Coupling concentration reductions

Membrane depolarization typically results from an increase in Na+ conductance. In addition, mobilization of intracellular Ca2+ from the endoplasmic or sarcoplasmic reticulum and the influx of extracellular Ca2+ appear to be elicited by ACh acting on muscarinic receptors (see Ch. 22). The resulting increase in intracellular free Ca2+ is involved in activation of contractile, metabolic and secretory events. Stimulation of muscarinic receptors has been linked to changes in cyclic nucleotide concentrations. Reductions in cAMP concentrations and increases in cGMP concentrations are typical responses (see Ch. 21). These cyclic nucleotides may facilitate contraction or relaxation, depending on the particular tissue. Inhibitory responses also are associated with membrane hyperpolarization, and this is a consequence of an increased K+ conductance. Increases in K+ conductance may be mediated by a direct receptor linkage to a K+ channel or by increases in intracellular Ca2+, which in turn activate K+ channels. Mechanisms by which muscarinic receptors couple to multiple cellular responses are considered later. 

All redox titration curves we have discussed here are independent of the total analytical concentration C of the redox couple. (This is not always the case in the Cr2072 / Cr 3+couple the reduction of one Cr2072- generates two Cr3+ions, which leads to a concentration-dependent redox titration curve.) Therefore, the above expressions precisely give Cs/ln (10) times the first derivative of the progress curve of the corresponding redox titration. You can convince yourself that this is so in exercise 5.10-1. 

Exogenous maltosyl bovine serum albumine at two concentrations. MBSA (Sigma Chemical lot 98F8120) is prepared from bovine albumin and maltose coupled via reductive amination.8 One mole of albumin contains 14 moles of disaccharide linked to lysine residues. The sugar content represents 5% of the molecule. The MBSA molecular weight and p/ are 71 kDa and 4, respectively. It was used without further purification. 

Ce02 host is substituted with either Sm or Gd (Cei-jcSm Oz Sj CSO, and Cei Gd 02 5, CGO), creating significant vacancy concentrations. Use of these ceria-based materials is limited by the redox characteristics of the Ce3+/4+ couple, with reduction occurring at temperatures above about 650 °C leading to a reduction of the ionic transport number. This in turn can lead to short circuits within the cell and hence a loss of performance. However, as conductivity in ceria-based compounds is sufficient at temperatures below 650 °C for fuel cell electrolytes, the issue is then one of suitably active cathodes, addressed in Section 2.1.3. 

Fig. 2.16 (a) In situ FT-IR study of methane oxidation in [C4mPy][NTf2] (b) plots of methane anodic currents at 0.9 V apphed vs. methane concentration and (c) plots of normalized oxygen reduction currents at -1.2 V vs. oxygen concentration, (d) Scheme of methane oxidation and coupled oxygen reduction in [C4mPy][NTf2]... 

If the initial concentration of Cu + is 1.00 X 10 M, for example, then the cathode s potential must be more negative than -1-0.105 V versus the SHE (-0.139 V versus the SCE) to achieve a quantitative reduction of Cu + to Cu. Note that at this potential H3O+ is not reduced to H2, maintaining a 100% current efficiency. Many of the published procedures for the controlled-potential coulometric analysis of Cu + call for potentials that are more negative than that shown for the reduction of H3O+ in Figure 11.21. Such potentials can be used, however, because the slow kinetics for reducing H3O+ results in a significant overpotential that shifts the potential of the H3O+/H2 redox couple to more negative potentials. 

The reduction potentials for the actinide elements ate shown in Figure 5 (12—14,17,20). These ate formal potentials, defined as the measured potentials corrected to unit concentration of the substances entering into the reactions they ate based on the hydrogen-ion-hydrogen couple taken as zero volts no corrections ate made for activity coefficients. The measured potentials were estabhshed by cell, equihbrium, and heat of reaction determinations. The potentials for acid solution were generally measured in 1 Af perchloric acid and for alkaline solution in 1 Af sodium hydroxide. Estimated values ate given in parentheses. 

Nitrite can be deterrnined by reaction with sulfanilamide to form the diazo compound, which couples with /V-(1-naphthyl)ethylenediamine dihydrochloride to form an intensely colored red azo dye. Nitrate can be deterrnined in a similar manner after reduction to nitrite. Suitable reducing agents are cadmium filings or hydrazine. This method is useful at a nitrogen concentration of 10 -lO " M. 

Now at some pH comparable to pK, two waves are observed, corresponding to the reduction of both HA and A. The currents are proportional to the concentrations of the electroreducible species. Because the pH and pK are known, the concentrations of HA and A in the bulk solution can be calculated. It is then found that the observed polarographic currents cannot be accounted for on tbe basis of the known bulk concentrations. It is concluded that the ratio of the concentrations at the electrode surface is different from the ratio of bulk concentrations, and this is a consequence of the coupling between the chemical and electrode processes. In the pyruvic acid system, HA can be converted to the hydroxy acid by the electrode... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

Formation of azo-type products might be troublesome. These by-products, arising from reduction of aromatic nitro compounds, usually are assumed to be derived from the coupling of intermediate nitroso and hydroxylamine compounds. The coupling problem is accentuated in reduction of nitroso compounds because of much higher concentrations. It can be alleviated by dropwise addition of the substrate to the hydrogenation and use of acidic media. 

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