EPA-660/4-75-001
APRIL 1975
Environmental Monitoring Series
Determination of Molecular
Hydrogen Sulfide
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Corvallis, Oregon 97330
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RESEARCH REPORTING SERIES
Research reports of the Office of. Research and Development,
U.S. Environmental Protection Agency, have been grouped into
five series. These five broad categories were established to
facilitate further development and application of environmental
technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in
related fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL MONITORING STUDIES
series. This series describes research conducted to develop new
or improved methods and instrumentation for the identification and
quantification of environmental pollutants at the lowest conceivably
significant concentrations. It also includes studies to determine
the ambient concentrations of pollutants in the environment and/or
the variance of pollutants as a function of time or meteorological
factors.
EPA REVIEW NOTICE
This report has been reviewed by the National Environmental
Research Center—Corvallis, and approved for publication. Mention
of trade names or commercial products does not constitute endorsement
or recommendation for use.
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EPA-660/4-75-001
APRIL 1975
DETERMINATION OF MOLECULAR
HYDROGEN SULFIDE
by
Thomas B. Hoover
Southeast Environmental Research Laboratory
National Environmental Research Center
Athens, Georgia 30601
Program Element 1BA027
ROAP 16ADN, Task 38
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
CORVALLIS, OREGON 97330
For Salo by the National Technical Information Service
U.S. Department of Commerce, Springfield, VA 22151
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ABSTRACT
The gas sparging technique and a new gas-sensing electrode were
evaluated for the determination of dissolved molecular hydrogen
sulfide at environmentally significant concentrations in water.
From the sparging experiments approximate coefficients were
obtained for the distribution of hydrogen sulfide between
nitrogen and distilled water, seawater, or municipal sewage
effluent. In the latter medium the volatility of hydrogen
sulfide was very much less than predicted from the pH-total
sulfide relationship. The electrode, consisting of various
semipermeable membranes, buffered electrolyte filling solution,
silver-silver sulfide crystal sensor, and lanthanum fluoride
internal reference electrode, gave a generally Nernstian
response to more than 0.1 mg/i of molecular hydrogen sulfide.
At lower concentrations the response was typically several
tenths of a volt per decade of concentration, but was not
reproducible among different samples or electrodes. Various
sources of the anomalous behavior were considered. The
electrode is recommended for in situ measurements of molecular
hydrogen sulfide at concentrations greater than 0.1 mg/1.
More work is needed to make it useful at lower concentrations.
This report was submitted in fulfillment of ROAP 16ADN, Task 38
by the Southeast Environmental Research Laboratorh, Athens,
Georgia, under the sponsorship of the Environmental Protection
Agency. Work was completed as of March 1974.
-------
CONTENTS
Sections Page
I Conclusions 1
II Recommendations 2
III Introduction 3
IV Gas Sparging 4
V Gas-Sensing Electrode 11
VI References 37
iii
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FIGURES
No. Page
1 Gas-*Sparging Apparatus 6
2 Electrode Construction 15
3 Response Curves — Citric Acid 19
Filling Solution, pH 6
4 Response Curves — Tris Buffer 20
Filling Solution
5 Response Curves — Phosphate Buffer 21
Filling Solution
6 Chart Recording — Chemplast PTFE 22
Membrane
7 Chart Recording — Orion PVC 23
Membrane
8 Chart Recording — Orion Teflon 24
Membrane
9 Calculated mV vs. pHgS for Figures 25
6, 1, and 8
10 Chart Recording — GE Silicone 26
Membrane
11 Electrode Response Rate Curves 30
iv
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TABLES
No. Page
1 Gas Sparging of Hydrogen Sulfide 8
2 Manufacturer's Recommended Electrode 13
Filling Solution
3 Electrode Data for Figures 3-10 17
4 Linear Calibrations of Hydrogen Sulfide 18
Electrode
5 Conditions for Electrode Rate Curves 31
Shown in Figure 11
6 Parameters of Equation 3 Obtained by 32
Fitting to Data for Curve lib
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ACKNOWLEDGMENTS
The interest and cooperation of Dr. Martin Frant and Mr. John
A. Krueger of Orion Research, Inc., in the preparation and
evaluation of the hydrogen sulfide gas-sensing electrode is
acknowledged with sincere thanks.
Most of the experimental work was performed by Mr. George D.
Yager.
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SECTION I
CONCLUSIONS
A. new, gas-sensing electrode responded logarithmically to
molecular hydrogen sulfide in water at concentrations
greater than 0.1 mg/1.
At hydrogen sulfide concentrations significantly less than
0.1 mg/1 the electrode usually gave an anomalous response of
several hundred millivolts per decade of concentration.
The same quantitative performance was obtained with a
variety of membranes and filling solutions.
Both the electrode and gas sparging techniques measured the
thermodynamic activity of hydrogen sulfide, which is a
function of the ionic strength of the solution. In seawater
the activity was approximately 2055 greater than the
stoichiometric concentration of the molecular species.
In sewage effluent the indicated concentration of molecular
hydrogen sulfide was very much less than the value calcu-
lated from the ionization constant, pH, and total sulfide
concentration, as added or determined by the methylene blue
method.
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SECTION II
RECOMMENDATIONS
The gas-sensing hydrogen sulfide electrode is recommended
for applications where the molecular species of hydrogen
sulfide in water must be measured directly, in situ, at
concentrations greater than 0.1 mg/1.
The wide millivolt range of the gas electrode and good
repeatability of response, at least for a particular
electrode and sample, suggest that the device might be
useful at very much lower levels of hydrogen sulfide if the
response were more predictable. The non-Nernstian behavior
at low concentrations of hydrogen sulfide may be related
both to thin film liquid diffusion and to interfacial
potential mechanisms. The kinetics and mechanism of the
silver sulfide solid-state sensor response should be studied
as a function of surface treatment and condition of the
crystal sensor.
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SECTION III
INTRODUCTION
Sulfides frequently occur at environmentally harmful levels
in the hypolimnion of lakes and fiords as a result of anoxic
decomposition of organic matter, and may also result from
industrial pollution. Evidence (1,2) indicates that the
toxicity to fish of dissolved molecular species of hydrogen
sulfide is greater than that of the ionic species in
equilibrium with it. To assess the ecological significance
of dissolved sulfides more accurately a direct analytical
method is needed for the molecular species in the
approximate concentration range of 10 yg/1 to 3 mg/1.
