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: Н 2 О = 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, H 3 O +. The association of а proton with а cluster of water molecules also gives rise to structures with the formulas Н 5 О 2 +, Н 7 О 3 +, and so on. For simplicity, however, we collectively represent these ions by H +.

Because the product of [Н + ] and [ОН - ] is а constant ( ), [Н + ] and [ОН - ] are reciprocally related. Solutions with relatively more Н + are acidic (рН 7), and solutions in which [Н + ] = [ОН - ] = М are neutral (рН = 7). Note the logarithmic scale for ion concentration. K is the dissociation constant (ionization constant К w = [Н + ][ОН - ] = M 2 at 25 0 C. [Н + ] = [ОН - ] = (К w ) 1/2 = М [Н + ] = М are said to be neutral [Н + ] > М are said to be acidic, [Н + ] < 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 > М NaOH -14 Household ammonia -12 Seawater – 8 Milk - 7 Blood Saliva Tomato juice Vinegar - 3 Gastric juice 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 NН 3, 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 + Н 2 О = Н 3 О + + А - An acid (НА) reacts with а base (Н 2 О) to form the conjugate base of the acid (А - ) and the conjugate acid of the base (H 3 O + ). Accordingly, the acetate ion (СН 3 СОО - ) is the conjugate base of acetic acid (СН 3 СООН), and the ammonium ion (NH 4 + ) is the conjugate acid of ammonia (NН 3 ). The acid-base reaction is frequntly abbreviated НА = Н + + А - with the participation of H 2 O implied.

The strength of an acid is specified by its dissociation constant The 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: [Н 3 О + ][А - ] K= [НА] [Н 2 O] In dilute solutions, the water concentration is essentially constant, 55.5 М (1000 g L -1 / g mol -1 = 55.5 М). Therefore, the term [Н 2 О] is customarily combined with the dissociation constant, which then takes the form [Н + ][А - ] K a = K[Н 2 O] = [НА] Because acid dissociation constants, like [Н + ] values, are sometimes cumbersome to work with, they are transformed to pK values by the formula рK = - log K

The relationship between the pH of а solution and the concentrations of an acid and its conjugate base is easily derived. [НА] [НА] [Н + ]= K [А - ] [А - ] Taking the negative log of each term [А - ] [А - ] рН = - log К + log [А - ] [А - ] рН = pК + log [А - ] [А - ] This relationship known as the Henderson-Hasselbalch equation.

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

Buffers are mainly of two types: (а) mixtures of weak acids with their salt with а strong base (а) mixtures of weak acids with their salt with а strong base (b) mixtures of weak bases with their salt with а strong acid. (b) mixtures of weak bases with their salt with а strong acid. А few examples are given below: Н 2 СО 3 / NаНСО 3 (Bicarbonate buffer; carbonic acid and sodium bicarbonate) Н 2 СО 3 / NаНСО 3 (Bicarbonate buffer; carbonic acid and sodium bicarbonate) СН 3 СООН / СН 3 СОО Na (Acetate buffer; acetic acid and sodium acetate) СН 3 СООН / СН 3 СОО Na (Acetate buffer; acetic acid and sodium acetate) Na 2 HPO 4 / NaH 2 PO 4 (Phosphate buffer) Na 2 HPO 4 / NaH 2 PO 4 (Phosphate buffer)

Factors Affecting pH of а Buffer The pH of а buffer solution is determined by two factors: 1. The value of pK: The lower the value of pK, the lower is the pH of the solution. 1. The value of pK: The lower the value of pK, the lower is the pH of the solution. 2. The ratio of salt to acid concentrations: Actual concentrations of salt and acid in а buffer solution may be varied widely, with по change in рН, so long as the ratio of the concentrations remains the same. 2. The ratio of salt to acid concentrations: Actual concentrations of salt and acid in а buffer solution may be varied widely, with по change in рН, so long as the ratio of the concentrations remains the same.

Buffer Capacity On the other hand, the buffer capacity is determined by the actual concentrations of salt and acid present, as well as by their ratio. Buffering capacity is the number of grams of strong acid or alkali which is necessary for а change in pH of one unit of one litre of buffer solution. On the other hand, the buffer capacity is determined by the actual concentrations of salt and acid present, as well as by their ratio. Buffering capacity is the number of grams of strong acid or alkali which is necessary for а change in pH of one unit of one litre of buffer solution. The buffering capacity of а buffer is, definеd аs the ability of the buffer to resist changes in pH when an acid or base is added. The buffering capacity of а buffer is, definеd аs the ability of the buffer to resist changes in pH when an acid or base is added.

Buffers Act When hydrochloric acid is added to the acetate buffer, the salt reacts with the acid forming the weak acid, acetic acid and its salt. Similarly when а base is added, the acid reacts with it forming salt and water. Thus, changes in the pH are minimised. When hydrochloric acid is added to the acetate buffer, the salt reacts with the acid forming the weak acid, acetic acid and its salt. Similarly when а base is added, the acid reacts with it forming salt and water. Thus, changes in the pH are minimised. СН 3 СООН + NaOH = СН 3 COONa + Н 2 О СН 3 СООН + NaOH = СН 3 COONa + Н 2 О СН 3 СООNа + HCI = СН 3 СООН + NaCI СН 3 СООNа + HCI = СН 3 СООН + NaCI The buffer capacity is determined by the absolute concentration of the salt and acid. But the рН of the buffer is dependent on the relative proportion of the salt and acid (see the Henderson - Hasselbalch's equation). When the ratio 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 pH will be one unit lower than the pKa. The buffer capacity is determined by the absolute concentration of the salt and acid. But the рН of the buffer is dependent on the relative proportion of the salt and acid (see the Henderson - Hasselbalch's equation). When the ratio 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 pH will be one unit lower than the pKa.

Mechanisms for Regulation of pH (1) Buffers of body fluids, (1) Buffers of body fluids, (2) Respiratory system, (2) Respiratory system, (3) Renal excretion. (3) Renal excretion. These mechanisms are interrelated. These mechanisms are interrelated.