Emf methods

Two methods of providing cathodic protection for minimizing corrosion of metals are in use today. These are the sacrificial-anode method and the impressed-emf method. Both depend upon making the metal to be protected the cathode in the electrolyte involved. 

For obtaining internal or external mobilities, the corresponding transport numbers are usually measured. There are several methods for determining transport numbers in molten salts that is, the Kleimn method (countercurrent electromigration method or column method), the Hittorf method (disk method), the zone electromigration method (layer method), the emf method, and the moving boundary method. These are described in a comprehensive review. ... 

The transference or transport number of an ion can be determined by (i) the analytical method (ii) the moving boundary method and (iii) the emf method. The first two methods will be dealt with here, but the third will figure in a later section. 

Subasri R, Mathews T, Swaminathan K, and Sreedharan OM. Microwave assisted synthesis of Laj xSrxCr03 (x=0.05, 0.15 and 0.30) and their thermodynamic characterization by fluoride emf method. J. Alloys Compounds 2003 354 193-197. 

Bostrom D. (1988). Experimental studies of (Ni,Mg)—and (Co,Mg)—olivine solid solutions by single crystal X-ray diffraction and solid state emf method. Ph.D. thesis. Department of Inorganic Chemistry, University of Umea, Umea, Sweden. 

An EMF method has been used to study the interaction of ZnCl2 with chloride ion in methanol. It is found that Kx (7.76 x 103 M) is less than K2 (1.74 x 104 M), a finding which is interpreted in terms of passing from an octahedral [ZnCl(MeOH)5]+ species to a tetrahedral [ZnCl2(MeOH)2] complex.967 Related studies have shown that the solubility of cadmium halides in water decreases with increasing pressure.968 969 Anionic [ZnCl3] and [ZnCU]2-species are present in zinc chloride battery electrolyte, and are responsible for the observed negative transference numbers for zinc in aqueous acidic chloride medium.970 In neutral... 

Oil sample number Emf Method Ab TBN Cond Method Bb TBN Back Cal Method Cb TBN Direct Cal Method Db TAN AH... 

The composition of these complexes and their stability constants have been determined for a large number of metal ions primarily with the use of emf methods (200, 201). The free hydrogen ion concentration and in some cases the free metal ion concentration are determined as functions of the stoichiometric hydrogen ion and metal ion concentrations. From measurements on series of solutions of different concentrations the number of metal atoms in a complex and its charge can be derived, but no information is obtained on the number of water molecules in the complex. Since emf measurements are influenced by changes in activity factors they have usually been done in an inert ionic medium of high concentration (3 M NaC104) and at low metal ion concentrations. The major complexes formed, however, have been found to be stable also in the concentrated solutions needed for X-ray diffraction measurements, and the stability constants determined seem to be... 

Wartenberg et al. (1937) have determined the heat of formation of CaCr04 using a heat of neutralisation calorimeter and have reported the value - 49.75 kCal for A//. They have not reported the limits of error in their measurement. But there is agreement between their value and the value reported in the present work (Prasad Abraham, 1970) taking into account the error of 3000 calories, estimated for the results ofWartenberg. Subsequently, Jacob et al. (1992) re-estimated the value by EMF method and reported a value of 47.68 1.2 kCal. 

Figure 2 compares the variation of a with solvent composition for related surfactants as obtained from emf (12) and conductivity data. The emf results (at 308.15 K) have made use of the Botre Equation (39), the validity of which has been questioned (40,41). However, the main characteristics of these results are that they qualitatively present the same micelle behavior, i.e. an increase of a with the addition of acetone or propanol to water. The increase is much faster with propanol using the emf method than with the conductivity method and as Miyagishi apparently could not determine an a value above mole fraction 0.02 we suspect some electrode problems in this particular case. In the water -f-acetone mixtures the two methods yield similar results. Nevertheless, we believe that the conductivity method is more reliable than the emf method. Finally, Mathews et al. (13) found no change of a with addition of organic molecules in apparent contradistinction with the present results. However, these authors used scarcely soluble additives (e.g., CCL ) and Figure 2 shows that a changes slowly with the addition of acetone or propanol. 

Physical Evidence. Spectrophotometric and EMF methods have been used to show that Np(V) will react with a number of cations to give readily detectable equilibrium concentrations of binuclear complexes such as Np(V) U(VI), Np(V) Fe(III), and Np(V) Cr(III) (89). This last complex, probably CrONpO ", forms or dissociates at a relatively low rate, presumably owing to substitution inertness at Cr or Np. The same complex is also formed when Cr(II) and Np(VI) are mixed the complex is not the only product since Cr(H20)e, Np(IV), and Np(V) are formed as well. 

