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SULFRAD-Stockholm- Conductivity

Time-resolved Conductivity in Pulse Radiolysis. Klaus-Dieter Asmus. SULFRAD-Stockholm- Conductivity. pulse of high-energy electrons. monochromator. amplifier. R •. cell. conductivity cell. time. x-y recorder. V a. SULFRAD-Stockholm- Conductivity.

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SULFRAD-Stockholm- Conductivity

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  1. Time-resolved Conductivity in Pulse Radiolysis Klaus-Dieter Asmus SULFRAD-Stockholm-Conductivity

  2. pulse of high-energy electrons monochromator amplifier R• cell conductivity cell time x-y recorder Va SULFRAD-Stockholm-Conductivity

  3. • / • yield of absorbing species is not known •OH + RSSR (RSSR)•+ + OH– RS• + RSOH RSH + RSO• Application of conductivity • / • no optical absorption H• + CCl4 H+ + Cl– + •CCl3 • / • in general, conductivity provides an additional, independent parameter in mechanistic studies SULFRAD-Stockholm-Conductivity

  4. general requirements applied voltage Va --- must not interfere with radiation chemical „geminate“ or other ion recombination process --- must not itself result in ion formation Ohm‘s law applies under all conditions only a negligible part of the ions produced / destroyed / altered as a result of the irradiation are collected at the electrodes SULFRAD-Stockholm-Conductivity

  5. Any change in concentration of charged species changes the conductance of the irradiated solution in the irradiation cell. The associated change in current manifests itself in a voltage change, and this is the actually measured parameter. What is measured ? SULFRAD-Stockholm-Conductivity

  6. Gc conductance RL load resistor voltage divider string Va e-beam Gc + Gc(t) cell VL,0 + VL(t) RL Gc(t) causes VL(t) SULFRAD-Stockholm-Conductivity

  7. Gc conductance RL load resistor voltage divider string Va e-beam Gc + Gc(t) VL,0 + VL(t) I = current I I I I RL SULFRAD-Stockholm-Conductivity

  8. conditions of operations: Rcell >> RL Gcell << GL DGcell(t) << GL and VL(t) = Gc(t) • Va • RL some mathematical correlations: G ~ 1 / R SULFRAD-Stockholm-Conductivity

  9. F S DGc(t) Dci| zi | mi = kc : cell constant kc • 103 i F : Faraday constant ci : concentration of ith ion in aqueous solution: l =mF [W-1cm2] zi : net charge of ith ion mi : mobility of ith ion [cm2 V–1 s–1] 1 S DGc(t) Dci| zi | li = kc • 103 i li : specific conductivity of ith ion Va• RL S DVL(t) = Dci| zi | li kc • 103 i VL(t) = Gc(t) • Va • RL SULFRAD-Stockholm-Conductivity

  10. Va• RL S DVL(t) = Dci| zi | li kc • 103 i •/• polarization induces a Helmholtz layer operating against the voltage •/• too low voltage reduces sensitivity below detection limit •/• too high voltage may cause electrolysis electrolysis changes chemical composition, and neutralizes charges •/• too high voltage may effect geminate and other ion recombinaion processes application of voltage causes polarization and eventually electrolysis typical voltages applied: 20 – 200 V SULFRAD-Stockholm-Conductivity

  11. Va• RL S DVL(t) = Dci| zi | li kc • 103 i AC voltage especially good for long-time measurements (>1 ms) time resolution limited by frequency electronically more difficult to handle DVL(t) signals must be rectified and recorded at same phase position capacitance effects at higher frequencies application of voltage causes polarization and eventually electrolysis damage control pulsed DC voltage(triggered by the pulse) SULFRAD-Stockholm-Conductivity

  12. Va• RL S DVL(t) = Dci| zi | li kc • 103 i load resistor RL must remain small (<<) compared to Rc ( = 1 / Gc) typically < 200 W cell constant kc d / A d : distance between electrodes A : area of electrodes typically < 0.5 – 1.0 change of charge zi typically ± 1.0 SULFRAD-Stockholm-Conductivity

  13. Va• RL S DVL(t) = Dci| zi | li kc • 103 i change in concentration typically 10–6 – 10–5 M specific conductivity Haq+ 315 W–1 cm2 (S cm2) at 18°C OHaq– 176 F– 46.5 NO3– 61.7 Na+ 43.5 NH4+ 64.5 typical anion (A–) or cation (Kat+) 50  20 SULFRAD-Stockholm-Conductivity

