Effective formation constant

The number K f = aY. Kf is called the conditional formation constant, or the effective formation constant. It describes the formation of MY" 4 at any particular pH. Later, after we learn to use Equation 12-6, we will modify it to allow for the possibility that not all the metal ion is in the form M"+. 

K j is the effective formation constant at a fixed pH and fixed concentration of auxiliary complexing agent. Box 12-2 describes the influence of metal ion hydrolysis on the effective formation constant. 

Box 12-2 Metal Ion Hydrolysis Decreases the Effective Formation Constant for EDTA Complexes... 

The effective formation constant for FeY- in the graph has three contributions ... 

Formation constants for EDTA are expressed in terms of [Y4-], even though there are six protonated forms of EDTA. Because the fraction (aY4 1 of free EDTA in the form Y4 depends on pH, we define a conditional (or effective) formation constant as K = aYj Kf = MY" 4 /[M"+ [EDTA], This constant describes the hypothetical reaction Mn+ + EDTA MY 1-4, where EDTA refers to all forms of EDTA not bound to metal ion. Titration calculations fall into three categories. When excess unreacted M"+ is present, pM is calculated directly from pM = — log M l+]. When excess EDTA is present, we know both [MY"-4] and [EDTA], so IM"+] can be calculated from the conditional formation constant. At the equivalence point, the... 

The greater the effective formation constant, the sharper is the EDTA titration curve. Addition of auxiliary complexing agents, which compete with EDTA for the metal ion and thereby limit the sharpness of the titration curve, is often necessary to keep the metal in solution. Calculations for a solution containing EDTA and an auxiliary complexing agent utilize the conditional formation constant K" = aM aY4- Kt, where aM is the fraction of free metal ion not complexed by the auxiliary ligand. 

EDTA (ethylenediaminetetraacetic acid) (H02CCH2)2NCH2CH2N-(CH2C02H)2, the most widely used reagent for complexometric titrations. It forms 1 1 complexes with virtually all cations with a charge of 2 or more, effective formation constant Equilibrium constant for formation of a complex under a particular stated set of conditions, such as pH, ionic strength, and concentration of auxiliary complexing species. Also called conditional formation constant. 

The weak acidity of catechol makes its effective formation constant much less than 1045-9 near physiological pH. However, any chelate effect should tend to make the formation constant for enterobactin in larger than / 3 for catechol. Thus 1045 can be regarded as a lower bound for the reaction Fe3+ + ent6" Fe(ent)3". 

Aqueous solution stability constants for the aryl substituted complexes have been determined. The ML3 complexes are highly stable at 25 °C the overall stability constant (/Ss) for the parent complex (17) R = H is 10 . At ligand to metal ratios > 1, the hgands prevent gallium hydrolysis even at millimolar concentrations and under slightly basic conditions the effective formation constant for ML3... 

To take into account the effect of pH on the free ligand concentration in a complexation reaction, it is useful to introduce a conditional or effective formation constant. Such constants are pH-dependent equilibrium constants that apply at a single pH only. For the reaction of Fe with oxalate, for example, we can write the formation constant Ki for the first complex as... 

It has been shown that the effects found are caused by specific solvation of both the PhAA ionogenic and other polar groups by the plasticizers used, as well as by the influence of ion-exchangers nature on the PhAA cations-anionic sites complex formation constants. 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

Table 4 Structural Effects on 1 1 CTC Formation Constants (Kf) Between Br2 and Olefins and the Respective Bromination Rates.

The reaction between Fe(IlI) and Sn(Il) in dilute perchloric acid in the presence of chloride ions is first-order in Fe(lll) concentration . The order is maintained when bromide or iodide is present. The kinetic data seem to point to a fourth-order dependence on chloride ion. A minimum of three Cl ions in the activated complex seems necessary for the reaction to proceed at a measurable rate. Bromide and iodide show third-order dependences. The reaction is retarded by Sn(II) (first-order dependence) due to removal of halide ions from solution by complex formation. Estimates are given for the formation constants of the monochloro and monobromo Sn(II) complexes. In terms of catalytic power 1 > Br > Cl and this is also the order of decreasing ease of oxidation of the halide ion by Fe(IlI). However, the state of complexing of Sn(ll)and Fe(III)is given by Cl > Br > I". Apparently, electrostatic effects are not effective in deciding the rate. For the case of chloride ions, the chief activated complex is likely to have the composition (FeSnC ). The kinetic data cannot resolve the way in which the Cl ions are distributed between Fe(IlI) and Sn(ll). 

Kinetic data exist for all these oxidants and some are given in Table 12. The important features are (i) Ce(IV) perchlorate forms 1 1 complexes with ketones with spectroscopically determined formation constants in good agreement with kinetic values (ii) only Co(III) fails to give an appreciable primary kinetic isotope effect (Ir(IV) has yet to be examined in this respect) (/ ) the acidity dependence for Co(III) oxidation is characteristic of the oxidant and iv) in some cases [Co(III) Ce(IV) perchlorate , Mn(III) sulphate ] the rate of disappearance of ketone considerably exceeds the corresponding rate of enolisation however, with Mn(ril) pyrophosphate and Ir(IV) the rates of the two processes are identical and with Ce(IV) sulphate and V(V) the rate of enolisation of ketone exceeds its rate of oxidation. (The opposite has been stated for Ce(IV) sulphate , but this was based on an erroneous value for k(enolisation) for cyclohexanone The oxidation of acetophenone by Mn(III) acetate in acetic acid is a crucial step in the Mn(II)-catalysed autoxidation of this substrate. The rate of autoxidation equals that of enolisation, determined by isotopic exchange , under these conditions, and evidently Mn(III) attacks the enolic form. 

Po and Sutin " have disputed both the extent of the catalytic effect of chloride ion reported by Wells and Salam" and the formation constant of 5.54 (25 °C, [Cl ] = 0.300 M, n = 1.00) for FeCl estimated thereby. Wells " has replied that the value of k2 of Po and Sutin at zero chloride concentration is artifically increased because of the presence of stabiliser in their peroxide, consequently masking the catalysis. 

The complex Cu(II)2(0-BISTREN) is much more acidic than the free Cu2+ ion, by a factor of more than three log units. This is primarily due to the presence of two Cu(II) ions, because the formation constant of the Cu2(OH)+ complex is not much less than that for the Cu2(0-BISTREN) complex with hydroxide. This is not a good indication of how well two free Cu2+ ions would bind hydroxide compared to the Cu2(0-BISTREN) complex, however, since one must take into account the dilution effect operative in the chelate effect to make the comparison more realistic (90). Thus, the formation constant for the Cu2OH+ complex above applies for the standard reference state of 1 M Cu2 +. In contrast, in 10 6 M Cu2+, for example, the pH at which Cu2(OH) + would form is raised from pH 5.6 to 11.6, ignoring the fact that Cu(OH)2(s) would precipitate out long before this pH as reached. By comparison, the acidity of the Cu2(0-BISTREN) complex is not affected by dilution and would still form the hydroxide complex at pH 3.9 if present at a 10"6 M concentration. 

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