Transport Numbers Measurements

It is unclear at this time whether this difference is due to the different anions present in the non-haloaluminate ionic liquids or due to differences in the two types of transport number measurements. The apparent greater importance of the cation to the movement of charge demonstrated by the transport numbers (Table 3.6-7) is consistent with the observations made from the diffusion and conductivity data above. Indeed, these data taken in total may indicate that the cation tends to be the majority charge carrier for all ionic liquids, especially the allcylimidazoliums. However, a greater quantity of transport number measurements, performed on a wider variety of ionic liquids, will be needed to ascertain whether this is indeed the case. 

When the correction terms of formula (48) can indeed be neglected, the transport ratios obtained from B.I.P. measurements must be the same as those from transport number measurements, the membrane being placed in a solution of both ions. This is found to be so as long as ions of the same kind are concerned, e.g. for Na+ and K+. For a larger... 

The methods of measuring the velocity of electrokinetic motion are fully described in some of the reviews mentioned above. They include (for cataphoresis) various forms of U-tube in which the motion of the boundary of the suspension is observed, transference methods similar to Hittorf s transport number measurements in electrochemistry, and microscopic cells in which the motion of individual particles is watched, due allowance being made for the motion of the suspending fluid in the opposite direction to the particles. Sumner and Henry s device1 of fixing a sphere on a fibre and observing its deflexion in a horizontal electric field is very ingenious, and not so frequently mentioned as other methods. 

Even the elementary presentation given here makes it clear that transport-number measurements in fused salts are based on the transfer of the fused salt from the anode to the cathode compartment. The quantities measured are weight changes, the motion of indicator bubbles, the volume changes, etc. Some basic experimental setups shown in Fig. 5.42 include the apparatuses of Duke and Laity, Bloom, Hussey, and other pioneers in this field. 

The migration of the electrolyte from the anode to the cathode compartment can also be followed by using radioactive tracers and tracking their drift. Since isotopic analysis methods are sensitive to trace concentrations, there is no need to wait for the electrolyte migration to be large enough for visual detection. The results of some transport-number measurements are given in Table 5.29. 

The transport number calculated using the membrane potential (static state transport number) is, as mentioned above, lower than that measured by electrodialysis (dynamic state transport number) at the same concentrations. This is due to neglecting water transport through the membrane in the measurement by membrane potential. When water transport through the membrane is corrected for by the following equation, the transport numbers measured by both methods are almost the same.13,14... 

Values for the association of nickel with sulphate for temperatures from 25 to 95°C and ionic strengths to 2.6 M were measured using a variety of pH titration, conductivity and transport number measurement techniques. Although the reported association constants are in rough agreement with those from other studies, there are insufficient experimental details (or results) to use this work in deriving a recommended value for AT. ... 

It is, of course, necessary to test the hydrogen flux in a real membrane under realistic conditions. This can also give significant information on transport processes in the material. It can even be the only way to measure hydrogen transport in materials where, for example, electronic conductivity is so high that transport number measurements by the EMF method or other electrical methods are impossible. 

When dealing with all the various types of conducting media, the total conductivity is the quantity that is easiest to measure, if one wants to distinguish between the different contributions of the various charge carriers then this requires a set of complementary transport number measurements which are often more complex in experimental terms. 

The similarity in the ionic transport mechanism in organic liquid electrolytes and solid polymer electrolytes is reflected in the ionic transport numbers measured in the two media. Table 3.5 lists the transport numbers for Li in LiC104 solutions in propylene carbonate (PC) and propylene carbonate/dimethoxy ethane (PC/DME) mixtures [26]. The t+ in PC/LiC104 is 0.28 which increases to between 0.40 and 0.50 with the addition of DME. This increase in t+ in PC/DME mixtures may reflect a change in the solvation characteristics of Li, and/or ionic species present, with the addition of DME. It is then possible that a range of cation transference numbers between 0.2 and 0.6 measured in polymer electrolytes is a reflection of the coordination properties of the particular polymer host with Li" and the nature of the ionic species present. 

