Design and testing of a computer-controlled square wave voltammetry instrument

Rochester Institute of Technology

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Thesis/Dissertation Collections

6-1-1987

Design and testing of a computer-controlled square

wave voltammetry instrument

Nancy L. Wengenack

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Recommended Citation

DESIGN AND TESTING OF A COMPUTER-CONTROLLED

SQUARE WAVE VOL

TAMMETRY

INSTRUMENT

by

Nancy L

.

Wengenack

J

une ,

1987

T

HESI S

SUBMITTED

IN PARTIAL

FULFI

LLMENT

OF THE

REQUIREMENTS FOR

T

HE DEGREE OF MASTER OF SCIENCE

APPROVED:

Paul Rosenberg

Project Advisor

G. A.

Jakson

Department Head

Gate A.

Gate

Library

Rochester Ins

titu te

of

T

e c hnol og y

Rochester, New York 14623

Title

of

Thesis

Design

and

_Testing

of a

Computer-Controlled

Square

Wave

_Voltammetry

Instrument

I

_Nancy

L.

Wengenack

_hereby

grant permission

to

the

Wallace

Memorial

_Library,

of

_R.I.T.,

to

reproduce

_my

thesis

in

whole or

in

part.

_Any

reproduction will not

be

for

commercial use or profit.

ACKNOWLEDGEMENTS

I

would

like

to

express

_my

thanks

to

_my

research

advisor,

Dr.

L.

Paul

_Rosenberg,

for

his

assistance with

this

work.

I

would also

like

to

thank

_my

graduate committee:

Dr.

B.

Edward

_Cain;

Dr.

Christian

_Reinhardt;

and

_especially

Dr.

Joseph

_Hornak;

for

their

_help

and guidance.

Special

thanks

to

Peter

J.

Michelsen

for

_many

helpful

discussions.

Financial

assistance

from

the

Department

of

Chemistry

and

Rochester

Institute

of

_Technology

in

the

forms

of a

_teaching

_{assistantship}

and graduate

_fellowship

List

of

Figures

Figure

1

-

DC

Polarography

a)

Potential

Waveform

b)

Current Response

Curve

10

Figure

2

-Important

Features

of a

Typical

Polarogram

11

Figure

3

-Current Reaction

to

_{Mercury Drop}

Growth

and

Fall

at

the

_Working

Electrode

15

Figure

4

-

Potential

Waveform

and

Current

Response

Curve

For

TAST

_Polarography

16

Figure

5

-

Current

Sampling

Scheme

For

TAST

Polarography

17

Figure

6

-

Potential

Waveform

_and

Current

Response

Curve

For

NPP

20

Figure

7

-

Capacitive

and

Faradaic

Current

Behavior

_During

a

Potential

Pulse

22

Figure

8

-

Potential

Waveform

and

Current

Response

Curve

For

DPP

24

Figure

9

-

Applied

Potential

Waveform

For

SWV

26

Figure

10

-a)

Square

Wave

Voltammetry Timing

Relationships

b)

Current

response curve

for

Square

Wave

_Voltammetry

28

Figure

11

-

Effects

of

Square

Wave

Amplitude

on

Difference

Current

Magnitude

a)

Esw=

5

mV

b)

Esw=

₁₅

_mV

c)

ESw=

30

mV

Solid

line

is

the

difference

current

31

Figure

12

-

Peak Width

_at

half

height

_versus

Square

Wave

Ampl

itude

32

Figure

13

-

Traditional

_versus

Postfix Notation

45

Figure

14

-

Front

_and

Rear

Panel

Configurations

_of

the

Model

264A

Polarographic

_Analyzer/

Stripping

Voltammeter

53

Figure

15

-

Model

303A

Static

Mercury

Drop

Electrode

Configuration

55

Figure

16

-

LabMaster

Daughter

Board

Conf iguration

. . . .57

Figure

17

-

A/D

LIST OF

FIGURES

(continued)

Figure

18

-Current Measurement

Scheme

for

SWV

72

Figure

19

-Computer-Controlled

Square Wave

Voltammetry

-

Block

Diagram

74

Figure

20

-

Potential

versus

Time

Collected

For

SWV

Using

ASYST

76

Figure

21

-Voltammogram For Model

System

Collected

Using

SQUARE

78

Figure

22

-Square

Wave

_Voltammetry

Run

on

Internal

Cell

83

Figure

23

-Collecting

a

Function

Generator

Created

Square

Wave

85

Figure

24

-

Potential

Data

Collection

with

SQUARE

While

_Sitting

at

One

Potential

86

Figure

25

-

Current

Data

_Collection

with

SQUARE While

Sitting

at

One

Potential

88

Figure

26

-

DPP

Run

_on

Internal

Cell

89

Figure

27

-

SWV

of

_ZnCl2

-Plotting

every

10th

Point

-Trial

#1

91

Figure

28

-

SWV

of

_ZnCl2

-Plotting

every

10th

Point

-Trial

#2

92

Figure

29

-

SWV

of

_ZnCl2

-Plotting

every

10th

Point

-Trial

#3

93

Figure

30

-

SWV

_of

ZnCl2

-

Signal

Averaging

94

Figure

31

-

Front

_and

Rear

Panel

Configurations

_of

_the

IBM

EC/225

96

Figure

32

-

Block

Diagram

_of

Analyzer

_with

IBM

EC/2252A

List

of

Tables

Table

I

-

Comparison

of

DPP

and

SWV

Current

Magnitude

36

Table

II

-

ASYST

Features

40

Table

III

-

Comparison

of

Compilation

and

Run

Times

for

FORTH

and

Several

Other

Languages

... .43

Table

IV

-

Data

Acquisition

Boards

Supported

_By

ASYST

47

Table

V

-Comparison

of

Run

Times

for

FORTH

and

BASIC

Control

Programs

102

List

of

Programs

Program

I

-WAVE:

