Conductance measurements acidity constant determination from

Fig. 7. Determination of the dissociation constant of acetic acid from the conductivity measurements of its solution.

The dissociation constants of weak acids are determined, in the described manner, according to conductivity measurements. From the diagram of the dissociation of acetic acid it follows that the intercept on the axis of ordinates log K c = log Kc = —4.756, from which the true dissociation constant Kt = = 1.75.1(T5 at 25° C. 

The value of the first constant was determined as 1-1 x IQ-2 by measurements of the conductivity of the free acid.3 The second constant was determined as 1-95 x 10 7 by conductivity measurements in solutions of NaH2P04.3 This value was confirmed by calculations from the neutralisation curve as determined by means of the hydrogen electrode.4 The value of the third constant, viz. 3-6 x 10-la, was first determined by measurements of the conductivity of ammonium phosphates and also by the distribution of the ammonia between water and chloroform.3 It was shown that this result was incompatible with the observed values of hydrion concentration during the later stages of neutralisation by a strong alkali. A calculation based on these values gave 3 = 3-0 x 10-12 in decimolar solutions.4... 

Comparison of dissociation constants of acids and bases derived from electrochemical or conductivity measurements in solvents of different dielectric constants are meaningless (25, 28) since equilibrium constants determined in this way always represent the overall equilibrium constant K = Xform- sep-... 

The determination of the second dissociation constant of a dibasic acid also requires a knowledge of the equivalent conductance of the intermediate ion HA, and if the value of K2 is large enough to be determined from conductance measurements, the further dissociation of HA "... 

The determinations of hydrolysis constants from conductance measurements cannot be regarded as accurate the assumption has to be made that the added free acid or free base has a negligible conductance. This is reasonably satisfactory if the acid or base is very weak, e.g., a phenol or an aniline derivative, but for somewhat stronger acids or bases, e.g., acetic acid, an appreciable error would be introduced it is sometimes possible, however, to make an allowance for the conductance of the added acid or base. 

To establish a pH scale, Sorensen chose a dilute hydrochloric acid solution for a standard. He took the concentration of hydrogen ions in such a solution to be given by aC, where C is the concentration of hydrochloric acid and a is a degree of dissociation determined from conductance measurements. His procedure had drawbacks first, there is evidence that the extrapolation procedure does not actually reduce the liquid-junction potential of Cell (3-6) to zero second, the hydrochloric acid is completely dissociated (dissociation constant about 1.6 x 10 ), and therefore the concentration of H is C rather than a somewhat smaller quantity. 

The particular application of the Debye-Hiickel equation to be described here refers to the determination of the true equilibrium constant K from values of the equilibrium function K at several ionic strengths the necessary data for weak acids and bases can often be obtained from conductance measurements. If the solution of the electrolyte MA is sufficiently dilute for the limiting law to be applicable, it follows from equation (40.12), for the activity coe cient of a single ionic species, that... 

Fig. 32). This procedure has been utilized for the determination of disBoei-ation constants of weak acids from conductance measurements. ... 

This may be illustrated by some experiments of Fanjung (Zeitsch physik Chem, 14, 673, 1894), who by means of conductivity measurements determined the dissociation constants (the Ostwald constant) for a series of weak acids in aqueous solutions at various piessures It appears to be a general conclusion that the process of ionisation is accompanied by a contraction, or decrease in volume Hence the Le Chatelier-Braun principle (embodied quantitatively in the above expression) predicts that on increasing the pressure the degree of dissociation should increase also, that is the dissociation constant should increase. Fanjung found this to be the case The term Vl - V2 in this case represents the contraction due to the transfer of 1 gram-mole from the unionised to the ionised state For acetic acid at 180 C Fanjung found that... 

The results of recent determinations of the thermodynamic ionization constants of weak acids determined from conductance measurements which have been interpreted substantially as described above, are given in Table VIII. The relations of some of these values to the molecular structures of the corresponding compounds are discussed in Chapter 21. 

The Ka value of -cyanobenzoic acid is 3.14. It has already been mentioned (Section IILB) that Widequist " found this acid to be rapidly hydrolysed in water (initially to phthalamic acid). He seems to have taken great care to minimize the effects of this on the conductivity measurements which were used to determine the dissociation constant, but it must be supposed that the above value is not of the highest accuracy possible for pX determinations. The pKa value for the para acid is about 3.55. Thus the general pattern for the effect of moving an electron-attracting group from para- to r / -position is followed, but the increase in acidity is not so marked as in most of the cases mentioned above. To decide whether the increase in acidity is reasonable requires correlation analysis by the extended Hammett equation (Section II.B) in the form of equation 15 ... 

D. Machines and T. Shedlovsky, The determination of the ionization constant of acetic acid, at 25°, from conductance measurements, JACS, 1932,54,1429-1438. 

Electrochemical methods for NO determination offer several features that are not available with spectroscopic approaches. Perhaps the most important is the capability of microelectrodes to directly measure NO in single cells in situ, in close proximity to the source of NO generation. Figure 2 shows sensors that have been developed for the electrochemical measurement of NO. One is based on the electrochemical oxidation of NO on a platinum electrode (the classical Clark probe for detection of oxygen) and operates in the amperometric mode [17]. The other is based on the electrochemical oxidation of NO on conductive polymeric porphyrin (porphyrinic sensor) [24]. The Clark probe uses a platinum wire as a working electrode (anode) and a silver wire serves as the counterelectrode (cathode). The electrodes are mounted in a capillary tube filled with a sodium chlo-ride/hydrochloric acid solution separated from the analyte by a gas-permeable membrane. A constant potential of 0.9 V is applied, and direct current (analytical signal) is measured from the electrochemical oxidation of NO on the platinum anode. In the porphyrinic sensor, NO is catalytically oxidized on a polymeric metalloporphyrin... 

Ostwald (2) combined the Arrhenius theory of electrolytic dissociation with the law of mass action and calculated the dissociation constants of various weak acids from the results of conductivity measurements. The existence of complex ions could be deduced from distribution experiments (3) and solubility behavior (4) as well as from rate studies, and several equilibrium constants were determined. 

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