Buffer solutions Lector Varikova T.O

A solutions that maintains a nearly constant pH when a small amount of an acid or a base is added to it is called a BUFFER SOLUTION.

Buffer solutions are very important in many areas of chemistry. For example, the pH of blood maintains a constant pH of 7,35 ÷ 7,45. Small changes in blood pH lead to serious illness or death.

Classification buffer systems Acidic buffer systems is made by mixing a weak acid with its conjugate base (For example, acetate buffer system CH3COOH/CH3COO-, hydrocarbonate buffer system H2CO3/HCO3- ); Вasic buffer systems is made by mixing a weak base with its conjugate acid (For example, ammonia buffer solution NH3∙H2O/NH4+); Salt buffer systems is made by mixing a normal and acid salts or two acid salts (For example, carbonate buffer solution NaHCO3/Na2CO3, phostates buffer solution NaH2PO4/Na2HPO4); Аminoacid and protein buffer systems is made by mixing a) weak «acid-protein» + salt its weak acid; b) weak «base -protein» + salt its weak base.

Thus the pH of the buffer solution does not change. Buffer action When an acid is added to a buffer it reacts with the base component of the buffer; When a base is added to the buffer it reacts with the acid component of the buffer. Thus the pH of the buffer solution does not change.

 

ammonia buffer solution: NH3(g) + H2O(l) ↔ NH3∙H2O(aq) ↔ NH4+(aq) + OH-(aq) NH4Cl(aq) → NH4+(aq) + Cl-(aq) when strong acid added H+ + NH3∙H2O ↔ NH4+ + HOH strong acid OH- + NH4+ ↔ NH4OH strong base

Phostates buffer solution Oxyhemaglobin buffer solution NaH2PO4 → Na+ + H2PO4- ↔ H+ + HPO42- Na2HPO4 → 2Na+ + HPO42- H+ + HPO42- ↔ H2PO4- OH- + H2PO4- ↔ HPO42- + H2O Oxyhemaglobin buffer solution HHbO2 ↔ H+ + HbO2- KHbO2 → K+ + HbO2- H+ + HbO2- ↔ HHbO2 OH- + HHbO2 ↔ H2O + HbO2-

Buffer action protein buffer system

Calculating the pH of buffer solutions For acidic and salts buffers For basic buffers

Buffer capacity  

Buffer capacity depends on: Concentration of the components in the buffer solution. Ratio of the concentration of the components. Nature of the buffer solution.

Blood plasma, as biological buffer Blood is a complex liquid that is composed of a cells and a fluid called plasma, which contains various dissolved solutes. The dissolved solutes in blood contain buffer systems which regulate body pH at a remarkably constant value of 7.4. The principal buffer system in blood consists of carbonic acid (H2CO3), hydrogen carbonate ions (HCO3-), carbonate ions (CO32-), and carbon dioxide (CO2).

Carbon dioxide is produced by the oxidation of foоds body tissues such as skeletal muscles and the liver. For example, the sugar glucose, C6H12O6, is oxidized according to the equation C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) Carbon dioxide produced by metabolism diffuses out of various tissues into blood and is transported to the lungs, where it is exhaled.

H2CO3(aq) + H2O(l) ↔ H3O+(aq) + HCO3-(aq) K1 CO2(aq) + H2O(l) ↔ H2CO3(aq) K2 CO2 (g) ↔ CO2(aq) K3 CO2(g) + 2H2O(l) ↔ H3O+(aq) + HCO3-(aq) K1K2K3=K4

The reaction represented by equation (1) indicates that the [H3O+] and the pH of blood depend only on the concentration of hydrogen carbonate dissolved in blood and on the partial pressure of gaseous carbon dioxide CO2(g), in the air spaces of the lungs. If the rate of metabolism increases — as it does during strenuous exercise - more carbon dioxide is produced by tissues, where it dissolves.

It is transported by blood from the tissues to the lungs, and exhaled as carbon dioxide gas. As a result, the partial pressure of carbon dioxide gas in the air space of the lungs increases, and equilibrium (1) shifts to the right. This shift increases the concentrations of hydronium ions and hydrogen carbonate ions in blood. But hydronium ions react with hydrogen carbonate ions to give carbonic acid in equilibrium (1). As a result, the position of equilibrium (4) scarcely alters.

If, on the other hand, the metabolic rate falls and less carbon dioxide produced in tissues, less carbon dioxide is exhaled, and equilibrium (1) shifts to the left. Since the reservoir of carbon dioxide gas in the lungs is large and can be changed rapidly by altering the breathing rate, the pH of blood does not change under normal conditions. The pH of blood is controlled by the ratio of [HC03-] to the partial pressure of CO2 in the air spaces of the lungs.

If excess H+(aq) ions enter blood, they react with HCO3- ions to form more CO2 gas, and the pH does not change appreciably. If, on the other hand, the hydrogen ion concentration in blood decreases, more CO2 gas dissolves in the blood, restoring the H+(aq) ion concentration to its normal level.

When the pH of blood falls below 7 When the pH of blood falls below 7.4, the condition is called acidosis, and when the pH rises above 7.4, the condition is called alkalosis. The pH of blood in severe diabetes can drop as low as 6.8, leading to coma and death.