ASYST

Control

Program

63

Program

II

-

SQUARE:

ASYST

Control

Program

67

Program

III

-Plotting

Programs

:

ASYST

Control

Programs

70

Program

IV

-

WAVE:

BASIC

Control

Program

100

List

of

Abbreviations

Used

A/D

-

Analog-to-Digital

Ag/AgCl

-

Silver/Silver

_Chloride

_Electrode

BNC

-

British

Navy

Connector

D/A

-Digital-to-Analog

DCP

-

DC

Polarography

DIP

-Dual

Inline

Package

DME

-Dropping Mercury

Electrode

DOS

-

Disk

Operating

System

DPP

-

Differential

Pulse

_Polarography

FFT

-

Fast

Fourier

Transform

GPIB

-

General

Purpose

Interface

Bus

HMDE

-Hanging

Mercury

Drop

Electrode

HPLC

-

High

Performance

Liquid

Chromotography

LIFO

-Last-in,

First-Out

Stack

NPP

-

Normal

Pulse

Polarography

PARC

-

Princeton

Applied

Research

Corporation

RPN

-

Reverse

Polish

Notation

SCE

-

Saturated

Calomel

Electrode

SMDE

-

Static

Mercury

Drop

Electrode

SWV

-

Square

Wave

Voltammetry

TAST

-

Current-Sampled

TABLE OF

CONTENTS

Abstract

1

1

.

0

Introduction

3

1.1

Basic

Cell

Design

7

1

.

2

DC

Polarography (DCP)

9

1.3

TAST

or

Current

Sampled

_Polarography

14

1.4

Normal

Pulse

_Polarography

(NPP)

19

1.5

Differential

Pulse

_Polarography

_(DPP)

23

1.6

Square Wave

_Voltammetry

(SWV)

25

1.7

ASYST

38

1

.

8

FORTH

42

1.9

Data

Acquisition

Boards

47

2.0

Purpose

48

3.0

Experimental

49

3.1

EG&G

PARC

Model

264A

Polarographic

Analyzer/

Stripping

Voltammeter

50

3.2

EG&G

PARC

Model

303A

Static

_Mercury

_Drop

Electrode

54

3.3

Tecmar

LabMaster

Data

Acquisition

Board

56

3.4

Hewlett-Packard

HP7475A

Plotter

60

3.5

IBM

PC

61

4.0

Results

and

Discussion

62

4

.

1

Control

Programs

62

4.2

Current

Measurement

Scheme

71

TABLE

OF

CONTENTS

(continued)

4.4

Model

System

Results

77

4.5

Instrument

Operation

79

4.6

Elimination

of

Noise

81

4.7

_Interfacing

the

IBM

EC/2252A

Voltammetric

Analyzer

95

4.8

Control

Programs

Written

in

BASIC

99

Suggestions

for

Future

Work

104

Conclusion

105

ABSTRACT

Square

Wave

_Voltammetry

is

_rapidly

_becoming

the

most

popular

and

versatile

electrochemical

technique

used

in

research

laboratories

today.

A

discussion

of

electroanalytical

_techniques,

which are

the

forerunners

to

square

wave

_voltammetry,

provides a

basis

for

understanding

the

theoretical

and experimental aspects of

SWV

which are

presented.

A

computer-controlled

square wave

_voltammetry

system

was

designed

and

built

_using

an

IBM

PC

computer,

a

Tecmar

data

acquisition

_board,

an

EG&G

PARC

Model

264A

Polarographic

_{Analyzer/Stripping}

_Voltammeter,

an

EG&G

PARC

Model

303A

Static

_Mercury

_Drop

_Electrode,

and

the

scientific software

_package,

_ASYST.

The

system was

tested

_by

_performing

computer-controlled square wave

voltammetry

on a

Zn

_system,

popular

for

standardization

of square wave analysis systems.

The

major problem encountered with

the

system was a

large

amount of noise

in

the

collected

data.

The

noise

was

_{substantially}

reduced

_by

various

data

_smoothing

techniques

which

included

signal averaging.

The

_universality

of

the

system was

_successfully

2A

tested

_by

_using

an

IBM EC/225

Voltammetric

Analyzer

and

a

homemade

electrolytic cell

for

the

analysis.

_Finally,

the

control programs were written

in

BASIC

as well as

superior

language

for

the

_interfacing

application

in

1.0

INTRODUCTION

The

roots of modern electroanalytical

_chemistry

date

back

to

the

_early

1920

's

when

Jaroslav

Heyrovsky,

a

Czechoslovakian

_chemist,

developed

DC

Polarography.

