Calculations potentiometry

It may happen that AH is not available for the buffer substance used in the kinetic studies moreover the thermodynamic quantity A//° is not precisely the correct quantity to use in Eq. (6-37) because it does not apply to the experimental solvent composition. Then the experimentalist can determine AH. The most direct method is to measure AH calorimetrically however, few laboratories Eire equipped for this measurement. An alternative approach is to measure K, under the kinetic conditions of temperature and solvent this can be done potentiometrically or by potentiometry combined with spectrophotometry. Then, from the slope of the plot of log K a against l/T, AH is calculated. Although this value is not thermodynamically defined (since it is based on the assumption that AH is temperature independent), it will be valid for the present purpose over the temperature range studied. 

In measurements of conductivity, no electrochemical reactions occur. Differences in conductivity are due to differences in the ionic strengths of solutions. An alternating potential is applied to the solution at a known potential. The current is measured and the conductivity in Siemens/cm calculated.16 In potentiometry, the analyte is presumed to undergo no electrochemical reaction. The potential at the electrode changes due to changes in potential across the surface of the membrane in a membrane electrode or at the electrode surface of a solid electrode. The most familiar example of a potentiometric electrode is the pH electrode. In amperometry, current does flow, due to reduction or oxidation of the substance being analyzed. 

For the different values of pAHX and pA H+ see the summary Table 4.5 later of pKa data in various solvents of low e in comparison with pAa(H20). The mutual agreement of pffHX values obtained by spectrophotometry, DVP, potentiometry and titration was reasonably good the typical form of the curves for titration of the dinitrophenols with TMG can be explained by homoconjugation and more especially by its influence on the potentiometric measurements, calculated on the basis of simple dissociation hence the major discrepancies in the spectrophotometric and potentiometric pK values. In order to... 

Stepwise formation constants have been determined in the system Hg2+/Cl-/diethylenetriamine (dien) and related systems by potentiometry. Thermodynamic parameters have been calculated and the contribution of the entropy term to complex stability discussed.208... 

From potentiometry results we can easily calculate the degree of complexation, which is the ratio of the concentration of complexed acid groups to the total concentration of PAA 0 =... 

Acid/hase potentiometry enables the surface charge density to be measured. This involves comparison of the titration curves obtained for the suspension of oxide at several different ionic strengths (10 10" M) with that of the electrolyte alone, followed by calculation of the net consumption of protons or hydroxyl ions (mol g ) at each pH. The data is presented as a plot of excess of acid or base (Fh - Toh ) mol g or mol m ) vs pH (adsorption isotherm) or as a plot of surface charge, cr, (coulombs m ) vs pH (charging curve) (Figure 10.5). 

Equilibrium constants have been measured for (41)-(45) using a variety of techniques spectrophotometry 49), conductivity, 170-NMR line broadening (50), isopiestic measurements, potentiometry, and others 51). From these data, speciation distribution estimates have been calculated 52). As the equilibrium constants for (41)—(45) are quite sensitive to ionic strength, so are the speciation distributions. Thus, for pH = 1, Cr(VI)T = 10-2 M and T = 25°C, the [H2Cr04] is calculated to be 29%at/ = 0 and 1% at I = 1.0M(52). Nevertheless, a considerable proportion ( 50%) of Cr(VI) in the pH range 1-6 is estimated to be in the HCr04 form. 

To apply equation (3) for calculation of the equilibrium constant K waves Ia and ic must both be limited by diffusion. To prove this the current is measured under conditions when it is 15% or less of the total limiting current and its dependence on the mercury pressure is followed. A diffusion current must, under these conditions, show a linear dependence on the square root of the height of the mercury column. Whenever possible, polarographic dissociation curves should be compared with data on dissociation obtained by other methods, e.g. potentiometry, N.M.R. or spectrophotometry. In the latter case it is important to show that the species responsible for a given polarographic wave is identical with that responsible for the observed absorption peak. 

The determination of the pAa of a nucleobase by potentiometry relies on differences in NaOH consumptions between titrations of HN03 carried out in the absence and presence of the compound whose p/fa is to be obtained (208). Curve-fit procedures using Newton-Gauss nonlinear least-squares programs are applied to calculate the pA"a value. 

Calc., calculated value dir. pot., direct potentiometry solv. extrn., solvent extraction coprecip., coprecipitation radiopol., radiopolarography. 

Firstly, conformational changes and/or the new formation of bonds in the complex (including rearrangements of bonds) were observed by means of circular dichroism and potentiometry as shown in Fig. 46. In the PGA-QOEI system, the a-helix of PGA is destabilized with aging. From the change in pH of the complex solution, it is found that even after the complex has been formed the further bindings are newly formed. In Fig. 46, ApH is the change of proton concentration calculated from the equation... 

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

When the value of m is obtained experimentally by a technique such as potentiometry, the calculation of free ligand concentration can be done by using the concept of average number of ligands per complex [3] (u)... 

