SULFRAD-Stockholm- Conductivity Time-resolved Conductivity in Pulse Radiolysis Time-resolved...

SULFRAD-Stockholm-ConductivitySULFRAD-Stockholm-Conductivity

Time-resolved Conductivity in

Pulse Radiolysis

Time-resolved Conductivity in

Pulse Radiolysis

Klaus-Dieter AsmusKlaus-Dieter Asmus Klaus-Dieter AsmusKlaus-Dieter Asmus

RR••

cell

pulse of high-energy electrons

monochromator

amplifier

x-y recordertime

conductivity cell

Va


Application of conductivity Application of conductivity

• / • no optical absorption

• / • in general, conductivity provides an additional, independent parameter in mechanistic studies

H• + CCl4 H+ + Cl– + •CCl3

• / • yield of absorbing species is not known

•OH + RSSR (RSSR)•+ + OH–

RS• + RSOH

RSH + RSO•


general requirements 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


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 ?


Gc conductance

RL load resistorvoltage divider stringvoltage divider string

cell

e-beam Gc + Gc(t)

VL,0 + VL(t)

Va

RL

Gc(t) causes VL(t) Gc(t) causes VL(t) SULFRAD-Stockholm-ConductivitySULFRAD-Stockholm-Conductivity

Gc conductance

RL load resistorvoltage divider stringvoltage divider string

e-beam

Gc + Gc(t)

VL,0 + VL(t)

Va


I

I

I

I

I = current

RL

some mathematical correlations:

VL(t) = Gc(t) • Va • RL

G ~ 1 / R G ~ 1 / R

conditions of operations:

Rcell >> RL Gcell << GL Gcell(t) << GL and


VL(t) = Gc(t) • Va • RL


in aqueous solution:

Gc(t)F

kc • 103 i

ci | zi | i

F 1cm2

1

kc • 103 i

ci | zi | i

kc : cell constant

F : Faraday constant

ci : concentration of ith ion

zi : net charge of ith ion

i : mobility of ith ion [cm2 V–1 s–1]

i : specific conductivity of ith ion

=

Gc(t) =

VL(t) =Va • RL

kc • 103 i

ci | zi | i


application of voltage causes polarization and eventually electrolysisapplication of voltage causes polarization and eventually electrolysis

VL(t) =Va • RL

kc • 103 i

ci | zi | i

•/• polarization induces a Helmholtz layer operating against the voltage

•/• too low voltage reduces sensitivity below detection limit

•/• too high voltage may cause electrolysiselectrolysis changes chemical composition, and neutralizes charges

•/• too high voltage may effect geminate and other ion recombinaion processes

typical voltages applied: 20 – 200 Vtypical voltages applied: 20 – 200 V


application of voltage causes polarization and eventually electrolysisapplication of voltage causes polarization and eventually electrolysis

VL(t) =Va • RL

kc • 103 i

ci | zi | 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

VL(t) signals must be rectified and recorded at same phase position

capacitance effects at higher frequencies


VL(t) =Va • RL

kc • 103 i

ci | zi | i

load resistor RL must remain small (<<) compared to Rc ( = 1 / Gc)

typically < 200

cell constant kc d / A d : distance between electrodesA : area of electrodes

typically < 0.5 – 1.0

change of charge zi

typically ± 1.0


VL(t) =Va • RL

kc • 103 i

ci | zi | i

change in concentrationchange in concentration typically 10–6 – 10–5 M

specific conductivity

specific conductivity

Haq+ 315 –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


VL(t) =Va • RL

kc • 103 i

ci | zi | i

typical conditions: Va = 100 V

RL = 50

kc = 0.8

| zi | = 1

VL(t) = 0.5 m V

sensitivity

Example I:

i = 380 S cm2 (315 +

65)

H• + CCl4 H+ + Cl– + •CCl3

ci = 2.1 • 10–7 M

Example II:

i = 10 S cm2

Tl+ + •OH Tl(OH)+

ci = 8.0 • 10–6 M

What is possible these days ?

