acids and bases. ph scale. buffer solutions. hydrolysis of...

Acids and bases. Acids and bases. pH scale. Buffer solutions. pH scale. Buffer solutions.

Hydrolysis of saltsHydrolysis of salts

NatalyaNatalya [email protected]@karazin.ua

Department of Physical ChemistryDepartment of Physical Chemistry

V.N. V.N. KarazinKarazin KharkivKharkiv National UniversityNational University

Medical Chemistry Medical Chemistry Module 1. Lecture 4Module 1. Lecture 4

25 January 202125 January 2021

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LLectureecture topicstopics

√ Acids and bases√√ Neutralization: Bases are AntacidsNeutralization: Bases are Antacids√√ Acidity of solutionsAcidity of solutions√√ Chemistry in action: antacids and the pH balance in your Chemistry in action: antacids and the pH balance in your

stomachstomach√√ The action of aspirinThe action of aspirin√√ AcidAcid--base indicatorsbase indicators√√ Hydrolysis of saltsHydrolysis of salts√√ Buffer solutionsBuffer solutions

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Acids and basesAcids and basesH+

Citrus fruit contains both citric acid and ascorbic acid

√√ Acids and basesAcids and bases have been defined in a number of different ways. have been defined in a number of different ways. In the simplest form an In the simplest form an acidacid can be described as a substance that yields can be described as a substance that yields

hydrogen ions (Hhydrogen ions (H++)) when dissolved in water: when dissolved in water:

and a and a basebase can be described as a substance that yields can be described as a substance that yields hydroxide ions (OHhydroxide ions (OH––))when dissolved in water:when dissolved in water:

Bases generally taste bitter; when dissolved in water, bases havBases generally taste bitter; when dissolved in water, bases have a slippery, e a slippery, soapy feel.soapy feel.

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Acids and basesAcids and bases√√ The "strength" of an acid is a measure of its tendency to lose aThe "strength" of an acid is a measure of its tendency to lose a proton proton

(H(H++) ) , and the "strength" of a base is similarly a measure of its ten, and the "strength" of a base is similarly a measure of its tendency dency to take up a proton.to take up a proton.

√√ Acids commonly usedAcids commonly used in the laboratory practice are: hydrochloric acid in the laboratory practice are: hydrochloric acid ((HClHCl), nitric acid (HNO), nitric acid (HNO33), acetic acid (CH), acetic acid (CH33COOH), sulfuric acid (HCOOH), sulfuric acid (H22SOSO44), ), and phosphoric acid (Hand phosphoric acid (H33POPO44). The first three acids are ). The first three acids are monoproticmonoprotic..Sulfuric acid is a Sulfuric acid is a diproticdiprotic acid. acid. CHCH33COOH is a weak acid.COOH is a weak acid.

√√ The most commonly used The most commonly used strong basesstrong bases in the laboratory are: sodium in the laboratory are: sodium hydroxide (hydroxide (NaOHNaOH), potassium hydroxide (KOH) and barium hydroxide ), potassium hydroxide (KOH) and barium hydroxide (Ba(OH)(Ba(OH)22). ).

√√ NaOHNaOH and KOH are and KOH are monobasic basesmonobasic bases, Ba(OH), Ba(OH)22 is is dibasicdibasic. The most . The most commonly used commonly used weak baseweak base is aqueous ammonia solution (NHis aqueous ammonia solution (NH44OH). OH).

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Neutralization: Bases are AntacidsNeutralization: Bases are AntacidsIf equal volumes of solutions of equal concentration are mixed, If equal volumes of solutions of equal concentration are mixed, the the

products are sodium chloride and water. This is an example of products are sodium chloride and water. This is an example of neutralizationneutralization, a chemical reaction in which the hydrogen ions from an , a chemical reaction in which the hydrogen ions from an acid combine with the hydroxide ions from a base to form water acid combine with the hydroxide ions from a base to form water molecules. molecules.

The formation of water can be represented like this.The formation of water can be represented like this.

