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Kharkov National Medical University

Understand the key concepts of acid-base equilibrium in biosystems including solutions, dissociation, pH values of biological liquids, and buffer systems of the organism. Learn about different types of solutions, dissociation constants, hydrolysis reactions, and buffer capacity.

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Kharkov National Medical University

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

  2. 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

  3. 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 CH3COOH →CH3COO– + H+ the equilibrium constant has the form

  4. 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

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

  6. pH value is the measure of the active acidity of the medium. For strong acid solution:pH=-lgCa(where Ca- is the molar concentration of acid) For weak acid solution: 1.pH=-lgCa∙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.

  7. BASE For strong base solution: pOH=-lgCb (where Cb- is the molar concentration of base) For weak base solution: 1. pOH=-lgCb∙α 2. pH+pOH=14; pH=14-pOH

  8. NaCl + HOH NaOH + HCl Na+ + Cl- + HOH Na+ + OH- + H+ + Cl- HOH H+ + OH-;pH=7 Hydrolyses reactions Hydrolyses reactionis 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 pH 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.

  9. Basic solution of salts.When the saltforms from the cation of a strong base and the anion of a weak acid, the cation does not affect the pH 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;pH>7 where Cs – is molar concentration of the salt

  10. Acidic solutions of salts.When the saltforms from the cation of a weak base and the anion of a strong acid, the anion does not affect the pH but the cation does. In the cation hydrolyses reaction, a small equilibrium concentration of H+ makes the solution acidic. NH4Cl + HOH NH4OH + HCl NH4+ + Cl- + HOH NH4OH + H+ + Cl- NH4++ HOH NH4OH + H+;pH<7

  11. pH values of biological liquids

  12. BUFFER SOLUTION Acidic Basic ABuffer Solution (System) is one that resists a change in its pH even when a strong acid or a base is added to it. Acidic Buffer:weak acid + its salt of a strong base HCN and NaCN Basic Buffer:weak base + its salt of a strong acid NH4OH and NH4Cl

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

  14. Buffer Capacity BUFFER CAPACITY is a number of moles of a strong acid or a strong base that is needed to be added to 1L of buffer solution in order to change its pH 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; ΔpH - the changing of pH

  15. BUFFER SYSTEMS OF THE ORGANISM • Carbonic acid-Bicarbonate Buffer: H2CO3 + NaHCO3 • Phosphate Buffer: NaH2PO4 + Na2HPO4 • Amino Acid and Protein Buffer 4. Hemoglobinous Buffer: HHb + KHb 5. Oxyhemoglobinous Buffer: HHbO2 + KHbO2

  16. Isoelectric Point pH at which aminoacid 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(pI). Cationic form Zwitterion Anionic form pH<pI pH=pI pH>pI

  17. Thank you for your attention!

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