Dissociation constant species

Indicator Chemical name Dissociation constants and colors of free indicator species Colors of metal-indicator complexes Applications... 

A species that can serve as both a proton donor and a proton acceptor is called amphiprotic. Whether an amphiprotic species behaves as an acid or as a base depends on the equilibrium constants for the two competing reactions. For bicarbonate, the acid dissociation constant for reaction 6.8... 

A more challenging problem is to find the pH of a solution prepared from a polyprotic acid or one of its conjugate species. As an example, we will use the amino acid alanine whose structure and acid dissociation constants are shown in Figure 6.11. 

The equihbrium constant of this reaction is 5.4 x 10 at 25°C, ie, iodine hydrolyzes to a much smaller extent than do the other halogens (49). The species concentrations are highly pH dependent at pH = 5, about 99% is present as elemental at pH = 7, the and HIO species are present in almost equal concentrations and at pH = 8, only 12% is present as and 88% as HIO. The dissociation constant for HIO is ca 2.3 x 10 and the pH has tittle effect on the lO ion formation. At higher pH values, the HIO converts to iodate ion. This latter species has been shown to possess no disinfection activity. An aqueous solution containing iodate, iodide, and a free iodine or triodide ion has a pH of about 7. A thorough discussion of the kinetics of iodine hydrolysis is available (49). 

Cyanide Complexes. Insoluble silver cyanide, AgCN, is readily dissolved in an excess of alkah cyanide. The predominant silver species present in such solutions is Ag(CN) 2) with some Ag(CN) 3 and Ag(CN) 4. Virtually all silver salts, including the insoluble silver sulfide, dissolve in the presence of excess cyanide because the dissociation constant for the Ag(CN) 2 complex is only 4 x 10 (see Cyanides). 

When substances ionise their neutral species produce positive and negative species. The ionisation constants are those constant values (equilibrium constants) for the equilibria between the charged species and the neutral species, or species with a larger number of charges (e.g. between mono and dications), l ese ionisation constants are given as pK values where pK = -log K and K is the dissociation constant for the equilibrium between the species [Albert and Serjeant The Determination of Ionisation Constants, A Laboratory Manual, 3rd Edition, Chapman Hall, New York, London, 1984, ISBN 0412242907]. 

The substrate concentration when the half maximal rate, (Vmax/2), is achieved is called the Km. For many simple reactions it can easily be shown that the Km is equal to the dissociation constant, Kd, of the ES complex. The Km, therefore, describes the affinity of the enzyme for the substrate. For more complex reactions, Km may be regarded as the overall dissociation constant of all enzyme-bound species. 

Figure 6-14 shows FhjS. Fhs, and Fs plotted against pH for an acid with pF, = 5.0 and pA"2 = 10.0. Evidently with such widely spaced dissociation constants the solution contains, at any one pH, significant fractions of only two species. The fraction of monoanion rises essentially to unity at one point. The pH at which the monoanion fraction achieves its maximum value is calculated by differentiating Eq. (6-74) and setting the result equal to zero this gives... 

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

Acid Dissociation Constants or Some Hydrated and Anhydrous Species ... 

Aspartic acid acts as a triprotic acid with successive dissociation constants of 8.0 x 10-3, 1.4 X 10-4, and 1.5 X 10-10. Depending upon pH, aspartic acid can exist in four different forms in water solution. Draw these forms and calculate the pH range over which each form is the principal species. 

From the equilibrium equations, expressions for the various receptor species can be derived and substituted into Equation 3.32. With conversion of all equilibrium association constants to equilibrium dissociation constants, a general binding expression results for radioactive CD4... 

The formation of a single complex species rather than the stepwise production of such species will clearly simplify complexometric titrations and facilitate the detection of end points. Schwarzenbach2 realised that the acetate ion is able to form acetato complexes of low stability with nearly all polyvalent cations, and that if this property could be reinforced by the chelate effect, then much stronger complexes would be formed by most metal cations. He found that the aminopolycarboxylic acids are excellent complexing agents the most important of these is 1,2-diaminoethanetetra-aceticacid (ethylenediaminetetra-acetic acid). The formula (I) is preferred to (II), since it has been shown from measurements of the dissociation constants that two hydrogen atoms are probably held in the form of zwitterions. The values of pK are respectively pK, = 2.0, pK2 = 2.7,... 

The factor at can be calculated from the known dissociation constants of EDTA, and since the proportions of the various ionic species derived from EDTA will be dependent upon the pH of the solution, a will also vary with pH a plot of log a against pH shows a variation of logoc = 18 at pH = 1 to loga = 0 at pH = 12 such a curve is very useful for dealing with calculations of apparent stability constants. Thus, for example, from Table 2.4, log K of the EDTA complex of the Pb2+ ion is 18.0 and from a graph of log a against pH, it is found that at a pH of 5.0, log a = 7. Hence from equation (30), at a pH of 5.0 the lead-EDTA complex has an apparent stability constant given by ... 

Thus, when the associated (protonated) and dissociated (conjugate base) species are present at equal concentrations, the prevailing hydrogen ion concentration [H+] is numerically equal to the dissociation constant, A. If the logarithms of both sides of the above equation are... 

FIG. 4 Thermodynamic equilibria for the interfacial distribution of a solute X which can be ionized n times, and X being its most acidic (or deprotonated) and its most basic (or protonated) forms, respectively. X and are the dissociation constants in the aqueous and organic phase, respectively, and P is the partition coefficient of the various species between the two phases. 

This time, the boundary line is independent of the Galvani potential difference across the interface, and the corresponding pH value can be regarded as the effective dissociation constant, pK eff, which appears experimentally due to the partitioning of the neutral species. For a monoprotic substance, pK eff can thus be defined as ... 

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]. 

In this book we will use the symbol K, for the dissociation constant of the El complex, and aA) for the dissociation constant of the ESI complex (or subsequent species). The reader should note that different authors used different symbols for these dissociation constants. Hence in the enzymology literature one may find the dissociation constant for the El complex symbolized as K Ka, KEi, etc. Likewise the dissociation constant for the ESI complex may be symbolized as aK, K, Kis> and KEsi-... 

A noncompetitive inhibitor is one that displays binding affinity for both the free enzyme and the enzyme-substrate complex or subsequent species. In this situation the binding affinity cannot be defined by a single equilibrium dissociation constant ... 

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