Not only will this happen in test tubes in the laboratory, Not only will this happen in test tubes in the laboratory, but also in your home and in almost every ecological niche but also in your home and in almost every ecological niche of our planet. For example, if you put lemon juice on of our planet. For example, if you put lemon juice on fifishsh, , you run an acidyou run an acid––base reaction. The acids found in lemons base reaction. The acids found in lemons neutralize the ammonianeutralize the ammonia--like compounds that produce the like compounds that produce the ““fifishy smell.shy smell.””

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Acidity of solutionsAcidity of solutionsQuantitative representation of Quantitative representation of acidity of solutionsacidity of solutions, i.e. the content , i.e. the content

of of hydrogen ions hydrogen ions (H(H++)) in solutionin solution, is the , is the рНрН valuevalue that equals to the that equals to the negative decimal logarithm of negative decimal logarithm of activity of hydrogen ionsactivity of hydrogen ions ::

In a dilute solution In a dilute solution activities are close to concentrationsactivities are close to concentrations ((γγ±± ≈≈ 1), 1), so it is possible to set so it is possible to set the the рНрН value of solution equals to analytical value of solution equals to analytical concentration of hydrogen ion:concentration of hydrogen ion:

In liquid water solution In liquid water solution autoionizationautoionization process takes place:process takes place:

It can be characterized by It can be characterized by equilibrium constant, equilibrium constant, KK::

+ +H HpH log log( )a c ±= − = − ⋅ γ

+HpH logc≈ −

+2H O H + OH , K−

2

H OH

H O

a aK

a+ −⋅

=

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The degree of water dissociation is very small, so the The degree of water dissociation is very small, so the aa((HH22O) O) value is constant and equation can be represented as follows:value is constant and equation can be represented as follows:

Constant Constant KKww is known as is known as ionic product of waterionic product of water. At 25. At 25 °°C the C the KKwwvalue is equal to 1.008value is equal to 1.008⋅⋅1010––1414. Usually this constant (. Usually this constant (KKww) is ) is represented as the negative decimal logarithm: represented as the negative decimal logarithm:

at 25at 25 °°C.C.At 25At 25°°C if the contents of hydrogen and hydroxyl ions in a solution C if the contents of hydrogen and hydroxyl ions in a solution

are equal, , then are equal, , then рНрН == pOHpOH == 7, such media is called 7, such media is called neutralneutral. . In In acidic solutionsacidic solutions , in , in alkaline mediaalkaline media ..

Acidity of solutionsAcidity of solutions

2H OH OH wa a K a K−⋅ = ⋅ =+

p logw wK K= − p 14wK =

H OHa a −=+

H OHa a −>+ H OHa a −<+

pOHpOH = = ––log[OHlog[OH––] and ] and pOHpOH + pH = + pH = ppKKww = 14= 14

The pH and The pH and pOHpOH values of diluted solutions are in range 0values of diluted solutions are in range 0––14. 14.

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Figure. The relationship between pH and the concentration of H+. As pH increases, [H+] decreases

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Figure. Common substances and their pH values

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CHEMISTRY in ActionCHEMISTRY in ActionAntacids and the pH Balance in Antacids and the pH Balance in

StomachStomachAn average adult produces between

2 and 3 liter of gastric juice daily. Gastric juice is a thin, acidic digestive fluid secreted by glands in the mucous membrane that lines the stomach. It contains, among other substances, hydrochloric acid (HCl). The pH of gastric juice is about 1.5, which corresponds to a hydrochloric acid concentration of 0.03 M – a concentration strong enough to dissolve zinc metal!!!

Figure. A simplified diagram of the human stomach

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The inside lining is made up of parietal cells, which are fused The inside lining is made up of parietal cells, which are fused together to form tight junctions. together to form tight junctions.

The interiors of the cells are protected from the surroundings bThe interiors of the cells are protected from the surroundings by y cell membranes. These membranes allow water and neutral cell membranes. These membranes allow water and neutral molecules to pass in and out of the stomach, but they usually molecules to pass in and out of the stomach, but they usually block the movement of ions such as Hblock the movement of ions such as H++, Na, Na++, K, K++, and , and ClCl––. .