time windowtime window 2-5 ns 20 – 50 s DC

1 s 100 ms AC


detectable ion pair concentration changesdetectable ion pair concentration changes

10–6 - 10–7 M10–6 - 10–7 M

conversion of one ion into another ion

conversion of one ion into another ion

H+ / anion(–) pairH+ / anion(–) pair

Water radiolysis


formation of conducting species:formation of conducting species:

H2O radiolyisradiolyis

eaq– , H+ , •OH , H• , H2 , H2O2

0 50 s

720 nm

cond.

consumption of conducting species:consumption of conducting species:

eaq– + H+ H•

eaq– + H2O H• / ½ H2 + OH–

OH– + H+ H2O

no conducting species remains

specific conductance of eaq–


basic solution; pH 9

basic solution; pH 9

0 100 s

720 nm

cond.

pulseeaq

– + H2O H• / ½ H2 + OH–

H2O eaq– + H+

OH– + H+ H2O

fast

fast

formation of eaq– is accompanied by

an instantaneous loss of an OH–

as eaq– decays it is replaced by an OH–

Since there is almost no net signal change,

(eaq–) must be about the same as (OH–)

(OH–) = 176 S cm2

(eaq–) = 183 ± 10 S cm2

H+ + OH– neutralization


k (H+ + OH–) 1.1 • 1011 M–1 s–1

N2O-saturated, pH = 4.6N2O-saturated, pH = 4.6

t1/2 260 ns

neutralization becomes of pseudo-first order[OH–] = 3 • 10–6 M

[H+] = 2.5 • 10–5 M

eaq– + N2O •OH + N2 + OH–

H2O eaq– + H+

t1/2 3.5 ns

(RSSR)•+ radical cations


N2O-saturated solutions of CH3SSCH3 N2O-saturated solutions of CH3SSCH3

= 0eaq– + N2O •OH + N2 + OH–

H+ + OH– H2O

H2O eaq– + H+

pH 8.05

pH 4.75RSOH + 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 cm2) by less conducting (RSSR)•+ ( 50 S cm2)

ca 50% of •OH yield (RSSR)•+ca 50% of •OH yield (RSSR)•+

•OH reaction with t-Bu2S


N2O-saturated solutions of t-Bu2S ; pH 3.3 N2O-saturated solutions of t-Bu2S ; pH 3.3

370 nm

•OH + t-Bu2S t-Bu2S•(OH)

Q: Is the presumed sulfuranyl radical intermediate neutral or charged (protonated or deprotonated) ?

t-Bu2S•(OH) (t-Bu2S)•+ + OH–

H+ + OH– H2O

A: Under experimental conditions the sulfuranyl radical intermediate is a neutral species which later decays into the radical cation / OH– ion pair


•OH reaction with sulfoxides

N2O-saturated solutions of (CH3)2SO N2O-saturated solutions of (CH3)2SO

•OH + (CH3)2SO •CH3 + CH3SO2H

CH3SO2– + H+ acidic solution:

H+ + OH– H2Obasic solution:

Net result in basic solution:

OH– (176 S cm2) is replaced by the less conducting CH3SO2– (42 S cm2)

pH 4.4

pH 9.0


Decarboxylation of methionine and hydrolysis of CO2

N2O-saturated solutions of methionine / pH 11 N2O-saturated solutions of methionine / pH 11

pH 10.8

pH 11.0CO2 + OH– HCO3

–

HCO3–

+ OH– CO32– + H2O

k = 8.5 • 103 M–1 s–1

•OH + CH3SCH2CH2CH(NH2)CO2– + OH–

NS

OH H

H

CO2

NS

H

H

CO2

+

CO2 + CH3SCH2CH2C•NH2

k 1011 s–1


The time-resolved conductivity technique is more complex than the corresponding optical detection technique

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

Nevertheless, time-resolved conductivity excellently complements optical detection and provides information

otherwise not accessible

SULFRAD-Stockholm- Conductivity Time-resolved Conductivity in Pulse Radiolysis Time-resolved...

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