The calculation of un-ionized hydrogen sulfide from total
sulfide and pH (3) requires corrections for temperature and
ionic strength. A method based on gas sparging of the
molecular species from a water sample, followed by
colorimetric determination of the total sulfide removed, was
developed under contract (U). The present report deals with
an evaluation of the sparge method, especially as applied to
polluted and saline samples, and a study of a new "gas-
sensing" electrode technique for determining molecular
hydrogen sulfide in water.
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SECTION IV
GAS SPARGING
REQUIREMENTS AND LIMITATIONS
Although gas sparging is, in principle, a relatively
straight-forward means of determining a gaseous species in
solution, it has some severe limitations. For the amount of
substance determined in the gas stream to bear a direct
relation to the concentration in the solution, it is
important that the solution not be altered appreciably by
the sparging process. In a well buffered solution of high
total sulfide content and high pH the amount of l^S removed
may be many times that actually present as the neutral
species in the solution at any instant without displacing
the equilibrium significantly, since the reservoir of
sulfide ion can maintain steady conditions. On the other
hand, in an unbuffered solution of low total sulfide
concentration the removal of even 10X of the neutral species
can alter the equilibrium appreciably. The latter situation
is more typical of the natural waters of interest.
Therefore, the sparging process must be restricted to
removing a small fraction of the species in the sample. A
very sensitive analytical technique is then required.
In addition, the sparge technique is ill-adapted to field
use, and the transportation of samples to a laboratory
imposes problems of sampling, storage, and preservation of a
component that is volatile, subject to oxidation, and in
labile equilibrium with ionic forms. Consequently, there
may be serious difficulties in relating the laboratory
determination to actual conditions in the field.
Use of the sparge technique involves the assumption that the
ionic activity coefficients and the distribution coefficient
of hydrogen sulfide between the sample and the sparge gas
are constant from sample to sample, or can be estimated with
sufficient precision.
ROCKETDYNE RESULTS
In the contract work at Rocketdyne (4) samples were sparged
with nitrogen and the removed i^S was trapped as CdS and
determined by the methylene blue colorimetric procedure.
With the use of 10-cm path-length microcells, a photometric
detection limit of 0.2 pg E^S was obtained. The
determination of H^S at a concentration of 1 yg/1, while
removing no more than 10X of the H->S in the sample,
therefore required a sample of at least two liters. Most
-------
calibrations were made in 0.01 M solutions of Na2S in
deionized water or in 3.5% NaClT Calculated molecular H2S
concentrations were 3-34 vg/1. Observed methylene blue
absorbances were compared with absorbances calculated
assuming ideal extraction and constant molar absorption
coefficient. On this basis, recoveries were 85-100X, with
relative standard deviations of 2-5%. Only two runs
simulated natural conditions, with low total sulfide (3
yg/1, essentially all in the molecular form) ; 85% recoveries
were obtained in each run. Because of the high recoveries
and agreement between runs in deionized water and in 3.5%
salt solution the contractor concluded that the method was
applicable to freshwater and seawater without making ionic
strength corrections.
EXPERIMENTAL
Some in-house tests of the sparge method were made to check
its applicability to several natural samples with the
objective of using it as a referee method for checking the
gas-sensing electrode. The apparatus, shown in Figure 1,
included a sparge chamber of a design essentially the same
as that used by Rocketdyne (4) , but with a liquid capacity
of one liter, as compared to the two-liter capacity of the
Rocketdyne chamber. Hydrogen sulfide was trapped by passing
the sparge gas through 5 ml of 0.02 N NaOH above a fine
sintered glass disk. Negligible sulfide was collected in a
second similar trap in series, demonstrating the efficiency
of collection. The volume of sparge gas was measured by
displacement rather than by flow rate. The methylene blue
determination was made by the procedure of Reference 4,
using 1-cm cells in a Beckman Model B spectrophotometer at
670 nm. The detection limit was about 1 yg of H2S and the
apparent molar absorbance based on the calibration curve
between 1x10-« M and 2.5 x 10-s M sulfide was 25 x 10'. The
test solution was sampled at intervals from the bottom of
the sparge column to provide a material balance and
determination of the distribution coefficient.
The initial sulfur concentration in the sample (Sip, mol/1)
was calculated using the total amount of sulfur in the
sample and in the trap after sparging. Analytical
concentrations in the sample and trap were determined from
the spectrophotometer readings and calibration curve, with
allowance for dilution of the specimen taken for analysis.
The activity of sulfide ion species, S*- in the sample was
calculated according to Equation 1, using ST and the
measured pH.
IS2'] = Sj/Kio'1^/!^) + (lO'/Y^) + (1/Y2)] (1)
-------
FIGURE 1. Gas-Sparging Apparatus
6
-------
K^ and K2 are the respective ionization constants for
hydrogen sulfide, taking pK, = 7.02 (5) and pK2 = 13.78
(6) . The activity coefficients of the singly- and doubly-
charged ions, YT and Y2 , respectively, were estimated from
a graph provided by Orion Research (7) for simple
electrolyte solutions having ionic strengths less than 1 M.
In seawater, Yn and Y2 were taken as r 68 and 0.16,
respectively (8). In sewage effluent, 0.95 and 0.90 were
used arbitrarily for Yj and Y2. The negative logarithm of
the activity of the molecular species of hydrogen sulfide
was calculated from ionization constants by Equation 2.
pH2S = 2 pH + pS2~ - pK-L - pK2 (2)
The apparent distribution coefficient, PD, was obtained by
dividing the molar concentration of molecular hydrogen
sulfide in the water by the molar concentration in the
sparge gas. The latter was obtained by dividing the moles
of sulfide collected in the trap by the corresponding number
of liters of nitrogen used. The concentration in the water
was the mean value calculated for samples taken at the
beginning and end of the sparge period. The material
balance, expressed on a percentage basis, was obtained by
dividing the total number of moles of sulfide initially in
the water into the sum of the moles of sulfide in the water
at the end of the run, that collected in the trap, and that
in water samples taken for analysis.
The results of several sparge runs are summarized in Table
1. In all but one run, 1.5 1 of nitrogen was used and two
or more intermediate samples were withdrawn. No more than
one fourth of the total sulfide in the sample was removed as
sparged hydrogen sulfide during a run and generally less
than ten percent was removed between successive analyses.