Although

his

peers

did

not

_immediately

recognize

its

significance,

Heyrovsky

eventually

won

the

Nobel

Prize

in

chemistry

for

the

development

of

DC

_Polarography

in

1959.

By

1950,

polarography

had

reached

its

peak of

popularity

and most scientists regarded

it

as an area

in

which

there

were

few

avenues

left

to

explore.

At

the

same

time,

other analytical

techniques

were

_experiencing

surges

in

popularity.

Atomic

absorption

_spectroscopy

had

recently been

developed

and was

_{being heavily}

promoted as

the

technique

of greatest promise

in

analytical

chemistry.

Gas

_{chromatography}

was a

technique

which

had

come

to

the

forefront

for

the

analysis of organic compounds.

This

combination of

factors

led

_many

scientists

to

believe

that

_polarography

was

_essentially

a

"dead"

technique

and

_very

few

scientists continued work

to

improve

voltammetric

techniques

(1).

Fortunately

for

electrochemists ,

electrochemistry

did

not

become

a

forgotten

technique,

but

a

_popular,

modern method

for

chemical analysis.

Between

1955

and

1965,

the

limitations

of atomic

absorption

_spectroscopy

was

found

to

be

a

_limiting

method

in

that

all

the

sample

being

analyzed must

be

ionized

in

a

flame.

Gas

chromatography

is

limited

to

those

substances which are

able

to

be

volatilized.

_Polarography

's

great advantage

over

these

two

techniques

is

that

the

species of

interest

is

examined

in

its

original chemical environment and

therefore

a

better

representation of

its

behavior

in

that

chemical environment

is

obtained.

During

the

same

time

_period,

revolutions

in

the

electronics

_industry

brought

about

the

development

of

low

cost operational amplifiers.

This

made

the

development

of

_relatively

inexpensive

commercial electroanalytical

analysis

instruments

possible.

The

combined effect of

these

developments

was a

resurgence of

interest

in

electroanalytical methods.

The

first

_widely

accepted commercial

instrument

for

polarography,

the

Model

170

Electrochemistry System,

was

introduced

_by

Princeton

Applied

Research

Corporation

in

1968

(2)

.

The

Model

170

was

revolutionary because

it

permitted

the

electrochemist

to

perform a wide

_variety

of

different

measurements on

the

same solution

_simply

_by

changing

a

few

front

panel controls.

No

prior

knowledge

of electronics

_by

the

user was

_necessary

and

this

gave

more opportunities

for

the

use of electroanalytical

to

_$12,000.

In

_1976,

PARC

again

introduced

a new system

-the

Model

174

_{Electrochemistry}

System.

The

Model

174

was

smaller

and contained much simpler

circuitry,

which

resulted

in

a

lower

price

tag.

The

total

cost

for

a

Model

174

was

_{approximately}

$2000.

This

low

cost allowed

many

research

laboratories

to

have

a

_working

electrochemistry

system

for

routine

_daily

use.

Recent

developments

in

computer

_technology

have

further

aided electrochemistry's popularity.

_Today,

complete on-line control and automation of

techniques

is

possible.

A

"new" electroanalytical

technique

which

is

rapidly

gaining

in

popularity

is

Square

Wave

Voltammetry

(SWV)

.

An

in-depth

analysis of

SWV

will

be

presented

later

in

this

_work;

SWV

is

_rapidly

_replacing

other

electroanalytical

techniques

for

_many

research applications.

Since

the

_popularity

of

SWV has

_only

_recently

begun

to

_develop,

_only

two

commercial

_instruments,

Bioanalytical

System's

BAS-100

and

Princeton

Applied

Research's

_384B,

were available

by

1985

(3).

For

this

reason,

many

researchers still construct

their

own

SWV

instrument

which

is

_usually

computer-controlled.

Computer-control

of

SWV

is

needed

because

of

SWV's

rapid

analysis methods.

In

addition

to

_fashioning

the

instrument,

the

control programs are most often written

research

was undertaken

to

create a computer-controlled

square

wave

_voltammetry

instrument

capable of routine

usage

in

the

typical

undergraduate/graduate

research

1.10

BASIC

CELL DESIGN

Square

wave

_voltammetry

is

performed

_by

_applying

a

potential

waveform

to

an electrolytic cell over a period of

time.

The

electrolytic cell

is

the

vessel

in

which

the

oxidation

or reduction

takes

place.

It

_typically

consists

of a

heavy-walled

glass container which can

hold

between

2mL

and

30mL

of

the

analyte solution

_being

studied

(4).

Several

holes

in

the

_cap

of

the

cell allow

the

introduction

of a purge

tube

through

which

_N2(g)

or

Ar(g)

is

bubbled

into

the

solution.

The

inert

gas

displaces

the

oxygen

in

the

solution which

is

_easily

reduced and would

interfere

with

detection

of reduction

current attributed

to

the

analyte.

The

cell contains

three

types

of electrodes which

are also

immersed

in

the

analyte solution and are used

to

put

the

analyte solution

in

contact with

the

potential waveform as well as measure

the

current

_resulting

from

the

reaction which occurs

in

the

cell.

