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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963

LLL
OR NL A-3261
Conf-670720-21
CESTI POICES
H0 $3.00 MN.
CURRENT CAPABILITIES IN ANALYSIS OF TRACE
SUBSTANCES: ELECTROANALYTICAL METHODS*
RECEIVED BY DTIE AUG 29 1967
... Myron T. Kelley, Director
Analytical Chemistry Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee
MASTER
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This paper will attempt to present the current capabilities of electro-
analytical methods in trace analysis. These techniques are particularly attrac-
tive for such analyses because many of them are readily adaptable to automated
and continuous measurements.
All of the electroanalytical methods are solution methods and, for them to
be applicable to solid or gaseous samples, suitable solutions must be prepared.
Electrolytic conductivity has been used to monitor water quality for many
years. While it is almost completely non-selective, it is widely used to monitor
total salt concentration because of its ease of measurement. It has also been
applied at times as a method of determining the endpoint of titrations. By
absorbing sulfur dioxide from the atmosphere in hydrogen peroxide, the conductivity
of the resulting sulfuric acid solution has been used as a continuous indication
of sulfur dioxide concentration. .
Glass electrodes have been in use for many years as a means of determining
hydrogen ion concentration. We have always been warned that under certain condi-
tions they would also be affected by the concentration of other lors, particularly
by sodium ions. As the mechanism of the development of the potential in glass
electrodes has become better understood, this knowledge has been applied to make
electrodes which are responsive to other ions than hydrogen. This mechanism is
now thought to involve an ion exchange process on the surface layer of the elec-
trode, and using this hypothesis as a basis it has been possible to construct
useful electrodes for a number of ions. Although these are popularly called
specific ion electrodes It would be more nearly correct to say ion-selective
*Research sponsored by the U. 8, Atomic Energy Commission under contract
with the Union Carbide Corporation.
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DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED
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electrodes. When two lons are present in a solution, the electrode; potential 18::.
given by the following equation:
E = E + Bar In ([^*] + k[B*])
where x (or frequently actually its reciprocal) is called the selectivity coeffi-
cient. It is apparent that the interference of a second ion will depend strongly
upon the relative concentrations of the two lons. Actually in all cases the quan- :
tity measured is the activity of the ion which is a function of the lonic strength
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and of any complexing agents which may be present. If true concentration measure-
ments are desired, such electrodes can be used as Indicator's in titrations.
Specific ion electrodes have been developed in a number of types. Glass
electrodes are available which are responsive to Na*, **, *£* and Agt as well
as to n*. Electrodes containing an insoluble precipitate in a silicon rubber
matrix are available using Agi, Agbr, and Agci precipitates. The loàide con-
taining electrode 18 particularly satisfactory and 18 responsive down to jodide
concentrations of 10-0 M. Solid single crystal membrane electrodes are available
for fluoride and sulfide measurement. The fluoride electrode works very satis-
factorily down to levels of 10-0 M, and the sulfide electrode 18 said to be
responsive down to 10*' M solutions of sulfide ion. A fourth type of specific
ion electrode uses a membrane saturated with an organic liquid ion exchanger
containing the ion to be measured. The membrane can be either an organic
dialysis membrane or a porous glass frit. Those using organic membranes are
made in the form of a kit so that the electrode can be renewed since the useful
life of a membrane 18 relatively short (perhaps one month). Very satisfactory
electrodes of this type are available for ca**, cu, divalent cations, No , and
cioj, and there are undoubtedly many others which can be made. While in general
the response time of these specific ion electrodes 18 slow compared to glass
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electrodes for pH measurement, they are fast enough that no real difficulty is
encountered.
Voltammetry, the measurenent of the current at an electrode as a function
of its potential, has found wide application as a method of measuring the con-
centration of ions. The most widely used form of this technique, known as
polarography, involves the use of a dropping mercury electrode, in which the
mercury flows contimously from a very small capillary and the small drops at
the opening of the capillary constitute a polarized electrode with a surface
which is being constantly renewed. Most commonly the potential applied to the
system 18 mire d.c. Which 18 scanned slowly through the potential range of
interest. Different lons are reduced at this elentrode at different potentials,
thus the method has considerable degree of selectivity which can frequently be
modified or enhanced by the proper cho..ce of complexing agents in the medium
selected. Ordinarily d.c. polarography is most useful in the concentration
range of 2009 - 20-3 M. Variations of the technique such as the use of derivative
taking circuitry or the use of a rapidly scanred potential with oscillographic
recording can extend the concentration range to 100 - 2007 M. By superimposing
a small a.c. potential on the d.c. potential applied to the dropping mercury elec-
trode and measuring the a.c. component of the current, one obtains ver;" sensitive
polarographic curves which have the form of derivatives of the conventional d.c.
polarograms but which suffer from having a high background caused by the capacity
of the electrode. By using a square wave a.c. potential arid suitable gating
circuitry, the capacitive component can be minimized and the sensitivity capabil-
ities much enhanced. It 18 thus claimed that sensitivities in the range of 100 M
are attainable, but this may be difficult to utilize fully because of impurity
levels in the large concentration of supporting electrolyte wich 18 required
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for this technique.
