Absolute Standard Hydrogen Electrode Potential Measured by Reduction of Aqueous Nanodrops in the Gas Phase

Abstract

In solution, half-cell potentials are measured relative to those of other half cells, thereby establishing a ladder of thermochemical values that are referenced to the standard hydrogen electrode (SHE), which is arbitrarily assigned a value of exactly 0 V. Although there has been considerable interest in, and efforts toward, establishing an absolute electrochemical half-cell potential in solution, there is no general consensus regarding the best approach to obtain this value. Here, ion-electron recombination energies resulting from electron capture by gas-phase nanodrops containing individual [M(NH3)6]3+, M = Ru, Co, Os, Cr, and Ir, and Cu2+ ions are obtained from the number of water molecules that are lost from the reduced precursors. These experimental data combined with nanodrop solvation energies estimated from Born theory and solution-phase entropies estimated from limited experimental data provide absolute reduction energies for these redox couples in bulk aqueous solution. A key advantage of this approach is that solvent effects well past two solvent shells, that are difficult to model accurately, are included in these experimental measurements. By evaluating these data relative to known solution-phase reduction potentials, an absolute value for the SHE of 4.2 +/- 0.4 V versus a free electron is obtained. Although not achieved here, the uncertainty of this method could potentially be reduced to below 0.1 V, making this an attractive method for establishing an absolute electrochemical scale that bridges solution and gas-phase redox chemistry.

Measuring the extent and width of internal energy deposition in ion activation using nanocalorimetry

“…Ions that dissociate via sequential fragmentation and that have known threshold dissociation energies and similar dissociation entropies have also been used [18 -20]. From the abundance of the product ions, information about both the average energy and the width of the energy deposition can be inferred.An alternative approach to measuring internal energy deposition uses nanometer-sized hydrated ions as "nanocalorimeters," to precisely measure the amount of energy that is deposited into these ions upon activation [21][22][23][24][25][26][27][28][29][30]. When these ions are activated, they sequentially lose water molecules.…”

“…The number of water molecules that are lost upon electron capture (EC) by large hydrated ions can be high. For example, Ca(H 2 O) 32 2ϩ loses 10 or 11 water molecules and Ru(NH 3 ) 6 (H 2 O) 55 3ϩ loses 17 to 19 water molecules upon EC [22,24]. From the sum of the threshold water molecule binding energies and the amount of energy that is partitioned into translational, rotational, and vibrational modes of the products for each lost water molecule, the internal energy deposited into the ion can be obtained.…”

With or without light: comparing the reaction mechanism of dark-operative protochlorophyllide oxidoreductase with the energetic requirements of the light-dependent protochlorophyllide oxidoreductase

“…A b s o l u t e r e d o x p o t e n t i a l s ( V ) o f r e l e v a n t r e d o x p a i r s , c o mp u t e d a t t h e DB 3 L Y P -D3 / 6 - (Donald et al, 2008) the absolute reaction potential of the standard hydrogen electrode in water is usually taken as 4.43 V. (Reiss & Heller, 1985) Redox half-reaction =4 ε =10 ε =20 ε =78.36 ε…”

Untitled

“…"apoPChlide" and "apoChlide" refer to PChlide and Chlide devoid of Mg2+. Despite ongoing controversy, (Donald et al, 2008) the absolute reaction potential of the standard hydrogen electrode in water is usually taken as 4.43 V. (Reiss & Heller, 1985) (Donald et al, 2008) the absolute reaction potential of the standard hydrogen electrode in water is usually taken as 4.43 V. (Reiss & Heller, 1985) Redox half reaction =4 …”