The

first

electrode,

the

counter or

_auxiliary

_electrode,

consists

of an

inert

_conducting

electrode such as a pool of

mercury

or a platinum wire.

The

counter electrode

is

the

electrode

to

which

the

potential waveform

is

applied.

The

_working

electrode

is

_typically

a

_{Hanging Mercury Drop}

Electrode

while

the

reference electrode

is

_usually

either

a saturated calomel electrode

(SCE)

or a

silver-silver

the

reduction or oxidation process

is

measured at

the

1.2

DC

POLAROGRAPHY

_(DCP)

To

_fully

understand

the

technique

of square wave

voltammetry

and

appreciate

its

_advantages,

it

is

necessary

to

examine

several

_{electroanalytical}

_techniques

which

preceded

SWV.

Some

of

the

characteristics

of

preceding

techniques

are

_incorporated

_into

_SWV

_while

other aspects of

these

_preceding

techniques

are

improved

upon

_by

SWV.

The

oldest

electrochemical

_technique,

DC

polarography

(DCP),

is

also

the

one

for

which

_Heyrovsky

won

the

Nobel

Prize.

The

voltage

input

in

DC

polarography

(Figure

la)

consists of a potential

_ramp

which

_linearly

increases

with

time

(5).

For

a

reduction

process,

the

potential

becomes

more negative with

time.

For

an oxidation

_process,

the

potential

becomes

more

positive with

time.

The

typical

current response curve

for

DCP

can

be

seen

in

Figure

lb.

Several

important

pieces of

information

can

be

obtained

from

the

current response curve.

The

residual

current seen

in

Figure

2

₍₆₎

is

small current which

passes

through

the

cell caused

_by

either capacitive

current or

impurities

in

the

solvent/electrolyte solution

used.

Capacitive

current will

be

discussed

in

Section

1.4.

The

_limiting

current

is

the

plateau current reached

)

0

E(init)

-*

Time

b)

₍₄₀

0

-:

0

Figure

1

-DC

_Polarography

a> potential waveform

b )

current response withi applied potential

(

reproduced

Qj 13 <_) c c .2 c CO 0" 3

l-\

O) i >*-CO a> o

sl/

OD a. o i

d

CO a' co <-> c CD ?J o CL "O CD

"5.

CL

<

d

i

LJ

e a T3 01

y

c 0 i_ a m L it L cn 0 i. in 01 i_ -j IB Ol ll ."^. >0 1 01 <N U c n 01 L L D 01 cn 4--t 01 U.

0C

in

in

*-current

is

therefore

_essentially

independent

of applied

potential.

A

quantitative measure of

the

amount of electroactive species

in

solution

is

given

_by

the

diffusion

current,

_id

.

The

diffusion

current

is

obtained

_by

_determining

the

average current

from

the

reduction of

the

analyte species and

_subtracting

the

residual

_(background)

current

from

this

average current

value.

_According

to

the

Ilkovic

equation

(see

Equation

1)

(4),

the

diffusion

current

is

_directly

proportional

to

concentration of

the

analyte

in

solution.

id

=

₆₀₇

n

C

D1/2m2/3t1/6

[1]

Where

_id

=

_diffusion

_current

_in

_microamperes

n = _{number of equivalents per mole of}

_the

electrode reactant

C

=

concentration of

the

electroactive material

in

millimoles per

liter

D

=

_the

diffusion

coefficient of

the

electroactive species

in

square centimeters per second

m = _{mass rate of}

flow

of

_mercury

through

a

capillary

in

milligrams per second

t

=

_lifetime

_{of a}

drop

in

seconds

A

qualitative piece of

information

can also

be

Electroactive

materials are

characterized

_by

their

half-wave potentials.

Ei/g

is

tne

point of

inflection

of

the

current-voltage

_curve,

one-half

the

distance

between

the

residual current and

the

final

_limiting

current

plateau,

as shown

in

Figure

2.

As

an

_example,

cadmium

has

an

E-|/2

of -0.60 volts versus

the

SCE

while

lead

has

an

E-j/g

of

-0.40 volts versus

the

SCE

(7).

Both

reductions were

carried out

in

a solvent system of

1.0

M

hydrochloric

1.3

TAST

or

CURRENT-SAMPLED

POLAROGRAPHY

An

inherent

problem with

DC

_polarography

is

that

large

fluctuations

in

current

occur.

These

current

fluctuations

are a

direct

reflection

of current

variations which occur

_during

the

lifetime

of a

_mercury

drop.

Figure

3

₍₆₎

shows

the

current growth which

results

from

the

growth and

fall

of

three

successive

mercury

drops

from

a

_dropping

_mercury

electrode

(DME)

.

These

large

current

fluctuations

make

the

accurate

measurement of

the

diffusion

current

difficult.

In

order

to

avoid

these

large

current

_{fluctuations,}

TAST

₍

or

Current-Sampled

_{Polarography )}

was

developed.

The

potential waveform

for

TAST

_polarography

is

_exactly

the

same as

that

for

DC

_polarography

(Figure

₄₎

₍₅₎

.