By using a single stationary drop of mercury the sensitivity of voltammetry
can be considerably enhanced for metals which are soluble in mercury by first
applying a potential beyond the reduction potential of the metal desired and
e)ectrolyzing for a considerable time. In this process a quantity of the metal
is concentrated in the surface of the drop. Then by sweeping the potential back
to a point below the reduction potential the metal is stripped out and the magni-
tude of the peak current is a measure of the amount present. Using this technique
it 18 possible to analyze trace metal.s down to 10? M or even lower.
For eituations in which mercury electrodes are not suitable, such as at
oxidizing potentials at which mercury dissolves, solid electrodes of platinum,
gold, pyrolytic graphite, wax-impregnated graphite, or pastes of carbon in inert
organic solvents have been used. The carbon and graphite electrodes are particu-
larly suitable when the analysis of organic materials is involved.
As previously pointed out, voltammetry 18 useful for those ions that are
electroreducible or oxidizable. Metals that are commonly analyzed by such
techniques at trace concentrations include po**, cut, ca**, zn**, Mnt*. So le
organic materials are also determinable by these techniques.
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The technique of chronopotentiometry, the measurement of current as a function
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of time at a fixed value of current, while it has potential applications in analysis,
has found little practical use. In fact, it can be said that for determination of
concentration there 18 little that can be done by chronopotenti.ometry that cannot
be done better and more easily by polarography.
.:: A very specialized application of voltammetry and one of considerable use
in environmental analysis 18 the analysis for dissolved oxygen. A number or very
succosmful commercial devices t'or this analysis are available. Some involve
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reduction of oxygen at a dropping mercury electrode, while others involve diffu-
sion of the oxygen through a membrane to a solid electrode. The membrane mini..
wizes the interference of other electroreducible substances. At least one very
simple device makes use of a consumable anode of such a material that. a self
generating cell is formed and no externally applied potential 18 necessary. A
very sensit.ve technique, but one limited to relatively pure water involves the
reaction of the oxygou with thallium followed by electrochemical deteraination
of the thellium hydroxide formed.
Coulometry, which 16 essentially an electrochemical titration technique,
would ordinarily not be thought of an a trace analysis technique, but by careful
attention to details it has been used for analysis of quite small guantities,
down to the range of a few micrograms or less.
Constant current coulometry lnvolves the electrochemical. generation of a
reagent which then reacts with the substance to be determined with the endpoint
being determined by such techniques as amperometry, potentiometry, or colorimetry.
The time reo "red to reach the endpoint then determines the magnitude of the titra-
tion. Many titrants such as I, Brg, C1, Ag, ca*, Fe2+, sn2+, T134, No2+, ce++,
OH", and #* have been successfully generated with 100% current efficiency.
In controlled-potential coulonetry the electrode 18 held at a constant
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potential by a potentiostat and the current required to convert a substance
from one state to another 18 integrated over the course of the titration. The
titration is concluded when the current falls to some preselected small fraction
of its initial value. Controlled-potential coulometry can be more selective tiran
constant current coulometry because the potential can be chosen such that one ion
will react but others will not.
Coulometry can be used to monitor substances in flowing liquid or gaseous
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streams by measuring the current required to keep a titration vessel at a given
endpoint as a constant flow of sample 18 added. Such a technique has been
applied to monitoring mercaptans or sulfides in gaseous streams or to monitoring
toxic amines (such as ethylene dianine) in the atmosphers.
Electroanalytical methods deserve their place along with other techniques
in the analysis of trece substances concerned in environmental health.
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LEGAL NOTICE
is
This report were prepared una nocount of Government sponsored work. Netther the United
Kated, not the contesten, nor any person voting on buhall at the Commdastaus
.4. Mai muy warranty or representation, a n d or implied, with respect to the nood
rhoy, completament, or wetlawn of the larmation contained to the report, or that we
of my Information, peratura, mother, or preocu declared in this report may not futringo
petrato y owned rates of
3. Apmamos muy lloblution with repoot to the wo of, or for damages rolling from the
w olny taformation, apparatua, method, or prona dinalowed in the poputto
! As wood to the abovo, spornoa notteg a boall of the Cimmineboo" motions may en
ļ piogu ar contractor of the Commission, as weploys of wala tatrantor, the stone ther
wol employee of contrator of the Communication, or program a much contractor properesh
dosaminates, or frontem nooooo ho, aby tutornatira primant to womployment on oontract
with the Commission, of His employment with mode contariator.
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