  14. Va• RL S DVL(t) = Dci| zi | li kc • 103 i typical conditions: Va = 100 V RL = 50 W kc = 0.8 | zi |= 1 Example I: DVL(t)= 0.5 m V Tl+ + •OH Tl(OH)+ H• + CCl4 H+ + Cl– + •CCl3 Sli = 380 S cm2 (315 + 65) S Dci= 2.1 • 10–7 M Example II: Sli = 10 S cm2 S Dci= 8.0 • 10–6 M sensitivity SULFRAD-Stockholm-Conductivity

  15. What is possible these days ? time window 2-5 ns DC 20 – 50 ms 1 ms AC 100 ms detectable ion pair concentration changes 10–6 - 10–7 M conversion of one ion into another ion H+ / anion(–) pair SULFRAD-Stockholm-Conductivity

  16. Water radiolysis formation of conducting species: radiolyis H2O eaq– , H+ , •OH , H• , H2 , H2O2 consumption of conducting species: 720 nm eaq– + H2O H• / ½ H2 + OH– cond. OH– + H+ H2O eaq– + H+ H• no conducting species remains 0 50 ms SULFRAD-Stockholm-Conductivity

  17. specific conductance of eaq– H2O eaq–+ H+ fast fast 720 nm formation of eaq– is accompanied by an instantaneous loss of an OH– cond. OH– + H+ H2O 0 100 ms pulse as eaq– decays it is replaced by an OH– eaq– + H2O H• / ½ H2 + OH– l(OH–) = 176 S cm2 Since there is almost no net signal change, l(eaq–) must be about the same as l(OH–) l(eaq–) = 183 ± 10 S cm2 basic solution; pH  9 SULFRAD-Stockholm-Conductivity

  18. H+ + OH– neutralization N2O-saturated, pH = 4.6 H2O eaq–+ H+ t1/2 260 ns eaq– + N2O •OH + N2 + OH– t1/23.5 ns [OH–] = 3 • 10–6 M neutralization becomes of pseudo-first order [H+] = 2.5 • 10–5 M k (H+ + OH–)  1.1 • 1011M–1 s–1 SULFRAD-Stockholm-Conductivity

  19. (RSSR)•+ radical cations H2O eaq–+ H+ pH 8.05 eaq– + N2O •OH + N2 + OH– Dl = 0 H++OH– H2O •OH + RSSR (RSSR)•+ + OH– RSOH + RS• pH 4.75 RSH + RSO• basic solution: OH– stable acid solution: instantaneous neutralization of OH– increase in conductivity replacement of H+ (l=315 S cm2) by less conducting (RSSR)•+ (l 50 S cm2) N2O-saturated solutions of CH3SSCH3 ca 50% of •OH yield (RSSR)•+ SULFRAD-Stockholm-Conductivity

  20. •OH reaction with t-Bu2S •OH + t-Bu2St-Bu2S•(OH) 370 nm H++OH– H2O Q: Is the presumed sulfuranyl radical intermediate neutral or charged (protonated or deprotonated) ? t-Bu2S•(OH) (t-Bu2S)•+ + OH– A: Under experimental conditions the sulfuranyl radical intermediate is a neutral species which later decays into the radical cation / OH– ion pair N2O-saturated solutions of t-Bu2S ; pH 3.3 SULFRAD-Stockholm-Conductivity

  21. •OH reaction with sulfoxides N2O-saturated solutions of (CH3)2SO •OH + (CH3)2SO •CH3 + CH3SO2H acidic solution: CH3SO2– + H+ pH 4.4 basic solution: H++OH– H2O pH 9.0 Net result in basic solution: OH– (l 176 S cm2) is replaced by the less conducting CH3SO2–(l 42 S cm2) SULFRAD-Stockholm-Conductivity

  22. Decarboxylation of methionine and hydrolysis of CO2 N2O-saturated solutions of methionine / pH  11 •OH + CH3SCH2CH2CH(NH2)CO2– + OH– k  1011 s–1 pH 10.8 CO2+ CH3SCH2CH2C•NH2 pH 11.0 CO2 + OH– HCO3– k = 8.5 • 103 M–1 s–1 HCO3– + OH– CO32– + H2O SULFRAD-Stockholm-Conductivity

  23. The time-resolved conductivity technique is more complex than the corresponding optical detection technique It involves more electronic and electrical parameters Any signal is based on contributions of at least two ions In water the major contributors are H+ and OH–, and not necessarily the ions of interest Nevertheless, time-resolved conductivity excellently complements optical detection and provides information otherwise not accessible SULFRAD-Stockholm-Conductivity

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