However,

the

current response curve

is

much smoother

than

that

of

DC

polarography.

The

reason

for

this

improvement

in

current response

is

the

current

_sampling

method.

The

current

is

sampled

just

before

the

end of

the

lifetime

of

each

_mercury

_drop

(Figure

5)

(8).

This

current

_sampling

allows

for

the

rejection of currents

_arising

from

the

growth and

fall

of

the

_mercury

_drop

and results

in

the

smooth response curve seen

in

Figure

4.

_{Consequently,}

a much more accurate representation of

the

diffusion

CDX&ENT

REACTION

TO MOP

CROVTB AND FALL

Figure

3

Current

growth

_during

three

successive of a

DME

(reproduced

from

Reference

6)

.

E(inlt)

Time*

Figure

4

Drops

dislodged.

experiment

uses a

_mechanically

controlled

_drop

time

which

allows

the

_drop

time

to

be

known

_exactly

and

therefore

become

independent

of

potential.

At

a

fixed

period,

ususally

5

to

20

milliseconds prior

to

the

fall

of

the

drop,

the

current

is

sampled.

This

current value

is

maintained until

the

next sample period

is

reached

5

to

20

milliseconds

into

the

next

_mercury

_drop

when

the

current

is

sampled again.

A

TAST

polarogram

is

therefore

built

from

a series of

_steps,

the

_rising

portion

corresponding

to

the

time

interval

where

the

current

is

sampled.

If

the

steps are

_extremely

close

_together,

achieved

_by

a short

_drop

time

or slow scan

_rate,

a smooth

1.4

NORMAL

PULSE

POLAROGRAPHY

_(NPP)

A

major advance

in

electroanalytical

techniques

was

realized when pulse methods were

introduced.

The

first

pulse method

_developed,

Normal

Pulse

_Polarography

(NPP)

,

has

a potential waveform and current response

curve as shown

in

Figure

6

(5)

.

The

potential waveform

is

obtained

_by

_maintaining

a constant potential and

applying

pulses of

_successively

_increasing

amplitude

to

successive

drops

of

_mercury

from

the

DME.

The

current

response curve

is

obtained

_by

_sampling

the

current

_during

the

last

few

milliseconds of each pulse.

Sampling

the

current near

the

end of each pulse

largely

eliminates capacitive or

_charging

current.

Capacitive

current arises

because

the

_working

electrode

is

held

at a potential

different

from

that

of

the

solution.

As

a

result,

the

electrode-solution

interface

acts as a capacitor.

The

charge of a capacitor consists

of an excess of electrons on one plate and a

_deficiency

of electrons on

the

other.

The

electrode-solution

interface

can

be

described

in

a similar manner.

When

a

pulse of potential

is

applied

to

the

electrode or when a

new

_mercury

drop

grows

_thereby

changing

the

surface area

of

the

interface,

current must

flow

to

charge or

discharge

the

capacitor.

Therefore,

each

time

a

b)

0

-+

Figure

6

-Normal

Pulse

_Polarography

a> _potential

waveform

b)

current response curve

_(reproduced

f

Reference

5)

.

mercury

drop

forms,

capacitive current

increases.

Sampling

the

current near

the

end of

the

lifetime

of a

pulse

has

the

same effect as

_sampling

the

current near

the

end of

the

lifetime

of a

_mercury

drop:

the

capacitive

current

is

at a

low

level

while

the

faradaic

current,

current

from

the

reduction of

the

_analyte,

remains at a

high

level.

The

measurement scheme

is

illustrated

in

Figure

7

(6).

The

applied voltage

is

shown

in

Curve

A.

The

resulting

capacitive and

faradaic

currents are shown

in

Curves

B

and

C

respectively.

Both

capacitive and

faradaic

current

increase

with

the

applied potential

pulses.

_However,

capacitive current

decays

_rapidly

with

time

while

faradaic

current

does

not.

_Sampling

the

current near

the

end of each potential pulse results

in

the

measurement of

faradaic

current at a

time

when

capacitive current

is

at

its

lowest

point.

This

improved

method of

faradaic

current measurement gives

NPP

greater

a)

T

b)

c)

sb>

<-

2

sec

_A

0.05

sec

_>

35

mV

-^

>kJ

Figure

7

-a) applied voltage

b)

_capacitive

_current

response

to

applied voltage c)

_faradaic

_current

r^pnn,E

to

applied voltage

_(reproduced

_in

1.5

DIFFERENTIAL PULSE

_POLAROGRAPHY

_(DPP)

A

further

improvement

in

the

measurement

of

faradaic

current

resulted with

the

development

of

Differential

Pulse

_Polarography

(DPP).

The

potential

waveform

for

DPP

superimposes pulses of equal

amplitude

on a

linear

potential

_ramp

as shown

in

Figure

8

₍₅₎

.

The

current

is

sampled once

just

prior

to

the

pulse and once

just

before

the

end of

the

pulse.

The

difference between

these

two

current samples

is

then

plotted versus

_potential,

resulting

in

the

peak-shaped response curve shown

in

Figure

8.

This

difference

method of current measurement results

in

even

better

discrimination

against

_charging

currents

than

in

NPP

where

the

larger

pulses give

larger

capacitive currents.

