Soret potential

The rapid temperature change of the electrode perturbs the equilibrium at the electrode-solution interface and causes a change in the potential of the electrode measured with respect to a reference electrode. The change in the open-circuit potential, A t, and its relaxation with time are used to obtain kinetic information about the electrode reaction. A number of different phenomena come into play to cause the potential shift with temperature (e.g., temperature dependence of the double-layer capacitance and the Soret potential arising from the temperature gradient between the electrode and the bulk electrolyte), but the response can be treated by a general master equation (40) ... 

Another straightforward (non-ILIT) way to ealibrate the relevant ferri-ferrocyanide thermal response is to construct a nonisothermal cell in which the temperatures of the reference electrode and working electrodes are independently controlled. Then dE° /dT + dV j/dT is directly and easily obtained. The Soret potential (see Sec. V.E), which arises because of the temperature gradient in the solution, will exist between the working and reference electrodes and will depend only upon the difference in temperature of the two electrodes and not upon the shape of the gradient. Note that it is inappropriate to use the response of an isothermal cell in which the temperature of both electrodes are the same—the thermal junction potentials are eliminated but the response... 

The Soret effect is effectively a liquid junction potential produced by a temperature gradient in a homogeneous electrolyte. The activity coefiicients of ions are generally not identical consequently, a temperature gradient produces a driving force, which will be opposed by potential. The implications of the Soret effect in an ILIT perturbation were discussed by Smalley et al. [5], who showed that there is a linear relationship between the Soret potential, A Fsoret, and the temperature difference between the electrode surface and the bulk solution, AT, i.e. ... 

Ftj V Sum of potentials effected by temperature differences within the system including the Soret potential created by the temperature difference between the working and reference electrodes... 

The e.m.f. of a thermogalvanic cell is the result of four main effects (a) electrode temperature, (b) thermal liquid junction potential, (c) metallic thermocouple and (d) thermal diffusion gradient or Soret. 

LDMS is particularly well suited for the analysis of porphyrins.35-39 The heme molecule—a 22 rc-electron conjugated protoporphyrin system (Figure 8.1)—is an efficient photo-absorber in the visible and near UV (with an absorption maximum—the Soret band—near 400nm). This feature, concurrently with its low ionization potential, warrants that direct LDMS will possess extremely low limits for heme detection. The uses of IR or UV LDMS for structural characterization of natural porphyrins and their metabolites, synthetic monomeric porphyrins (e.g., used in photodynamic therapy), porphyrin polymers, and multimeric arrays, have been well documented.41148 In addition fast atom bombardment MS has been used to characterize purified hemozoin, isolated from the spleens and livers of Plasmodium yoelii infected mice.49... 

Fig. 22. UV-visible spectra of recombinant Rhodnius NPl-NO at pH 5.5 as a function of applied potential. In order of decreasing Soret band heights +200, —60, —70, —80, —90, —100, —110, and —400 mV vs Ag/AgCl, respectively (add 205 mV for potential vs NHE). Inset) Nernst plot of the spectroelectrochemicEil data. Reprinted with permission from Ref 49).

I. 4-methoxyacetophenone (30 //moles) was added as an internal standard. The reaction was stopped after 2 hours by partitioning the mixture between methylene chloride and saturated sodium bicarbonate solution. The aqueous layer was twice extracted with methylene chloride and the extracts combined. The products were analyzed by GC after acetylation with excess 1 1 acetic anhydride/pyridine for 24 hours at room temperature. The oxidations of anisyl alcohol, in the presence of veratryl alcohol or 1,4-dimethoxybenzene, were performed as indicated in Table III and IV in 6 ml of phosphate buffer (pH 3.0). Other conditions were the same as for the oxidation of veratryl alcohol described above. TDCSPPFeCl remaining after the reaction was estimated from its Soret band absorption before and after the reaction. For the decolorization of Poly B-411 (IV) by TDCSPPFeCl and mCPBA, 25 //moles of mCPBA were added to 25 ml 0.05% Poly B-411 containing 0.01 //moles TDCSPPFeCl, 25 //moles of manganese sulfate and 1.5 mmoles of lactic acid buffered at pH 4.5. The decolorization of Poly B-411 was followed by the decrease in absorption at 596 nm. For the electrochemical decolorization of Poly B-411 in the presence of veratryl alcohol, a two-compartment cell was used. A glassy carbon plate was used as the anode, a platinum plate as the auxiliary electrode, and a silver wire as the reference electrode. The potential was controlled at 0.900 V. Poly B-411 (50 ml, 0.005%) in pH 3 buffer was added to the anode compartment and pH 3 buffer was added to the cathode compartment to the same level. The decolorization of Poly B-411 was followed by the change in absorbance at 596 nm and the simultaneous oxidation of veratryl alcohol was followed at 310 nm. The same electrochemical apparatus was used for the decolorization of Poly B-411 adsorbed onto filter paper. Tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte when methylene chloride was the solvent. 

