
of tensides that formmicells because of their amphipathic
molecules
MICELLARcolloid solutions
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Cl–
protein
anion, e.g. Cl–
cation, e.g. Na+
molecules of watercharged groups of the macromolecule
Solubility of hydrophilic macromolecules (e.g. proteins)
Solubility of a protein is enabled by interactions of polar groups on the molecular with surface with water dipoles (both type dipole-ion and hydrogen bonds). Several layers of orientated water molecules represent the solvation shell (hydration shell) of a protein molecule.
When salts are present, their ionsalso interact with polar groups of theprotein (ion-ion and ion-dipole interactions) so that an diffuse layer of ions – electric dilayer – intermingled with water molecules surrounds the macromolecule. .
Molecular colloid solutions
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Stability of protein solutions – the dependence on salt concentration
When the concentration of ions in the solution is low, the diffuse layer is large and the protein molecules exhibit maximum repulsion – the colloid dispersion is stable. Some protein sorts are soluble only in solutions with low concentrations of salts (the salting-in effect).
When the concentration of ions is large, the electrostatic repulsion that prevents colloidal particles from coagulating becomes inefficient – protein molecules are coagulated by adding high amounts of electrolytes (salting-out).The competition for water molecules that form the hydration shell also cannot be neglected.
Salting-inoccurs only whensome sorts of proteins are dissolved
Salting-out
Salt concentration
Solubility
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Stability of aqueous molecular colloid solutions depends primarily on the existence of a hydration shell and on the net electric charge of dispersed molecules.
The more concentrated molecular colloid solutions are less stable than those at low concentrations.
Molecular colloid dispersions can be destabilized so that the solute aggregates and precipitates by means of
– eliminating the charge of the solute,e.g. by a change of pH of the solution,
– abstracting the solute from its solvation shellby addition of a solvent that binds water
being easily hydrated (acetone, ethanol),
– exclusion of both stabilizing factorby addition of large amount of a salt
(increase of the ionic strength – salting-out).
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Micellar colloid solutions
A micelle
H2OH2O
H2O
H2O
H2O H2O
H2O
H2O
H2O
H2OH2O
H2O
Hydrophobic parts
Molecules of tenside
Hydrophilic parts
are solutions of tensides - low-molecular compounds thathave amphiphilic (diphilic) character. Their name is derived from the ability to diminish surface tension of liquids.
The molecules of tensides (surfactants) have two parts: – a long or voluminous hydrocarbon chain (hydrophobic part),and – a polar (hydrophilic) group.
Although tensides are low-molecular, they associateto form colloid particles – micelles:
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At low concentration of a tenside, the molecules are adsorbedat the phase boundary; some free molecules are dispersedin the liquid (true solution).
At concentrations higher thancritical micellar concentrationthe molecules of tenside formmicelles.
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Classification of tensides according to the polar group:
polyethylene glycolglycosides
phospholipids
quaternary ammonium cations (with s long
aliphaticd chain)
soaps alkyl sulfates
(alkanesulfonates)bile acids
Anionic tensides
Cationic tensides
Amphoteric tensides
Non-ionogenic tensides
negative electric charge
positive electric charge
both negative and positive charge
polar group without any charge
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Anionic tensidesSoaps are produced by alkaline hydrolysis (saponification) of fats and vegetable oils (triacylglycerols).
Washing powders (detergents) are mixtures of synthetic tensides and several additives.
sodium alkyl sulfate
sodium alkanesulfonate
O SO
OO Na
SO
OO Na
C
O Na
O
sodium stearate
Ostearate anion acts as a tenside
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Cationic tensides
Tetraalkylammonium salts (one or two lon-chain alkyls)
N+ Cl-
R1
R2
Antiseptics and disinfectants with a considerable cleaning effect
X–
N+ R2
R1
R3
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C
CH2
CH
CH2
O
O P
O
O
O CH2CH2
O
N
CH3
CH3
CH3
C
O
OPhosphatidylcholine
Phospholipids
Phospholipids are extremely important as constituents of lipidic dilayers of biomembranes.
Natural tensides of the human body
Bile acids
Both phospholipids and bile acids enable dissolutionof another bile constituent – cholesterol.
Bile acids are excreted by the liverinto the bile. They act as potentemulsifiers of lipids in the smallintestine facilitating lipid digestionand absorption of all liposolublecompounds.
Cholic acid
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Effect of tensides
Micelles in solution of tensides are able to absorb limited amount of hydrophobic compounds into their hydrophobic inner.Micells keep their size in the range required for colloid dispersions (less than 500 nm).
If the concentration of micelles remains constant and the amount of hydrophobic compounds is too large, the tenside molecules cover the surface of hydrophobic droplets and stabilize the resulting crude dispersion (emulsion)..
Solubilizing effect Emulsifying effect
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Liposomes are used for transport of hydrophilic pharmaceuticals (even nucleic acids) into cells;They enter cells by means of endocytosis.
LiposomesAqueous innerof the liposome
Lipidic dilayer,a phospholipid membrane prepared arteficially by sonicationof phospholipid colloid dispersion
