Kharkov National Medical University Department of Medical and

Kharkov National Medical University Department of Medical and Bioorganic Chemistry Medical Chemistry Lecture 3 for students of medical faculty ACID-BASE EQUILIBRIUM IN BIOSYSTEMS

A solution consists of one or more dissolved substances called solutes, and the medium in which the solutes are uniformly distributed in the form of molecules or ions, called the solvent. Aqueous solutions (with water as the solvent) are most important for us electrolytes - compounds whose aqueous solutions conduct electricity Solutions nonelectrolytes - compounds whose aqueous solutions do not conduct electricity

Arrhenius introduced the concept of the degree of dissociation. By the degree of dissociation of an electrolyte is meant the ratio of the number of its molecules that have broken up into ions in the given solution to the total number of its molecules in the solution For the dissociation of acetic acid CH 3 COOH → CH 3 COO– + H+ the equilibrium constant has the form

The meaning of Kw: In any aqueous solution at 25 C, no matter what it contains, the product of [H+] and [OH-] must always equal 10 -14

A neutral solution, where [H+] = [OH-] An acidic solution, where [H+] [OH-] A basic solution, where [OH-] [H+][OH-] = Kw=10 -14

p. H value is the measure of the active acidity of the medium. For strong acid solution: of acid) p. H=-lg. Ca (where Ca- is the molar concentration For weak acid solution: 1. p. H=-lg. Ca∙α α-is the degree of dissociation. Degree of dissociation is the ratio of the number of molecules of electrolyte that have been broken up into ions in solution to the total number of its molecules in the solution. 2. K-is the dissociation constant. Dissociation constant is the ratio of concentrations of the ions that are the dissociation products to the concentration of the undissociated molecules.

BASE For strong base solution: concentration of base) For weak base solution: 1. p. OH=-lg. Cb (where Cb- is the molar p. OH=-lg. Cb∙α 2. p. H+p. OH=14; p. H=14 -p. OH

Hydrolyses reactions Hydrolyses reaction is the reaction of ions with water. When we dissolve a salt in water, its solution may be neutral, basic, or acidic. Neutral solutions of salts. When neither the cation nor the anion undergoes hydrolyses reactions, the p. H of the water is not affected and the solution remains neutral. These are salts that are formed from the neutralization of a strong acid and a strong base. Na. Cl + HOH Na. OH + HCl Na+ + Cl- + HOH H+ + OH-; Na+ + OH- + H+ + Cl- p. H=7

Basic solution of salts. When the salt forms from the cation of a strong base and the anion of a weak acid, the cation does not affect the p. H but the anion does. In the anion hydrolyses reaction, a small equilibrium concentration of OH- makes the solution basic. KCN + HOH KOH + HCN K+ + CN- + HOH K++ OH- + HCN CN- + HOH OH- + HCN; p. H>7 where Cs – is molar concentration of the salt

Acidic solutions of salts. When the salt forms from the cation of a weak base and the anion of a strong acid, the anion does not affect the p. H but the cation does. In the cation hydrolyses reaction, a small equilibrium concentration of H+ makes the solution acidic. NH 4 Cl + HOH NH 4 OH + HCl NH 4+ + Cl- + HOH NH 4++ HOH NH 4 OH + H+ + Cl- NH 4 OH + H+; p. H<7

p. H values of biological liquids MEDIUM Blood p. H 7. 35 -7. 45 Cerebrospinal fluid Saliva Gastric juice Pancreatic gland secret Urine Lacrimal fluid Milk Bile fluid 7. 35 -7. 45 6. 35 -6. 85 1. 2 -3. 0 7. 5 -8. 0 4. 8 -7. 5 7. 4 6. 6 -6. 9 5. 4 -6. 9

DISTURBANCE IN ACID-BASE BALANCE p. H<7. 35 Decrease of blood p. H>7. 45 Increase of blood p. H ACIDOSIS Variation of normal ALKALOSIS treatment by 4% blood p. H value 7. 35 -7. 45 treatment by 5% by 0. 3 units causes a death ascorbic acid Na. HCO 3 RESPIRATORY ACIDOSIS METABOLIC ACIDOSIS RESPIRATORY ALKALOSIS METABOLIC ALKALOSIS

A Buffer Solution (System) is one that resists a change in its p. H even when a strong acid or a base is added to it. BUFFER SOLUTION Acidic Basic Acidic Buffer: weak acid + its salt of a strong base HCN and Na. CN Basic Buffer: weak base + its salt of a strong acid NH 4 OH and NH 4 Cl

Henderson-Hasselbalch Equation For acidic buffer solution: For basic buffer solution:

Buffer Capacity BUFFER CAPACITY is a number of moles of a strong acid or a strong base that is needed to be added to 1 L of buffer solution in order to change its p. H for unity. where, β- is the buffer capacity; Ca, Cb - the molarities of the added acid or a base correspondingly; Va, Vb - the volumes of the added acid or a base correspondingly; Δp. H - the changing of p. H

BUFFER SYSTEMS OF THE ORGANISM 1. Carbonic acid-Bicarbonate Buffer: H 2 CO 3 + Na. HCO 3 2. Phosphate Buffer: Na. H 2 PO 4 + Na 2 HPO 4 3. Amino Acid and Protein Buffer 4. Hemoglobinous Buffer: HHb + KHb 5. Oxyhemoglobinous Buffer: HHb. O 2 + KHb. O 2

Isoelectric Point p. H at which amino acid or a protein exists as zwitterion i. e. number of positive charges equals number of negative charges in the molecule so that the molecule is electrically neutral is called isoelectric point of aminoacid or protein (p. I). Cationic form p. H<p. I Zwitterion p. H=p. I Anionic form p. H>p. I