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Response to Reply to Comment on Can Water Store Charge?
In their response to our comment1 on their article “Can Water Store Charge?”,2 Ovchinnikova and Pollack (O&P) argue that Corti and Colussi’s (C&C) considerations apply to O&P’s experiments in an electrolysis cell filled with 10 mM NaCl and, therefore, could not be used to dismiss those performed in ultrapure water (“Type 1 by ASTM D1193-06 standard”, as quoted in ref 2). C&C’s analysis only requires freely migrating ions in the electrolyte filling the cell. In fact, at variance with O&P’s claim, the “ultrapure water” used in their experiments grossly fails to meet the resistivity standard: F = 18 MΩ cm at 298 K. The electrical current, i, that should have circulated through O&P’s electrolysis cell consisting of 1.0 cm 1.0 cm electrodes separated by 5.4 cm of “Type 1 ASTM ultrapure water” connected to a 4 V dc power source is (from Ohm’s law) 41 nA rather than the i ≈ 50 μA value reported by O&P (lower curve of Figure 3 in ref 2). Thus, all of O&P’s experiments involved more or less dilute electrolyte solutions. The higher conductivity of O&P’s ultrapure water was likely due to the HCO3-/Hþ introduced by the dissolution of atmospheric CO2(g), as we previously suggested.1 Let us calculate how long it should have taken to ‘store charge’ in O&P’s cell. The cell, dealt with as a capacitor of capacitance
C = εε0 (area/separation) = 1.3 10-12 F, in series with a resistance of R = 4 V/(5 10-5 A) = 8 104 Ω, should have been charged with a transient current, given by eq 1, iðtÞ ¼
ð1Þ
in less than a microsecond: τ = RC = 104 ns. The circulation of a steady current after 10 min implies that the cell behaves as a parallel RC circuit, in which the capacitor is instantly charged but current keeps flowing indefinitely through an electrolyte of resistance R. As noted above, the steady i ≈ 50 μA current measured by O&P is, however, ∼103 times larger than expected. Admittedly, some charge is capacitively stored in the cell: Q = CV = 5.4 10-17 F or ∼1 elementary charge per 5.6 1015 water molecules. Horacio R. Corti* Departamento de Fı´sica de Materia Condensada, Centro At omico Constituyentes, Comisi on Nacional de Energı´a At omica (CNEA), and INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, (1428), Buenos Aires, Argentina Agustin J. Colussi Environmental Science and Engineering, California Institute of Technology, Pasadena, California 91125
(1) Corti, H. R.; Colussi, A. J. Langmuir 2009, 25, 6587–6589. (2) Ovchinnikova, K.; Pollack, G. H. Langmuir 2009, 25, 542–547.
Langmuir 2009, 25(18), 11203–11203
V expð -t=τÞ R
Published on Web 08/20/2009
Received June 5, 2009
DOI: 10.1021/la902030p
11203