Using (7) the cell emf method and (2) the electrode-potential method, calculate the equilibrium constants for the following chemical reactions. (Use the values for standard electrode potentials in Table 12-1.)... 

Since the determination by theoretical methods of activities or activity coefficients for strong electrolytes is limited to very dilute solutions, experimental methods must be invoked to find y for m > 10 molal. We shall briefly describe some of the methods in use here the discussion is closely patterned after Section 3.13. The use of emf methods for the same purpose is described in Section 4.10, after proper background material has been developed. Once again, the choice of P = 1 bar serves as the standard pressure in this case the activity coefficient Pi introduced in Section 3.4 is to be used. Otherwise the quantity comes into use. 

Fuller and Newman [96] reported an elegant solution to the problem of obtaining electroosmotic drag coefficients ( transport numbers of water in their parlance) in ionomeric membranes under conditions of vapor phase equilibration. Their EMF method is based on the membrane potential which arises across a membrane sample exposed at each end to different water activities. The potential difference A is determined by (A) according to [96] ... 

Both activity scales are thermodynamically equally well defined. In constant ionic medium, activity (= concentration) can frequently be determined by means of emf methods. 

An operational definition endorsed by the International Union of Pure and Applied Chemistry (lUPAC) and based on the work of Bates determines pH relative to that of a standard buffer (where pH has been estimated in terms of p"H) from measurements on cells with liquid junctions the NBS (National Bureau of Standards) pH scale. This operational pH is not rigorously identical to p H defined in equation 30 because liquid junction potentials and single ion activities cannot be evaluated without nonthermodynamic assumptions. In dilute solutions of simple electrolytes (ionic strength, I < 0.1) the measured pH corresponds to within 0.02 to p H. Measurement of pH by emf methods is discussed in Chapter 8. 

The aqueous system H+ HV042 Mo042 has been investigated by EMF methods and NMR... 

Electron spin resonance. 353 Electrophoretic mobility effect of ionic strength on, 242 effect of particle size on, 247 EMF method, 84 Enthalpy of adsorption, 318-320 at constant cto, 320 EpHL method, 82 Equiadsorption point, 73 Equilibrium constant, 52, 588 Ethers, surface charging in the presence of, 10... 

Only a CCC-based IEP is reported in [3005]. The pH of minimum stability is termed the IEP in [3006]. References [3007-3009] report PZCs derived from AFM results. A nonstandard method (second-harmonic generation) was used in [651], and the purity of the surface was problematic. The inflection point-based PZC obtained in the presence of acetate is reported in [3010]. The EMF method was used in [3011] and the ISFET method in [3012,3013]. 

Huang, K., Xia, Y. and Liu, Q. (1994) Rapid determination of electronic conductivity in MgO-partially stabilized ZrO2 electrolytes using EMF method. Solid State Ionics, 73, 41-8. 

Tiwari, B.L. (1987) Thermodynamic properties of liquid Al-Mg alloys measured by the EMF method. Metall. Trans. A, 18, 1645-51. 

A summary of the extensive results on ionization constants obtained by Harned and associates in the Yale laboratories, using the emf method as just described, is given in Table II. 

For most equilibrium constant measurements the amount of a reactant or product present is found as a concentration, e.g. spectrophotometric or conductance analyses. However, pH and some emf methods determine the activity of a species directly rather than a concentration, and so corrections for non-ideality for these species will not be necessary. But, there are also some situations where, although the basic experimental measurement is an activity, subsequent calculations involve stoichiometric relations given in concentrations. Unless care is taken, the final equilibrium constant could end up involving terms in activities and concentrations, i.e. is mixed. Here corrections for non-ideality will still have to be made. Specific cases will make this clearer. 

There are three main techniques for measuring transport numbers emf methods and the Hittorf and moving boundary methods described in Sections 11.19.1 and 11.19.2 to 11.9.4. 

There are two main methods for determining transport numbers, both of which were developed early in the study of conductance. They are the Hittorf method and the moving boundary method. The emf method has been described in Section 9.21. 

EMF Method, The ionic product can also be obtained from emf measurements on voltaic ceils. The principle of the method is to prepare an alkaline solution of known hydroxide-ion concentration, and then to determine the hydrogen-ion concentration by dipping into the solution a hydrogen electrode and a reference electrode, then measuring the emf (Chapter 8). Instead of the hydrogen electrode one may, of course, use a glass electrode or any other electrode which responds to the hydrogen-ion concentration. 

Table A-47 log for the reaction Th" + HF(aq) ThF + studied using liquid-liquid extraction, and emf methods.

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