Kharkov National Medical University

1. Kharkov National Medical University Departmentof Medical and Bioorganic Chemistry Medical Chemistry Lecture 3 ACID – BASE EQUILIBRIUM IN BIOLOGICAL SYSTEMS

2. SOLUTIONS Solutions are of great importance in the life and practical activities of man. A solution is a homogeneous mixture in which the components are uniformly intermingled. Solutions can be liquids or solids. All of the biological liquids – blood, lymph, inter­cellular lymph, etc. – are solutions. All biochemical processes in organism occur in aqueous solutions.

3. 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

4. There are two classes of electrolytes: strong electrolytes and weak electrolytes. Strong electrolytes dissociate virtually completely in aqueous solutions. Weak electrolytes dissociate only partly in aqueous solutions, and dynamic equilibrium sets in between the undissociated molecules and the ions in the solution.

5. 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

6. Cells of living organisms contain60-80%of water. Water is the most common amphoteric substance. Amphoteric substance - a substance which can behave either as an acid or as a base. H2O ↔ H+ + OH-Water autoionization

7. The concentration of each ion at 25C has been found by experiment to be10-7 mol/L. At 25C [H+][OH-] = Kw (a constant) Kw =[10-7] [10-7] = 10-14 Kw - theion-product of waterat 25C or the dissociation constant

8. 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

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

10. p scale p scale is based on common logarithms (base 10 logs). The pH scale represents the negative exponent of 10 as a positive number. The symbolpmeans–lg H is molar concentration of H+ ([H+]) pH=-lg[H+] Therefore, a solution of pH = 1 has [H+] = 10–1 M and pure water has pH = 7 ([H+] = 10–7M) [H+]=10-pH

11. A way of expressing [OH–] is pOH: pOH = –lg[OH–] [H+][OH–] = 10–14 If we now take -lg of both sides of the equation, we have –lg[H+][OH–] = –lg10–14 ( lg(AB) = lgA + lgB) -lg[H+] - lg[OH-] = -lg10-14

12. Since –lg = p and lg10-14= -14, the equation is pH + pOH = 14 In acidic solution pH <7 In neutral solution pH=7 In basic solution pH>7

13. 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.

14. Task 1. What is the pH of a 0.03M solution of HCl? Solution. pH = -lg Ca = -lg 0.03 = -lg 3∙10-2 = = -(lg 3∙10-2) =-(lg 3+lg10-2) = -(lg 3 - 2) = = -lg 3+2 = 2 – lg 3 = 2 - 0.477 = 1.523 Task 2. A weak monoprotic acid is 10% ionized in solution. What is the pH of a 0.1M solution of this acid? Solution. α=0.1; pH = -lg C∙α = -lg 0.1 ∙ 0.1 = = -lg 0.01 = -lg 10-2 = 2

15. 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

16. Task 3. What is the pH of a 0.018M solution of the strong base Ca(OH)2? Solution. [OH-] = 0.018 ∙ 2 = 0.036 mol/L, pOH = -lg Cb = -lg 3.6 ∙10-2 = 2 – lg 3.6 = = 2 - 0.55 = 1.45; pH = 14 – pOH = 14 - 1.45 = 12.55 Task 4. What is the pH of a 0.03M solution of NH4OH, if  = 0.2? pOH = -lg Cb∙α = -lg 3∙10-2∙0.2 = - lg 6∙10-3 = = 3 - 0.778 = 2.222; pH = 14 – pOH = 14 - 2.222 = 11.778

17. 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.

18. 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

19. 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

20. pH values of biological liquids

21. 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

22. Mechanism of Buffer action Acetic buffer:CH3COOH + CH3COONa CH3COONa + HCl = CH3COOH + NaCl CH3COO- + H+→CH3COOH CH3COOH + NaOH = CH3COONa + HOH CH3COOH + OH- → CH3COO- + HOH Ammonia buffer:NH4OH + NH4Cl H+ + NH4OH → H2O + NH4+ OH- + NH4Cl → NH4OH + Cl-

23. Calculating the pH of Buffer Solutions Acetic buffer:CH3COOH + CH3COONa CH3COOH CH3COO- + H+ CH3COONa CH3COO- +Na+ [CH3COO-]=[salt]; [CH3COOH]=[acid] or

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

25. 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 1 unit. 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

26. Buffer capacity depends on • the initial concentrations of components • the ratio between concentrations of components • The more are the initial concentrations of components of buffer system, the more is its buffer capacity. ; now we add 5 mol of HCl: 1. ofHCl ;now we add 5 mol of HCl: 2. of HCl The changing of pH is less in the last case.

27. 2. The more are the initial concentrations of components of buffer system, the more is its buffer capacity. 1. ; now we add 10 mol of HCl: of HCl the correlation between the components 1.5 times changed 2. ; now we add 10 mol of HCl: of HCl the correlation between the components changed more than twice

28. 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

29. HPO42- + H+ H2PO4- HCO3- + H+ H2CO3 H2PO4- + OH- H2O + HPO42- H2CO3 H2O + CO2 (g) OH- + H2CO3 H2O + HCO3- Carbonic acid-Bicarbonate buffer H2CO3 + NaHCO3 Phosphate bufferNaH2PO4 + Na2HPO4

30. :NH2 – CH – COOH R +NH3 – CH – COO – R Zwitterion (internal salt or dipolar ion) Amino Acid Buffer NH2 – CH – COOH R +NH3 – CH – COO – R +NH3 – CH – COOH R + H+ +NH3 – CH – COO – R NH2 – CH – COO- R + OH- + H2O

31. 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

33. ACIDOSIS ALKALOSIS RESPIRATORY METABOLIC RESPIRATORY METABOLIC Due to accumulation of organic acids (keto acids, lactic acid) in the tissues and the blood and excessive loss of bicarbonate ions Due to the loss of a large amounts of acid equivalents and increase in bicarbonate ions concentration Due to reduced removal of CO2 because of hypoventilation Due to sharply intensified pulmonary ventilation, accompanied by a rapid removal of CO2 Bronchial asthma; Edema; Emphysema; Traumatic asphyxia; Severe pneumonia; Suppression of respiratory center by medicines or drugs – opiates, barbiturates, alcohol, etc. Diabetes; Starvation; Fever; Gastrointestinal diseases; Variety of shocks (cardiogenic shock, burn shock, wound shock, etc.) Renal insufficiency Psychic excitation, Hysteria, Narcosis, Encephalitis, Excitation of respiratory center by medicines-theophylline, etc., Inhalation of pure oxygen Uncontrollable vomiting, Intestinal obstruction

34. Thank you for your attention!