A Rapid, Accurate and Simple Coulometric Method for C & S ... - Behr

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Feature
A Rapid, Accurate and Simple
Coulometric Method for C & S
Analysis
Author
Jürgen Behr
behr Labor-Technik GmbH
Düsseldorf
Germany
It is important that the determination
of carbon and sulphur is accurate and
precise, to meet stringent quality
control demands. Coulometry is a
versatile and simple method of
determination; small amounts of
analyte can be used, but it is also
suited to high analyte contents.
This article examines the
coloumetric process and looks at a
coulometric method, used for the
analysis of both Carbon and Sulphur.
Sue Fakes
ILM Features Editor
Accurate and precise determination of carbon and sulphur,
especially when present at high levels in diverse sample
matrices, are analytically challenging requirements.
Quality control of product materials often requires high
accuracy. Although the coulometric technique finds application
with even small amounts of analyte, the accuracy,
precision and simplicity of coulometry are unrivalled at
high analyte contents.
Coulometry, being an absolute method based on
Faraday’s Law, does not require calibration with reference
standards. The ability of modern electronics to accurately
measure and integrate current-time functions permits
coulometry to be applied even with relatively small
amounts of sample material [1,2,3,4,5]. Coulometry finds
application in micro and trace analysis of elements such as
C, H, O, Cl, Br, I and S in organic and inorganic matrices.
Indeed, the coulometric titration of carbon has been
increasingly accepted as the method of choice in numerous
applications [6,7,8,9]. Coulometric analysis is excellently
suited to the determination of total carbon in the following
sample materials:
• silicon carbide and tungsten carbide
• ferrous and non-ferrous metals
• cement
• soil
• ceramic materials
• minerals
• coal, etc.
• total carbonate and non-carbonate carbon in sedimentary
rock [10].
A typical coulometric system, applicable to determination
of both carbon and sulphur, consists of a high temperature
furnace, a microcoulometric current controller/current-time
integrator, a sensitive indicating electrode as detector and
associated computational and data storage capabilities
(Fig. 1).
Figure 1: A typical coloumetric system
A furnace for a system designed for both carbon and sulphur
analyses contains two separate combustion tubes for
each analyte, as well as an internal combustion tube for
pre-purification of the oxygen supply. Carbon or sulphur
samples are typically combusted in the respective furnace
tube at a temperature of 1,350 ˚C in an oxygen stream.
Excess oxygen exiting from the sample insertion port prevents
the entry of atmospheric CO2 into the furnace. On
exiting the furnace, the combustion gases are passed
through dust traps and reagent tubes, which remove moisture
and interferants (CO2 would interfere in the sulphur
determination and oxides of sulphur would interfere in the
carbon determination). The CO2 or SO2 produced as com-
bustion products of the carbon or sulphur content in the
sample are then quantitatively absorbed in the electrolyte
of the coulometric titration cell through employment of a
high-speed gas dispersion stirrer.
The detection principle is based upon the change of pH
in the titration cell, which results from the dissolution of
the CO2 or SO2. In the case of SO2, hydrogen peroxide is
used to oxidise the SO2 to sulphuric acid, which produces
a pronounced pH alteration. A glass electrode with built-in
reference is used to detect the pH deviation. Response
time is enhanced, while sensitivity to electronic interference
and electromagnetic perturbation is minimised by
use of a preamplifier mounted directly on the pH electrode.
Since the potential of the pH electrode is temperature
dependent, the temperature of the titration cell is waterjacketed
for maintenance of temperature constancy. In
addition to the pH electrode, the titration cell is provided
with a platinum cathode. The cathodic electrode reactions,
which proceed at 100% current efficiency in the electrolytes
utilised are:
2H2O + 2e - 2OH - + H2 (1)
2H3O + +e - 2H2O + H2 (1a)
for the carbon determination.
Thus, the cathodic electrode reaction is used to quantitatively
neutralise the alkalinity shift produced by the dissolution
of the carbon dioxide.
