Monobasic acids dissociation

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

It was found already by Cattelain that the 3-thioxo derivatives behave as monobasic acids that can be titrated on phenolphthalein and he considered them as more acid than the analogous 3,5-dioxo-triazines. This assumption was recently confirmed by determining the dissociation constants. Just as with 6-azauracil, it was possible to demonstrate, by comparing the dissociation constants of the V-methyl derivatives of all the thioxo analogs, that with the 3-thioxo compounds too, dissociation proceeds first at the NH group in position 3 122... 

Some compounds exhibit pH behavior in which a bell-shaped curve is obtained with maximum instability at the peak [107]. The peak corresponds to the intersection of two sigmoidal curves that are mirror images. The two inflection points imply two acid and base dissociations responsible for the reaction. For a dibasic acid (H2A) for which the monobasic species (HA-) is most reactive, the rate will rise with pH as [HA-] increases. The maximum rate occurs at pH = (pA) + pK2)/2 (the mean of the two acid dissociation constants). Where an acid and base react, the two inflections arise from the two different molecules. The hydrolysis of penicillin G catalyzed by 3,6-bis(di-methylaminomethyl)catechol [108], is a typical example. For a systematic interpretation of pH-degradation profiles, see the review papers by van der Houwen et al. [109] and Connors [110]. 

Using the dihydrogen salt, NaH2As03, von Zawidzki2 determined the equivalent conductivity in the presence of N/32 arsenious acid in order to diminish hydrolytic dissociation, and concluded that it resembled the salt of a monobasic acid. At extreme dilutions the equivalent conductivity increased, apparently owing to hydrolysis and oxidation. This view that arsenious acid is essentially a feeble monobasic acid is supported by Thomsen s thermochemical values for the heats of neutralisation of the acid (see p. 140). 

The identicalness of the ionization sites in a linear polyelectrolyte (Tanford, 1961) stimulated the interest of Walter and Jacon (1994) in a possible relationship between Kz and M of ionic polysaccharides displaying the characteristic titration curve of a weak, monobasic acid. Without any theoretical assumption, Eq. (S.4) was derived from simple algebra by combining elementary principles of the dissociation theory of weak acids with polymer segment theory ... 

The heat of dissociation, Qd, probably is positive, since dissociation diminished with rise of temperature. This agrees with the fact that the heat of neutralisation, Qa, of the first hydrogen ion, viz. 14-8 Cals., is somewhat greater than that of a completely dissociated strong monobasic acid, viz. 13-5 Cals. The following calculation also shows a quantitative agreement —3... 

All the acids mentioned so far produce one hydrogen ion per molecule when dissociating these are termed monobasic adds. Other monobasic acids are perchloric acid (HC104), hydrobromic acid (HBr), hydriodic acid (HI) etc. 

In general, if the dissociation of a monobasic acid HA takes place according to the equilibrium... 

EXPERIMENTAL DETERMINATION OF THE DISSOCIATION EQUILIBRIUM CONSTANT. OSTWALD S DILUTION LAW The dissociation equilibrium constant and the degree of dissociation at a given con-, centration are interlinked. To find this correlation let us consider the dissociation of a weak monobasic acid. The dissociation reaction can be written as... 

In a M solution of a strong monobasic acid (supposing that the dissociation is complete) the hydrogen-ion concentration is 1 mol -1. On the other hand, in a M solution of a strong monovalent base the hydroxyl-ion concentration is lmol -1, thus the hydrogen-ion concentration is 10"14 mol C-. The hydrogen-ion concentration of most of the aqueous solutions dealt with in chemical analysis (other than concentrated acids, used mainly for dissolution of samples), lies between these values. 

Cryoscopic measurements of the sodium and potassium salts give values for the molecular weights equal to half those corresponding to the simple formula MFe4(NO)7S3, and thus indicate that the salts are completely dissociated into the ions M and Fe4(NO)7S 3 also electric conductivity measurements show that the salts are derived from a monobasic acid, and that they have the simple molecular formula —3... 

If the dissociation constants for any two. successive stages are sufficiently different it is sometimes feasible to apply the methods employed for monobasic acids the conditions under which this is possible will be considered with reference to a dibasic acid, but the general conclusions can be extended to more complex cases. If H2A is a dibasic acid for which Ki, the dissociation constant of the first stage, ... 

Neutralization of Mixture of Two Monobasic Acids.—An expression for the variation of the pH during the whole course of the neutralization of a mixture of two monobasic acids by a strong base can bo derived, but as it is somewhat complicated, simplifications arc made which are applicable to certain specific conditions. Let ai and an be the initial concentrations of the two acids HAi and HAn, whose dissociation constants are ki and kn suppose that at a certain stage of the neutralization a concentration h of strong base MOH has been addeH to the mixture of acids. If the salts formed when the acids are neutralized arc completely dissociated, then at any instant... 

A 0.046 N solution of the potassium salt of a weak monobasic acid was found to have a pH of 9.07 at 25° calculate the hydrolysis constant and degree of hydrolysis of the salt, in the given solution, and the dissociation constant of the acid. 

Approximate Methods for Dissociation Constants.—Approximate, but more rapid, methods, similar to those used for simple monobasic acids and monoacid bases, have been frequently employed to determine dissociation constants of ampholytes. Upon taking logarithms, the equation for Ki may be written as... 

The procedure just described has been used for the determination of the dissociation constants of a number of weak monobasic acids. It has also been extended to the study of polybasic acids and amino-acids. 

Thus if QH+] is calculated under the assumption that the acid in pure solution behaves as a monobasic acid, and if K2 is known, it can be seen immediately whether the second dissociation may be neglected. For example, if K2 equals 10 and [H ]] is calculated to be 10 , the second dissociation need not be considered. On the other hand, if K2 is 10 and the calculated value of [H+] is 10 , then [A ] likewise equals 10 so that the roughly corrected value of is... 

For the sake of simplicity we shall assume that phenolphthalein behaves as a monobasic acid with a dissociation constant of 10 . Let us calculate the hydroxyl ion concentration resulting from the addition of 0.1 c.c. of 1% phenolphthalein to 10 c.c. of 0.0001 N sodium hydroxide solution. This corresponds to a con-... 

Figure 6.11 shows the titration curve for adipic acid, hexane-1,6-dioic acid with p Ti = 4.41 and pK2 = 5.28 at 25°C. Here the curve resembles that of a monobasic acid with one extended buffer region, and only one end-point. There is thus a clear distinction between this titration curve and those for malonic and phosphoric acids. The two dissociations for this acid cannot be treated as independent of each other, and analysis of the curve must consider both equilibria simultaneously. 

For the dissociation of a monobasic acid SL in water (L2O) the isotope effect is given by Eqn. 39. 

P-acid is a strong monobasic acid and its dissociation constant is 8.0 X 10 -. Paris and Merlin (15) have reported that the melting point of... 

The stability constant given in Eq. 4 is an association constant. A dissociation constant is numerically equal to the reciprocal of the corresponding association constant. It is common practice to quote dissociation constants for acids (particularly organic acids), while association constants are used for complexes. For a monobasic acid, HL. there are no further complications. But for polybasic acids, the numerical order of the two sets is reversed. For example, for a dibasic acid, the first stepwise dissociation constant is equal to the reciprocal of the second stepwise association constant and vice versa. 

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