Material balances on total sulfur were between 80 and 120X.
The results are best compared in terms of the derived
distribution coefficient for hydrogen sulfide between the
aqueous solution and nitrogen. The mean value of the
distribution coefficient obtained from five runs in de-
ionized water was 2.18, with a standard deviation of 0.43.
In reference 4, consistent recoveries were obtained on the
basis of a distribution coefficient of 2.50, which was
obtained from solubility measurements at very much higher
concentrations of hydrogen sulfide (9). For three runs in
seawater we found a mean value for PD of 1.79, with a
standard deviation of 0.2U. A value lower than that in
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Table 1. GAS SPARGING OF HYDROGEN SULFIDE
Run
1. Deionized
Water
1.14 ppm H0S
2. Deionized
Water
2.69 ppm H2S
3. Deionized
Water
2.12 ppm H,S
4 . Deionized
Water
2.53 ppm H0S
5. Deionized
Water
1.75 ppm H_S
6 . Seawater
1.03 ppm H S
Sparge
Vol. (I)
0
0.5
1.0
1.5
0
0.5
1.0
1.5
0
0.5
1.0
1.5
0
0.5
1.0
1.5
0
2.0
0
0.5
1.0
1.5
PH
7.59
7.65
7.70
7.76
6.58
6.64
6.74
6.84
6.89
6.94
7.02
7.06
6.93
7.00
7.07
7.15
6.96
7.12
6.85
7.09
7.23
7. .41
s
MxlO5
16.4
16.2
15.1
15.2
11.0
10.8
9.8
8.7
11.1
9.3
9.2
8.7
13.8
12.1
10.5
10.8
9.9
8.4
9.8
9.4
7.4
7.3
H2S
MxlO5
2.11
1.87
1.56
1.41
5.00
4.73
3.99
3.24
3.94
3.11
2.82
2.54
4.70
3.81
3.02
2.84
3.26
2.16
3.04
2.11
1.34
0.89
Collected
H2S(ymoles)
. 70
O 00
• M *J
1 ? 7R
10.62
8 17
7.75
6 46
9 67
8.96
6.88
24 6
6.92
5.46
2 88
Distribution
Coeff .
2.90
1.74
2.05
2.08
2.15
1.79
Mat ' 1 .
Bal.
98.7%
113.4%
99.1%
97.2%
110.1%
91.6%
00
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Table 1 (continued). GAS SPARGING OF HYDROGEN SULFIDE
Run
7. Seawater
1.35 ppm H2S
8 . Seawater
0.64 ppm H2S
9 . Sewage
Effluent
74.5 ppm H2S
0 . Sewage
Effluent
112.5 ppm H S
^£
Sparge
Vol. (I)
0
0.3
.6
.9
1.2
1.5
0
0.3
.6
.9
1.2
1.5
0
0.5
1.0
1.5
0
0.5
1.0
1.5
pH
6.45
6.70
6.93
7.13
7.36
7.52
7.60
7,64
6.45
6.67
6.73
6.79
6.74
6.84
6.94
7.03
STOT
MxlO
9.85
9.90
9.10
8.90
20.6
15.0
15.2
15.0
450
510
540
482
830
750
750
725
H2S
MxlO5
3.97
3.10
2.12
1.56
1.88
1.36
1.16
1.08
219
216
217
184
331
274
246
215
Collected
HpS (ymoles)
7.03
6.18
5.38
5.00
3.70
2.42
2.49
1.85
1.76
1.51
1.30
1.25
1.20
2.97
2.60
2.76
Distribution
Coeff .
1.55
2.04
111
475
Mat ' 1 .
Bal.
117.8%
79.6%
107.8%
88.4%
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deionized water is expected since neutral molecules are
generally salted out of the high ionic strength seawater.
The activity coefficient of neutral ion pairs in seawater
has been estimated as 1.13 at 25° C. The ratio of the
measured distribution coefficients (2.18/1.79) corresponds
to an activity coefficient for molecular hydrogen sulfide in
seawater of 1.22. This result has a large uncertainty both
because of the statistical uncertainty of the experimental
values on which it is based and because the calculation of
the distribution coefficient in seawater is sensitively
dependent on the values assumed for the ionic activity
coefficients, which have not been measured directly.
The values indicated in Table 1 for the distribution
coefficient of molecular hydrogen sulfide between nitrogen
and sewage effluent are highly anomalous. The very large
numbers calculated for the coefficient probably mean that
only a small fraction of the total sulfide determined by the
methylene blue procedure is free to participate in the ionic
equilibrium. In calibrations of the sulfide ion-selective
electrode in the same sewage effluent medium, anomalously
low values of sulfide ion activity have been observed at pH
less than 6. Because of these effects in sewage effluent
and the probable dependence of PD upon ionic strength of the
sample, gas sparging does not provide an independent and
reliable check of the gas-sensing electrode in media of
uncertain composition.
10
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SECTION V
GAS-SENSING ELECTRODE
PRINCIPLES
The use of a potentiometric sensor for the activity of
dissolved gases dates from the carbon dioxide electrode
(10,11), which has found biological and medical
applications. Since then, ammonia (12) and, more recently,
sulfur dioxide and NOx (13) electrodes have become
commercially available. These sensors are based on the
principle discussed in detail previously (13) of allowing
the dissolved molecular species to diffuse through a
permselective membrane that is impermeable to ionic species.
In the interior of the probe the molecules react with the
filling solution to form characteristic ions that are sensed
conventionally. For example, carbon dioxide and ammonia
each change the pH of the filling solution and are detected
by a conventional glass electrode inside the body of the
probe. Selectivity is obtained by choice of the filling
solution and of the permselective membrane. Recently
Ruzicka and Hansen (14) have described a gas-sensing
electrode in which an air gap replaces the diffusion
membrane. Surface tension presumably holds a film of
electrolyte on the surface of the sensing electrode.
For the measurement of hydrogen sulfide, a silver sulfide
solid state probe for detecting product sulfide ions in the
filling solution is a logical choice. Two electrodes based
on the ammonia-sensing electrode design, were made on
special order by Orion Research, Inc. The contractor
developed the filling solution, suggested the use of
lanthanum fluoride as an inner reference electrode, and
furnished two types of membranes used in the commercial
ammonia and sulfur dioxide probes.
