Standard potential mixtures

It is possible to titrate two substances by the same titrant provided that the standard potentials of the substances being titrated, and their oxidation or reduction products, differ by about 0.2 V. Stepwise titration curves are obtained in the titration of mixtures or of substances having several oxidation states. Thus the titration of a solution containing Cr(VI), Fe(III) and V(V) by an acid titanium(III) chloride solution is an example of such a mixture in the first step Cr(VI) is reduced to Cr(III) and V(V) to V(IV) in the second step Fe(III) is reduced to Fe(II) in the third step V(IV) is reduced to V(III) chromium is evaluated by difference of the volumes of titrant used in the first and third steps. Another example is the titration of a mixture of Fe(II) and V(IV) sulphates with Ce(IV) sulphate in dilute sulphuric acid in the first step Fe(II) is oxidised to Fe(III) and in the second jump V(IV) is oxidised to V(V) the latter change is accelerated by heating the solution after oxidation of the Fe(II) ion is complete. The titration of a substance having several oxidation states is exemplified by the stepwise reduction by acid chromium(II) chloride of Cu(II) ion to the Cu(I) state and then to the metal. 

Studies of pzc in mixed solvents were also carried out by Blaszczyk etal n using the dipping method. They worked in mixtures offormamide and NMF and estimated the shift of the standard potential of the hydrogen electrode, of the surface dipole potential atHg, and of the liquid junction potential. 

In the chronopotentiogram of mixtures (see Fig. 3.58), the reactive components will yield different inflection points if their standard potentials show a sufficient difference (at least 0.1 V). To take a simple example, let us consider a reversible electrode process for both ox and ox2 in a solution of the supporting electrolyte. Then eqn. 3.72 is simply valid for the first reacting oxj with up to the first inflection point however, beyond this point the last traces of exj... 

Note that in all ion interaction approaches, the equation for mean activity coefficients can be split up to give equations for conventional single ion activity coefficients in mixtures, e.g., Eq. (6.1). The latter are strictly valid only when used in combinations that yield electroneutrality. Thus, while estimating medium effects on standard potentials, a combination of redox equilibria with H " + e 5112(g) is necessary (see Example 3). 

This process becomes much more probable if a composition is moistened, again pointing out the variety of problems that ean be created if water is added to a magnesium-eontaining mixture. The standard potential for the Cu " /Mg system is +2.72 volts, indicating a very spontaneous proeess. Therefore, Cu, Pb, and other readily-redueible metal ions must not be used in magnesium-containing compositions. 

The standard potential of the hydrogen electrode is taken as zero at all temperatures by convention [24, 25]. H does not refer to isotopically pure hydrogen H but to a mixture of Jh, and jH (deuterium, at the levels of natural... 

Several investigations have been made of the reduction of cobalt(II) to cobalt(O) in molten salt media. Eor a eutectic melt of LiCl-KCl at 450°C[10], a 1 1 NaCl-KCl melt at 450°C[11], and a MgCh-NaCl-KCl (50 30 20 mol%) mixture at 475 °C [12], the apparent standard potentials for the cobalt(II)-cobalt(0) couple have been deduced to be —1.207 V, — 1.277 V, and—1.046 V, respectively, each with respect to a chlorine-chloride ion reference electrode. 

Electrochemical studies of plutonium in NaCl-based melts are less common than that in LiCl—KCl mixtures. In a recent paper by Lambertin etal. the standard potentials for the Pu(III)/Pu(0) couple were determined from cyclic voltammetry data in equimolar NaCl—KCl and CaCl2... 

Table VIII. Values of the Standard Potentials (on the Molal Scale) and the Corresponding Variations of the Ion-Size Parameter, a0, with the Standard Deviations of a(EIJl)/mV for x = 10, 30, and 50 Mass Percent Monoglyme—Water Mixtures at 298.15°K...

As the potential is scanned from positive to negative, the reduction of Ox 1 takes place first. As the potential is made even more negative, Ox2 begins to be reduced, then Ox3, and so on. Thus, at the applied potential E, only Ox will be reduced, but at the more negative potential 2, simultaneous reduction of Ox and Ox2 will take place. In order to determine these two species separately, measurements at two potentials must be made. In order to do that, the two potentials have to be at least 180 mV apart. Given an electrochemical window of 2.5V, we can see that the maximum number of electroactive species that can be accommodated is not more than 13, provided that their standard potentials are equally spaced. In reality, the number of different species that can be selectively determined in a mixture by using the selection of the applied potential is 4-6 at most. Thus, the choice of applied potential offers only a very limited selectivity and is used only to complement other modes of selectivity. In the context of the equivalent electrical circuit (Fig. 7.8), this strategy would be represented by the same potential applied to all resistive channels... 

The standard potential of the silver-silver bromide electrode has been determined from emf measurements of cells with hydrogen electrodes and silver-silver bromide electrodes in solutions of hydrogen bromide in mixtures of water and N-methylacetamide (NMA). The mole fractions of NMA in the mixed solvents were 0.06, 0.15, 0.25, and 0.50, and the dielectric constants varied from 87 to 110 at 25°C. The molality of HBr covered the range 0.01-0.1 mol kg 1. Data for the mixed solvents were obtained at nine temperatures from 5° to 45°C. The results were used to derive the standard emf of the cell as well as the mean ionic activity coefficients and standard thermodynamic constants for HBr. The information obtained sheds some light on the nature of ion-ion and ion-solvent interactions in this system of high dielectric constant. 

