C02-bicarbonate system

There are two explanations for the response of the hgh-antibody system to extremely low current densities. First, it must be recalled that the hormone, antibody, and complex are multivalent. The extreme cases are the charge of -5 on the antibody near the upstream boundary and +17 on the protonated, two-hormone complex near the downstream boundary. In the C02/bicarbonate system, all species except are monovalent. 

As the subject of acid-base physiology is developed, it will become apparent that carbon dioxide and bicarbonate play a crucial role. Carbon dioxide is not itself an acid but in aqueous solution it reacts with water to yield hydrogen ions (Table 1.4 reaction 1) and such a chemical is an acid. It also yields bicarbonate and this is the corresponding (conjugate) base. The C02-bicarbonate system is thus a buffer pair in which we will refer, rather loosely, to CO2 as the buffer acid and bicarbonate as the buffer base. Typical values for the concentrations in arterial blood are [CO2] = 1.2 m M and [HCO3 ] = 24 m M. 

Not only is the concentration of carbon dioxide under physiological control, so also is the concentration of the other component of the buffer pair, the bicarbonate. This is controlled in the kidney, which may increase or decrease the excretion of bicarbonate as components of physiological control mechanisms. Since both components of the buffer pair are under physiological control, it is possible for the C02-bicarbonate system to act as a perfect buffer and completely to restore the pH of the internal environment to normal. Examples of such contributions will appear repeatedly as the subject of acid-base physiology unfolds. 

Be familiar with and be able to perform calculations on the bicarbonate-C02 buffer system. 

Bc2 produced by flash photolysis of seawater decays by parallel first- and second-order reactions. The environmentally important exponential decay is a pseudo first-order reaction of Br2 with the carbonate/ bicarbonate system in seawater. A chemical speciation model for the free ions and ion-pairs in seawater and in solutions at seawater ionic strength allowed us to measure the dependence of the pseudo first-order rate term, a, on individual C02-containing species. A predictive equation based on reaction of Br2 with free C03 and the hgC03°, NaC03 and CaC03 ion pairs accounts for the mean seawater a at pH 8.1 within experimental uncertainty. The reaction productfs) are unknown. 

In addition to the phase behavior and the mixing state of the water/C02 biphasic system, the pH value must be considered as an important process parameter. In general, the system has quite an acidic nature due to the formation and dissociation of carbonic acid (Scheme 1). The second dissociation, of bicarbonate to a proton and carbonate, lies far to the left and does not contribute strongly to the pH value. Typically, the pH of unbuffered water at elevated pressures of CO2 is in the range of 2.8-3.0 [19]. 

At first sight, one might discount the importance of carbon dioxide and bicarbonate as buffers of the extracellular fluid. Since the pK is 6.1 and the extracellular fluid is normally at a pH of about 7.4, the pK is more than one pH unit from body pH and, as we have already seen, this indicates that as a chemical buffer in a closed system, this buffer pair is inefficient. The importance of the C02-bicarbonate pair in vivo is that the system is not closed it is called open . The body is not isolated from the environment as is the case for a fluid in a flask. Both members of the buffer pair can be excreted from or retained within the body. CO2, which is the equivalent of buffer acid, can be blown off by the lungs by hyperventilation or retained within the body by hypoventila-... 

The effects of liquid and gas flow rates for this process are given by Equation 3-13 using the exponents for liquid-film-controlled systems. For an unpromoted 25 wt % K2CO3 solution at a liquid rate of 40 gpm/ft with a gas capacity factor (Fg) of 0.94 Ib - /ft s, industrial experience indicates that the overall K a values will be approximately one-sixth of the values for the standard C02/Na0H system given in Tables 3-3 through 3-6. These values apply for 20% conversion to bicarbonate in the solvent at a temperature of 235""F. The K a value decreases as the percent conversion to bicarbonate increases at the rate of K a oc 1.207 — 1.08 B, where B is the mol fraction of K2CO3 present as bicarbonate. 

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 resulting ferrous bicarbonate is not particularly stable in condensate, being affected by heat, pH, and the partial pressure of C02 in the steam-C02 condensate mix. Various secondary reactions subsequently take place, resulting in the formation of ferrous ox ferric oxides, hydroxides, and carbonates throughout the condensate system. 