The relation between the activities of sulfide ion and
molecular hydrogen sulfide is pH-dependent and requires that
the filling solution be well buffered with respect to pH.
The use of a lanthanum fluoride reference electrode, which
is selective for fluoride ion, eliminates any direct
response to other acid gases, particularly carbon dioxide,
that might also diffuse through the membrane. One of the
porous diffusion membranes was made of TeflonR and the
other, polyvinylchloride. The hydrophobic character of the
plastics prevents penetration either by the inner filling
solution or the external test solution. Consequently, the
actual diffusion barrier consists of air trapped in the
pores, and the membranes should be almost ideally osmotic.
11
-------
The combination of features made a probe that was expected
to be almost perfectly selective for molecular hydrogen
sulfide. The specifications for the filling solution are
given in Table 2.
Although the gas-sensing electrode is expected to be highly
specific for hydrogen sulfide, several aspects of its
performance were recognized as being potentially limiting:
• Hydrogen sulfide and its ionic products are readily
subject to oxidation by air (5) . Ascorbic acid was
included in the filling solution to guard against
such reactions. Similar precautions may be needed in
handling standard and test solutions.
• As mentioned above, the filling solution must be
buffered with respect to pH to maintain a constant
fraction of the total sulfur in contact with the
sensing crystal as the sulfide ionic species.
Further study may be needed to determine the optimum
pH and the extent of buffering required. A filling
solution with a high pH would tend to convert
substantially all the diffusing hydrogen sulfide to
sulfide ions, thereby providing the lowest detection
limit, but the response on going from higher to lower
levels of H2S in the test solution might be expected
to be very slow because of the very low concentration
of molecular H2S (at high pH) inside the probe,
available for diffusion. At equilibrium, the
activity of molecular H2S must be the same on both
sides of the membrane; the quantity that must diffuse
across the membrane, in either direction, when
changes are made in the external solution, must
therefore be much greater when the filling solution
is at higher pH.
• Since the diffusion membrane is nearly ideal
osmotically, significant amounts of solvent (water
vapor) may be transferred across it if the sample and
filling solutions differ appreciably in osmotic
strength. The desirability of pH buffering and
protection against oxidation led to a recommended
filling solution (Table 2) that had an ionic strength
of 1.2. This is much greater than the value for most
samples of interest and would promote the transfer of
water to the interior of the probe.
• The membrane will probably have a finite life,
possibly limited by plugging, either by solid debris
in the samples or by Ag2S from the probe electrode.
12
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Table 2. MANUFACTURER'S RECOMMENDED ELECTRODE
FILLING SOLUTION
0.2 M Citric acid
0.1 M Asorbic acid
0.001 M Sodium fluoride
pH 6.0
Ionic strength 1.3
Osmolality, 0.9, approx.
13
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ELECTRODE CONSTRUCTION
The construction of gas-sensing electrodes has also been
described elsewhere in considerable detail (13) . The
elements are shown schematically in Figure 2. The most
critical feature is the close proximity of the gas-permeable
membrane to the sensing electrode. The volume of filling
solution in the liquid film between these elements must be
as small as possible to provide a reasonable response rate
since concentration changes are effected by diffusion. The
filling solution also provides an essential electrical
pathway between the sensing and reference electrodes. The
latter can be located at some distance from the sensing
element since mixing is not critical and the ionic
concentration (P-) to which the reference electrode responds
is virtually constant.
TEST METHODS
Tests and calibrations were made at room temperature (22 -
25° C), under nitrogen in closed systems to minimize
oxidation of the sulfide. Two basic procedures were used:
• measured additions of a stock solution of sodium
sulfide to a constant pH test solution, and
• additions of acid or alkali to vary the pH of a
solution with a relatively high and constant
concentration of total sulfide.
The test vessel was a 100-ml, round-bottom, multinecked
flask, with magnetic stirring bar, nitrogen bleed
(adjustable either above or below the solution surface), one
or two gas-sensing electrodes, glass electrode, double-
junction reference electrode with a nitrate filling solution
(Orion Model 90-02-00) and, usually, a sulfide ion-selective
electrode (Orion Model 9ft-16). Potentials were recorded
with a Leeds & Northrup Model 7421 Digital pH Meter. The
glass electrode was calibrated in buffers at pH 6.87 and
7.42, using the double-junction reference. The meter was
then standardized on zero millivolts and millivolt readings
were taken for the two buffers. During a run all electrode
potentials were recorded in millivolts and the glass
electrode results were converted to pH. In some runs, the
response of the gas-sensing electrode was recorded on the X-
Y chart recorder of a Princeton Applied Research Model 170
Electrochemistry System. The output of the pH meter,
connected to the hydrogen sulfide electrode, was applied to
the external X-axis input of the recorder. At the same
time, the glass and reference electrodes were plugged into
14
-------
Reference electrode
Filling solution
Sensing electrode
-Spacer
'Membrane
FIGURE 2. Electrode Construction
15
-------
the jacks of the Model 170r which recorded pH directly on
the Y-axis. For rate-of-response curves a ramp voltage was
applied to the dummy cell of the Model 170 to provide a
vertical Time Scale axis.
Stock sodium sulfide solutions were calibrated daily
iodometrically, using ASjO, as the primary standard. The
concentration of total sulfide was calculated from the
volumes of test solution and added stock solution, making
allowance for dilution by further additions of acid or base,
as appropriate. The sodium sulfide solutions were added
from 5- to 10-yl Grunbaum pipets or from a 1-ml Mohr pipet
calibrated in 0.01 ml. For the various solutions, pH2S was
calculated from pH and total sulfide by equations 1 and 2.
RESULTS
The manufacturers indicated that the hydrogen sulfide
electrodes should give a Nernstian response (30 mV per
decade at 25° C) between pI^S 1 and 8 (15) . Subsequently,
they have published a response curve (13) indicating a break
in the curve about pH2S 7.5. The response approaches a
constant millivolt output at pIS 8.
In my tests of the electrodes I almost invariably found a
break between pH2S 6 and 7. Instead of approaching a
constant output, the voltage increased sharply (became less
negative) , often 100-300 mV per decade, and did not reach a
constant value even at pl^s 9. Because of this unexpected
and anomalous response the major evaluation effort was
directed toward finding the source of the anomaly and
defining the range of reliable operating conditions. The
former objective was not reached and many experiments at low
levels were poorly reproducible.
