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Effects of buffers on hydrogen evolution at iron electrodes
SHORT COMMUNICATION EFFECTS
OF BUFFERS ON HYDROGEN IRON ELECTRODES
EVOLUTION
AT
T. HURLEN, S. GUNVALDSEN and F. BLAKER Department of Chemistry, University of Oslo, Blindern, Norway (Received 14 December
1983)
In unbuffered aqueous solution, the hydrogen evolution at iron electrodes is usually first order in hydrogen-ion dependence at pH below about 7 and pH independent at higher pHC1-41. This indicates that hydrogen ions and water molecules are the main electroactive reactants for this reaction in acid and alkaline solution, respectively. In buffered solution, however, also the acid component of the buffer might be such a reactant. Information about this is often needed in corrosion research, but appears rather hidden in the literature. Some pertinent studies have therefore been made and are briefly described below. Figures 14 show results of cathodic polarization measurements on high-purity iron electrodes in deoxygenated solutions of of various salt and buffer
ElsceIV
Fig. 2. Cathodic polarization curves for iron electrodes in deoxygenated solutions of x M NaHCO, -t (x/10) M Na2C03 + (1 - x) M KC1 (pH = 9.5) at 25°C. Values of x at the curves. No stirring.
E (see) /V
Fig. 1. Cathodic polarization nerves for iron electrodes in deoxygenated solutions of x M HAc + x M NaAc + (1 - x) M NaCl (pH = 4.7) at 25°C. Values of x at the curves. No stirring.
content at 25°C. Most of these results, Figs l-3, are cathodic sweep (1 mV s-l) polarization curves, starting at the stationary open-circuit corrosion potential of the iron electrode after grinding, etching and about 1 h conditioning of it in the test solution. Other results (Fig. 4) are stationary polarization points (partly Wagner-Traud corrected) purely for the hydrogen evolution reaction at such iron electrodes. All solutions were made from A.R. quality acids and salts and twice distilled water. In acetate buffered acid solution (Fig. l), a buffercontent dependent and stirring dependent transition occurs between two different Tafel lines. The Tafel line obeyed at low current densities coincides with the one 1163
1164
T. HURLEN,S.GUNVALDSENAND
F. BLAKER
2_ \
\
3_
L_
I
5_
I
-1.0 E bce)/V
-0.9 ElscellV
I
-0.8
Fig. 3. Cathodic polarization curves for iron electrodes in deoxygenated solutions of x M NH&I +x M NH3 + (1 -x) M KC1 (pH = 9.7) at 25°C. Values of x at the curves. No stirring.
Fig. 4. Stationary polarization data for hydrogen evolution at iron electrodes in 0.5 M H3BOx + 0.1 M KOH (PH = 8.1) at 25°C. Dashed line represents hydrogen evolution from solvent water. No stirring.
for hydrogen evolution from hydrogen ions in unbuffered solution of pH 4.7[1-4]. The Tafel line obeyed at high current densities coincides with the one for hydrogen evolution from solvent water in unbuffered salt solution. The current for transition between the lines fits with estimations assuming a limiting diffusion of proton-delivering acetic acid to the electrode. Hence, no sign appears of any electroactive participation by acetic acid, but only of its concentration and transport limited ability of preserving the pH at the electrode. In buffered alkaline solution (Figs 24), the rate of hydrogen evolution increases with the buffer concentration when this exceeds a certain limit. This indicates that there is a direct electroactive participation by the acid component of the buffer, and that this participation exceeds the one by the solvent water when the buffer concentration (or the concentration of its acid component) exceeds the limit noted. In the present
cases, this limit is largely around IO-’ M. It may be that also acetic acid behaves somewhat like this, but that this be hidden by the overwhelming participation of hydrogen ions in the acid solutions used in testing the acetate buffer (Fig. I).
REFERENCES M. Stern, J. electrochem. SIX. 102, 609 (1955). T. Hurlen, Acra them. wand. 14, 1533 (1960); Ekctrochim. Acta 8, 609 (1963). K. J. Vetter, Elec~orAemL~l Kinetics. Academic Press, New York (1967). A. J. Appleby, H. Kita, M. Chemla and G. Bronotil, in Encyrlopedia of Elerrrochemistry of the Elements (Edited by A. J. Bard), Vol IX, Part A, p. 383, Dekker, New York (1982).