The platinum anode, at which the oxidation reaction proceeds,
is separated from the titration cell by a ceramic
diaphragm, which prevents intermixing of the electrolytes
in the two cells.
Figure 2: Diagrams showing the process
At the start of determination, the pH within the titration
cell (Fig. 3) is adjusted automatically to a specified initial
value. For carbon analysis, a basic pH of approximately 10
is employed. This initial pH is exactingly established electrolytically,
and exactingly measured by the pH electrode.
The amount of electricity (in Coulombs, determined as the
integral of titration current over time) required to restore
the pH to its initial value, after the deviation resulting from

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absorption of the combustion gas, is directly proportional
(Faraday’s Law) to the mass of CO2 or SO2, which has dissolved
in the titration cell. A simple stoichiometric computation
then enables calculation of the percentage of carbon
or sulphur in the sample mass.
Figure 3: Graph of pH against Charge (mC)
The electrolyte employed for the carbon determination is a
barium perchlorate solution. On introduction of the carbon
dioxide into the cathode chamber of the titration cell, the
following reaction leads to a decrease in the cell pH:
Ba 2+ + CO2 + 2OH – BaCO3 + H2O (2)
Ba 2+ + CO2 + 3H2O BaCO3 + 2H3O + (2a)
The electrolyte employed for the sulphur determination is
a sodium sulphate solution containing hydrogen peroxide,
which has been adjusted to an initial pH of approximately
4. On dissolution in the titration cell (cathode chamber),
the sulphur dioxide is converted to sulphuric acid according
to the following equation:
SO2 + H2O2 H2SO4 (3)
The corresponding reduction on the cathode electrode proceeds
according to equation (1), so that electronically generated
sodium hydroxide tends to change the pH of the
solution until the initial value is reached.
Within the anode chamber, the complementary reaction
to equations (1) and (1a) proceeds at the platinum
electrode:
3H2O 2H3O + + 1/2O2 + 2e – (4)
2OH – H2O + 1/2O2 + 2e – 4a)
for the carbon analysis. Reaction (4) also takes places
within the anode chamber of the sulphur apparatus.
A solid phase of barium carbonate within the anode
cell is used to neutralise the ions H3O + arising according
to eq. (4). Similarly, in the case of sulphur determination
the acid H2SO4 is precipitated by means of ZnO to ZnSO4
in order to maintain the pH within the cell at a constant
level.
It is thus evident that the carbon or sulphur analytes in
the samples, which are quantitatively converted to their
respective oxides, result in an acidic shift of the pH in the
titration cell electrolyte. By means of the electrolysis reactions
(1), the pH can be restored exactly to its initial value.
The amount of carbon or sulphur in the sample is then
obtained from the coulometric current-time integral.
Coulometric analysis conducted under conditions of
100% current efficiency is an absolute method and as
such requires no calibration. Nonetheless, for quality control
purposes, the accuracy and precision should be verified
using reference standards. For the carbon determination,
calcium carbonate is well suited as a standard, while
for the sulphur determination, silver sulphate is
recommended.
The coulometric procedure using a ceramic tube furnace
has been successfully applied to the determination of
sulphur in
• oils, greases, petrochemicals
• ferrous and non-ferrous metals
• plastics
• raw materials and natural products containing sulphur,
such as wood,etc.
The electronic and control unit of the coulometer consists
of
• a very precise pH detection system,
• a digitally controlled current source,
• a coulometer (for integration of the titration current-time
curve), and
• a combustion-gas pump and gas splitter, the operation
of which is coupled to the pH in the titration cell.
The potential of a glass pH electrode is used as a signal to
control the level of titration current and to detect the titration
end point. A preamplifier is mounted directly atop the
glass electrode to enhance response time and minimise
effects of electromagnetic perturbations. The analog output
of the preamplifier is digitalised in a 20 bit A/D converter
and then input into the micro-controller.
The titration current is regulated quasi continuously (in
256 constant-current steps) and controlled by a PID-controller.
The PID parameters are user-programmable by
means of system software installed in a conventional PC.