Figures 3, 4, 5, and 9, showing manual plots of potential
(millivolts) vs. pH2S, are presented with an unconventional
ordering of scales to provide a direct comparison with
tracings of the X-Y recorder (Figures 6, 7, 8, and 11).
16
-------
Table 3. ELECTRODE DATA FOR FIGURES 3-10
Fig.
3a
3b
4a
4b
4c
5a
5b
6, 9a
7, 9b
8, 9c
10
(gas)
Electrode
B
A
B
A
B
B
A
B
B
B
B
B
Membrane
Silicone (Radiometer)
PVC (Orion , SO- ) reused
11 M it ii
Teflon (Chemplast)
used once
PVC (Or ion, SO2) 10 days
PVC-used after 3b
ii ii ii n *
Teflon (Chemplast)
PVC (Or ion ,802)
Teflon (Orion,NH3)
Silicone (GE)
PVC(Orion,SO2)
Filling Solution
Citric acid, pH 6 . 0
n n n n
Tris, pH 7.0
Tris, pH 7.8
» i n
Phosphate, resin-
treated
• M
Tris, pH 7.0
i ii n
n n ii
i n n
n n n
Test Solution
Citric acid
1 M Potassium chloride
Deionized water
Seawater
n
Phosphate
n
Tris
n
n
n
Deionized water
(sparge)
-------
Table 4. LINEAR CALIBRATIONS OF HYDROGEN SULFIDE ELECTRODE
Fig.
3a
3b
4a
4b
4c
5a
5b
9a
9b
9c
(gas)
Range pKUS
2.7 - 4.6
4.2 - 7.1
4.4 - 7.6
3.4 - 5.6
3.4 - 4.9
3.5 - 5.2
3.9 - 7.3
3.3 - 6.1
4.3 - 5.8
4.3 - 5.8
4.1 - 5.7
Slope
mV/decade
30.7
36.7
40.5
31.3
16.5
33.7
30.8
28.5
26.9
27.5
35.6
Rel. Std.
Dev. of Slope (%.}
0.7 %
.7
1.2
2.7
1.3
1.6
1.0
2.0
1.2
0.6
1.5
No. of
points
7
7
11
6
5
8
7
8
16
8
13
Std. Dev.
(pH2S)
0.03
.05
.15
.11
.03
.08
.08
.13
.08
.02
.28
-------
-mV
500
^400
300
7
6
I
5
4
I
,• • •
FIGURE 3.
Response Curves
Solution, pH 6
19
~ Citric Acid Pilling
-------
-mV
500
Uoo
300
PH2S 8
FIGURE 4. Response Curves
Solution
— Tris Buffer Filling
20
-------
-mV
500
400
3 PH2S
• •»
FIGURE 5. Response Curves — Phosphate Buffer Filling
Solution
21
-------
I
-mV
600
-500
— 400
1.2
8
pH
FIGURE 6. Chart Recording — Chemplast PTFE
Membrane
22
-------
-mV
-500
-40C
1,2
1.0
,8
pH
FIGURE 7. Chart Recording — Orion PVC Membrane
23
-------
-mV
— 500
-40C
8
FIGURE 8. Chart Recording — Orion Teflon Membrane
24
-------
-mV
500
400
pH2S
Curve a shifted
5 units to left
Curve b shifted
2 units to left
FIGURE 9. Calculated mV vs.
7f and 8
25
for Figures 6,
-------
-mV
-500
— 400
12
1.0
.8
PH
FIGURE 10. Chart Recording — GE Silicone
Membrane
26
-------
Experimental Factors
Filling Solution —
The principal function of the filling solution, as indicated
above, is to provide a constant pH medium for the ionization
of hydrogen sulfide so that the free sulfide ions detected
by the sensing electrode will bear a fixed relation to the
molecular species. Lower limits of detection, in the
presence of sufficient total sulfide, have been reported as
10-»» M (16) and 10~20 M (17) free sulfide ion. At pH less
than 8, substantially all the sulfide is in the molecular
form, so that the practical detection limit for ^S may be
the same as for total sulfide. This limit was given as 10-'
M, provided ascorbic acid was used to prevent oxidation of
the sulfide (18) .
Examples of the response with the Orion citric acid filling
solution at pH 6 are shown in curves a and b of Figure 3.
The latter had one of the longest ranges of linear response
that was found, although one outlier point was observed.
The slope of curve b (36.7 mV/decade - Table 4), however, is
considerably greater than the theoretical Nernstian value of
30 mV/decade.
A filling solution with the following composition was also
prepared to provide a medium with a higher pH and also a
smaller ionic strength:
0.1 M Tr is (hydroxymethyl) ami nome thane
0.1 M Ascorbic acid
0.001 M Sodium fluoride
This solution was adjusted to pH 7.0 in Figures Ua, 6, 1, 8,
and 10, and to pH 7.8 in Figures Hf b and c. Figure 4a also
showed a long linear range of response, but with
considerable scatter, and also showed a greater than
Nernstian slope (40.5 mV/ decade) . There was no clear
difference in suitability of the two filling solutions.
Ionic Strength of the Sample —
Because the electrode membrane should function nearly
ideally as an osmotic membrane, most tests were made in
solutions having the same composition as the respective
electrode filling solution (except for the addition of
sulfide and, usually, omission of sodium fluoride) . This
was the case in Runs 3a, 6, 7, 8, and 10. To compare the
response in various media Run 3b was made in 1 M potassium
chloride (approximately the same ionic strength as the
citric acid filling solution) , Runs 4b and 4c in seawater
27
-------
(ionic strength greater than that of the tris filling
solution) , and Run la in deionized water. Run 4a
corresponded closest to the behavior described by the
manufacturer (10, 11), suggesting that osmotic transport of
water in the direction of hydrogen sulfide diffusion might
have aided the response. However, if sufficient solvent
were transported to alter significantly the concentration at
the silver sulfide crystal surface, a less-than-Nernstian
response would be expected in low ionic-strength test media
and high response slope in media of high ionic strength. On
the contrary, the distilled water run (Ua) had a large slope
and one of the seawater runs (He) had the smallest slope.
