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

R cell pulse of high-energy electrons monochromator amplifier x-y recorder time conductivity cell VaVa SULFRAD-Stockholm- Conductivity

Application of conductivity / no optical absorption / in general, conductivity provides an additional, independent parameter in mechanistic studies H + CCl 4 H + + Cl – + CCl 3 / yield of absorbing species is not known OH + RSSR(RSSR) + + OH – RS + RSOH RSH + RSO SULFRAD-Stockholm- Conductivity

general requirements applied voltage V a --- 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

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

G c conductance R L load resistor voltage divider string cell e-beam G c +  G c (t) V L,0 +  V L (t) VaVa RLRL  G c (t) causes  V L (t) SULFRAD-Stockholm- Conductivity

G c conductance R L load resistor voltage divider string e-beam G c +  G c (t) V L,0 +  V L (t) VaVa SULFRAD-Stockholm- Conductivity I I I I I = current RLRL

some mathematical correlations:  V L (t) =  G c (t) V a R L G ~ 1 / R conditions of operations : R cell >> R L G cell << G L  G cell (t) << G L and SULFRAD-Stockholm- Conductivity

 V L (t) =  G c (t) V a R L SULFRAD-Stockholm- Conductivity in aqueous solution:  G c (t) F k c 10 3  i  c i | z i |  i   F   1  cm 2  1 k c 10 3  i  c i | z i | i k c : cell constant F : Faraday constant c i : concentration of i th ion z i : net charge of i th ion  i : mobility of i th ion [cm 2 V –1 s –1 ] i : specific conductivity of i th ion =  G c (t) =  V L (t) = V a R L k c 10 3  i  c i | z i | i

SULFRAD-Stockholm- Conductivity application of voltage causes polarization and eventually electrolysis  V L (t) = V a R L k c 10 3  i  c i | z i | 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 typical voltages applied: 20 – 200 V

SULFRAD-Stockholm- Conductivity application of voltage causes polarization and eventually electrolysis  V L (t) = V a R L k c 10 3  i  c i | z i | i damage control pulsed DC voltage (triggered by the pulse) especially good for long-time measurements (>1  s) AC voltage time resolution limited by frequency electronically more difficult to handle  V L (t) signals must be rectified and recorded at same phase position capacitance effects at higher frequencies

SULFRAD-Stockholm- Conductivity  V L (t) = V a R L k c 10 3  i  c i | z i | i load resistor R L must remain small (<<) compared to R c ( = 1 / G c ) typically < 200  cell constant k c  d / A d : distance between electrodes A : area of electrodes typically < 0.5 – 1.0 change of charge zizi typically ± 1.0

SULFRAD-Stockholm- Conductivity  V L (t) = V a R L k c 10 3  i  c i | z i | i change in concentration typically 10 –6 – 10 –5 M specific conductivity H aq  –1 cm 2 (S cm 2 ) at 18°C OH aq – 176 F – 46.5 NO 3 – 61.7 Na NH typical anion (A – ) or cation (Kat + ) 50  20

SULFRAD-Stockholm- Conductivity  V L (t) = V a R L k c 10 3  i  c i | z i | i typical conditions: V a = 100 V R L = 50  k c = 0.8 | z i | = 1  V L (t) = 0.5 m V sensitivity Example I:   i = 380 S cm 2 ( ) H + CCl 4 H + + Cl – + CCl 3   c i = –7 M Example II:   i = 10 S cm 2 Tl + + OH Tl(OH) +   c i = –6 M

What is possible these days ?time window2-5 ns 20 – 50  s DC 1  s 100 ms AC SULFRAD-Stockholm- Conductivity detectable ion pair concentration changes 10 – –7 M conversion of one ion into another ion H + / anion (–) pair

Water radiolysis SULFRAD-Stockholm- Conductivity formation of conducting species: H2OH2O radiolyis e aq –, H +, OH, H, H 2, H 2 O  s 720 nm cond. consumption of conducting species: e aq – + H + H e aq – + H 2 O H / ½ H 2 + OH – OH – + H + H 2 O no conducting species remains

specific conductance of e aq – SULFRAD-Stockholm- Conductivity basic solution; pH   s 720 nm cond. pulse e aq – + H 2 O H / ½ H 2 + OH – H 2 O e aq – + H + OH – + H + H 2 O fast formation of e aq – is accompanied by an instantaneous loss of an OH – as e aq – decays it is replaced by an OH – Since there is almost no net signal change, (e aq – ) must be about the same as (OH – ) (OH – ) = 176 S cm 2 (e aq – ) = 183 ± 10 S cm 2

H + + OH – neutralization SULFRAD-Stockholm- Conductivity k (H + + OH – )  M –1 s –1 N 2 O-saturated, pH = 4.6 t 1/2  260 ns neutralization becomes of pseudo-first order [OH – ] = 3 10 –6 M [H + ] = –5 M e aq – + N 2 O OH + N 2 + OH – H 2 O e aq – + H + t 1/2  3.5 ns

(RSSR) + radical cations SULFRAD-Stockholm- Conductivity N 2 O-saturated solutions of CH 3 SSCH 3  = 0 e aq – + N 2 O OH + N 2 + OH – H + + OH – H 2 O H 2 O e aq – + H + pH 8.05 pH 4.75 RSOH + RS OH + RSSR (RSSR) + + OH – RSH + RSO basic solution: OH – stable increase in conductivity acid solution: instantaneous neutralization of OH – replacement of H + (  315 S cm 2 ) by less conducting (RSSR) + ( 50 S cm 2 ) ca 50% of OH yield (RSSR) +

OH reaction with t-Bu 2 S SULFRAD-Stockholm- Conductivity N 2 O-saturated solutions of t-Bu 2 S ; pH nm OH + t-Bu 2 S t-Bu 2 S (OH) Q: Is the presumed sulfuranyl radical intermediate neutral or charged (protonated or deprotonated) ? t-Bu 2 S (OH) (t-Bu 2 S) + + OH – H + + OH – H 2 O A: Under experimental conditions the sulfuranyl radical intermediate is a neutral species which later decays into the radical cation / OH – ion pair

SULFRAD-Stockholm- Conductivity OH reaction with sulfoxides N 2 O-saturated solutions of (CH 3 ) 2 SO OH + (CH 3 ) 2 SO CH 3 + CH 3 SO 2 H CH 3 SO 2 – + H + acidic solution: H + + OH – H 2 O basic solution: Net result in basic solution: OH – (  176 S cm 2 ) is replaced by the less conducting CH 3 SO 2 – (  42 S cm 2 ) pH 4.4 pH 9.0

SULFRAD-Stockholm- Conductivity Decarboxylation of methionine and hydrolysis of CO 2 N 2 O-saturated solutions of methionine / pH  11 pH 10.8 pH 11.0 CO 2 + OH – HCO 3 – HCO 3 – + OH – CO 3 2– + H 2 O k = M –1 s –1 OH + CH 3 SCH 2 CH 2 CH(NH 2 )CO 2 – + OH – CO 2 + CH 3 SCH 2 CH 2 C NH 2 k  s –1

SULFRAD-Stockholm- Conductivity 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