THEME: Acid-base equilibrium in

biological systems. Buffer solutions.

Plen

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), [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

According to а definition coined in the 1880s by Svante Arrhenius, an acid is а substance that can donate а proton, and а base is а substance that can donate а hydroxide ion. This definition is rather limited. For example, it does not account for the observation that NН3, which lacks an ОН- group, exhibits basic properties. In а more general definition, which was formulated in 1923 by Johannes Britinsted and Thomas Lowry, an acid is а substance that can donate а proton (as is the Arrhenius definition), and а base is а substance that can accept а proton. Under the Bronsted-Loury definition, an acid - base reaction can be written as

НА + Н2О = Н3О+ + А-

An acid (НА) reacts with а base (Н2О) to form the conjugate base of the acid (А-) and the conjugate acid of the base (H3O

+). Accordingly, the acetate ion (СН3СОО- ) is the conjugate base of acetic acid (СН3СООН), and the ammonium ion (NH4

+ ) is the conjugate acid of ammonia (NН3). The acid-base reaction is frequntly abbreviated НА = Н+ + А- with the participation of H2O implied.

The strength of an acid is specified by its dissociation constantThe equilibrium constant for an acid - base reaction is expressed as а dissociation constant with the concentrations of the "reactants" in the denominator and the concentrations of the "products" is the numerator: [Н3О

+][А-]

K= ---------------- [НА] [Н2O]

In dilute solutions, the water concentration is essentially constant, 55.5 М (1000 g L -1/18.015 g mol-1 = 55.5 М). Therefore, the term [Н2О] is

customarily combined with the dissociation constant, which then takes the form [Н+][А-]Ka = K[Н2O] = -------------

[НА] Because acid dissociation constants, like [Н+] values, are sometimes cumbersome to work with, they are transformed to pK values by the formulaрK = - log K

The relationship between the pH of а solution and the concentrations The relationship between the pH of а solution and the concentrations of an acid and its conjugate base is easily derived. of 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--]] [А[А--]]рН = рН = ppК + К + log log ------------------ [[АА--]]This relationship known as the Henderson-Hasselbalch equation. This relationship known as the Henderson-Hasselbalch equation.

BUFFERSBUFFERS

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

Buffers are mainly of two types: Buffers are mainly of two 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:• НН22СОСО33 / N / NаНСОаНСО33 (Bicarbonate buffer; (Bicarbonate buffer;

carbonic acid and sodium bicarbonate)carbonic acid and sodium bicarbonate)• СНСН33СООНСООН / / СНСН33СООСОО Na (Acetate Na (Acetate

buffer; acetic acid and sodium acetate)buffer; acetic acid and sodium acetate)• NaNa22HPOHPO44/ NaH/ NaH22POPO4 4 (Phosphate buffer)(Phosphate buffer)

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.

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. Buffering capacity is the number of ratio. 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 litre of buffer solution.one litre 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.

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 interrelated. These mechanisms are interrelated.