Buffer solutions.

ass. prof. I. R. Bekusass. prof. I. R. Bekus

Plan

1.Ionization of water.2.Acid-base theory.3.Buffer solutions.4.Buffer in blood.

Water is а neutral molecule with а slight tendency to ionize. We usually express this ionization as:Н2О = Н+ + ОН-

There is actually no such thing as а free proton (Н+) in solution. Rather, the proton is associated with а water molecule as а hydronium ion, H3O

+. The association of

а proton with а cluster of water molecules also gives rise to structures with the formulas Н5О2

+, Н7О3+, and

so on. For simplicity, however, we collectively represent these ions by H+.

Because the product of [Н+] and [ОН-] is а constant (10-14 М2), [Н+] and [ОН-] are reciprocally related. Solutions with relatively more Н+ are acidic (рН < 7), solutions with relatively more ОН- are basic (рН >7), and solutions in which [Н+] = [ОН-] = 10 -7 М are neutral (рН = 7). Note the logarithmic scale for ion concentration. K is the dissociation constant (ionization constant)

Кw = [Н+][ОН-] =10 -14 M2 at 25 0C.[Н+] = [ОН-] = (Кw)1/2 = 10-7 М [Н+] = 10-7 М are said to be

neutral[Н+] > 10-7 М are said to be

acidic, [Н+] < 10-7 М are said to be basic. Most physiological solutions have

hydrogen ion concentrations near neutrality.

рН = - log[H+] The pH of pure water is 7.0, Acidic solutions have рН < 7.0 Basic solutions have рН > 7.0.1 М NaOH -14Household ammonia -12Seawater – 8Milk - 7Blood  - 7.4Saliva - 6.6Tomato juice - 4.4Vinegar - 3Gastric juice - 1.51 М НСl - 0

The relationship between the pH of а solution and the concentrations of The relationship between the pH of а solution and the concentrations of an acid and its conjugate base is easily derived. an acid and its conjugate base is easily derived.

[НА] [НА] [Н[Н++]= ]= KK ---------- ---------- [[АА--]]Taking the negative log of each term Taking the negative log of each term [А[А--]]рН = - рН = - log log К + К + log log ------------------ [А[А--]]

This relationship known as the Henderson-Hasselbalch equation, This relationship known as the Henderson-Hasselbalch equation, that is often used to perform the that is often used to perform the calculations required in preparation of buffers for use in the laboratory, or other applications. calculations required in preparation of buffers for use in the laboratory, or other applications.

BUFFER SOLUTIONSBUFFER SOLUTIONS

Buffers are solutions which can Buffers are solutions which can resist changes in pH by resist changes in pH by addition of acid or alkali.addition of acid or alkali.

Buffers are mainly classified of two Buffers are mainly classified of two types: types: ((аа) mixtures of weak acids with their ) mixtures of weak acids with their salt with salt with аа strong base strong base (b) mixtures of weak bases with their (b) mixtures of weak bases with their salt with salt with аа strong acid. strong acid. АА few examples are given below: few examples are given below:НН22СОСО

33 / N / NаНСОаНСО33 (Bicarbonate (Bicarbonate

buffer;carbonic acid and sodium buffer;carbonic acid and sodium bicarbonate)bicarbonate)СНСН

33СООНСООН / / СНСН33СООСОО Na Na (Acetate (Acetate

buffer; acetic acid and sodium acetate)buffer; acetic acid and sodium acetate)NaNa

22HPOHPO44/ NaH/ NaH

22POPO44 (Phosphate buffer)(Phosphate buffer)

Acid–Base ConceptAcid–Base Concept1)1) The Arrhenius theoryThe Arrhenius theory

ACIDACID - a substance that provides H+ ions in water - a substance that provides H+ ions in water

BASEBASE - - a substance that provides OH- ions in watera substance that provides OH- ions in water

2) 2) The BrønstedThe Brønsted--Lowry TheoryLowry Theory

All Brønsted–Lowry bases have one or more lone pairs of electrons:

3) The Lewis Acids and Base theory3) The Lewis Acids and Base theoryLEWIS LEWIS ACIDACID An electron-pair acceptor An electron-pair acceptor

LEWIS LEWIS BASEBASE An electron-pair donor An electron-pair donor

AcidsAcids

Lemon juiceLemon juice contains contains citric acid,citric acid, and and vinegar vinegar containscontains ethanoic acid.ethanoic acid.

Some strong acids Some strong acids areare hydrochloric acid, sulphuric hydrochloric acid, sulphuric acid acid andand nitric acid. nitric acid.

Some weak acids Some weak acids areare ethanoic ethanoic acid, citric acid acid, citric acid andand carbonic acid. carbonic acid.

There are many acids present in our everyday lives.

AlkalisAlkalis

When the When the oxides of some oxides of some metalsmetals dissolve in dissolve in waterwater they they make an make an alkali solution.alkali solution.

Alkalis Alkalis react with react with acidsacids and and neutralise them. them.

Many everyday substances are alkalis. They feel soapy. They are corrosive.

Alkalis Alkalis

AlkalisAlkalis are present in many are present in many cleaning substancescleaning substances in use in in use in our homes. our homes.