The HThe H++ ions come from carbonic acid (Hions come from carbonic acid (H22COCO33) formed as a ) formed as a result of the hydration of COresult of the hydration of CO22, an end product of metabolism:, an end product of metabolism:

Antacids and the pH Balance in Antacids and the pH Balance in StomachStomach

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The purpose of the highly acidic medium within the stomach is toThe purpose of the highly acidic medium within the stomach is to digest digest food and to activate certain digestive enzymes. Eating stimulatefood and to activate certain digestive enzymes. Eating stimulates Hs H++ ion ion secretion.secretion. A small fraction of these ions normally are reabsorbed by the A small fraction of these ions normally are reabsorbed by the mucosa. mucosa.

About half a million cells are shed by the lining every minute, About half a million cells are shed by the lining every minute, and and a healthy a healthy stomach is completely relined every three days or so.stomach is completely relined every three days or so.

However, if However, if the acid content is excessively highthe acid content is excessively high, the constant influx of H, the constant influx of H++

ions through the membrane back to the blood plasma ions through the membrane back to the blood plasma can cause muscle can cause muscle contraction, pain, swelling, inflammation, and bleedingcontraction, pain, swelling, inflammation, and bleeding. .

One way to temporarily One way to temporarily reduce the Hreduce the H++ ion concentrationion concentration in the stomach is in the stomach is to take to take an an antacidantacid..


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The major function of antacids is to neutralize excess The major function of antacids is to neutralize excess HClHCl in gastric juice.in gastric juice. The The reactions by which antacids neutralize stomach acid are as follreactions by which antacids neutralize stomach acid are as follows:ows:

The COThe CO22 released by most of these reactions increases gas pressure in treleased by most of these reactions increases gas pressure in the he stomachstomach, causing , causing the person to belchthe person to belch. .

The fizzing that takes place when an AlkaThe fizzing that takes place when an Alka--Seltzer tablet dissolves in water is Seltzer tablet dissolves in water is caused by carbon dioxide, which is released by the reaction betwcaused by carbon dioxide, which is released by the reaction between citric acid and een citric acid and sodium bicarbonate:sodium bicarbonate:


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The mucosa of the stomach is also damaged by the action of The mucosa of the stomach is also damaged by the action of aspirinaspirin, the chemical name of which is acetylsalicylic acid. Aspirin is, the chemical name of which is acetylsalicylic acid. Aspirin is itself a itself a moderately weak acid:moderately weak acid:

The Action of AspirinThe Action of Aspirin

In the presence of the high concentration of HIn the presence of the high concentration of H++ ions in the stomach, ions in the stomach, this acid remains largely this acid remains largely nonionizednonionized. .

A relatively A relatively nonpolarnonpolar molecule, acetylsalicylic acid has the ability to molecule, acetylsalicylic acid has the ability to penetrate membrane barriers that are also made up of penetrate membrane barriers that are also made up of nonpolarnonpolarmolecules. molecules.

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However, inside the membrane are many small water pockets, and However, inside the membrane are many small water pockets, and when an acetylsalicylic acid molecule enters such a pocket, when an acetylsalicylic acid molecule enters such a pocket, it ionizes into it ionizes into HH++ and acetylsalicylate ions.and acetylsalicylate ions. These ionic species become trapped in the These ionic species become trapped in the interior regions of the membrane.interior regions of the membrane. The continued buildup of ions in this The continued buildup of ions in this fashion weakens the structure of the membrane and eventually caufashion weakens the structure of the membrane and eventually causes ses bleeding. bleeding. Approximately 2 Approximately 2 mLmL of blood are usually lost for every aspirin of blood are usually lost for every aspirin tablet taken, an amount not generally considered harmful. tablet taken, an amount not generally considered harmful.

However, the action of aspirin can result in severe bleeding in However, the action of aspirin can result in severe bleeding in some some individuals. individuals.

It is interesting to note that the presence of alcohol makes It is interesting to note that the presence of alcohol makes acetylsalicylic acid even more soluble in the membrane, and so facetylsalicylic acid even more soluble in the membrane, and so further urther promotes the bleeding.promotes the bleeding.

The Action of AspirinThe Action of Aspirin

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AcidAcid--base indicatorsbase indicatorsAn An acidacid--base indicator base indicator (we will write as (we will write as HInHIn)) is a substance which is a substance which

varies colorvaries color of the solution of the solution according to the hydrogen ion concentrationaccording to the hydrogen ion concentrationof its environment. of its environment.

