Proton transfer conductance in aqueous solution. Part 1.—Conductance of concentrated aqueous alkali metal hydroxide solutions at elevated temperatures and pressures
01 Jan 1971-Transactions of The Faraday Society (The Royal Society of Chemistry)-Vol. 67, pp 132-148
TL;DR: In this paper, the electrical conductivity of aqueous solution of KOH, NaOH and LiOH was investigated and it was shown that with increasing concentration there is a transition in the primary mechanism of conductance in these solutions from the proton transfer mechanism to the hydrodynamic mechanism.
Abstract: Measurements of the electrical conductivity of aqueous solutions of KOH, NaOH and LiOH are presented within the ranges 25–200°C, 1–3000 atm and 0.1–6.68 molal. These indicate that with increasing concentration there is a transition in the primary mechanism of conductance in these solutions from the proton transfer mechanism to the hydrodynamic mechanism. In LiOH solutions, however, saturation occurs before this transition is established. It is suggested that at high concentrations most of the water molecules are dominated by their proximity to an ion and so cannot participate in the proton transfer mechanism of conductance by the hydroxyl ion. This mechanism is disrupted most by KOH and least by LiOH at a given concentration in excess of 1 molal, and this is related to the greater ionic association of the latter solute.
With increasing concentration of KOH the Walden product becomes more nearly independent of temperature and pressure. Data for the viscosity of water have been surveyed and a table covering the ranges 10–200° and 1–3000 kg cm–1 is presented. The pressure dependence of the conductance of these solutions is virtually independent of concentration to the unexpectedly high value of about 1 molal, but it alters markedly in the range 1–2 molal. The observations are discussed qualitatively.
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TL;DR: Voltage-gated proton channels represent a specific subset of proton channel that have voltage- and time-dependent gating like other ion channels, but differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion.
Abstract: Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H+ channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H+ch...
654 citations
TL;DR: The classical Grotthuss model has been recently questioned and new mechanisms and ideas regarding proton transfer are briefly discussed and some of the questions that need to be addressed in the near future are discussed.
Abstract: This review article is divided into three sections. In Section 1, a short biographical note on Freiherr von Grotthuss is followed by a detailed summary of the main findings and ideas present in his 1806 paper. Attempts to place Grotthuss contribution in the context of the science done at his time were also made. In Section 2, the modern version of the Grotthuss mechanism is reviewed. The classical Grotthuss model has been recently questioned and new mechanisms and ideas regarding proton transfer are briefly discussed. The last section discusses the significance of a classical Grotthuss mechanism for proton transfer in water chains inside protein cavities. This has been an interesting new twist in the ongoing history of the Grotthuss mechanism. A summary and discussion of what was learned from probably the simplest currently available experimental models of proton transfer in water wires in semi-synthetic ion channels are critically presented. This review ends discussing some of the questions that need to be addressed in the near future.
494 citations
TL;DR: The latest research progress related to electrically rechargeable Zn-air batteries is compiled, particularly new key findings in the last five years (2013-2018), and recommendations regarding the testing routines and materials design are provided.
Abstract: The century-old zinc-air (Zn-air) battery concept has been revived in the last decade due to its high theoretical energy density, environmental-friendliness, affordability, and safety. Particularly, electrically rechargeable Zn-air battery technologies are of great importance for bulk applications like electric vehicles, grid management, and portable electronic devices. Nevertheless, Zn-air batteries are still not competitive enough to realize widespread practical adoption because of issues in efficiency, durability, and cycle life. Here, following an introduction to the fundamentals and performance testing techniques, the latest research progress related to electrically rechargeable Zn-air batteries is compiled, particularly new key findings in the last five years (2013-2018). The strategies concerning the development of Zn and air electrodes are in focus. The design of other battery components, namely electrolytes and separators are also discussed. Poor performance of O2 electrocatalysts and the lack of the long-term stability of Zn electrodes and electrolytes remain major challenges. Finally, recommendations regarding the testing routines and materials design are provided. It is hoped that this up-to-date account will help to shape the future research activities toward the development of practical electrically rechargeable Zn-air batteries with extended lifetime and superior performance.