Integration of the titration current is performed by means
of a 16 bit processor within the controller module. The
same micro-processor also controls A/D converters, the
current source and the initial processing of raw data. The
completion of the computations and display of the analysis
parameters and findings in both numerical and graphical
form is done by a personal computer.
The following four analytical parameters may be optionally
displayed as functions of time on the monitor: (Fig. 4).
Figure 4: Four analytical parameters displayed as functions of time
• the pH at the start of the analysis. The value may be
specified by the user by entry into the computer program.
The value actually achieved will be displayed.
• the real-time pH-value in the titration cell
• the titration current
• the mass of analyte, and optionally
• the temperature of the combustion furnace.
A mathematical treatment of the time-dependent rate of
analyte mass flow into the titration cell offers the prospect
of employing the coulometric analyser for online determinations
[4]. Furthermore, evaluation of the analyte mass
flow as a function of time discloses useful information pertaining
to the process of sample combustion, which may
be used to optimise the combustion process. Realisation
of these possibilities by means of software programming is
in development at the present time.
The software used for operation of the instrument, the
signal processing, data computation and display as storage
of analysis findings development of measuring data
operates under Windows 98 as well as Windows NT
4.0. Other commercially available application software
operating under Windows, such as data base or spreadsheet
programs, can be used for
• data management and recording,
• statistical analysis.
• GLP control
• LIMS
The system conveniently allows for the direct communication
of the sample weight from the analytical balance to
the computer by means of a serial cable.
Optionally, the high temperature (max. 1,550 ˚C) ceramic
tube furnace can be replaced by an infrared quartz-tube
furnace (max. temp 1,100 ˚C) coupled to a two-zone downstream
resistance furnace. The IR furnace has rapid heating
rates and enables execution of temperature-time programs
(a sequence of up to five increasing temperature
gradients and five temperature plateaus are freely programmable).
The infrared furnace requires no cooling
water for operation. Rapid cooling by means of ‘heat pipes’
enables the furnace to return to room temperature within
minutes after completion of a program which ran to maximum
temperature. The downstream constant temperature
resistance furnaces ensure complete oxidation and
removal of interferences in various applications, such as
the determination of
• TC,TIC and TOC in aqueous, sludge and solid samples
• organic components in dust
• Diesel soot and motor emissions in accordance with the
German standard methods
• Compliance with workplace air safety standards through
determination of carbon compounds and elemental carbon
(particulate) content
• adsorbable organically bound carbon in exhaust gases.
General technical characteristics:
Analysis time: typically 2-5 minutes, depending on
weight and nature of sample
Analytical range Carbon: 5 ppm – 100%, depending on
sample weight
Sulphur: 10 ppm – 10%, depending on
sample weight
Precision better than 0.05%
References
[1] Lingane, J.J., Anal. Chem.26 (1954), 522-626
[2] Abresch,K., Claasen, J., Die Coulometrische Analyse,
Weinheim, 1961
[3] Ehrenberger, F., Quantitative Organische
Elementaranalyse. VCH, Weinheim 1991, New York,
Basle, Cambridge
[4] Kupka, J.,Luitjens, M.,Grinewitschus, V., Dalsaß, K.,
Fresenius Z. Anal. Chem. 347 (1993), 87-91
[5] Luitjens, M., Kupka, J., Gherban, D., Baumgarten, E.,
Fresenius Z. Anal. Chem. 357 (1997), 592-592
[6] Staats, G., Fresenius Z. Anal. Chem. 731 (1984),
761-764
[7] Frank, D., Staats, G., Fresenius Z. Anal. Chem. 327
(1987), 456-460
[8] Kupka, J., Lottmann, J., Stremming, H., Stahl u. Eisen
113 (1993) No. 9, 121-124
[9] Ohnemüller, W., Solf, A., Zement-Kalk-Gips 59 (1973),
200-202
[10] Herrmann, A. G., Knacke, D., Z. Anal. Chem. 266 (1973)
196-201
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