ABSOLUTE AQUEOUS REDOX POTENTIALS (via a new link between aqueous and gaseous properties) Raji Heyrovská Institute of Biophysics, Academy of Sciences of the Czech Republic, 135 Kralovopolská, 612 65 Brno, Czech Rep. Email:
[email protected]
Presented at: 9. Pracovní setkání fyzikálních chemiků a elektrochemiků , Brno, 29. a 30. června 2009 (http://www.chemi.muni.cz/~libuse/setkani2009/index.html) (9th Workshop of Physical Chemists and Electrochemists, Brno, 29th and 30th June 2009)
on the occasion of the 3 main anniversaries, 1) 90th: foundation of Masaryk Univ. (MUNI), 2) 90th: foundation of Mendel Univ. of Agriculture & Forestry (MUAF), 3) 50th: award of the Nobel Prize in 1959 to Professor J. Heyrovský, and also 3 more anniversaries, 4) 70th: survival of MUNI & MUAF in 1939, 5) 40th: survival of MUNI & MUAF again in 1969 and 6) 20th: “ velvet revolution” in 1989. *********
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1. INTRODUCTION “ Chemistry is electric” (Bk: M.E. Bowden, CHF, 1997) Tomato and Lemon batteries: http://www.funsci.com/fun3_en/electro/electro.htm http://www.youtube.com/watch?v=AY9qcDCFeVI
Electrolyte: acidic juice, Electrodes: copper (+) and zinc (-). Chemical reactions at the two half cells: (1) Cu++ + 2e- ==> Cu and (2) Zn ==> Zn++ + 2 e- ; the overall reaction is Zn + Cu++ ==> Zn++ + Cu. The cell voltage is the “ potential difference“ between the two half cells. Electrochemists have chosen by convention, the potential of the standard hydrogen electrode (S.H.E.) as zero. The redox reaction for this half cell is 2H + + 2e- H 2. The standard potentials, Eo(S.H.E.) of Cu and Zn are +0.34 and -0.76 V and the cell voltage is, E = +0.34 - (-0.76) = +1.10 V. (Note: The unit of potential is named volt, in honor of Volta)
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1. INTRODUCTION (contd.)
Latest news on S.H.E. : “ ..universal reference electrode for which, under standard conditions, the standard electrode potential (H+/H2) is zero at all temperatures: Last update: 2009-06-16; http://goldbook.iupac.org/S05917.html
There have been attempts in the past to obtain the absolute value of the potential of the S.H.E. (Bard et al [1]). Trasatti (1986) [2] suggested, by theoretical calculations, the value 4.44 (+/-) 0.04V. Recently, Donald et al (2008) [3] obtained for S.H.E. (by electron capture on gaseous phase nano-drops) the value, 4.20 (+/-) 0.4 V. The “ new observation here” that Eo varies linearly with the ionization potentials (I) for many groups of elements, has given the “ absolute potential of the S.H.E., (EOI = 0)” (which confirms the value in [3]) (and can be taken as the final answer). Thereby, the “ absolute redox potentials, have been obtained as the sum, EOabs = EO + EOI = 0” , (Heyrovska, [4]).
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2. LINEAR DEPENDENCE OF AQ. STD. POTENTIALS (EO) ON IONIZATION POTENTIALS (I)
The data in [1] have been used to plot Eovs I in Fig. 1, where I = I (sum) is the sum of the 1st, 2nd, .. ionization potentials, as the case may be, depending on the value of n, (ox + ne- = red). Eo = kaqI - EoI=0; absolute aq. redox potential = Eoabs = Eo + EoI=0 EoI=0 = Abs. potl. of S.H.E. = 2.87 V (H-/H: halogens) & 4.20 V (H+/H: all others) (Author: Raji Heyrovska, Brno Conference, 29-30 June 2009)
Aq. standard. potentials (vs S.H.E.), Eo /V
Aq. Gp.(i), Cu, Ag, Au, Tl, n = 1 4
Gp.(ii), F, Cl, Br, I, n = 1
3
Gp.(iii), H, Li$, Na, K, Rb, Cs, n = 1 Gp.(iv), Ge, Sn, Pb, Se, Pd, n = 2
2
Gp.(v), Fe, Co, Ni, Cd, n = 2 1
Gp.(ix), N, P, Cr, Ga, n = 3
0
Gp.(vii), O, S, Te, n = 2 Gp.(viii), Be, Mg, Ca, Sr, Ba, Ra, n = 2
-1
Gp.(xi), B, Al, Sc, Y, La, n = 3 -2
Gp.(xii), Ti, Zr, n = 4 -3
Gp.(vi), Mn, Zn, n = 2
-4
Gp.(x),Al$, Eu, n = 3
-5
(Tl, n = 1), Cu, Pb, Cd, Zn, Mn, Ba (n = 2), Al (n = 3): $E1/2 (J. H. & D.I., 1935) - - - , Cu, Ba, n = 2, E1/2
0
20
40
60
Ionization potentials, I
80
/V
Fig. 2. Linear dependence of aq. std. potls. on I.
