HCO3 carbonic acid dissociation

One of the most important buffer systems in the human body is that which keeps the pH of blood around 7.4. If the pH of blood fall below 6.8 or above 7.8, critical problems and even death can occur. There are three primary buffer systems at work in controlling the pH of blood carbonate, phosphate, and proteins. The primary buffer system in the blood involves carbonic acid, H COj and its conjugate base bicarbonate, HCO3. Carbonic acid is a weak acid that dissociates according to the following reaction ... 

The Solubility of Carbon Dioxide in Brines. Information required to describe the carbonate system in a brine includes the solubility of carbon dioxide, the extent of carbonic acid formation, and the distribution of the anionic carbonic acid dissociation products HCO3 and C0 . ... 

In clean natural water the pH can be calculated from the content of free CO2 and hydrogen carbonates using the expression for the first dissociation constant of carbonic acid. Dissociation to the 2nd degree can be neglected as its effect becomes significant only as pH > 8.3. Due to the inaccurate determination of free CO2 the calculation provides only rough results. On the contrary, from a known value of pH and the content of HCO3 the content of free CO2 can be calculated. 

The CO2 content in water is influenced by the same microorganisms as in the case of O2 but in an opposite direction. Free CO2 is present in water mostly as a dissolved gas. Only a small fraction (0.7%) reacts with water to form carbonic acid which dissociates to ions H and HCO3. Carbonic acid converts difficultly soluble carbonates of calcium and magnesium into readily soluble bicarbonates. Thus, so-called bonded (carbonate) CO2 is transformed into a semi-bonded (bicarbonate) form which is utilizable for... 

These reactions describe the dissolution of atmospheric carbon dioxide (CO2) in water (reaction 1) and the subsequent reaction forming carbonic acid (2). Once in solution, carbonic acid dissociates forming bicarbonate (HCO3-) and a hydrogen ion (H+ reaction 3). Subsequently, this bicarbonate ion may dissociate to form a carbonate ion and a further hydrogen ion (4). Concurrently, calcium hydroxide (Ca(OH)2) dissolves in water forming calcium and hydroxyl ions (OH- 5). 

The enzyme carbonic anhydrase promotes the hydration of COg. Many of the protons formed upon ionization of carbonic acid are picked up by Hb as Og dissociates. The bicarbonate ions are transported with the blood back to the lungs. When Hb becomes oxygenated again in the lungs, H is released and reacts with HCO3 to re-form HgCOj, from which COg is liberated. The COg is then exhaled as a gas. 

The value of Ka for the reaction in Equation (6.37) is 4.3 x 10 7, so carbonic acid is certainly a very weak acid. The hydrogen carbonate anion HCO3 could dissociate further, according to... 

This is a simplified version of the more general expression in Chapter 3. The influence of [OH ] and [H ] on the TA is small and can often be ignored. The borate contributes about 3% of the TA, and if not determined independently, can be estimated from the apparent boric acid dissociation constants and the salinity, relying upon the relative constancy of composition of sea salt. This correction would give the carbonate alkalinity (CA) CA = [HCO3 ] + 2[C03 ]... 

The concentrations of the various forms of C02 present in an aqueous phase are temperature dependent and extremely sensitive to pH (the concentrations also depend on the presence of other solutes, which presumably is a small effect for the cell wall water). For instance, the equilibrium concentration of C02 dissolved in water divided by that of C02 in an adjacent gas phase, Cc 7cccv decreases more than two-fold from 10°C to 40°C (Table 8-3 the decreasing solubility of C02 as the temperature increases is a characteristic of dissolved gases, which fit into the interstices of water, such space becoming less available as molecular motion increases with increasing temperature). This partition coefficient is not very pH dependent, but the equilibrium concentration of HCO3- in water relative to that of dissolved C02 is markedly affected by pH. In particular, C02 dissolved in an aqueous solution can interact with water to form carbonic acid, which then dissociates to form bicarbonate ... 

Insight into the conductivity is provided by measuring the electrical conductivity of aqueous ionic solutions (Fig. 22.20 this topic is referred to in Chapters 11 and 15). The conductivity of pure water, multiply distilled to remove all impurities, is about 0.043 X lO (O cm) h Exposed to the air, pure water dissolves CO2, which forms carbonic acid, H2CO3 dissociation produces H30 and HCO3, which increase the conductivity to about 1 X 10 (O cm) . As ionic solutes are added to water, the conductivity increases rapidly a 1.0-M solution in NaOH has conductivity of about 0.180 (El cm) at 25°C. The conductivity depends strongly on both concentration and ionic species. The concentration dependence is summarized by the molar... 

Distal tubular hydrogeu ion secretion accounts for the remaining 50% of net acid excretion (Fig. 51-2). In the distal tubular cell, CO2 combines with water in the presence of intracellular carbonic anhydrase to form carbonic acid, which dissociates to H and HCO3. The... 

Sodium carbonate can be titrated to give end points corresponding to the stepwise additions of protons to form HCO3- and CO2. (Carbonic acid, H2CO3, dissociates in acid solution to CO2—its acid anhydride—and H2O in acid solution.) The Kb values should differ by at least 10 to obtain good separation of the equivalence point breaks in a case such as this. 

The concentration of dissolved CO2 in sea water is relatively small because CO2 reacts with water to form a weak acid, carbonic acid (H2CO3), which rapidly dissociates (within milliseconds) to form HCO3 and COf-(eqn [I]). 

Carbonate chemistry was incorporated into the model in order to verify realistic geochemical conditions. First, the carbonate chemistry is characterized by the dissociation of carbonic acid yielding HCO3 and COf. The two dissociation constants K,. and K2c are defined as follows... 

The problem of sodium and potassium hydroxides effect on the formation of efflorescence was explained by Dow and Glasser [356]. Solubility of CO in water is increasing with pH, that means with the concentration of sodium and potassium. CO2 in solution at pH slightly higher than 10 occurs mainly in the form of CO3 ion. In the carbonic acid solution, the main component of its dissociation is HCO3 ... 

Buffer response the bicarbonate buffering system is the first line of defence, HCO3 combines with the protons to form (carbonic acid) H2CO3 which dissociates to form CO2 and H2O... 

As an example of an acid-base equilibrium problem, consider water in equilibrium with atmospheric carbon dioxide. The value of [COj (aq)] in water at 25°C in equilibrium with air that is 390 parts per million COj (close to the concentration of this gas in the atmosphere) is 1.277 X 10 mol/L. The carbon dioxide dissociates partially in water to produce equal concentrations of H+ and HCO3" ... 

The carbonic acid molecules immediately dissociate in water to form hydrogencarbonate ions, HCO3, and oxonium ions, H30 ... 

Given values for Cf, AT], K2 and an. we can solve Eqs. (5.22) to (5.24) for concentrations of the carbonate species as a function of pH. Carbonate species concentrations computed for Cr- 10" ° are plotted in Fig. 5.2. Also shown are concentrations of H and OH. Important features of the diagram include (1) the pH regions of dominance of H2CO3, HCO3, and C03 with increasing pH (2) the fact that crossovers of their concentration curves occur where pH = pAf for each dissociation step (3) the pH dependence of the distribution plot is independent of C and (4) concentrations of H+ and OH" are independent of Cp We will return to this figure later in the chapter when we discuss acidity and alkalinity. 

Because HCO3 is also a weak acid, a second dissociation can take place to produce another hydronium ion and the carbonate ion, C03 . 

---