171 citations
TL;DR: It is proposed that the smaller g(H) in the RR dimer is the consequence of a different organization and dynamics of the H-bonded network of water molecules inside the pore of the channel, resulting in a slower proton transfer and multiple pore occupancy by protons.
Abstract: Proton conductivities in bulk solution ( λ H ) and single-channel proton conductances ( g H ) in two different stereoisomers of the dioxolane-linked gramicidin A channel (the SS and RR dimers) were measured in a wide range of bulk proton concentrations ([H], 0.1–8000mM). Proton mobilities ( μ H ) in water as well as in the SS and RR dimers were calculated from the conductivity data. In the concentration range of 0.1–2000mM, a straight line with a slope of 0.75 describes the log ( g H )-log ([H]) relationship in the SS dimer. At [H]>2000mM, saturation is followed by a decline in g H . The g H -[H] relationship in the SS dimer is qualitatively similar to the [H] dependence of λ H . However, the slope of the straight line in the log( λ H )-log([H]) plot is 0.96, indicating that the rate-limiting step for proton conduction through the SS dimer is not the diffusion of protons in bulk solution. The significant difference between the slopes of those linear relationships accounts for the faster decline of μ H as a function of [H] in the SS dimer in relation to bulk solution. In the high range of [H], saturation and decline of g H in the SS dimer can be accounted for by the significant decrease of μ H in bulk solution. At any given [H], g H in the RR dimer is significantly smaller than in the SS. Moreover, the g H -[H] relationship in the RR stereoisomer is qualitatively different from that in the SS. Between 1 and 50mM [H], g H can be fitted with an adsorption isotherm, suggesting the presence of a proton-binding site inside the pore (pK a ≈ 2), which limits proton exit from the channel. At 100 mM g H increases linearly with [H]. The distinctive shape of the g H -[H] relationship in the RR dimer suggests that the channel can be occupied simultaneously by more than one proton. At higher [H], the saturation and decline of g H in the RR dimer reflect the properties of μ H in bulk solution. In the entire range of [H], protons seem to cross the SS and RR channels via a Grotthuss-like mechanism. The rate-limiting step for proton transfer in the SS dimer is probably the membrane-channel/bulk solution interface. It is also proposed that the smaller g H in the RR dimer is the consequence of a different organization and dynamics of the H-bonded network of water molecules inside the pore of the channel, resulting in a slower proton transfer and multiple pore occupancy by protons.
119 citations
TL;DR: The limiting molar conductance Λ0 and ion association constants of dilute aqueous NaOH solutions were determined by electrical conductance measurements at temperatures from 100 to 600°C and pressures up to 300 MPa.
Abstract: The limiting molar conductances Λ0 and ion association constants of dilute aqueous NaOH solutions (<0.01 mol-kg−1) were determined by electrical conductance measurements at temperatures from 100 to 600°C and pressures up to 300 MPa. The limiting molar conductances of NaOH(aq) were found to increase with increasing temperature up to 300°C and with decreasing water density ρw. At temperatures ≥400°C, and densities between 0.6 to 0.8 g-cm−3, Λ0 is nearly temperature-independent but increases linearly with decreasing density, and then decreases at densities <0.6 g-cm−3. This phenomenon is largely due to the breakdown of the hydrogen-bonded, structure of water. The molal association constants K
Am
for NaOH(
aq
) increase with increasing temperature and decreasing density. The logarithm of the molal association constant can be represented as a function of temperature (Kelvin) and the logarithm of the density of water by
$$\begin{gathered} log K_{Am} = 2.477 - 951.53/T - (9.307 \hfill \\ - 3482.8/T)log \rho _{w } (25 - 600^\circ C) \hfill \\ \end{gathered} $$
which includes selected data taken from the literature, or by
$$\begin{gathered} log K_{Am} = 1.648 - 370.31/T - (13.215 \hfill \\ - 6300.5/T)log \rho _{w } (400 - 600^\circ C) \hfill \\ \end{gathered} $$
which is based solely on results from the present study over this temperature range (and to 300 MPa) where the measurements are most precise.
89 citations