Figure 11. The experimental apparatus for dynamic transport number measurements.

The specific conductivities of concentrated solutions are by one order of magnitude higher than those of salts in water i 3-57 With increasing concentration the conductivity decreases and passes through a minimum for 0.04 M solutions. Transport number measurements on sodium solutions have shown that the equivalent conductance of the anion (the solvated electron) has a minimum value at 0.04 M, while the equivalent conductance of the metal ion decreases continuously with increase in concentration . The blue colour of dilute metal solutions is due to the short wave length tail of a broad absorption band with its peak at 15,000 A. The... 

The high conductivities of the solutions of alkali chlorides in nqiolten iodine monochloride would permit the assumption that the chloride ions possess abnormally high mobilities. This has been shown to occur in arsenic (III) chloride and antimony (III) chloridei s by the results of transport number measurements. A chain-conduction mechanism involving chloride ions may be in operation. 

The Ag+ ions move to the cathode, while the electrons move to the anode. Therefore in the Agl phase all the current is carried by Ag+ ions, and the transport number measured by deposition of silver at the cathode is unity. The excess sulphur liberated at the phase boundary AggS/AgI eventually reacts... 

Open circuit voltage of concentration cell for transport number measurements The equation above can be used to measure the average oxygen ion transport number, by measuring the open circuit voltage (OCV) over a sample exposed to a small, well-defined gradient in partial pressure of oxygen ... 

The determination of electrophoretic velocities may be carried out experimentally by the use of methods suitable for transport number measurements. Moving boundary techniques have proved useful despite the problem of a difficulty in selecting suitable indicator ions. Reliable estimates of electrophoretic velocities make possible the determination of zeta-potentials. Since colloids migrate at characteristic rates under the influence of an electric field, electrophoresis provides an important means of separation. Coatings, such as rubber or graphite, may be deposited on metal electrodes by this means and additives to these may be co-deposited. 

DC and AC conductivity analysis on the Mg(II) and Pb(II) electrolytes were carried out using non-blocking (Mg or Pb) and blocking electrodes. The Mg(II) electrolytes showed no evidence of Mg(II) motion and appear to be virtually pure anion conductors. The Pb(II) electrolytes appeared to be good conductors of Pb(II) as well as halide ions. An initial estimate of the transport number of Pb(II) in PbBr. (PE0)2q is 0.6-0.7 at 140 C. We must caution that these transport number measurements are preliminary estimates. It is a major undertaking to measure definitive transport numbers, and that work is not yet begun. 

The conductance of a pure electrolyte solution is (normally) the sum of two contributions, the conductance of the cations and the conductance of the (equivalent number of) anions. Transport number measurements separate these two effects and enable the individual contributions to be calculated. For this reason they have made a valuable contribution to electrolyte theory. For other properties of electrolytes, such as activities, this separation into ionic contributions is impossible the activity coefficient of an individual ion cannot be measured, a fact which causes difficulties in the definition of pH and similar concepts. 

The transport numbers, U and ta, of an electrolyte are defined as the fractions of the current carried, respectively, by the cation and anion. The calculation of these quantities from transport number measurements implies a knowledge of the nature of the ion carriers present, and this usually presents no difficulties. For a pure KCl solution, for instance, the ion carriers can confidently be stated to be and Cl ions, and the value of ta = 0.5102 for a 0.10 mol dm" solution of KCl at 25 shows that the fractions of current carried by and Cl" are 0.4898 and 0.5102, respectively. Combined with the measured molar conductivity of the salt at this concentration, A = 128.9012" cm the molar conductivity of the ion in this solution is 0.4898 x 128.90 = 63.14 and that of Cl" is 65.76 cm. ... 

Table 2.1 Values of Li transference number obtained by the Bruce-Vincent method for polymer electrolytes consisting of 1M LiCI04 in PEO (5 x 10 g/mol) and 10% w/w of corresponding filler. Transport numbers measured by AC technique with Sprensen-Jackobsen formula lay within the range 0.28-0.41, with the same order. For noncomposite electrolyte it is 0.16. Data from the author s own work...

---