In all calculations of pH^S, the activity coefficient of
hydrogen sulfide was taken as unity, regardless of the ionic
strength of the medium. As mentioned in the discussion of
gas sparging, this assumption probably does not contribute
more than 0.3 log unit error in
Diffusion Membranes—-
The electrodes were supplied with two commercial types of
diffusion membranes: a polyvinylchloride (PVC) membrane
(Orion Cat. 95-6U-04) and a TeflonR membrane (Orion Cat. 95
10-04) . In addition, membranes were cut from TFE medium
filter membranes (Chemplast 75-M) , paper-backed silicons
rubber (Radiometer MEM-213) , and General Electric dimethyl
silicone film. The silicone films are of interest because
they transport hydrogen sulfide by condensed-phase, rather
than gaseous, diffusion. The silicone films were cemented
across the end of the electrode holder with Dow Corning
silicone adhesive.
The membranes used for the various runs represented in the
charts are summarized in Table 3. Two major qualitative
observations were made: the silicone films reached steady-
state response more slowly than the gaseous diffusion
membranes, and the high-permeability membranes (TFE filter
and PVC) occasionally became flooded with liquid. When a
continuous liquid pathway was established through the
membrane the electrode responded to sulfide ion rather than
to hydrogen sulfide species.
The useful life of each membrane was not determined since
the major emphasis was on optimizing conditions for the
lowest detection limit. Most runs were made with fresh
membranes. Run 4c_, however, was recorded with a PVC
membrane that had been in place for ten days and used for
four prior runs. Except for the flooding, alluded to above,
there were no indications of membrane failure.
28
-------
Rate of Response—
The rate at which the electrode responded to abrupt changes
in concentration of hydrogen sulfide was recorded for
various levels within the range studied above. Tracings of
the recordings are shown in Figure 11. Curves a - e were
recorded immediately following the run shown in Figure 7, in
which an Orion PVC membrane was used; curves f and g_ were
recorded after the run shown in Figure 8, using an Orion
Teflon membrane; and curves h - k were obtained following
the run shown in Figure 10, using a methyl silicone film.
In all cases, the change in hydrogen sulfide concentration
was made by pH adjustment between the values shown in Table
5.
As expected, the response was qualitatively faster in the
more concentrated solutions, reaching steady potentials in
about a minute (curves c and ij . Empirical fitting of
Equation 3 to these curves was attempted as a means of
obtaining improved estimates of the final potential, E£, and
a measure of the diffusion constant, K, for the various
membranes.
E = Ef + B In [1 - A exp (-Kt)] (3)
Equation 3 was derived on the basis of a Nernstian-type of
response of potential, E, to the concentration of sulfide
ion reaching the sensor surface at time, t, by linear
diffusion. Constant B is the Nernst factor and A is the
ratio of the initial concentration difference providing the
diffusion potential, to the final (sample) concentration at
the electrode. Equation 3, above, is equivalent to equation
8 of reference 13, with 1/K = (Im/Dk) [ 1 + (dC^dC) ] of the
reference.
Several methods of non-linear least-squares fitting of
Equation 3 to the data for curve b of Figure 11 showed that
the four constants could not be evaluated with any
certainty. Within the 2-mV estimated precision of the
measured values of E a very wide range of parameters gave an
acceptable fit, as shown in Table 6. The Simplex (19) and
Fletcher-Powell (20) procedures are systematic techniques
for minimizing the variance of fit, but each was converging
very slowly when the calculations were stopped. The best
fit that was found was by a rather lucky guess in a trial-
and-error approach but there is no assurance that it was the
least-squares "best11 fit. The expected values of the
constants were obtained as follows: The curve was visually
extrapolated to long times to give Ef; B is the theoretical
Nernstian value in millivolts per unit natural logarithm of
29
-------
U)
o
I 40 Sec (solid)
10 Sec (broken,
-mV
FIGURE 11. Electrode Response Rate Curves
-------
Table 5. CONDITIONS FOR ELECTRODE RATE CURVES
SHOWN IN FIGURE 11
Curve
ApH
ApH^S
Following calibration
a
b
c
d
e
11.
9.
8.
<8.
9.
5 -
4 -
8 -
5 -
9
8
8
9
3 -11
shown
.4
.8
.5
.3
.1
in Figure
8.
6.
5.
5.
6.
7
41 -
37 -
82 -
49 -
26 -
6
5
5
6
7
.37
.82
.49
.26
.99
Following calibration shown in Figure 8
f 7.8 - 11.0 4.86 - 7.97
g 11.0 - 11.7 7.97 - 8.70
Following calibration shown in Figure 10
h >11.1 - 5.9 8.04 - 3.85
i 5.9 - 2.7 3.85 - 3.80
j 2.7 - 7.3 3.80 - 4.42
k 7.3 - 7.8 4.42 - 4.82
31
-------
Table 6. PARAMETERS OF EQUATION 3 OBTAINED BY FITTING
TO DATA FOR CURVE lib
Procedure
Constants
Variance of Fit
(mV2)
Simplex
Fletcher-
Powell
Trial &
Error
Interpola-
tion
Trial &
Error
Expected
Value
-E
500
498
467
457
453
480
f
.7
.6
.0
.8
.6
B
33
35
67
120
168
13
.94
.49
.1
.3
.6
A
1.0047
1.0025
0.859
.6415
.5
0.715
k
0.001371
.001616
.0072
.01208
.0143
40
3
3
3
1
0
.33
.32
.14
.037
.229
700
32
-------
concentration; A was calculated from the estimated initial
and final concentrations of I^S (Table 5) ; and K was very
crudely estimated for free-air diffusion of gaseous HjS by
equation 4.
K = DA/PDVT (4)
In equation 4, D is the diffusion coefficient (0.1), A the
area (0.1 cm2), T, thickness (0.1 cm), V, liquid volume
(1x10-3 cm'), and PD is the distribution coefficient
measured in the sparge runs (2.5). It is clear, that the
curve-fitting approach did not give useful estimates of the
physical parameters, although the very large discrepancy in
K may indicate that liquid diffusion is actually the rate-
limiting process.
Gaseous Sampling—
Because the electrode is truly "gas-sensing" it should be
capable of measuring hydrogen sulfide in air, or other
gases, as well as in solution. To test its gas analysis
capabilities, the electrode was placed at the top of the
sparge column (Figure 1) containing initially 1.91 x 10-* M
sodium sulfide in deionized water. In two runs with the
same electrode the solution was sparged with 1.5 to 8 1 of
nitrogen at two pH levels (6.5 and 5.4). The results are
not plotted but are summarized in the final lines of Tables
3 and 4. The data cover a relatively narrow range of
(4.1 to 5.7) and show considerable scatter but generally
show a linear response (potential vs. log concentration)
with a somewhat greater than Nemstian slope.
