Electro osmosis in the system ion‐exchange membrane — sulphuric acid solution

Abstract

Electro osmotic phenomena in NAFION-120 or MRF-26 sulfonic acid membrane bisecting the same H,SO, solution were investigated. It was found that the transference numbers of hydrogen ions, tH in the NAFION membrane remain nearly constant and close to one even at a concentration of 1.8 mol/kg H,O of the external H,SO, solution, and in MRF-26 membranewhen the concentration reaches 1.5 mol/kg H,O. The transport numbers of water and consequently the volume electro osmotic flow in NAFION membrane change with increasing H,SO, concentration in a similar way as fH, however, in MRF-26 membrane the changes are remarkable in the whole concentration range. ZUSAMMENFASSUNG: Es wurden die elektroosmotischen Erscheinungen in NAFION-120 und MRF-26 Sulfonsaure-Membranen, die dieselbe H,SO,-Lbsung abtrennen, untersucht. Die Uberfuhrungszahlen von Wasserstoffionen (FH) in den Membranen sind konstant und nahe 1 in der NAFION-Membran bis zu einer Konzentration von 1,8 mol/kg H,O in der H,SO,-Losung und in der MRF-Membran bis 1,5 mol/kg H,O. Die Massenuberfuhrungszahlen von Wasser und die elektroosmotischen Volumenstromungen in der NAFION-Membran andern sich mit zunehmender H,SO,-Konzentration ahnlich wie die TH Zahlen. Im Fall der MRF-Membran treten dagegen Anderungen in dem ganzen Konzentrationsbereich auf.

Unidimensional Approximation of the Diffuse Electrical Layer in the Inner Volume of Solid Electrolyte Grains in the Absence of Background Ions

“…Our theory of autodissociative double layers began as an attempt of explaining the catalytic interaction between solid acid catalysts and solid cellulose, and it successfully explained a number of previously known experimental facts . The theory keeps gaining supportive evidence: In the present paper it is shown that generation of negative charge on pore walls of a sulfonic ionomer membranes due to dissociation of the acid groups can adequately (even semi‐quantitatively) explain both high cation selectivity of sulfonic ionomer membranes, and decline of this selectivity at high concentration of a strong acid background solution …”

“…Coincidentally, the predicted proton concentration at the strongly acidic pore wall in thin pores is 3–4 M (Figure a, pore radius 1 nm, pK 1) is comparable to the concentration of H 2 SO 4 in solution (2 M) at which this proton selectivity decrease occurs in Nafion, which is similarly narrow‐pored . Proton selectivity in MRF‐26 begins deteriorating at a lower H 2 SO 4 concentration (1.5 M).…”

“…[20] The theory keeps gaining supportive evidence: In the present paper it is shown that generation of negative charge on pore walls of a sulfonic ionomer membranes due to dissociation of the acid groups can adequately (even semi-quantitatively) explain both high cation selectivity of sulfonic ionomer membranes, and decline of this selectivity at high concentration of a strong acid background solution. [36] While our model assumes complete absence of the background electrolyte, its predictions can be expected to be approximately valid for the case where the background electrolyte concentration is very low (i. e., lower than the predicted near-surface proton concentration by at least an order of magnitude). It would be interesting to compare the model predictions to experimental data from literature, but there are not many known experiments with appropriate design for such comparison.…”

“…In our opinion, practical evidence of autodissociative EDL formation in pores of a solid electrolyte can be easily observed in electroosmotic phenomena. In electroosmosis of sulfuric acid solutions through porous Nafion-120® and MRF-26 membranes bearing sulfonic groups, the proton transference number very close to unity can be observed, and this number begins decreasing rapidly when H 2 SO 4 concentration in the solution exceeds 1.5-2 M. [36] There are two facets to this phenomenon. First, the fact that the electroosmosis occurs indicates directly that there exists electrically charged liquid inside the pores (i. e., the existence of an EDL).…”

“…Within our autodissociative EDL theory, this negative charge arises from dissociation of the sulfonic groups at the pore surface. However, this selectivity deteriorates at higher H 2 SO 4 concentration in the solution, [36] and a possible explanation is that at high concentration this dissolved acid begins to suppress the dissociation of the solid acid according to the Le Chatelier principle. The resulting decrease of the magnitude of negative charge at the pore surface leads to less likely electrostatic rejection of anions by the pores, and the proton transference number begins decreasing.…”