Indicator, Indicator, HInHIn, and its conjugate base, In, and its conjugate base, In––, must have distinctly , must have distinctly different colors. In solution, the acid ionizes to a small extendifferent colors. In solution, the acid ionizes to a small extent:t:

Color of the solution of the indicator is determined by the ratiColor of the solution of the indicator is determined by the ratio of the o of the concentrations of the colored forms: concentrations of the colored forms:

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AcidAcid--base indicatorsbase indicatorsIt is thus possible to determine the pH value of a solution by oIt is thus possible to determine the pH value of a solution by observing bserving

the color of a suitable indicator when it is placed in that soluthe color of a suitable indicator when it is placed in that solution. tion. In the simplest case of existing of two forms of indicator it isIn the simplest case of existing of two forms of indicator it is possible to possible to

approximate the actual state of equilibrium between the forms (approximate the actual state of equilibrium between the forms (HIn ↔ H+ + In–) using dissociation constant, using dissociation constant, KK: :

[H ][In ] ,[HIn]

K+ −

=[In ]pH p log[HIn]

K−

= +

If the pK value of indicator is known, and the ratio of In– and HIn of the indicator in the given solution is measured, the pH of the solution can be evaluated.

Indicators can be utilized for determination of the pH of the solution in the range of the pH values in: pH p 1K= ±

This interval of the pH values is called the transition or useful range of the indicator.

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Figure. Solutions containing extracts of red cabbage (obtained by boiling the cabbage in water) produce different colors when treated with an acid and a base. The pH of the solutions increases from left to right

AcidAcid--base indicatorsbase indicatorsMany acid-base indicators are plant pigments. For example, by boiling

chopped red cabbage in water we can extract pigments that exhibit many different colors at various pHs (Figure).

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Table. Colors and useful ranges for some pH indicatorsTable. Colors and useful ranges for some pH indicators

AcidAcid--base indicatorsbase indicators

Determination of the pH using indicators

Due to the ability of indicators to change a color with pH of solution they are employed for determination of the acidity of solutions.

Universal indicator is a mixture of several indicators displaying a variety of colors over a wide pH range. Usually it is used as a test paperthat changes color in accordance with the solution pH value.

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Test papers of universal indicatorTest papers of universal indicator are used only for an are used only for an approximate pH of the solutionapproximate pH of the solution. For precise determination of . For precise determination of the pH value, the pH value, colorimetriccolorimetric or or potentiometricpotentiometric methodsmethods are are used.used.

AcidAcid--base indicatorsbase indicators

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Hydrolysis of saltsHydrolysis of saltsThe term The term salt hydrolysissalt hydrolysis describes describes the reaction of an anion or a the reaction of an anion or a

cationcation of a salt, or both, with water.of a salt, or both, with water. Salt hydrolysis usually affects the pH Salt hydrolysis usually affects the pH of a solution.of a solution.

(1) (1) The salts composed of an alkali or alkaline earth metal ion and The salts composed of an alkali or alkaline earth metal ion and the the residue of a strong acidresidue of a strong acid do not undergodo not undergo hydrolysis and their solutions are hydrolysis and their solutions are assumed assumed to be neutralto be neutral. .

(2) Another situation is observed when dissolved (2) Another situation is observed when dissolved salt is formed by a salt is formed by a weak acid or (and) a weak baseweak acid or (and) a weak base..

For example, the dissociation of sodium acetate, that is strong For example, the dissociation of sodium acetate, that is strong electrolyte, in water proceeds according to equation: electrolyte, in water proceeds according to equation: CH3COONa →Na+(aq) + CH3COO– (aq).

The sodium ion does not react with water. The acetate ion CH3COO–

has an affinity for H+ ions. The hydrolysis reaction of this anion is given by equation: CH3COO– (aq) + H2O ↔ CH3COOH (aq) + OH– (aq). Due to Due to formation of OHformation of OH–– ions in this reaction the solution of sodium acetate will ions in this reaction the solution of sodium acetate will be be basicbasic. .