The determination of the standard potential of the Ag-AgI electrode in these mixtures is based on Owen s method (10), which involves the use of the buffered cell (Cell 1) to prevent the air oxidation of I to I2. Using nearly identical molalities of HOAC, NaOAC, and KI and assuming that all electrolytes other than HOAC are completely dissociated, the emf of Cell 1 was measured in the various solvent mixtures, and the data... 

No comparison exists for the standard potentials in terf-butanol-water mixtures or in anhydrous tert-butanol. However, the trends in the data obtained here in terf-butanol are similar to the trends observed in this work in ethanol-water and in anhydrous ethanol. See Figures 1 and 2 in which the standard potentials E° for the silver-silver bromide elec-... 

Oxidation-Reduction Indicators.—A reversible oxidation-reduction indicator is a substance or, more correctly, an oxidation-reduction system, exhibiting different colors in the oxidized and reduced states, generally colored and colorless, respectively. Mixtures of the two states in different proportions, and hence corresponding to different oxidation-reduction potentials, will have different colors, or depths of color every color thus corresponds to a definite potential which depends on the standard potential of the system, and frequently on the hydrogen ion concentration of the solution. If a small amount of an indicator is placed in another oxidation-reduction system, the former, acting as a potential mediator, will come to an equilibrium in which its oxidation-reduction potential is the same as that of the system under examination. The potential of the given indicator can be estimated from its color in the solution, and hence the potential of the system under examination will have the same value. 

The shape of the curve for an oxidation-reduction titration depends on the nature of the system under consideration. The titration curve in Fig. 7 is symmetric about the equivalence point because the molar ratio of oxidant to reductant is equal to unity. An asymmetrical curve results if the ratio differs from this value. Solutions containing two oxidizing or reducing agents yield titration curves containing two inflection points if the standard potentials for the two species are different by more than approximately 0.2 V. Fig. 8 shows the titration curve for a mixture of iron(II) and titanium(III) with cerium(rV). The first additions of cerium are used by more readily oxidized titanium(III) ion, thus, the first step in the titration curve corresponds to titanium and the second to iron. 

The application of standard electrode potential data to many systems of interest in analytical chemistry is further complicated by association, dissociation, complex formation, and solvolysis equilibria involving the species that appear in the Nemst equation. These phenomena can be taken into account only if their existence is known and appropriate equilibrium constants are available. More often than not, neither of these requirements is met and significant discrepancies arise as a consequence. For example, the presence of 1 M hydrochloric acid in the iron(Il)/iron(llI) mixture we have just discussed leads to a measured potential of + 0.70 V in 1 M sulfuric acid, a potential of -I- 0.68 V is observed and in 2 M phosphoric acid, the potential is + 0.46 V. In each of these cases, the iron(II)/iron(III) activity ratio is larger because the complexes of iron(III) with chloride, sulfate, and phosphate ions are more stable than those of iron(II) thus, the ratio of the species concentrations, [Fe ]/[Fe ], in the Nemst equation is greater than unity and the measured potential is less than the standard potential. If fomnation constants for these complexes were available, it would be possible to make appropriate corrections. Unfortunately, such data are often not available, or, if they are, they are not very reliable. 

We can calculate the electrode potential of a system from standard potential data. Thus, for the reaction under consideration, the titration mixture is treated as if it were part of the hypothetical cell... 

Tomita et al. studied CO2 reduction at a Pt electrode in mixtures of H2O and AN under 1 atm CO2. Figure 3 shows the result of their constant current electrolysis of CO2 reduction at 5 mA cm". The major product is (COOH)2 in low water concentration region, and changes to HCOOH and H2 with the increase of water concentration. The electrode potential is as negative as -3.2 V vs. Fc/Fc (ca. -2.7 V vs. SHE), much more negative than the standard potential of CO2 /CO2. The electrode surface is fully... 

Photocatalytic oxidation of lactic acid (600) on platinized Ti02 or CdS is strongly regioselective (Scheme 6.295).1553 Irradiation in the presence of Pt/TiCE at 360 420 nm leads to the cleavage oxidation products acetaldehyde and carbon dioxide, whereas pyruvic acid is exclusively obtained by irradiation of the Pt—CdS mixture at 440 520 nm. Since the standard potential of oxidation of acetic acid is known to be more positive than the valence band edge of CdS (CdS has a less positive valence band edge than does Ti02), both catalysts readily oxidize aliphatic alcohols and the CdS photocatalyst is apparently capable of specific oxidation. 

The Standard Potential of Chlorine. Measurements of the potentials of galvanic cells without liquid junctions from which the standard potential of chlorine may be deduced have been made by Lewis and Ruppert40 who used, as one electrode, platinum over which a mixture of chlorine and nitrogen was bubbled, and, as reference, a calomel electrode and hydrochloric acid as electrolyte. The arrangement may be represented by... 

Table III. Standard Potentials of the Silver-Silver Chloride Electrode, at 25°, in Water-Dioxane Mixtures...

The quinhydrone electrode can be used for the potentiometric determination of pH. The solution to be measured is saturated with quinhydrone, an equimolar mixture of quinone (Q) and hydroquinone (HQ), and the potential of the solution is measured with a platinum electrode. The half-reaction and its standard potential are as follows ... 

Obviously, it is easier to make the calculations using the half-reaction we have the most information about in essence, the potential of that half-reaction must be calculated anyway during the calculation using the other half-reaction. The calculations illustrate that in a mixture, the concentrations of all species at equilibrium are such that the potential of each half-reaction is the same. Note that the potential will be close to the standard potential ( °) of the half-reaction in which there is an excess of the reactant in this case, there is an excess of Fe " ". 

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