At

the

same

_time,

DPP's

peak-shaped

output offers advantages over

the

current response curves

seen previously.

The

peak

is

symmetric about

the

E

and

its

height

is

_directly

proportional

to

the

concentration

of electroactive species.

The

peak-shaped output makes

accurate qualitative and quantitative

determinations

a)

0

E(init)

Time

b)

0

0

o

Figure

8

-Differential

Pulse

_Polarography

_a)

potential

waveform

b)

current response curve

_(reproduced

_from

1.6

SQUARE WAVE

VOLTAMMETRY

_(SWV)

Many

scientists

believe

that

the

newest

electrochemical

_technique,

Square

Wave

_Voltammmetry

,

may

become

a replacement

for

_DPP,

which

i6

_currently

the

most

popular electroanalytical

technique.

SWV

began

in

1957

when

G.C,

Barker

(9)

applied a square wave

to

a single

drop

of

_mercury

from

a

DME.

_Ramaley

and

Krause

(10)

developed

the

_theory

for

SWV

_by

_modifying

Barker's

treatment

of

the

technique,

however

SWV

was not used

routinely

until

1977

when

J.H.

Christie,

John

A.

_Turner,

M.

Vukovic,

and

R.A.

_Osteryoung

(11)

resurrected

the

technique.

The

waveform used

for

square wave

_voltammetry

consists of a symmetrical square wave which

is

superimposed on a staircase waveform as shown

in

Figure

9

(12).

One

of

the

square wave pulses occurs

in

conjunction with a staircase

_step

while

the

other occurs

at a point one

half

_way

through

the

step

and continues

until

the

end of

the

step.

The

first

pulse

is

called

the

"forward"

pulse since

it

occurs

in

the

same

direction

as

the

scan.

The

second pulse

is

said

to

be

the

"reverse"

pulse since

its

magnitude

is

applied

in

a

direction

opposite

to

the

direction

of

the

scan.

The

reverse pulse

causes re-oxidation of

the

reduced product

that

remained

at

the

electrode surface and was

formed

by

the

forward

IT-PLUS

:step

I

EQUALS

ME

Tl

Figure

9

-Applied

excitation

in

Square

Wave

_Voltammetrv

The

_relationship

between

time

and

the

potential

applied

to

the

electrolytic

cell

in

SWV

can

be

seen

in

Figure

10a.

is

the

time

for

one

square

wave cycle or

one

staircase

_step

in

seconds.

The

square

wave

_frequency

in

Hz

is

_1/^

.

Eswis

tne

height

of

the

square wave

pulse

in mV,

where

_2Esw

is

equal

to

the

peak-to-peak

amplitude.

_Estep

is

the

staircase

_step

size

in

mV

(12).

Using

these

values.

the

scan rate

for

a

SWV

experiment

can

be

calculated

from

the

equation:

Scan

Rate

_(mV/sec)

=

Estep

(mV)

[2]

Y

(sec)

The

experimental parameters used

for

this

research

result

in

a scan rate as

low

as

67

mV/sec.

_(Esep

=

₂

_mV

and

X-

0.03

sec.)

This

scan rate

is

_considerably

faster

than

the

1

to

10

mV/sec rate of other pulse

techniques.

The

current

is

sampled at

two

points

_during

each

square wave

cycle,

once near

the

end of

the

forward

pulse

and again near

the

end of

the

reverse pulse.

This

current

_sampling

technique,

as

discussed

_earlier,

effectively

discriminates

against capacitive

current.

The

_resulting

currents are plotted as either

the

forward

current versus

time

₍

for

reduction processes

)

, as

the

reverse current versus

time

₍

for

oxidation processes

)

,

or as

the

difference

of

the

forward

and

reverse

currents

1

1

\r ,

rr

>

^STvI

2

'1

V

f

Figure

10

-a)

Square

Wave

_Voltammetry

_timinq

relationships

b>

current, response curve

(reproduced

in

against

the

potential measured

in

the

electrolytic cell.

A

typical

current

response

curve

for

SWV

can also

be

seen

in

Figure

10b.

The

major

advantage

of

SWV

is

its

speed.

A

complete

potential scan can

be

performed with each

_drop

of

mercury.

Therefore

a more rapid scan can

be

done

and

the

signal-to-noise

ratio

of

the

current-potential curve can

be

improved

_by

repetitive

_scanning

and signal averaging.

Janet

_Osteryoung

₍₃₎

found

_that,

with

_DPP,

a scan of

500

mV required

250

seconds

(

more

than

4

minutes

)

to

complete,

while square wave

_voltammetry

required

_only

10

seconds

to

complete

the

same analysis.

Early

SWV

experiments were

hindered

_by

the

use of a

Dropping

Mercury

Electrode

(DME).

When

a

DME

is

used,

mercury

drops

are

_continually

_growing

and

_falling

off

during

the

experiment,

resulting

in

an electrode of

constantly

changing

surface area.

The

_changing

surface

area causes

the

current

baseline

to

drift

and makes

the

determination

of an accurate net current

difficult.

