Carbonate buffer system, equilibrium

The principal buffer system in blood serum is based on the equilibrium between carbonic acid, H2C03(aq), and the hydrogen carbonate ion, HCO3 . Carbonic acid is unstable, however. It is also in equilibrium with carbon dioxide. Therefore, a second equilibrium reaction is involved in the hydrogen carbonate buffer system in the blood the reaction between carbon dioxide and water to produce carbonic acid, and its reverse. The two equilibrium reactions are summarized below. 

Table 4A2.1. Parameters for calculating the effect of pressure change on carbonate buffer system reactions and values of equilibrium constants at P = 0 and 300 bar ...

Under the conditions of temperature and ionic strength prevailing in mammalian body fluids, the equilibrium for this reaction lies far to the left, such that about 500 CO2 molecules are present in solution for every molecule of H2CO3. Because dissolved CO2 and H2CO3 are in equilibrium, the proper expression for H2CO3 availability is [C02(d)] + [H2CO3], the so-called total carbonic acid pool, consisting primarily of C02(d). The overall equilibrium for the bicarbonate buffer system then is... 

The pH of the blood is maintained by a finely tuned buffering system, consisting primarily of hydrogen carbonate ion (HC03 ) and H30+ in equilibrium with water and C02. 

Buffers are extremely important in biological systems. The pH of arterial blood is about 7.4. The pH of the blood in your veins is just slightly less. If the pH of hlood drops to 7.0, or rises above 7.5, life-threatening problems develop. To maintain its pH within a narrow range, blood contains a number of buffer systems. The most important buffer system in the blood depends on an equilibrium between hydrogen carbonate ions and carbonate ions. Dissolved carbon dioxide reacts with water to form hydrogen carbonate ions. 

Two types of investigations of the alkaline hydrolysis of chlorpyrifos in sediment/water systems were made, all at pH s between 10.6 and 10.8. First, studies were conducted in which the pH was adjusted (using a carbonate buffer) immediately upon mixing the sediments (EPA-23 and EPA-26) with the chlorpyrifos solution. Second, a study using EPA-26 was made in which the alkaline buffer was not added until three days after mixing the sediment with the chlorpyrifos solution. Three days represents a time which is long with respect to the achievement of sediment-water equilibrium for this system, yet short compared to the neutral hydrolysis half life (-50 days). 

By comparing the actual composition of sea water (sediments + sea -f- air) with a model in which the pertinent components (minerals, volatiles) with which water has come into contact are allowed to reach true equilibrium, Sillen in 1959 epitomized the application of equilibrium models for portraying the prominent features of the chemical composition of this system. His analysis, for example, has indicated that contrary to the traditional view, the pH of the ocean is not buffered primarily by the carbonate system his results suggest that heterogeneous-equilibria of silicate minerals comprise the principal pH buffer systems in oceanic waters. This approach and its expansion have provided a more quantitative basis for Forchbammer s suggestion of 100 years ago that the quantity of the different elements in sea water is not proportional to the quantity of elements which river water pours into the sea but is inversely proportional to the facility with which the elements in sea water are made insoluble by general chemical actions in the sea. 

Effect of Holding One s Breath on Blood pH The pH of the extracellular fluid is buffered by the bicarbonate/carbonic acid system. Holding your breath can increase the concentration of C02(g) in the blood. What effect might this have on the pH of the extracellular fluid Explain by showing the relevant equilibrium equation(s) for this buffer system. 

Figure 15.5. Buffer intensity versus pH for some heterogeneous systems and for the homogeneous dissolved carbonate system. Buffer intensities 0ct (dissolved carbonate, Cr = 10 M) /Scacoj (carbonate solution in equilibrium with calcite), (carbonate solution in equilibrium with pco2 atm), /3an kaoi (solution in equilibrium with...

Appendix 4.2 Equations for calculating the equilibrium constants of the carbonate and borate buffer system... 

It is apparent from the pK values that neither equilibrium can serve as a buffer system at the physiological pH of 7.4. However, carbonic acid (the proton donor) is in equilibrium with dissolved CO2, which in turn is in equilibrium with gaseous CO2 ... 

A pH of 7.4 is maintained in blood partly by a carbonic acid-bicarbonate buffer system based on the following equilibrium ... 

The equilibrium pH of the solutions was estimated for stirred cell atmospheres as shown in Table A5.1. Both N2 and instrument air can be used to pressurise the stirred cell solutions. Due to its inert nature N2 is commonly used for this purpose. However, when a carbonate buffer is used in a system, the buffer depends on the partial pressure of CO 2 in the air above the solution. To provide this partial pressure instrument air can be used. When the system is operated at high pressures such as in NF, the partial pressure of CO 2 increases. The equilibrium pressure calculation was carried out for background solution without pH adjustment (0.5 mM CaCh, 1 mM NaHCOs, 20 mM NaCl) in the absence of organics. 

One very important buffer solution is human blood An equilibrium between carbonic acid (H2CO3) and its conjugate base bicarbonate (HCOsi helps blood to maintain a relatively constant pH of around 7.4. The carbonic acid buffer system is created by carbon dioxide (CO2) dissolved in blood carbon dioxide reacts with water (H2O) to form carbonic acid. Since the amount of carbon dioxide in the blood depends on the rate at which you breathe, your blood pH is influenced by your breathing rate. Your body can... 

The PCO2 of a sample is typically determined by measuring changes in the pH of a bicarbonate solution that is isolated from the sample by a COj-permeable membrane but remains in equilibrium with the COj. The bicarbonate and CO2, as carbonic acid, form a pH buffer system, and, by the Henderson-Hasselhalch equation, hydrogen ion concentration is proportional to the pCOj in the sample. This measurement is done with either a pH electrode or a dye indicator in solution. 

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