Use of concentration cells

These are cells where there are two chemically identical electrodes with the solutions around the electrodes at different concentrations. The electrode compartments must be kept physically separate generally by means of a salt bridge. The net reaction is the equivalent of transferring the solute from one electrode compartment to the other. This is effected by means of the electrode reactions and not by any actual transfer. The following problem illustrates this. 

What is the overall reaction and the expression for the emf for each of the following cells  

Calculate the emf of the second cell, ignoring non-ideality. Indicate how non-ideality can be taken into account. 

Pay particular attention there is no AG for a concentration cell because the /r s refer to the same species in solution and cancel out. 

Since there are no equilibria involved in either electrode compartment the actual ionic strength is found direcdy from the concentrations, and the mean activity coefficients for each of the electrode compartment solutions can be calculated from the Debye-Hiickel equation. 

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The Determination of Transference Numbers from the Potentials of Concentration Cells. Another use of concentration cells, which involves the principles already discussed in this chapter, is that of the determination of transference numbers. Since a cell without liquid junctions of the type... 

USE OF CONCENTRATION CELLS WITH AND WITHOUT LIQUID JUNCnONS... 

Use of concentration cells with and without liquid junctions in the determination of transport numbers... 

The EOD coefficient ( drag) is defined as the ratio of the flux of water through the membrane to the flux of protons in the absence of a concentration gradient of water [224]. EOD increases with current density and often exceeds the ability of the membrane to redistribute water by back diffusion. A hydrodynamic model for electroosmosis has been developed [225] which treats ions as spherical particles moving in a continuous viscous medium. The model describes the variation of the EOD within polystyrene-based model membranes quite well. Several methods have been utilized to measure EOD coefficients including streaming potential measurements [226,227], the use of concentration cells [224,228], water flux measurements [191], DMFC experiments [229,230], and NMR spectroscopy [231]. Plots of n rag values for selected membranes are shown in Fig. 30. 

The loss of HGPRT and APRT activities by freezing and thawing of the cells has been dealt with previously and can be prevented by the use of concentrated cell suspensions and the addition of PRPP (1). The presence of nucleotidase activity in the cell extracts causes the rapid degradation of the nucleotides, produced in the assay, to their corresponding nucleosides. 

The concentration cell techniques involve the use of concentration cells. Bouridah et aL (1986) measured the electromotive force (EMF) of suitable cells under conditions where no current was passed, and obtained the anion transference number from... 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

The horizontal dispersion of a plume has been modeled by the use of expanding cells well mixed vertically, with the chemistry calculated for each cell (31). The resulting simulation of transformation of NO to NO2 in a power plant plume by infusion of atmospheric ozone is a peaked distribution of NO2 that resembles a plume of the primary pollutants, SO2 and NO. The ozone distribution shows depletion across the plume, with maximum depletion in the center at 20 min travel time from the source, but relatively uniform ozone concentrations back to initial levels at travel distances 1 h from the source. 

The extent of ion permselectivity displayed by a membrane can be expressed quantitatively by the transference numbers [88] for cations (t+) and anions (t ) within the membrane. Transference numbers can be determined potentiometrically by using a concentration cell [88], in which the membrane to be evaluated separates two electrolyte solutions that contain the same salt but at different concentrations. For a 1 1 salt, the membrane potential (E ,) is given by... 

The simplest and most sensitive assays for detecting clastogenic (i.e. chromosomal breaking) effects involve the use of mammalian cells. Cultures of established cell lines (e.g. Chinese hamster ovary) as well as primary cell cultures (e.g. human l)nnphocyte) may be used. After exposure to a range of chemical concentrations in the presence and absence of an appropriate metabolic activation system, the cell cultures are treated with a spindle inhibitor (e.g. vinblastine) to accumulate cells in a metaphaselike stage of mitosis. Cells are harvested at appropriate times and chromosome preparations are made, stained with DNA-specific dye and the metaphase cells are analysed under the microscope for chromosome abnormalities. 

A similar approach is the use of diffusion cells. In this case, the liquid is held in a container that has a capillary of fixed length and diameter through which the vapor over the liquid diffuses. The vapor exiting the capillary is swept into a flow of gas to provide the gas mixture this approach has been used to prepare mixtures of terpenes in air, for example (Larsen et al., 1997). The concentration of the gas can be varied by using capillaries of varying internal diameter and length. 

The level of enzyme needed can influence the choice of preparation used for the study. Microsomal preparations from cell cultures allow the use of higher concentrations of active enzyme per unit volume than use of whole cells or cell lysates. The use of whole, viable cells allows the use of longer incubation times but at a lower enzyme concentration per unit volume. In addition, adequate oxygen transfer and nutrient concentrations are needed to maintain culture viability. These requirements impose limitations on cell concentration. In addition, microsomes cannot be efficiently prepared from all cultured cell types. We have found that standard microsome preparation procedures as used for human or rodent liver were unsuitable for isolating active enzymes from human lymphoblasts, and this appears to be a general property of cultured cell lines. Specific catalytic activities in microsomes were lower than for whole cell lysates. This loss of activity appears to happen in other mammalian cell systems which has led to the common use of whole cell lysates.With human lymphoblasts, shortening the length of... 

One important application of the Nernst equation is the measurement of pH (and, through pH, acidity constants). The pH of a solution can be measured electrochemically with a device called a pH meter. The technique makes use of a cell in which one electrode is sensitive to the H30+ concentration and the second electrode serves as a reference. An electrode sensitive to the concentration of a particular ion is called an ion-selective electrode. One combination is a hydrogen electrode connected through a salt bridge to a calomel electrode. The reduction half-reaction for the calomel electrode is... 

The first estimate of the transference number of the electron was contributed by C. A. Kraus in 1914 (25). Using the concentration cell,... 

Similar approaches are used for most steady-state measurement techniques developed for mixed ionic-electronic conductors (see -> conductors and -> conducting solids). These include the measurements of concentration-cell - electromotive force, experiments with ion- or electron-blocking electrodes, determination of - electrolytic permeability, and various combined techniques [ii-vii]. In all cases, the results may be affected by electrode polarization this influence should be avoided optimizing experimental procedures and/or taken into account via appropriate modeling. See also -> Wagner equation, -> Hebb-Wagner method, and -> ambipolar conductivity. 

A variation on the whole-cell biotransformation theme is the use of permeabilized cells. Whole cells can be rendered permeable to small molecules yet remain essentially intact by contacting them for a short time with low concentrations of solvents. This process has the effect of making holes in the cellular membrane while leaving enough of the cell membrane and cell wall intact to still contain the enzymes and other macromolecules. The permeabilized cells can then be treated much like immobilized enzymes. This technique is especially useful when transport issues are found to be limiting the reaction. 

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