Presently,

SWV

uses more stable

_working

electrodes such

as

the

Static

_Mercury

_Drop

Electrode

_(SMDE)

or

_Hanging

Mercury

Drop

Electrode

(HMDE)

.

These

two

types

of

electrodes

deposit

a

_mercury

drop

on

the

_tip

of

the

electrode

before

analysis

begins

and

the

_drop

remains

there

without

_changing

size until

the

analysis

is

complete.

Frequencies

used

for

square wave

_voltammetry

typically

range

from approximately

1

Hz

to

120

Hz.

This

frequency

range allows square wave experiments

to

be

_up

to

100

times

faster

than

other pulse

techniques

(12).

SWV's

speed gives

the

advantage of an

increased

sample

throughput

when compared with other

techniques

and

therefore

makes

it

a prime candidate

for

use

in

process

control applications.

Both

the

square wave amplitude and

the

_step

height

have

an effect on

the

_resulting

current versus potential

plot.

The

difference

current

is

always an

_essentially

symmetric peak-shaped

_function;

_however,

for

small square

wave

amplitudes,

it

is

smaller

than

either

the

forward

or

reverse currents.

The

effect of square wave amplitude on

the

components'

currents and on

the

difference

current

can

be

seen

in

Figure

11.

_Only

for

nEsw

>

~15mV

does

the

reverse current

in

the

_vicinity

of

the

half

wave

potential

become

negative and

the

difference

current

become

larger

in

magnitude

than

the

forward

current

(13).

This

suggests

that

the

analytical response could

be

increased

_by

using

a

larger

square wave

amplitude,

but

increased

amplitudes

lead

to

a

_broadening

of

the

observed

current peak as shown

in

Figure

12

(13)

.

The

_step

height,

Estep

does

not effect

the

too 100 o -100

nCE-Ew3 (mV)

ZOO

(A)

nE=5mV.

_(B)

_nE- 15mV.

_(C)

_r=30mV. Soldk

kDifference currentfunction.

Figure

11

-Effect

of square wave amplitude on current

current

peaks.

The

forward

and

reverse

current peaks

will

increase

with an

increase

in

_step

height

and

decrease

with a

decrease

in

_step

height.

The

net

increase

in

the

reverse

peak will

be

opposite

in

direction

but

equal

in

magnitude

to

the

increase

of

the

forward

peak.

_Therefore,

the

net

difference

peak will

remain unchanged with a change

in

_{step height.}

As

well as

_having

the

_ability

to

generate analytical

information,

SWV

can also generate

information

about

kinetic

processes

_occurring

at

the

electrode.

The

forward

and reverse currents

_may

be

analyzed

to

give

kinetic

information

like

that

given

in

a cyclic

voltammogram,

in

which a symmetric

forward

and

reverse

current curve

indicates

that

the

reduction process

is

reversible while an antisymmetric

forward

and

reverse

current curve

indicates

that

the

reduction process

is

irreversible.

If

the

rate of electron

transfer

at

the

electrode surface proceeds

fast

enough

to

maintain

the

surface concentrations of reactants and products

_very

close

to

their

equilibrium values,

the

reaction

is

said

to

be

reversible.

If

these

equilibrium values cannot

be

maintained,

the

reaction

is

said

to

be

irreversible.

Square

wave

voltammetry,

it

is

believed,

will

be

very

advantageous when solid electrodes are used.

Differential

pulse

_polarography

is

slow and

therefore

allows

undesirable surface reactions

to

occur when solid

electrodes

are used.

The

high

effective scan rates of

SWV

decrease

the

_quantity

of charge

_passed,

which

in

turn

lessens

the

_possibility

of

undesirable

surface reactions

occurring

at

the

electrode.

Another

major advantage of

SWV

is

its

excellent

rejection of

background

currents.

Background

currents

in

voltammetric

techniques

are

_usually

large

and

therefore

are a critical

factor

in

the

determination

of

detection

limits.

Square

wave

_voltammetry,

on

the

other

_hand,

rejects currents

that

are

_largely

independent

of applied

potential.

As

suggested

_by

Figure

10

;

the

net current

in

the

_limiting

current region

is

_exactly

zero.

In

trace

analysis,

oxygen reduction often produces

_unacceptably

large

and unstable

background

currents.

_Using

square

wave

voltammetry,

the

_limiting

current plateau

for

oxygen

reduction

_{automatically}

subtracts

these

background

currents.

The

square wave net signal

is

also

insensitive

to

currents

_arising

from

convective mass

transport

as

long

as

the

characteristic

time

of

that

transport

is

large

in

comparison with

the

voltammetric pulse width

(3).

As

a

result,

the

net square wave signal

is

_relatively

insensitive

to

fluctuations

in

flow

rate and

therefore

is

a prime candidate

for

use as a

detector

for

HPLC.

as a

replacement

for DPP.

The

_sensitivity

of

SWV

was

believed

to

be

reduced

for

_irreversibly

reduced

substances

as

compared

with

_reversibly

_reduced

_material.

Howard

Siegerman

₍₁₄₎

of

EG&G

PARC

wondered

in

1980

"if

SWV

doesn't

resemble

ac

_polarography

_in

_its

inferiority

relative

to

DPP

for

the

analysis

of

_irreversibly

reduced

substances.