OF BUFFERS ON HYDROGEN IRON ELECTRODES
EVOLUTION
AT
T. HURLEN, S. GUNVALDSEN and F. BLAKER Department of Chemistry, University of Oslo, Blindern, Norway (Received 14 December
1983)
In unbuffered aqueous solution, the hydrogen evolution at iron electrodes is usually first order in hydrogen-ion dependence at pH below about 7 and pH independent at higher pHC1-41. This indicates that hydrogen ions and water molecules are the main electroactive reactants for this reaction in acid and alkaline solution, respectively. In buffered solution, however, also the acid component of the buffer might be such a reactant. Information about this is often needed in corrosion research, but appears rather hidden in the literature. Some pertinent studies have therefore been made and are briefly described below. Figures 14 show results of cathodic polarization measurements on high-purity iron electrodes in deoxygenated solutions of of various salt and buffer
ElsceIV
Fig. 2. Cathodic polarization curves for iron electrodes in deoxygenated solutions of x M NaHCO, -t (x/10) M Na2C03 + (1 - x) M KC1 (pH = 9.5) at 25°C. Values of x at the curves. No stirring.
E (see) /V
Fig. 1. Cathodic polarization nerves for iron electrodes in deoxygenated solutions of x M HAc + x M NaAc + (1 - x) M NaCl (pH = 4.7) at 25°C. Values of x at the curves. No stirring.
content at 25°C. Most of these results, Figs l-3, are cathodic sweep (1 mV s-l) polarization curves, starting at the stationary open-circuit corrosion potential of the iron electrode after grinding, etching and about 1 h conditioning of it in the test solution. Other results (Fig. 4) are stationary polarization points (partly Wagner-Traud corrected) purely for the hydrogen evolution reaction at such iron electrodes. All solutions were made from A.R. quality acids and salts and twice distilled water. In acetate buffered acid solution (Fig. l), a buffercontent dependent and stirring dependent transition occurs between two different Tafel lines. The Tafel line obeyed at low current densities coincides with the one 1163
1164
T. HURLEN,S.GUNVALDSENAND
F. BLAKER
2_ \
\
3_
L_
I
5_
I
-1.0 E bce)/V
-0.9 ElscellV
I
-0.8
Fig. 3. Cathodic polarization curves for iron electrodes in deoxygenated solutions of x M NH&I +x M NH3 + (1 -x) M KC1 (pH = 9.7) at 25°C. Values of x at the curves. No stirring.
Fig. 4. Stationary polarization data for hydrogen evolution at iron electrodes in 0.5 M H3BOx + 0.1 M KOH (PH = 8.1) at 25°C. Dashed line represents hydrogen evolution from solvent water. No stirring.
for hydrogen evolution from hydrogen ions in unbuffered solution of pH 4.7[1-4]. The Tafel line obeyed at high current densities coincides with the one for hydrogen evolution from solvent water in unbuffered salt solution. The current for transition between the lines fits with estimations assuming a limiting diffusion of proton-delivering acetic acid to the electrode. Hence, no sign appears of any electroactive participation by acetic acid, but only of its concentration and transport limited ability of preserving the pH at the electrode. In buffered alkaline solution (Figs 24), the rate of hydrogen evolution increases with the buffer concentration when this exceeds a certain limit. This indicates that there is a direct electroactive participation by the acid component of the buffer, and that this participation exceeds the one by the solvent water when the buffer concentration (or the concentration of its acid component) exceeds the limit noted. In the present
cases, this limit is largely around IO-’ M. It may be that also acetic acid behaves somewhat like this, but that this be hidden by the overwhelming participation of hydrogen ions in the acid solutions used in testing the acetate buffer (Fig. I).
REFERENCES M. Stern, J. electrochem. SIX. 102, 609 (1955). T. Hurlen, Acra them. wand. 14, 1533 (1960); Ekctrochim. Acta 8, 609 (1963). K. J. Vetter, Elec~orAemL~l Kinetics. Academic Press, New York (1967). A. J. Appleby, H. Kita, M. Chemla and G. Bronotil, in Encyrlopedia of Elerrrochemistry of the Elements (Edited by A. J. Bard), Vol IX, Part A, p. 383, Dekker, New York (1982).