Oxalic acid dissociation constants

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. 

Dicarboxylic acids have two dissociation constants, one for the initial dissociation i into a monoanion and one for the second dissociation into a dianion, i-or oxalic acid, H02C—COoH, the first ionization constant has p/Cal = 1.2 and the second ionization constant has pKa2 = 4.2. Why is the second carboxyl group so much less acidic than the first ... 

If the dissociation constant of the acid HA is very small, the anion A- will be removed from the solution to form the undissociated acid HA. Consequently more of the salt will pass into solution to replace the anions removed in this way, and this process will continue until equilibrium is established (i.e. until [M + ] x [A-] has become equal to the solubility product of MA) or, if sufficient hydrochloric acid is present, until the sparingly soluble salt has dissolved completely. Similar reasoning may be applied to salts of acids, such as phosphoric(V) acid (K1 = 7.5 x 10-3 mol L-1 K2 = 6.2 x 10-8 mol L-1 K3 = 5 x 10 13 mol L-1), oxalic acid (Kx = 5.9 x 10-2 mol L-K2 = 6.4 x 10-5molL-1), and arsenic)V) acid. Thus the solubility of, say, silver phosphate)V) in dilute nitric acid is due to the removal of the PO ion as... 

With a knowledge of the pH at the stoichiometric point and also of the course of the neutralisation curve, it should be an easy matter to select the appropriate indicator for the titration of any diprotic acid for which K1/K2 is at least 104. For many diprotic acids, however, the two dissociation constants are too close together and it is not possible to differentiate between the two stages. If K 2 is not less than about 10 7, all the replaceable hydrogen may be titrated, e.g. sulphuric acid (primary stage — a strong acid), oxalic acid, malonic, succinic, and tartaric acids. 

Dissolution of Sparingly Soluble Salts. Obtain precipitates of calcium carbonate and calcium oxalate in test tubes by reacting the relevant salts. Decant the solutions and pour an acetic acid solution onto the moist precipitates. What happens Repeat the experiment, but use hydrochloric acid instead of the acetic acid. Write the equations of the chemical reactions in the molecular and net ionic forms. Explain the results obtained on the basis of the dissociation constants of the acids and the solubility product. 

The kinetics of the 1 1 substitution of aqua Mo with NCS- and HC2Oj have been studied in trifluoromethanesulfonic acid solutions, 7 = 0.10M (CF3S03Na), with the Mo reactant in greater than ten-fold excess (to avoid higher complex formation).30 First-order equilibration rate constants, /ccq, determined by the stopped-flow method can be expressed as in equation (1). At 25 °C ki for the formation is 590 M-1 s-1, and i for the reverse reaction is 0.21 s-1. With oxalate the rate law is equation (2), where K is the acid dissociation constant for H2C204 to HC2Oj, which is believed to be the reactant. In this case, at 25 °C, k2 for formation is 43 M-1 s-1 and is 4.7 x 10-3 s 1. 

The inductive effect of one carboxyl group is expected to enhance the acidity of the other. In Table 18-4 we see that the acid strength of the dicarboxylic acids, as measured by the first acid-dissociation constant, K1, is higher than that of ethanoic acid (Ka = 1.5 X 10-5) and decreases with increasing number of bonds between the two carboxyl groups. The second acid-dissociation constant, K2, is smaller than Ka for ethanoic acid (with the exception of oxalic acid) because it is more difficult to remove a proton under the electrostatic attraction of the nearby carboxylate anion (see Section 18-2C). 

Oxalic acid dissociation constants were selected from the critical compilation of Kartell and Smith (J ). At 25.0 C and 1 molar Ionic strength the selected results are ... 

The radical -COOH (CAS 2564-86-5) has only a separate fleeting existence. The acid dissociation constant of-COOH has been measured using electron paramagnetic resonance spectrocopy. The carboxyl group tends to dimerise to form oxalic acid. 

Dicarboxylic acids have two dissociation constants, one for the initial dissociation into a monoanion and one for the second dissociation into a dianion. For oxalic acid,... 

Nikolaeva NM, Antipina VA (1972) The dissociation constants of oxalic acid in water at temperatures from 25 to 90 °C. Izv Sib Otd Akad Nauk SSSR Ser Khimicheskikh Nauk 6 13-17 (in Russian)... 

Pinching GD, Bates RG (1948) Second dissociation constant of oxalic acid from 0 to 50 °C, and the pH of certain oxalate buffer solutions. J Res Natl Bur Standards 40 405-416... 

Attribution of chemical reactions to experimentally accessible dissociation constants is indicated by Equations (l)-(7) (shown here for oxal-acetic acid, the same assignments are used for a-ketoglutaric acid) ... 

Kinetics and mechanisms of complex formation have been reviewed, with particular attention to the inherent Fe +aq + L vs. FeOH +aq + HL proton ambiguity. Table 11 contains a selection of rate constants and activation volumes for complex formation reactions from Fe " "aq and from FeOH +aq, illustrating the mechanistic difference between 4 for the former and 4 for the latter. Further kinetic details and discussion may be obtained from earlier publications and from those on reaction with azide, with cysteine, " with octane-and nonane-2,4-diones, with 2-acetylcyclopentanone, with fulvic acid, and with acethydroxamate and with desferrioxamine. For the last two systems the various component forward and reverse reactions were studied, with values given for k and K A/7 and A5, A/7° and A5 ° AF and AF°. Activation volumes are reported and consequences of the proton ambiguity discussed in relation to the reaction with azide. For the reactions of FeOH " aq with the salicylate and oxalate complexes d5-[Co(en)2(NH3)(sal)] ", [Co(tetraen)(sal)] " (tetraen = tetraethylenepentamine), and [Co(NH3)5(C204H)] both formation and dissociation are retarded in anionic micelles. 

Che and Kustin studied complexation 438 results for oxalic and malonic acids are in Table 27. From previous relaxation data 07 and their own results, they concluded that the rate constants are more consistent with a normal dissociative pathway if VOL formation from [VO(OH)]+ is assumed. 

The effect of exchange of lactic, mandelic and sulfosalicylic acids on the relaxation of solvent protons gave rate constants (k) of exchange from 1.73 to 0.701 mol-1 s-1.642 Kinetics of complex formation with mandelic (HMDA) and vanillomandelic acids (HVMDA) gave rate constants (1.09 x 103 and 1.13 x 103 mol-1 s 1 for MDA- and VMDA ) consistent with a dissociative (Eigen) mechanism.438 As in the case of oxalic and malonic acids (Section 33.5.5.5.ii Table 27), species with coordinated hydroxyl are labilized. 

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