In

both

SWV

and ac

_polarography

a

_rapidly

changing

modulation

signal

is

applied

to

the

_working

electrode,

and

the

slow electron

transfer

kinetics

associated with

_irreversibly

reduced substances prevents

one

from

_determining

these

materials with

the

same

sensitivity

as

DPP."

However,

in

_1982,

Robert

_Osteryoung

(5)

demonstrated

that

SWV

was

four

times

more sensitive

than

DPP

for

_reversibly

reduced substances and

three

time

more sensitive

than

DPP

for

_irreversibly

reduced

substances

(see

Table

₁₎

.

It

seems

that

as

SWV

technology

continues

to

_develop

it

becomes

an even

better

rival with

DPP

for

routine

_laboratory

analysis.

Since

the

electronic

_circuitry

required

for

square

wave

_voltammetry

must

have

short response

_times,

the

standard practice of

_integrating

a

_long

time

for

each

sample

to

reduce noise

in

the

measured current signal

cannot

be

used

(12).

For

this

reason,

many

scientists

who engage

in

SWV

research make use of

SWV's

_ability

to

lpeak(SwV)/lp

eak(DPP)

i

Theoretical Calc

ulations

sw

sw

frequency

frequen

_-y

=

10

Hz

=

1

00

r

lz

Reversible Case

1.3

4.0

irreversible

Case

1.1

-T ~T

Condi tt

ons

_Delay

between

pulses

_DPP),

Is.

Pulse

width

50

ms

(DP

_-'),

100

ms

(one

full

square wave

cycle)

Pulse

amplitude

(DPP)

50

mV;

Esw

25

it

V

Change

in

pot.ential

between

p

jlses

(DPP)

10 mV,

stec

height

(SWV)

1 0

mV

Tab

1

e

I

-I

Pea

k <

SWV

) / I

pea

k ( DPP )

.

Theor

et

i

c a

1

Calc

ui.nnr,

Multiple

scans can

be

done

on a

sample

in

less

time

than

it

would

take

to

perform one scan with

DPP.

These

scans

can

be

signal

_averaged,

_reducing

the

noise component

while

_increasing

the

signal

component.

The

signal-to-noise ratio

increases

as

the

square root of

the

number of

1.7

ASYST

The

software

package

chosen

for

use

in

_creating

the

control programs

for

the

interface

was

_ASYST,

a major

data

analysis

package

which

was

_introduced

_in

₁₉₈₄

_by

Adaptable

_Laboratory

_Software,

a

_{Rochester-based}

_company.

ASYST

was

designed

_especially

to

meet

the

_programming

needs of

scientists

and

engineers.

The

ASYST

data

analysis product

is

marketed

_by

Macmillan

Software

and

runs on

the

IBM

personal

computer and

compatibles

which

are equipped with either an

Intel

8087

₍

IBM

PC

and

PC-XT

₎

or an

80287

₍

IBM PC-AT

₎

math

coprocessor

(15).

The

hardware

configuration

must

include

384

Kbytes

of

RAM

or

_more,

two

disk

drives,

and an

IBM

color graphics video

interface

board

(regular

or enhanced

_version)

or a

Hercules

graphics card.

ASYST

is

divided

into

four

separate modules each of

which

handle

different

aspects of

data

analysis.

The

first

_module,

Systems

Graphics

and

_Statistics,

covers

graphics

_operations,

_statistics,

_array

_operations,

programming

control

_structures,

and

RS-232

communications.

Module

_{2, Analysis,}

provides a complete

set of

data

reduction

routines.

Module

3.

Acquisition,

supports

_A/D,

_D/A,

and

digital

data

acquisition and

control.

_Finally,

Module

4,

GPIB/IEEE-488

.

interfaces

module.

ASYST

contains

_many

pre-written

software

tools

such

as a

spreadsheet-like

data

editor and

interactive

graphics which can

be

used

_{independently}

or

incorporated

into

custom application programs written

_by

the

user.

The

data

acquisition and analysis module supplies

prewritten

tools

such as

the

_ability

to

_simply

input

analog

information

from

instruments,

the

_ability

to

find

the

local

minima on a

_waveform,

or a

_highly

optimized

Fast

Fourier

Transform

_(FFT)

(17).

ASYST

provides

the

user with a wide

_variety

of

interactive

commands

specifically

suited

to

analysis problems.

_Simply

_typing

"STACK.

DISPLAY

CR" allows

the

user

to

view

the

contents

of

the

stack at

_any

time.

ASYST

allows arrays

to

be

manipulated as

_easily

as

scalars.

The

arrays can

be

multiplied,

divided

or edited

with

_simplicity

and without

the

usual

_necessity

of

the

use of

loops.

ASYST

also provides a convenient error

display.

Words

and parameters

_may

be

entered one per

line,

or multiple words and parameters

_may

be

entered on

the

same

line

if

separated

_by

spaces.

Spaces

are

the

delimiters

in

ASYST,

so words must

be

separated

_by

spaces.

If

a

line

contains more

than

one

_word,

the

words

are executed

sequentially

from

left

to

right.

As

each