NOTE Some recirculation of ammonia and amine takes place within the overall boiler plant system, although at higher pH much of the ammonia is lost at the deaerator vent. In practice, this recirculation coupled with low er-than-theoretical C02 liberation (as a result of the incomplete breakdown of sodium bicarbonate when present in the boiler) typically results in a reduced amine-demand for any particular boiler pressure. This reduced demand, compared with the apparent demand, results in real cost savings. 

Because C02 is a volatile acid, it can rapidly be changed by the respiratory system. If a respiratory acid-base disturbance is present for minutes to hours it is considered an acute disorder while if it is present for days or longer it is considered a chronic disorder. By definition, the metabolic machinery that regulates HC03 results in slow changes in serum bicarbonate and all metabolic disorders are chronic. This means that there are six simple acid-base disorders as outlined in Table 25-1.2... 

The absorption technique using hot potassium carbonate has also been developed to capture C02 (Probstein and Hicks, 1990). The chilled ammonia process is another solvent-based C02 capture technology where ammonia carbonate slurries are used to capture 90% of the C02 in the gas stream mixture gas forming ammonia bicarbonate in the process. A pilot-scale chilled ammonia unit for 5 MW equivalent flue gas capture is under construction by ALSTOM and EPRI. Although this process is developed for a combustion system, the results will provide valuable information for the future development of such a process for hydrogen production. According to ALSTOM, commercial products on chilled ammonia process will be available by 2010 (Alstom, 2007). 

There are two ways of dealing with the bicarbonate buffer system. The first uses the Henderson-Hasselbalch equation and an effective pKa of 6.1. If there is more base (HCO 3) than acid (C02), the pH will always be bigger than the pKa. This is usually the case physiologically (pH = 7.4 pKa = 6.1) so that on a molar basis there is always more than 10-fold more HCO 3 than C02. 

You might be wondering why the bicarbonate buffer can buffer effectively at pH 7.4 when its pKa is 6.1. The answer is that it doesn t buffer all that well. What makes it unique and the major buffer system of the blood is that C02, being a gas, can be exhaled by the lungs. Exhaling C02 is equivalent to exhaling protons. 

The most fundamental process dealing with the activation of C02 involves the hydration of C02 to produce bicarbonate and the reverse dehydration of bicarbonate to produce C02. These processes are of biological and environmental significance since they control the transport and equilibrium behavior of C02. The spontaneous hydration of C02 and dehydration of HCO3 are processes that are too slow and must therefore be catalyzed by metal complexes in order to expedite the overall conversion rate. In biological systems, a series of enzymes, the carbonic anhydrases, are the efficient catalysts and can accelerate the reactions by up to 7 orders of magnitude. The mechanism of this... 

The pH of a bicarbonate buffer system depends on the concentration of H2C03 and I I(X)3, the proton donor and acceptor components. The concentration of H2C03 in turn depends on the concentration of dissolved C02, which in turn depends on the concentration of C02 in the gas phase, called the partial pressure of C02. Thus the pH of a bicarbonate buffer exposed to a gas phase is ultimately determined by the concentration of HC03 in the aqueous phase and the partial pressure of C02 in the gas phase (Box 2 4). 

FIGURE 11-33 Chloride-bicarbonate exchanger of the erythrocyte membrane. This cotransport system allows the entry and exit of HCOf without changes in the transmembrane electrical potential. Its role is to increase the C02-carrying capacity of the blood. 

In recent years there has been a growing interest in the chemistry of carbon dioxide, stimulated by current anxieties about alternative petrochemical feedstocks. One aspect under active exploration involves carbon dioxide activation via coordination to a transition metal, and indeed transition metal ions do form C02 complexes.177 The number of simple and reasonably stable complexes is still relatively small and usually limited to low oxidation state metal ions. There are many systems where C02 is used as a reagent giving rise to systems which, while not true C02 complexes, may simplistically be viewed as the products of insertion into metal-ligand bonds, e.g. reaction (9), where if L = H, formates are produced if L = OH, carbonates or bicarbonates result L = NR2 yields dialkylcarbamates and if L = R, carboxylate products result. Much of this area has recently been reviewed and will not be considered further.149... 

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