The different terms of Equation 1 were obtained as follows— /ce, formal potential of the Ce(IV)-Ce(III) couple in the medium, was taken from publications [Ce(IV)]a and [Ce(IV)]o have been measured by direct absorption spectrophotometry [Ce(III)] was calculated by difference between total cerium, titrated by potentiometry, and tetravalent cerium [Bk(IV)]a was calculated from the solvent beta counting, allowing for the measured distribution coefficient of Bk(IV) [Bk(III)] was determined by subtracting the [Bk(IV)]a value from the aqueous counting in all cases [Ce(III)]o and [Bk(III)]o were found to be negligible. 

The EDTA analog, N,N,N ,N -tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), forms stable complexes with lanthanides Ln(TPEN) as shown in the work [111]. The stability constant for Am complex of the similar composition was calculated from potentiometry titration in the work [33]. The [M(TPEN)] complex is the major and the [M(OH)] is the minor components from the... 

Octadentate anion DTPA forms stable complexes with Np in solution. In contrast to EDTA, the absorption band of [Np(DTPA)] with maximum at 983 nm is very broad and has low intensity. Based on the spectrophotometry stability constant logy5i=30.33 0.12 was calculated [47,74]. In the work [122], the similar value, logy5i=29.29 0.02, was obtained using the same technique. Ion exchange measurement yields the value logy5i=30.96 [53]. Potentiometry titration gives very similar value for Pu(lV), logy5i=29.49 0.10 [123]. 

The complexation of pentavalent neptunium with pyridinecarboxylic acids was investigated by different methods including spectrophotometry, potentiometry and solvent extraction [18,42,101,127-130]. Depending on the technique, complexation model used in calculations and experimental conditions, different values of stability constants were obtained (Table 8). 

Complexation of Np with NCS has been extensively studied using different experimental techniques. In the work [57], stability constants were calculated from spectrophotometry and potentiometry data for ionic strength 1=9 (Table 12). The stability constant logy0i=O.32 was obtained using solvent extraction technique 1=2 M) [60], Since the isothiocyanate ion is able to reduce Np(V), the special attention was given in this study to maintain the Np oxidation state. 

Construct a coulometric titration curve of 100.0 mL of a 1 M H2SO4 solution containing Fe(ll) titrated with Ce(lV) generated from 0.075 M Ce(lll). The titration is monitored by potentiometry. The initial amount of Fe(II) present is 0.05182 mmol. A constant current of 20.0 mA is used. Find the time corresponding to the equivalence point. Then, for about 10 values of time before the equivalence point, use the stoichiometry of the reaction to calculate the amount of Fe produced and the amount of Fe + remaining. Use the Nemst equation to find the system potential. Find the equivalence point potential in the usual manner for a redox titration. For about 10 times after the equivalence point, calculate the amount of Ce " produced from the electrolysis and the amount of Ce + remaining. Plot the curve of system potential versus electrolysis time. 

The pKa value is numerically equal to the pH value at which a molecule is half dissociated and the calculation or determination of the pK value(s) of a potential candidate drug should be one of the flrst physical properties determined. The approximate pK of the molecule of interest can be estimated by referring to literature values for substituents and similar molecules.A range of experimental techniques such as potentiometry, solubility determinations or ultraviolet-visible spectroscopy may be used for determination of the true pK value(s). 

The pKa values are also temperature-dependent, often in a nonlinear and unpredictable way. Samples measured by potentiometry are therefore held at a constant temperature bath and therefore, pK value should be quoted at a specific temperature. Often a temperature of 25°C is chosen to reflect room temperature whereas this may be quite different from the body temperature. Percent ionization at different temperatures can be calculated as ... 

The technique is generally unaffected by the state (ionic, imdissociated, sometimes complexed) of the analyte to be titrated. For example, the direct potentiometric determination of pH in a solution of a weak acid reports only the hydrogen ion concentration. Since the major portion of the acid is present in the undissociated form, direct potentiometry can not provide data yielding the total acid concentration. Potentiometric titration involves titrating the acid solution with a standard base, determining the equivalence point volume of standard base solution used, and calculating the total weak acid concentration from the stoichiometric data. 

If Uq is measured at a temperature different from 813 °C, we can calculate V applying the determined Q. It is not possible to measure all four components in water gas directly using gas potentiometry. But it should be possible theoretically on the basis of the Equations (25-33) to (25-35) to determine the two important parameters Vand Q by simple measurements at two temperatures. In order to determine K T has to be as far as possible from 1086 K (813 °C) and both measurements must be very exact. 

In the review of Baes and Mesmer [1976BAE/MES] an upper hmit of logj < -11.7 was calculated from the solubility data of [1964NAB/KUD] while Neck and Kim [2001NEC/KIM] selected an estimated value of logn, /3° = -(11 + 1) in their review. The present review does not recommend an equilibrium constant for the complex Th(OH)3 whieh is eonsidered to be a minor speeies in both potentiometrie and solubility studies. 

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