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Hydrolysis of saltsHydrolysis of saltsThe equilibrium constant for the hydrolysis reaction of CHThe equilibrium constant for the hydrolysis reaction of CH33COONa is COONa is

determined by dissociation constant of acetic acid and ionic prodetermined by dissociation constant of acetic acid and ionic product of duct of water:water:

3

3

[CH COOH] [HO ][CH COO ]

wh

acid

KKK

−

−⋅

= =

(3) When a salt derived from a strong acid and a weak base dissolves in water, the solution becomes acidic.

For example, in solution of NH4Cl the dissociation process gives NH4+

and Cl– ions: NH4Cl → NH4+ (aq) + Cl– (aq).

The ammonium ion NH4+ is the conjugate acid of the weak base

NH3 and reacts with water molecule: NH4+ (aq) + H2O ↔ NH3 (aq) +

H3O+ (aq).Because this reaction produces H3O+ ions, the pH of the solution

decreases. The equilibrium constant (hydrolysis constant) for this process is given by equation:

3 3

4

[NH ] [H O ][NH ]

wh

base

KKK

+

+⋅

= =

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Hydrolysis of saltsHydrolysis of salts(4) For (4) For salts formed by a weak acid and a weak basesalts formed by a weak acid and a weak base, both the , both the cationcation

and the anion hydrolyze. The hydrolysis constant in this case isand the anion hydrolyze. The hydrolysis constant in this case isdetermined by both dissociation constants of the acid and the bdetermined by both dissociation constants of the acid and the base:ase:

wh

acid base

KKK K

=⋅

If Kbase > Kacid then the solution must be basic because the anion will hydrolyze to a greater extent than the cation. At equilibrium, there will be more OH– ions than H+ ions.

If Kbase for the anion is smaller than Kacid for the cation, the solution will be acidic because cation hydrolysis will be more extensive than anion hydrolysis.

If Kbase is approximately equal to Kacid, the solution will be nearly neutral.

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Buffer solutionsBuffer solutionsBuffer solutionsBuffer solutions are solutions are solutions with ability to keep constant the with ability to keep constant the рНрН

valuevalue at (1) dilution or (2) addition of at (1) dilution or (2) addition of small amounts of a strong acid or a small amounts of a strong acid or a strong basestrong base. .

Usually buffer solution consists of a weak acid (weak base) and Usually buffer solution consists of a weak acid (weak base) and salt salt of this acid (base) which is strong electrolyte, e.g. CHof this acid (base) which is strong electrolyte, e.g. CH33COOH + COOH + CHCH33COONa COONa –– acetate buffer; NHacetate buffer; NH44OH + NHOH + NH44Cl Cl –– ammonia buffer, etc. ammonia buffer, etc.

In general form it is possible to say, that In general form it is possible to say, that the buffer solution consists the buffer solution consists from conjugated acid and base. from conjugated acid and base.

The pH value of a buffer solutionThe pH value of a buffer solution may be calculated using quantities may be calculated using quantities of the components forming it, for example, for acid buffer:of the components forming it, for example, for acid buffer:

oHAoMeA

pH p log cKc

= −

pK – negative decimal logarithm of the acid dissociation constant; co –initial concentrations of the acid and its salt in the solution. This equation is known as Henderson-Hasselbach equation.

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Ability of buffer solutions to keep the Ability of buffer solutions to keep the рНрН valuevalue (see Figure) at (see Figure) at addition of a strong acid or a base is called addition of a strong acid or a base is called buffer actionbuffer action. .

Buffer solutionsBuffer solutions

As a measure of buffer action As a measure of buffer action the buffer capacitythe buffer capacity is used. is used.

Buffer capacityBuffer capacity is an added amount of a strong acid or a is an added amount of a strong acid or a strong base, which addition to one liter of a buffer solution strong base, which addition to one liter of a buffer solution changes the changes the рНрН value to unity.value to unity.

Figure. A comparison of the change in pH when 0.1 mol HCl is added to pure water and to an acetate buffer solution

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The pH of the blood plasma is maintained at about 7.40 by several buffer systems but the most important is the bicarbonic buffer system. This pH value is dependent upon two coupled reactions.

