Liquid water can be described as a dielectric [1] with the wide band gap ε g = 6.9   eV [2] where Fermi level ε F as an identifier of water oxidation-reduction potential (ORP) is easily varied by action of the galvanic cell with the strongly polarized one electrode and the other in the quasi-equilibrium with water solution. Such cell works as ORP changer of any aqueous solution by a forced shift of Fermi level ε F in the band gap at an expense of insignificant deviation ( | z | < 10 − 12 ) of water composition H 2 O 1 − z from the stoichiomentric one ( z = 0 ) [1]. This variation is occupied between two allowed local electron states in the band gap of liquid water such as an occupied-by-electron energy level ε OH of hydroxide anion OH⁻ and the vacant one ε H 3 O of hydroxonium ion H₃O⁺.

These electronic levels are located symmetrically nearby the band-gap middle with the energy difference between them of 1.75 eV [3]. This theoretical concept allows eliminating the inconsistencies [4] in reconciling the electrochemical properties of these well-known aqueous ions in the frame of electronic band structure. Then, Fermi level ε F as an electrochemical potential indicates the tendency of liquid water to donate or accept proton. If ε F is high, there is a strong tendency for liquid water to donate protons, i.e. it is reducing. Opposite, if ε F is low in the aqueous medium, there is the strong tendency for that to accept protons when this matter is oxidizing [1].

It is interesting to understand how galvanic modifying of seawater can be useful for a practical application of such approach. The solution to this problem is the subject of the given work.

We have shown in [3] that a weak aqueous solution of any atomic impurity i (at its little concentration) has a local electronic level ε i in the band gap.

At the same time, such inherent constituents of liquid water as ions of hydroxonium H₃O⁺ and hydroxide OH⁻ emerge there due to the self-dissociation of water molecules in the reversible chemical reaction [5]

2H 2 O ↔ H 3 O + + OH − . (1)

These water constituents have the electronic levels [3] ε H 3 O = − 5.575   eV and ε OH = − 7.325   eV located symmetrically nearby the middle of the band gap as shown in Figure 1(a). The electronic levels of sodium ε Na and chlorine ε Cl of their little impurities in liquid water are shown here too. They are obtained by Equations (1):

ε Na = ε SHE − e E Na − k B T ln ( [ Na 0 ] s / [ Na + ] ) = − 3.29   eV (2)

ε Cl = ε SHE − e E Cl + k B T ln ( [ Cl 0 ] / [ Cl − ] ) = − 7.7   eV (3)

where k B is Boltzmann constant equal to 8.617 × 10⁻⁵ eV/K; T is Kelvin temperature; e is the charge of electron; [ Na 0 ] s = 3.3 × 10 − 4 M is the solubility of neutral sodium atoms in liquid water [1] ; [ Na + ] is its ionic mole fraction and [ Cl − ] is the one of chlorine equal to 1.0 M; E Na = − 2.71   V is the standard ORP of sodium in the aqueous solution and E Cl = + 1.39   V is the one of chlorine [5] ; [ Cl 0 ] ≡ [ HClO ] = 0.09   M is the mole fraction of chlorine atoms in the aqueous solution at P Cl 2 = 1   atm , pH = 1 , T = 300   K [6].

The electronic levels in Equations (1) and (2) are obtained according to standard electrode reactions [5] :

Na + + e − ↔ Na   ( E Na = − 2.71   V ) , (4)

Cl 2 + 2 e − ↔ 2 Cl −   ( E Cl = + 1.39   V ) . (5)

The correspondent Fermi levels ε F ( 4 ) and ε F ( 5 ) are shown in Figure 1(b) and obtained from Equation

ε F ( n ) = ε SHE − e E ( n ) (6)

The Standard Hydrogen Electrode (SHE) is described by the electrode reaction [5]

2H 3 O + + 2 e − ↔ H 2 + 2 H 2 O   ( E SHE = 0   V ) (7)

with the mole fractions: [ H 3 O ] = K H 2 [ H 2 ]   ( P H 2 = 1   atm ) ~ 2 × 10 − 11 M and [ H 3 O + ] = 1.0   M [3]. Here K H 2 ~ 2 × 10 − 19 M is the constant of hydrated dissociation of hydrogen molecule in water up to H∙H₂O. Then in [3], we have obtained ε SHE = − 6.21   eV . These estimations agree with the experimental data [7] shown in Figure 2 for Total Electron Yield (TEY) of the X-ray Absorption (XA) by water with ε g = 7.0   eV and the vacant electron levels: ε H 3 O , ε Na , and ε c denoted by dotted lines where ε c is the bottom of conduction band. The electron yield of salt NaCl into the identification of ε H 3 O level is positive due to the synergetic effect of ions [ Na + ] and [ Cl − ] on separating radioactive pairs [ H 3 O + , OH − ] into basic and acidic micro-domains of the salt solution. The electron yield of acid HCl into the same identification is negative due to the inhibitory action of hydroxonium ions [ H 3 O + ] onto radioactive formation of pairs [ H 3 O + , OH − ] in the acidic solution.