MECHANISM OP THE ANOMALOUS RESPONSE
As indicated above, a large part of the effort was directed
toward understanding and correcting the extremely non-
Nernstian response usually observed at values of pH^S
greater than 5 or 6. The usual low-concentration limitation
of ion-selective electrodes is a lack of response, or near-
zero slope, rather than the great increase in slope observed
in this work. Several possible causes of this phenomenon
were considered.
33
-------
Crystal Defects
In a recent paper, Morf, Kahr, and Simon (21) reported
similar behavior by Ag«S and Agl electrodes. This is the
only independent confirmation of the sharp break in
potential response that the writer has found. They
interpreted the results in terms of crystal surface defects
that strongly adsorbed free silver ions. Their data were
explained by an assumed defect activity equivalent to 3 x
10""6 M free silver ions in solution. In the present study,
the corresponding defect activity was approximately 10"^4 M,
which was the activity of free sulfide ions at the potential
break. A consequence of the theory, which was not tested,
is that the detection limit for hydrogen sulfide might be
lowered by increasing the pH of the filling wolution to pH 9
or greater, thtos increasing the relative concentration of
sulfide ion. That modification would be expected to slow
the response time of the electrode at low concentrations of
hydrogen sulfide. The theory of Simon and co-workers (21)
does not seem to account for the well established response
of the silver sulfide electrode to extremely low activities
of free sulfide ion (17) or silver ion (22) in the presence
of at least 10~4 M total sulfide or complexed silver.
Heavy Metal Poisoning
The resemblance of the electrode response to potentiometric
titration curves suggested that the first traces of hydrogen
sulfide to diffuse through the semi-permeable membrane were
precipitated by heavy metal contaminants in the filling
solution. That mechanism would not account, however, for
the same sort of response on going from higher to lower
concentrations of hydrogen sulfide. Baumann (23) lowered
the detection limit of the electrode for sulfide ions by a
factor of three by ion-exchange pretreatment of her buffers.
For the runs shown in Figure 5 the phosphate-buffered
filling solution was pre-treated by Chelex ion exchange
resin. In one case (curve b) the response is more nearly
linear but could not be reproduced and the difference
between the runs is unaccountable.
Carrier Mobility
The possibility was considered that the response of the
electrode might be limited by the concentration or mobility
of charge carriers in the silver sulfide crystal. The
resistance of the silver sulfide electrode in combination
34
-------
with various reference electrodes (lanthanum fluoride, Orion
double junction, silver-silver chloride) was measured by the
technique of Eckfeldt and Perley (24). The lowest apparent
resistance was 11 Mfl. This high dc resistance tends to
confirm the conclusion (25), based on ac impedance
measurements, that the silver sulfide electrode acquires a
potential by capacitive coupling with the solution rather
than by ion exchange.
Surface Condition of the Crystal
By rigorously controlled preconditioning of a copper sulfide
electrode, copper ions were determined kinetically at
concentrations well below the equilibrium detection limit
(26). A similar approach may be the key to lower detection
limits with the silver sulfide electrode. In this work the
electrode commonly became dull and dark colored after a few
runs. This did not necessarily affect the response, but
when erratic results were obtained at relatively high levels
of hydrogen sulfide the crystal was re-polished by rubbing
with a few milligrams of Cab-0-Sil silica on a dry, hard
filter paper. Some improvement in response was noted.
CALIBRATION
Despite the anomalous response of the gas-sensing electrode
at very low levels of hydrogen sulfide the response
generally was Nernstian for pH->S less than 5.5, (i.e., H2S
activity greater than 0.1 mg/1). Since this lower limit is
close to the organoleptic detection limit (27), the
electrode may well have a significant application in the
examination of anoxic or polluted natural water or
wasfeewater.
For the practical calibration of the electrode, stock sodium
sulfide solution was added to commercial pH buffers.
Buffers for pH 4.01 (potassium hydrogen phthalate) and pH
9.18 (borax), when made 1 x 10~3 M in sodium sulfide, had pH
values of 4.6 and 9.7, respectively. The corresponding
calculated values of P^S were 3.203 and 6.0. The latter
value depended, within 0.08 unit, on the exact pH of the
resulting solution, in the six preparations made, while the
former was independent of small pH variations (within 0.08
pH unit). Calibrations over a period of three weeks for
various filling solutions and membranes gave the following
values for the mean slope, AmV/ApH2S : 32.1, 30.9, 15.1,
15.5, 30.6, 48.0, 27.2, 24.3, 151. The last value indicates
35
-------
that the potential break occurred at a pI^S less than 6.0
(the higher calibration point) and corresponds to Run
-------
SECTION VI
REFERENCES
1. Adelman, I. R. and L. L. Smith, Jr. Toxicity of
Hydrogen Sulfide to Goldfish (Carassius auratus) as
Influenced by Temperature, Oxygen, and Bioassay
Techniques. J. Pish. Res. Bd., Canada. 29; 1309-1317,
1972, and references cited therein.
2. Doudoroff, P. Water Quality Requirements of Pishes and
Effects of Toxic Substances. In: The Physiology of
Fishes, Volume 2, Brown, M. E. (ed.). New York,
Academic Press, Inc., 1957. p. 423.
3. Standard Methods for the Examination of Water and
Wastewater, 13th edition. Taras, M. J., A. E.
Greenberg, R. D. Hoak, and M. C. Rand (eds.). New York,
American Public Health Association, 1971. p. 554.
4. Offner, H. G. Determination of Trace Quantities of
Hydrogen Sulfide in Aqueous Solutions. North American
Rockwell Corp., Washington, DC. Final Report Contract
14-12-807. FWPCA, U. S. Department of Interior,
March 1970. 32 p.
5. Chen, K. Y. and J. C. Morris. Kinetics of Oxidation of
Aqueous Sulfide by 0?. Environ. Sci. Technol. £: 529-
537, June 1972.
6. Stephens, H. P. and J. W. Cobble. Thermodynamic
Properties of the Aqueous Sulfide and Bisulfide Ions and
the Second lonization Constant- of Hydrngen Sulfide over
Extended Temperatures. Inorg. Chem. 3.0_(3): 619-625,
1971.
