Acid equilibrium constant

The equilibrium constant Ka is called, logically enough, the acid equilibrium constant of the weak acid HB. The Ka values of some weak acids (in order of decreasing strength) are listed in Table 13.2. The weaker the acid, the smaller the value of Ka. For example, HCN (Ka = 5.8 X 10-10) is a weaker acid than HN02, for which Ka = 6.0 X 10-4. 

The concentration of H+ ion in a buffer can be calculated if you know the concentrations of the weak acid HB and its conjugate base B . These three quantities are related through the acid equilibrium constant of HB ... 

K. See Equilibrium constant Ka. See Acid equilibrium constant See Base equilibrium constant Kc. See Equilibrium constant Kf. See Formation equilibrium constant Kr See Equilibrium constant K,p. See Solubility product constant K . See Water ion product constant K-electron capture The natural radioactive process in which an inner electron (n = 1) enters the nucleus, converting a proton to a neutron, 514 Kelvin, Lord, 8... 

The pH of a buffer solution depends on the weak acid equilibrium constant and the concentrations of the weak acid and its conjugate base. To show this, we begin by taking the logarithm of the acid equilibrium constant ... 

Uioxane. 72.166, 174-176 Dipeptkles. 263 Diphenyidichlorosilane. 133 Dipole moment. 208.217 Diprotic acids, equilibrium constants and retention bctors in hetaeric chromatography. 241... 

These surface acid equilibrium constants differ from their solution counterparts in that they reflect both an intrinsic reactivity of the particular O-H bond and an electrostatic free energy of moving H+ to and from a charged surface ... 

Solution equilibrium studies410 have identified both 1 1 and 1 2 complexes found between V02+ ions and oxalic acid, but only 1 1 complexes for phthalic, maleic, succinic, and adipic acids. Equilibrium constants for the formation and subsequent hydrolysis of these complexes have been determined. VO(H2L) (H5L = trihydroxy-glutaric acid) is the principal complex species in mixed aqueous acetone solutions but minor amounts of (VO)3(H2L)2 are also formed 411 the stability of VO(H2L)-increases with increasing acetone content. Solvent effects are also evident in vana-dium(iv)-quercetin system 412 the protonated ligand RH2 forms VOR+ and VR2 + in methanol at pH 3 but only VOR+ in aqueous methanol. 

The acid equilibrium constant KL involves the COOH termination the base equilibrium constant K2 involves the amine termination. At the isoelectric point (pH = (l/2)(pRi pR2) the amino acid is a zwitterion or betaine The proton is detached from the carboxylate end and is attached at the amine end. Dipeptides are formed by the addition of any two amino acids and a loss of a water molecule, for example ... 

The acidity equilibrium constants for each form (with the enolate ion) are both small, but they are not the same. 

One of the first reaction series studied involved triethylamine reacting with a series of methyl esters of substituted benzoic acids. A plot of the logarithm of the rate constant (Infe) versus the logarithm of the acid equilibrium constant (InRa) was linear. " In mathematical form, this is Eq. [2] where m is the slope and b the intercept. 

Boric acid is a relatively weak acid compared to other conunon acids, as illustrated by the acid equilibrium constants given in Table 4. Boric acid has a similar acid strength to sihcic acid. Calculated pH values based on the boric acid equihbrium constant are significantly higher than those observed experimentally. This anomaly has been attributed to secondary equilibria between B(OH)3, B(OH)4, and polyborate species. Interestingly, the aqueous solubihty of boric acid can be increased by the addition of salts such as potassium chloride and sodium sulfate, but decreased by the addition of others salts, such as the chlorides of lithium and sodium. Basic anions and other nucleophiles such as fluorides and borates significantly increase boric acid solubility. 

Table 4 Acid equilibrium constants of some common acids...

We can now calculate the surface acidity equilibrium constants (equations 20 and 21). There are five species, =FeOH, =FeOH, =FeO", H , OH", that are interrelated by the two acidity mass law constants (equations 20 and 21), by the ion product of water Kw = [H l [OH"]) and two mass balance equations ... 

C) Acidity is a thermodynamic property, and the acid equilibrium constant, K, is a measure of the relative concentrations of species in the protonated and unprotonated form. As most organic acids are weak acids, meaning they are present mostly in the protonated form at equilibrium, the < 1. Since the pK = -log K, the pKj values will be greater than 0, with the larger pK representing a weaker acid. If this is not clear, review text section 1-13. 

A comparable constant for hafnium was not calculated owing to a lack of data for hydroxo and chloro complexing. A similar value has been obtained for the same constant by Ermako et al. (169, 170) by ion exchange techniques under vastly different conditions, namely, metal ion concentrations of 2 x 10 mole/liter and 0.23 M perchloric acid. Equilibrium constant values of logAz, = 20.8 and ogK = 20.3 were obtained. At high metal ion concentrations (0.01-0.1M) there is evidence for formation of a 2Zr NTA complex in which hydrolyzed zirconium atoms are bridged by the nitriloacetate ion (172). 

Knowledge Required (1) The interpretation of acid equilibrium constants, K. (2) Meaning of terms associated with weak and strong acids and bases. (3) Brpnsted-Lowry acid-base definition. 

Negative logarithm of the acidity equilibrium constant = -logio [Al ]... 

Table 6 lists association constants between porphyrins and nucleic acids. Equilibrium constants between Hi(TMpyP-4) (52) and Cu(TMpyP-4) (88) with a synthetic polymer poly(dG-dC) were estimated to be 7.7 x lO M and 8.0 x 10 M, respectively, based on McGhee-von Hippel analysis.The equilibrium constants were ionic-strength dependent. It was observed that at 2 M ionic strength, the Free-base poqrhyrin 52 almost completely dissociates from DNA, poly(dA-dT) and poly(dG-dC). Therefore electrostatic interactions are the important driving force. Kinetic studies were carried out for the above system... 

Escolar D, Haro MR, Saucedo A, etal. (1996) Determining acid equilibrium-constants by means of chromatic parameters. Applied Spectroscopy 50 1290-1294. 

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