pH of carbonate buffer solutions at different concentrations

This post was last edited by qugd on 2016-9-26 at 13:49. The carbonate buffer system is a typical buffer system for the alkaline range of binary weak acids; its parameters can be calculated directly using the second dissociation constant of carbonic acid and its concentration. When the concentration is not very high, the calculated values are quite close to the actual values. Find a book on analytical chemistry and do the calculations by yourself. Below are the basic aspects of simple ionization equilibrium and concentration calculations. In aqueous solutions, there exists a complex equilibrium among carbonic and bicarbonate compounds, which is generally represented by the following reaction equations: CO2 + H2O → H2CO3; H2CO3 → H+ + HCO3-; HCO3- → H+ + CO32-. These three equations can be combined into one equation: CO2 + H2O → (H2CO3) → H+ + HCO3- → 2H+ + CO32-. Since one molecule of CO2 can produce only one molecule of H2CO3, one molecule of H2CO3 can ionize to produce only one molecule of HCO3-, and one molecule of HCO3- can ionize to produce only one molecule of CO32-, the sum of the concentrations of CO2, HCO3-, and CO32- in a system remains constant at a value equal to C. In other words, [H2CO3]/[CO2]·[HCO3-]/[H2CO3]·[CO32-]/[HCO3-] = 1 (A). Additionally, [H2CO3]/[CO2] = K1 (B) and [HCO3-]/[H2CO3] = K2 (C). By using these three fractions from equation (A) as dependent variables and [H+] as the independent variable, and by solving these three equations, it is possible to determine the concentrations of CO2, HCO3-, and CO32- at different [H+] levels (or pH values) in a system. The values of K1 (4.45×10⁻⁷) and K2 (4.96×10⁻¹¹) at 25°C are known; by plotting these three fractions on the vertical axis and the pH value on the horizontal axis, the relationship between carbonic compounds in aqueous solutions and pH at 25°C is obtained, as shown in Figure 1. http://www.mining120.com/WebEditor/uploadfile/20100902113400825.jpg Figure 1: Relationship between the ionization of carbonic acid and pH value (25°C). As can be seen from Figure 1, when pH < 4.4, only CO2 is present in the solution; when pH > 8.35, no CO2 is present in the aqueous solution ； The bicarbonate ion HCO3‑ can exist only when the pH is between 4.4 and 12.0; at a pH of 8.35, the concentration of HCO3‑ is at its highest, accounting for about 98% ； CO32‑ is present in aqueous solutions only when pH > 8.35