7. Instruction Manual: Sulfide Ion Electrode Model 94-16.
Orion Research, Inc.,.Cambridge, Mass., 1968. p. 14.
8. Garrells, R. M. and M. E. Thompson. A Chemical Model
for Sea Water at 25° C and One Atmosphere Total
Pressure. Amer. J. Sci. 260: 57-66, January 1962.
9. Wright, R. H. and O. Maass. The Solubility of Hydrogen
Sulfide in Water from the Vapor Pressure of the
Solutions. Can. J. Res. 6_: 94-101, 1932.
10. Stow, R. W., R. F. Baer, and B. F. Randall. Rapid
Measurement of the Tension of Carbon Dioxide in Blood.
Arch. Phys. Med. 38; 646-650, October 1957.
37
-------
11. Severinghaus, J. W. and A. F. Bradley. Electrodes for
Blood p(>2 and pCO2 De-termination. J. Appl. Physiol.
13: 515-520, 1958.
12. Thomas, R. F. and R. L. Booth. Selective Electrode
Measurement of Ammonia in Water and Wastes. Environ.
Sci. Technol. 7: 523-526, June 1973.
13. Ross, J. W., J. H. Riseman, and J. A. Krueger.
Potentiometric Gas Sensing Electrodes. J. Pure Appl.
Chem. J»: 473-487, 1973.
14. Ruzicka, J. and E. H. Hansen. New Potentiometric Gas
Sensor. Air-gap Electrode. Anal. Chim. Acta.
(Amsterdam). 69(1): 129-141, 1974.
15. Krueger, J. A. Orion Research, Inc. Personal
communication, July 1973.
16. Ross, J. W., Jr. Solid-state and Liquid Membrane Ion-
Selective Electrodes. In: Ion-Selective Electrodes,
Durst, R. A. (ed.). Washington, DC. National Bureau of
Standards, Special Publication 314, November 1969. p.
77.
17. Light, T. S. and J. L. Swartz. Analytical Evaluation of
the Silver Sulfide Membrane Electrode. Anal. Lett.
1(13) : 825-836, 1968.
18. Bock, R. and H. J. Puff. Bestimmung von Sulfid mit
einer Sulfidionen-empfidlichen Electrode [Determination
of Sulfide with a Sulfide Ion-Selective Electrode].
Fresenius1 Z. Anal. Chem. (Leipzig) . 240; 381-386,
1968.
19. Morgan, S. L. and s. N. Deming. Simplex Optimization of
Analytical Chemical Methods. Anal. Chem. 46 (9): 1170-
1181, August 1974.
20. Cooper, L. and D. Steinberg. Introduction to Methods of
Optimization. Philadephia, Pa., W. B. Saunders Co.,
1970. p. 169-173.
21. Morf, W. E., G. Kahr, and W. Simon. Theoretical
Treatment of the Selectivity and Detection Limit of
Silver Compound Membrane Electrodes. Anal. Chem.
46(11): 1538-1543, September 1974.
38
-------
22. Durst, R. A. Analytical Techniques and Applications of
Ion-Selective Electrodes. In: Ion-Selective
Electrodes, Durst, R. A. (ed.). Washington, DC.
National Bureau of Standards, Special Publication 314f
November 1969. p. 402.
23. Baumann, E. W. Determination of Parts per Billion
Sulfide in Water with the Sulfide-Selective Electrode.
Anal. Chem. 46(9): 1345-1347, August 1974.
24. Eckfeldt, E. L. and G. A. Perley. Measurement of and
Effect of Temperature on Electrical Resistance of Glass
Electrodes. J. Electrochem. Soc. 98; 37-47, 1951.
25. Brand, M. J. D. and G. A. Rechnitz. Mechanistic Studies
on Crystal-Membrane Ion-Selective Electrodes. Anal.
Chem. 42.: 478-483, April 1970.
26. Blaedel, W. J. and D. E. Dinwiddie. Study of the
Behavior of Copper Ion-Selective Electrodes at
Submicromolar Concentration Levels. Anal. Chem. MO) •
873-877, June 1974.
27. Standard Methods for the Examination of Water and
Wastewater. 13th edition. Taras, M. J., A. E.
Greenberg, R. D. Hoak, and M. C. Rand (eds.). New York,
American Public Health Association, 1971. p. 336.
39
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
i. R 'ORT NO.
ExA-660/4-75-001
3. RECIPIENT'S ACCESSIOf*NO.
4. Ti .LE AND SUBTITLE
DETERMINATION OF MOLECULAR HYDROGEN SULFIDE
5. REPORT DATE
March 1975
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
THOMAS B. HOOVER
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southeast Environmental Research Laboratory
College Station Road
Athens, Georgia 30601
10. PROGRAM ELEMENT NO.
1BA027
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The gas sparging technique and a new gas-sensing electrode were
evaluated for the determination of dissolved molecular hydrogen sulfide
at environmentally significant concentrations in water. From the
sparging experiments approximate coefficients were obtained for the
distribution of hydrogen sulfide between nitrogen and distilled water,
seawater, or municipal sewage effluent. In the latter medium the
volatility of hydrogen sulfide was very much less than predicted from
the pH-total sulfide relationship. The electrode, consisting of
various semipermeable membranes, buffered electrolyte filling solution,
silver-silver sulfide crystal sensor, and lanthanum fluoride internal
reference electrode, gave a generally Nernstian response to more than
0.1 mg/£ of molecular hydrogen sulfide. At lower concentrations the
response was typically several tenths of a volt per decade of
concentration, but was not reproducible among different samples or
electrodes. Various sources of the anomalous behavior were considered.
The electrode is recommended for in situ measurements of molecular
hydrogen sulfide at concentrations greater than 0.1 mg/£. More work
is needed to make it useful at lower concentrations.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS c. COS AT I Field/Group
chemical analysis, hydrogen
sulfide, electrodes,
permselective membranes
dissolved gas,
molecular hydrogen
sulfide, sparging,
gas-sensing
05A
18. DISTRIBUTION STATEMENT
Release unlimited
19. SECURITY CLASS (ThisReport)
21. NO. OF PAGES
40 + vi
20. SECURITY CLASS (Thispage)
22. PRICE
EPA Form 2220-1 (9-73)
------- |