Jd is the mass current caused by gradients of the chemical potential p( = pA - Pb) and Jt is the mass current due to the Soret effect in an inhomogeneous temperature field T [76]... 

The undesirable additional potential differences produced by the temperature differences between both halfcells are the following The potential difference established by the temperature gradient in the conductor Ma, this component is described by the - Thomson effect, and the potential difference that is produced in the salt bridge that can be described by thermodiffusion (-> Soret effect). 

As is well known, metalloporphyrins exhibit intense Soret bands around 420 nm and weaker Q bands between 500 and 600 nm. These absorption bands are assigned to the tt-it excitation in origin [61]. Because these absorptions are observed in the visible region and their molar extinction coefficients are very large, metalloporphyrins are potentially useful as photosensitizers. In particular, zinc(II)... 

Simultaneous heat and mass transfer plays an important role in various physical, chemical, and biological processes hence, a vast amount of published research is available in the literature. Heat and mass transfer occurs in absorption, distillation extraction, drying, melting and crystallization, evaporation, and condensation. Mass flow due to the temperature gradient is known as the thermal diffusion or Soret effect. Heat flow due to the isothermal chemical potential gradient is known as the diffusion thermoeffect or the Dufour effect. The Dufour effect is characterized by the heat of transport, which represents the heat flow due to the diffusion of component / under isothermal conditions. Soret effect and Dufour effect represent the coupled phenomena between the vectorial flows of heat and mass. Since many chemical reactions within a biological cell produce or consume heat, local temperature gradients may contribute in the transport of materials across biomembranes. 

It has been shown recently that the mitochondrial electron transport system contains at least three different fe-type cytochromes 178). Two of these cytochromes are found in complex III, and under appropriate conditions are reducible with substrates. The third 6-type cytochrome was discovered by Davis et al. 178), and shown to fractionate exclusively into complex II. At 77°K, the cytochrome 6 of complex II exhibits a double a band at 557.5 and 550 nm, a prominent band at 531 nm, and a Soret band at 422 nm (Fig. 29). Cytochrome 6557.5 appears to have a low reduction potential. It is not detectably reduced by succinate in either complex II or respiratory particles, but its dithionite reduced form is rapidly oxidized by either fumarate or ubiquinone. The role of this cytochrome in mammalian mitochondria is not known. Davis et al. 178) have suggested that it might be an electron entry point for an unknown ancillary tributary of the respiratory chain. Further, Bruni and Racker 179) have shown that a preparation of cytochrome 6 is required for reconstitution of succinate-ubiquinone reductase activity (see below). 

Split-Soret Cytochrome. The split-Soret cytochrome (Ssc) and NapB proteins have no relationship except the fact that both contain a single stacked parallel diheme motif Ssc is a dimer of two identical 26.3 kDa subunits, which has so far been isolated only from the D. desulfuricans ATCC 27774, a species that can use sulfate or nitrate as terminal electron acceptors. Each monomer contains two c-type hemes with bis-histidinyl coordination and redox potentials of —168 and —330 mV. The name of this cytochrome derives from the fact that the reduced form displays a spht-Soret band with maxima at 420 mn and a shoulder at415mn. 

The bleaching of P-700 upon illumination corresponds to the production of an oxidized species that has been proposed to be a chlorophyll a dimer. The same bleaching can be obtained by chemical oxidation and titrates with a midpoint potential of about 0.45 V [49,50). The total differential spectrum of P-700 photooxidation presents, in addition to a major band at about 700 nm, a minor band at 685 nm and another in the Soret region at 435 nm. This spectrum has been attributed to chlorophyll a [51], although the presence of a new chlorophyll species in P-700-enriched preparations has been claimed [52]. 

In general, the diffusive mass flux is composed of diffusion due to concentration gradients (chemical potential gradients), diffusion due to thermal effects (Soret diffusion) and diffusion due to pressure and external forces. It is possible to include the full multicomponent model for concentration gradient driven diffusion (Taylor and Krishna, 1993 Bird, 1998). In most cases, in the absence of external forces, it is... 

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