Kitchen cleaners are alkalineKitchen cleaners are alkaline because they contain because they contain ammoniaammonia or or sodium hydroxidesodium hydroxide, which , which attack grease.attack grease.

Calcium hydroxide and sodium hydroxide are strong alkalis. The most recognisable and common weak alkali is ammonia.

Buffer CapacityBuffer Capacity On the other hand, the buffer capacity is On the other hand, the buffer capacity is

determined by the actual concentrations of determined by the actual concentrations of salt and acid present, as well as by their salt and acid present, as well as by their ratio. ratio. Buffering capacity is the number of Buffering capacity is the number of grams of strong acid or alkali which is grams of strong acid or alkali which is necessary for necessary for аа change in pH of one unit of change in pH of one unit of one liter of buffer solution.one liter of buffer solution.

The buffering capacity of The buffering capacity of аа buffer is, definеd buffer is, definеd ааs the ability of the buffer to resist changes s the ability of the buffer to resist changes in pH when an acid or base is added.in pH when an acid or base is added.

Definition of buffer capacity

A buffer absorbs strong acid and base through the two reactions shown on the left side of our diagram: A- + H3O+ => HA + H2O HA + OH- => A- + H2OThe buffer will stop working when either one of its components (HA or A-) is exhausted, and therefore cannot neutralize any more strong acid or strong base. The most effective buffering solutions are those which have similar concentrations of HA and A- because then the buffer has the capacity to absorb both acid and base with the same effectiveness. Although the pH of a buffer depends only on the ratio [HA]/[A-], the ability of the buffer to absorb acid or base depends on the overall value of [HA] and [A-]. For instance, above we found a pH change of -0.02 units (from 7.20 to 7.18) when we added 0.010 moles of HCl to 1L of a buffer in which [HA] = [A-] = 0.50 M.

Supposed that we had a buffer with [HA] = [A-] = 5.0 M. How much HCl would we need to add to get a pH change of -0.02 units? The answer is 10x as much as we found above, or 0.10 moles of HCl. This is summarized in this diagram:

A ten-fold increase in the concentration of our buffering agents increased the ability to absorb acid, i.e. the buffer capacity, ten fold. The buffer capacity is directly proportional to the concentration of our buffering agents.

Buffers ActBuffers Act When hydrochloric acid is added to the acetate buffer, the salt When hydrochloric acid is added to the acetate buffer, the salt

reacts with the acid forming the weak acid, acetic acid and its reacts with the acid forming the weak acid, acetic acid and its salt. Similarly when salt. Similarly when аа base is added, the acid reacts with it base is added, the acid reacts with it forming salt and water. Thus, changes in the pH are minimised.forming salt and water. Thus, changes in the pH are minimised.

СНСН33СООН + NaOH = СНСООН + NaOH = СН33COONa + НCOONa + Н22ОО СНСН33СООСООNNа + HCI = СНа + HCI = СН33СООН + NaCIСООН + NaCI The buffer capacity is determined by the absolute The buffer capacity is determined by the absolute

concentration of the salt and acid. But the concentration of the salt and acid. But the рНрН of the buffer is of the buffer is dependent on the relative proportion of the salt and acid (see dependent on the relative proportion of the salt and acid (see the Henderson - Hasselbalch's equation). When the ratio the Henderson - Hasselbalch's equation). When the ratio between salt and acid is 10:1, the pH will be one unit higher between salt and acid is 10:1, the pH will be one unit higher than the pKa. When the ratio between salt and acid is 1:10, the than the pKa. When the ratio between salt and acid is 1:10, the pH will be one unit lower than the pKa.pH will be one unit lower than the pKa.

Mechanisms for Regulation of pHMechanisms for Regulation of pH

1.1.      Buffers of body fluids, Buffers of body fluids, 2.2.      Respiratory system, Respiratory system, 3. Renal excretion. 3. Renal excretion.

These mechanisms are These mechanisms are interrelated. interrelated.

Acidic solutions have a high H+ Acidic solutions have a high H+ concentration. Base solutions have a concentration. Base solutions have a low H+ concentration. The pH scale is low H+ concentration. The pH scale is used to indicate the acidity or alkalinity used to indicate the acidity or alkalinity of a solution. Pure water with an equal of a solution. Pure water with an equal number of hydrogen and hydroxide ions number of hydrogen and hydroxide ions has a pH of 7.has a pH of 7.

Factors Affecting pH of Factors Affecting pH of аа BufferBuffer The pH of The pH of аа buffer solution is determined by buffer solution is determined by

two factors:two factors: 1. The value of pK: The lower the value of 1. The value of pK: The lower the value of

pK, the lower is the pH of the solution.pK, the lower is the pH of the solution. 2. The ratio of salt to acid concentrations: 2. The ratio of salt to acid concentrations:

Actual concentrations of salt and acid in Actual concentrations of salt and acid in аа buffer solution may be varied widely, with buffer solution may be varied widely, with попо change in change in рНрН, so long as the ratio of , so long as the ratio of the concentrations remains the same.the concentrations remains the same.

Thank you for attentionThank you for attention