First, the equilibrium of gaseous carbon dioxide dissolved in the blood and water producing the carbonic acid:

CO2(aq) + H2O ↔ H2CO3(aq).The amount of carbon dioxide in the blood is coupled to the The amount of carbon dioxide in the blood is coupled to the

amount present in the lungs. amount present in the lungs. Second, the equilibrium between carbonic acid and Second, the equilibrium between carbonic acid and

bicarbonate ion:bicarbonate ion:HH22COCO33 ((aqaq) ) ↔↔ HCOHCO33

−− ((aqaq) + H) + H++ ((aqaq), ), ppKK = 6.37.= 6.37.These reactions lead to the presence in solution the conjugThese reactions lead to the presence in solution the conjugate ate

pair HCOpair HCO33−−/H/H22COCO33 that forms the buffer system. The fact that the pH that forms the buffer system. The fact that the pH

of normal blood is 7.40 implies that [HCOof normal blood is 7.40 implies that [HCO33−−]/[H]/[H22COCO33] = 20. The ] = 20. The

excess of an acid in the blood is fixed by interaction with excess of an acid in the blood is fixed by interaction with hydrocarbonatehydrocarbonate ion, the excess of a base ion, the excess of a base –– by interaction with by interaction with carbonic acid.carbonic acid.

Buffer solutions in the organismBuffer solutions in the organism

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HydrophosphateHydrophosphate buffer systembuffer system is formed by is formed by hydrophosphatehydrophosphateandand dihydrophosphatedihydrophosphate ions. ions.

There is an There is an protolyticprotolytic equilibrium between these two ions:equilibrium between these two ions:

HH22POPO44−− ↔↔ HH++ + HPO+ HPO44

22−−, , ppKK == 7.21.7.21.

If If рНрН value of the blood is 7.40, the ratio of the concentrations value of the blood is 7.40, the ratio of the concentrations of the ions [HPOof the ions [HPO44

22−−]/[H]/[H22POPO44−−] is 1.55] is 1.55 :: 1.1.

Buffer solutions in the organismBuffer solutions in the organism

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In the erythrocytes the pH is 7.25, the principal buffer systemsIn the erythrocytes the pH is 7.25, the principal buffer systems are are bicarbonate HCObicarbonate HCO33

−−/H/H22COCO33 and and hemoglobin systemshemoglobin systems. . As a very rough approximation, we can treat it as a weak As a very rough approximation, we can treat it as a weak

monoproticmonoprotic acid of the form acid of the form HHbHHb that dissociates in solution:that dissociates in solution:

HHb(aqHHb(aq) ) ↔↔ HH++(aq(aq) + ) + HbHb––((aqaq),),

HHbHHb represents the hemoglobin molecule and represents the hemoglobin molecule and HbHb–– the conjugate the conjugate base of base of HHbHHb. . OxyhemoglobinOxyhemoglobin (HHbO(HHbO22), formed by the combination of ), formed by the combination of oxygen with hemoglobin, is a stronger acid than oxygen with hemoglobin, is a stronger acid than HHbHHb::

HHbOHHbO22(aq) (aq) ↔↔ HH++(aq(aq) + HbO) + HbO22–– (aq).(aq).

Buffer solutionsBuffer solutions

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ReferencesReferences1. Chang1. Chang R. Chemistry. 10R. Chemistry. 10--th edition. NY: McGrawth edition. NY: McGraw--Hill, 2010. Hill, 2010.

11701170 p.p.2. 2. ChangChang R. R. GeneralGeneral ChemistryChemistry: : TheThe EssentialEssential ConceptsConcepts. 6. 6--thth

eeditiondition. NY: McGraw. NY: McGraw--Hill, 2011. 853Hill, 2011. 853 p.p.3. Allen J.P. Biophysical Chemistry. Blackwell Publishing, 2008.3. Allen J.P. Biophysical Chemistry. Blackwell Publishing, 2008.

492492 p.p.4. Atkins P., de Paola J. Physical Chemistry for the Life 4. Atkins P., de Paola J. Physical Chemistry for the Life

Sciences. Sciences. W.H.FreemanW.H.Freeman Publishers, 2006. 624 p.Publishers, 2006. 624 p.5. 5. S.S. V.V. EltsovEltsov,, N.N. A.A. VodolazkayaVodolazkaya. Practical Medical Chemistry: . Practical Medical Chemistry:

manual / manual / KhKh.: V..: V. N.N. KarazinKarazin KharkivKharkiv National University, 2018. National University, 2018. 196 p.196 p.

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