100
(i) (ii) (iii) (iv)
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All the straight lines in Fig. 1 follow the simple equation, Eo = kaqI – EoI=0 ; Eoabs = Eo + EoI=0 (= kaqI)
(1a,b)
A bsolu teaq . std. po tE en ti= alk s, I /V abs aq
where kaq is the slope, “ EoI=0 is the absolute potential of the S.H.E.” and “ Eoabs is the absolute redox potential” . Eoabs = EoI=0 when Eo = 0 (Eq. 1b). For Gp. VIIA (halogens, (1/2)X 2/X -), EoI=0 = 2.87 (+/-) 0.01 V, and for the others, EoI=0 = 4.20 (+/-) 0.03 V, which correspond to the (1/2)H 2/H and H +/H redox equilibria resply. Fig. 2 shows plots of Eoabs (= kaqI) vs Eo. The slopes (kaq) of the lines have been used as criteria for classifying the redox couples into groups (i) to (xii) (see: the box in Fig. 1). 7
o
6 5 y=1.003x +4.204 R2 =0.999
4 3
y= 0.999x+2.871 R2 =0.999
2 1 0 -5
-4
-3
-2
-1
0
1
2
3
o
A queou sstand ardpoten tials, E (vs. S .H .E .) /V Fig. 2: A bsoluteaq. std .
o p otentials, E alculatedaskaqI) vs abs (c
5 o
E.
3. DEPENDENCE OF POLAROGRAPHIC HALF-WAVE POTENTIALS (E1/2) ON IONIZATION POTENTIALS (I)
Fig. 3 shows the E1/2 values for Cu, Pb, Cd, Zn, Mn, Ba (n = 2 for all) Al (n = 3) (Heyrovsky, Ilkovic [5a,b]) and Tl (n = 1) (Heyrovsky, Kuta [6]). The causes for the differences between Eo and E1/2 in some cases (here, Ba and Al) have been explained by Lingane [7]. A q .G p .(i),C u ,A g ,A u ,T l,n=1 G p .(ii),F ,C l,B r,I,n=1
3
o
A q . stan d ard .p o te n tials(vsS E .H .E /V .),
4
G p .(iii),H ,L i$ ,N a ,K ,R b ,C s ,n=1 2
G p .(iv ),G e ,S n ,P b ,S e ,P d ,n=2
1
G p .(v ),F e ,C o ,N i,C d , n=2 G p .(ix ),N ,P ,C r,G a ,n=3
0
G p .(v ii),O ,S ,T e , n=2 G p .(v iii),B e ,M g ,C a ,S r,B a ,R a ,n=2
-1
G p .(x i),B ,A l,S c ,Y ,L a ,n=3
-2
G p .(x ii),T i,Z r,n=4 G p .(v i),M n ,Z n ,n=2
-3
G p .(x ),A l$ ,E u ,n=3
-4 -5 0
(T l,n=1 ),C u ,P b ,C d ,Z n ,M n ,B a(n=2 ), A l(n=3 ):$ E 1 /2(J .H .& D .I., 1 9 3 5 ) ---,C u ,B a ,n=2 ,E 1 /2 2 0
4 0
6 0
Io n izatio np o te n tials, I
8 0
/V
1 0 0
(i) (ii)
o
F ig .3 .L in e ard e p e n d e n c eo fE o nI. R e dfille dc irc le s :E a lu e s 1 /2v
(iii) (iv ) (v ) (v ii)
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ACKNOWLEDGEMENTS
The author thanks the Institute of Biophysics for institutional grant Nos. AV0Z50040507 and AV0Z50040702 of the ASCR and the Organizers of this conference for the opportunity given to present the “ new” scientific results. LITERATURE
• Bard, A.J., et al.: Standard potentials in aqueous solutions, Marcel Dekker, 1985, NY. • Trasatti, S.: Pure & Appl. Chem. 58 (1986) 955. • Donald, W.A., et al.: J. Am. Chem. Soc. 130 (2008) 3371. • Heyrovska, R.: submitted • a) http://nobelprize.org/nobel_prizes/chemistry/laureates/1959/; b) Heyrovsky, J., Ilkovic, D.: Chem. Listy, XXIX (1935) 234; Collection VII (1935) 198. 6. Heyrovsky, J., Kuta, J.: Principles of Polarography, Publishing House of the CAS, 1965, Prague. 7. Lingane, J.J.: J. Am. Chem. Soc. 61 (1939) 2099. 7
