Proton and deuteron mobility in normal and heavy water solutions of electrolytes

Abstract

Proton and deuteron mobility in normal and heavy water solutions of some alkali halides (NaCI, KCl, KBr and KI) and tetra-alkylammonium bromides (where alkyl is Me, Et, Pr or Bu) is measured polarographically and the diffusion coefficients calculated from a corrected IIkovic equation. The results are analysed in terms of the effect of electrolyte concentration on the excess or Grotthuss mobility of the hydrogen and deuterium ion. The results raise some questions about the accepted mechanism for the transport of the proton and deuteron in solution.Earlier studies 1-5 on proton mobility in aqueous electrolyte solutions using polarography gave useful information about the degree of hydration of the proton and the structure of water in the presence of electrolytes. In this paper we give a more detailed analysis as a result of further work on deuteron mobility in heavy water solutions of electrolytes. EXPERIMENTAL Apparatus, purification techniques and special procedures are described elsewhere. 1-5 As was noted previously4 it is important to purify all salts thoroughly before use. For example, different samples of A.R. KCI gave different results before purification and the most marked difference was observed in the case of the tetra-alkylammonium salts. After purification the order of the diffusion currents for the hydrogen ion in the presence of the tetra-alkylammonium salts was altered from that reported previou~ly,~ fig. l(e) and (f).However, the half-wave potentials remained unchanged. Tetramethylammonium bromide (Kodak) was recrystallised three times from distilled de-ionised water and dried under reduced pressure at 110°C for 1 week. Tetraethylammonium bromide (Kodak) and tetrapropyIammonium bromide (Kodak) were recrystallised three times from super-dry ethanol and methanol respectively, and dried under reduced pressure at 110°C. Tetrabutylammoniuin bromide (Kodak) was recrystallised five times from dry acetone, and dried under reduced pressure at 80°C. The purity of the recrystallised tetra-alkylammonium salts was determined by titration against silver nitrate using the Mohr method as percentage bromide and gave the following results, Me4NBr 100.00 %, Et4NBr 100.00 %, Pr4NBr 99.88 %, Bu4NBr 99.69 %. The alkali halides (A.R.) were recrystallised twice from distilled deionised water and dried under reduced pressure at 110°C for at least a week. In two previous publications 4-5 and in this work all salts were exhaustively purified as described above and the diffusion coefficients for the proton and deuteron so obtained were reproducible within 1 %.The DzO, 99.75 % by weight, was obtained from the Australian Institute of Nuclear Science and Engineering and was made 1 . 1 7~ mol dm3 in DCI by adding a small quantity of a solution of DCl in D 2 0 (Stohler Isotope Chemicals). The concentration of DCl was determined by potentiometric titration with sodium tetraboraie. The solution of DCl in D20 was de-aerated with high purity nitrogen and stored in a glass dispensing vessel under nitrogen. To make up the solutions a q...

Nanometer‐Scale Water‐ and Proton‐Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes

“…The enhanced proton mobility observed with 4-hydroxy-TEMPO in D 2 O compared to in H 2 O likely arises from a small fraction of 4-hydroxy-TEMPO probing the protons bound to sulfonic acid groups. The estimated D p for the proton near the sulfonic acid groups from the ξ ~270×10 −3 measured with 4-amino-TEMPO is ~ 2.8×10 −9 m 2 s −1 , which is a value comparable to D p ~2×10 −9 m 2 s −1 estimated from conductivity [29,34,35] Thus, the observation of an increased ξ for 4-amino-TEMPO and 4-hydroxy-TEMPO in D 2 O unequivocally demonstrates that a significant fraction of proton remains bound to the sulfonic acid groups, not readily exchanged by deuteron, and that these bound proton species diffuse > 5 times faster than water. The existence of tightly bound, yet highly mobile protons offer experimental support to the Grotthuss type mechanism for cooperative proton transport in Nafion membranes.…”

“…Therefore, if H + is replaced by D + , and incorporated into HDO or H 2 O, the measured ξ should have remained equal as in H 2 O or slightly decreased, as the mobility of HDO is nearly the same as H 2 O, [33] while the abundant D 2 O and D + diffuses 1.2-1.5 times slower than H + and H 2 O in bulk. [29,34,35] Thus, the observation of an increased ξ for 4-amino-TEMPO and 4-hydroxy-TEMPO in D 2 O unequivocally demonstrates that a significant fraction of proton remains bound to the sulfonic acid groups, not readily exchanged by deuteron, and that these bound proton species diffuse > 5 times faster than water. The existence of tightly bound, yet highly mobile protons offer experimental support to the Grotthuss type mechanism for cooperative proton transport in Nafion membranes.…”

Nuclear Quantum Effects in Water and Aqueous Systems: Experiment, Theory, and Current Challenges

“…For the solvation of large biomolecules (94,95), small molecules (96)(97)(98), and ions (99) proton transfer by a factor of ~3 (121,122). Experimentally proton diffusion has been observed to 14 be slowed by a factor of ~1.5 upon deuteration (123). Hence the excess proton in water diffuses ~4.1 times faster than water itself whereas D + diffuses faster by only a factor of ~3.6 in D 2 O.…”

Communication: Improved ab initio molecular dynamics by minimally biasing with experimental data

“…Excess protons migrate both through shuttling along hydrogen-bond connected water molecules (Grotthuss hopping) 12 and vehicular diffusion of a H 3 O + hydronium ion. Experimental values give a 4:1 ratio of excess proton to water diffusivities, 33,37 whereas AIMD simulations usually yield large ratios between 70:1 (BLYP) and 31:1 (BLYP-D2), 4,38 mostly due to the slow underlying water diffusion. AIMD simulations using the BLYP functional can give a reasonably accurate excess proton diffusivity of 0.45 Å 2 /ps vs. 0.93 Å 2 /ps in the experiment, but with an incorrect balance of transport mechanisms.…”