Carbonic acid second dissociation constant

Table 9 includes data on the first dissociation constants of seven weak acids it will be recalled that we expect these to fall into class III. The table includes the second dissociation constants of five acids, phosphoric, sulfuric, oxalic, malonic, and carbonic, which fall into class IV, while the amino acids glycine and alanine provide four examples that should fall into class II. 

There are four reactions that deal explicitly with the H+ ion (Table 3.3) one is the dissociation constant for water (Kw), two are the first and second dissociation constants of carbonic acid (K and K2), and the fourth deals with the dissociation of the bisulfate (HSOJ) ion (iFbisuifate)-... 

It is interesting that the presence of a charged atom in a molecule induces a fractional charge not only on the other atoms of the chain but also on those atoms which are near by in space. Thus if the electrostatic attraction were conducted only through the carbon skeleton of a molecule, we should expect to find the dissociation constants of fumaric and maleic acids to be identical. In actual fact the first dissociation constants are somewhat similar, but there is a considerable difference in the second dissociation constants. A possible explanation of this phenomenon is that in the case of the mono acid salts of maleic acid, the near-by negative charge on the carboxylate group hinders the removal of the second proton. 

For the calculation of pHg it is possible to derive the following equation from the equation for the second dissociation constant of carbonic acid K2 and the solubility product of calcium carbonate, K ... 

W G Kolthoff IM and Bosch W, The influence of neutral salts on acid-salt equilibria. III. The second dissociation constant of carbonic acid and the influence of salts on the activity of the hydrogen ions in a bicarbonate-carbonate mixture, Rec. Trav. Chim. Pays-Bas Belg., 47, 819-825 (1928). 

K = second dissociation constant of carbonic acid, that is,... 

Salomaa has used an ingenious procedure for separating the transfer effect in his studies of the first and second dissociation constants of carbonic and sulphurous acids in H2O-D2O mixtures. We have already seen (p. 38) that only 0.3% of dissolved carbon dioxide is in the form of H2CO3, and there is some evidence that the proportion of H2SO3 is also small. The product of the first and second dissociation constants thus represents the equilibrium constant for the process... 

Second dissociation constant of carbonic acid (K ) in seawater as a function of temperature and salinity (after Roy et at, 1993,1996 DOE, 1994). 

Ascorbic acid is a white, crystalline compound, with a melting ppetroleum ether. In aqueous solution the hydrc en atom of the enol group on carbon atom 3 dissociates, yielding a solution with a pH of about 3. Thus, in aqueous solution, ascorbic add behaves as a monobasic acid, forming salts containing one monovalent metal atom or equivalent. In alkalme solution the hydrogen of the enol group on carbon atom 2 dissociates and is i laced by metal. The of the first dissociation constant is 4.17 and that of the second dissociation constant is 11.57. 

VI.12. QUASI-THERMODYNAMIC SECOND DISSOCIATION CONSTANT OF CARBONIC ACID IN SEA WATER... 

Kq, and = the activity product for calcite, and the first and second dissociation constants for carbonic acid in infinitely dilute aqueous solutions. 

OH- solutions in DMSO-water mixtures. For acetone, the ionisation ratios were measured by spectrophotometry, but in other cases an indirect kinetic method was used. This latter is based on monitoring the rates of detritiation of a standard labelled carbon acid (HS1) both in the presence and absence of a second acid HS2 (ketone). The dissociation of HS2 brings about a decrease in hydroxide ion concentration, and, since the rate of detritiation of HS1 is proportional to [OH ], the consequent decrease in rate can be related to [(S2) ]/[HS2]. Data listed in Table 7 exhibit large variations with structure, far larger than those expected from ionisation rate constants if an enolate-like transition state were assumed (see p. 34). 

Step 8. The in situ value of the second apparent dissociation constant of carbonic acid in seawater calculated by Equations 20, 21, and 22 is 10 . ... 

When this is the case, the heat of reaction must be quite independent of the nature of the anion and of the cation, aa these are not affected by the reaction. This is clearly true for nitric and hydrochloric acids with all the bases given in the table. For sulphuric and carbonic acids, however, the conditions for the validity of the theory are apparently not fulfilled. In the first case, the heat of dilution of sulphuric acid amounts to 2000 cal., and this amount must be subtracted from the figure given in the table, as it is evolved when the alkali and acid are mixed. In the second case, carbonic acid is so weak an acid that it is practically undissociated. The heat necessary for the dissociation into ions therefore uses up part of the heat of neutralisation. From the table it follows that the electrolytic dissociation of J mol. HgCOg requires 13700 — 10200 = 3500 calories. The constant heat of neutrahsation 13700 cal. is the heat of ionisation of water, i.e, the quantity of heat required for the dissociation of water, and liberated on the combination of its ions. 

At the second equivalence point, the solution consists of carbon dioxide and sodium chloride. The carbon dioxide can be treated as a simple weak acid having a dissociation constant After excess hydrochloric acid has been introduced, the dissociation of the weak acid is repressed to the point where the hydronium ion concentration is essentially that of the molar concentration of the strong acid. 

Where is the total concentration of acid species and A, and K2 are the first and second stepwise dissociation constants of the acids. This equation can be used to compute the buffer index of a polyprotic acid as long as successive dissociation constants differ by at least 20 times (this assures a calculation error of 5% or less). In other words, for a diprotic acid K2fK should be less than 0.05 (cf. Butler 1964). Thus, for example, Eq. (5.114) may be used to compute the buffer index due to species of carbonic acid, for which A = 10 and K2 = 10" °, or/ for species of silicic acid, for which a , = lO- and K, = 10... 

The equilibrium constant for this is deduced from the solubility product of CaCOa (K3), and the dissociation constants of carbonic acid to the first degree (A j) and to the second degree (A a) the equilibrium constant (K) is thus given by the relation K =. ... 

C is the concentration of heme iron in the solution, j8 is the buffering power of hemoglobin per heme, K4 is the first dissociation constant of carbonic acid, q is the solubility of carbon dioxide, and A is a constant. The first term on the left represents the negative charge carried by the protein, which is very nearly linear in pH in the range 6.5 < pH < 7.8 and the second term on the left represents the concentration of bicarbonate ion. /3 may be taken as 2.9 equivalents per mole iron per pH unit, the value obtained for horse hemoglobin from the results of Ger.man and Wyman (56). The value of the constant A depends on the amount of base present in the solution. 

Thanks are due to Dr G. L. Pickard, Director of the Institute of Oceanography at the University of British Columbia, for permission to include bis tables for the conversion of 20 C chlorosity to salinity. The tables given for calculating the carbonate balance of sea water largely follow the Buch presentation as given by H. W. Harvey in his book The Chemistry and Fertility of Sea Waters (Cambridge University Press, 1957). Thqr have been extended and some have been changed so as to incorporate a more recent estimate of the second carbonic acid dissociation constant by J. Lyman. 

TTie first dissociation constant is typically greater than the second i.e., K i > K,2- For example, the weak unstable carbonic acid [H2CO3) can lose one proton to form bicarbonate anion (HC03 ) and lose a second to form carbonate anion [C03 "). Both Ka values are small, but K,] > K. ... 

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