The population [H₃O] and [OH] of the energy levels ε H 3 O and ε OH by electron and hole respectively can be defined by the proportions of the species concentrations [ H 3 O + ] / [ H 3 O ] and [ OH ] / [ OH − ] as shown in [3] [8]. They are given by Maxwell-Boltzmann distribution of electrons and holes in the corresponding energy levels [3] :

[ H 3 O + ] / [ H 3 O ] = exp [ ( ε H 3 O − ε F ) / k B T ] (8)

[ OH ] / [ OH − ] = exp [ ( ε OH − ε F ) / k B T ] (9)

Using Equations (8) and (9), we can transform the index

z = 0.009   M − 1 { [ H 3 O ] + [ H 3 O + ] − [ OH − ] − [ OH ] } (10)

of non-stoichiometric water H 2 O 1 − z saline by NaCl and electrically charged ( [ H 3 O + ] ≠ [ OH − ] ) to the form

z = 0.009   M − 1 K w { Δ ch + exp [ α ch + ( ε F − ε H 3 O ) / k B T ]           − exp [ − α ch + ( ε OH − ε F ) / k B T ] } (11)

where Δ ch = ( [ H 3 O + ] − [ OH − ] ) / K w , α ch = ln ( 1 + Δ ch 2 / 4 + Δ ch / 2 ) , and the dissociation constant K w = 10 − 14 M 2 at T = 300   K is defined by the known dissociation ratio [9]

[ H 3 O + ] ⋅ [ OH − ] = K w (12)

for the mole fractions of hydroxonium [ H 3 O + ] and hydroxide [ OH − ] ions.

As seen in Equation (11), the non-stoichiometric index z of seawater at Δ ch = 0 is controlled by ε F . Then, we can present Equation (11) in the form

z = 1.68 × 10 − 24 { exp [ 38.76 Δ F / eV ] − exp [ − 38.76 Δ F / eV ] } (13)

at k B T = 0.0258   eV , ε H 3 O = − 5.575   eV , ε OH = − 7.325   eV , and ε Fs = − 6.45   eV [3] for Δ F = ε F − ε Fs .

Then, Equation (13) allows plotting Fermi level in seawater as a function of its non-stoichiometry z shown in Figure 3 by the green dotted line for [ H 3 O + ] = [ OH − ] = 10 − 7 M , [ H 2 ] = 1.3 × 10 − 3 M , [ O 2 ] = 2.7 × 10 − 4 M , P H 2 = P O 2 = 1   atm , T = 300   K [10], [ H 3 O ] ~ 2 × 10 − 11 M , and [ OH ] ~ 8 × 10 − 13 M [3].

The electro-reduced seawater can be easily obtained by galvanic cell under the voltage of ~2 V between the strongly polarized anode and the quasi-equilibrium cathode. In this cell, the negative bulk charge is appeared near polarized electrode due to occupation of ε H 3 O by electron and joining of hydroxonium radicals [ H 3 O ] to the hydroxide anions [ OH − ] in forming hydrated electrons [ ( H 2 O ) 2 − ] .

Then, increasing content of different charges in seawater can block the formation of gas-hydrates there because these charges (as kinetic inhibitors of this process) shift the equilibrium curve of gas-hydration towards low temperatures.

At the same time, the negative bulk charge [ ( H 2 O ) 2 − ] keeps seawater in the stable hypo-stoichiometric state with high mole fraction of hydroxide anions ( [ OH − ] ~ [ H 3 O ] ≫ 10 − 7 M ) as proton acceptors.

Opposite, the hydroxonium radicals [ H 3 O ] as electron donors are very active antioxidants more effective than gaseous hydrogen.

It is shown that the region of non-stoichiometric composition of water ( H 2 O 1 − z ) is limited by the very narrow interval | z | < 10 − 14 where Fermi level is changed in the band gap up to 1.5 eV. At the strong polarization of cathode in the electrochemical cell, the high mole fraction of formed hydroxyls ( [ OH ] ≫ 10 − 7 M ) will essentially decrease content of ions in seawater by forming neutral species [ NaClO ] .

It is shown that the electron yield of NaCl into the identification of energy level ε H 3 O is positive due to the synergetic effect of ions [ Na + ] and [ Cl − ] on separating radioactive pairs [ H 3 O + , OH − ] into basic and acidic micro-domains of seawater. The electron yield of acid HCl into the same identification is negative due to inhibitive action of hydroxonium ions [ H 3 O + ] onto radioactive formation of pairs [ H 3 O + , OH − ] in the aqueous solution of acid.

The author thanks the Russian Foundation of Basic Research (RFBR) for supporting this work (grant #19-08-00149a) and appreciates his colleagues for active discussing all the aspects of galvanic modifying the chemical properties of seawater.

The author declares no conflicts of interest regarding the publication of this paper.

Shimkevich, A. (2019) On the Galvanic Modification of Seawater. World Journal of Condensed Matter Physics, 9, 112-121. https://doi.org/10.4236/wjcmp.2019.94009