4o PDPETRO, Campinas, SP
21-24 de Outubro de 2007
2.3.0174-2 – 1
THE USE OF EDTA IN ALKALINE ZINC ACETATE SOLUTIONS AS A
TRAPPING MIXTURE FOR HYDROGEN SULPHIDE
Ian Hovell1 (UFRJ), Rogério F. Lacerda2 (UFF), Luis Augusto Medeiros Rutledge1 (UFRJ), Krishnaswamy
Rajagopal 1(UFRJ)
1
Laboratório de Propriedades do Petróleo, LATCA, Escola de Química,Universidade Federal do Rio de Janeiro,
Centro de Tecnologia, Sala I-122,CP 68542 Cidade Universitária - CEP: 21941-909, Brasil, [email protected].
2
Universidade Federal Fluminense, Departamento de Engenharia Química e do Petróleo
Hydrogen sulphide, (H2S), is a highly toxic gas and is increasingly found in offshore reservoir fluids and it is
important to have reliable and accurate analytical procedures. Normally the standard USEPA 376.2 procedure is
used for the determination of hydrogen sulphide in formation water and live-oil samples. In this method, H2S is
trapped in aqueous alkaline solutions and the sulphides are then analysed by the methylene blue (MB) method.
This procedure however requires immediate analysis of the sample, as the precipitates are unstable. Zinc acetate
produces acetic acid during the absorption of H2S thus reducing the effectiveness of the trapping solution,
making the control of initial and final pH value of the trapping solution necessary. The precipitate formed was an
irregular gelatinous mass, which adhered to the glass walls and sintered glass of the gas wash bottle necessitating
acid solvation of the precipitate, to free the H2S, for analysis resulting in loss of precision in measurements. We
propose to stabilize the sample with an alkaline solution of a chelating agent, disodium
ethylenediaminetetraacetate salt (EDTA). H2S was liberated from a number of reservoir fluids by pressure
reduction and was trapped in aqueous alkaline solutions of zinc acetate and EDTA. The resulting zinc sulphide
precipitate was analysed using a HACH UV-visible spectrometer following the MB method. The calibration of
the spectrophotometer was checked using standard sodium sulphide solutions. The sulphide precipitate in the
original trapping mixture decomposes and disperses in a few hours while with the proposed chelating agent
system, the precipitate remained stable and in suspension even for several days. This method of temporary
stabilization can be readily adapted into standard methods used for the analysis of H2S content found in reservoir
fluids and other process streams.
Key words: Hydrogen sulphide trapping, Hydrogen sulphide analysis, Methylene Blue method,
spectrophotometry, EDTA
1. INTRODUCTION
The Methylene Blue method is an analytical end-method used for measuring low concentrations of sulphides
in aqueous solutions. It involves measuring the absorption of light of a known wavelength (665 λ) in a
spectrophotometer. When this occurs due to the formation of a colored reaction product, the concentration can be
accurately measured by a spectrophotometer. The degree of absorption or strength of color is proportional to
concentration provided the light path is kept fixed. The best known application, first described by Johnson and
Nishita, (1952), of this technique is the methylene blue method for measuring hydrogen sulphide and other
reduced sulphur compounds. Where an acidified solution of N, N-dimethyl-p-phenyldiamine is added to a
potassium dichromate solution and in the presence of sulphide ions, the methylene blue dye is formed, see
figure 1.
The optical density of the methylene blue solution is determined spectrophotometrically, and the hydrogen
sulphide concentration is determined from a calibration curve. The MB reaction is highly specific for sulphide at
low concentrations.
The MB method is not, however, without problems. Strong reducing agents (e.g., sulfur dioxide) will inhibit
the MB colour development and even solutions containing several micrograms of sulphide per millilitre show a
similar effect and must be diluted to eliminate colour inhibition before analysis.
Aqueous samples of hydrogen sulphide also present problems since they are readily volatilized, when the pH
of the solution is below 7.0. At 0 oC 437 cm3 of H2S (g) will slowly equilibrate by dissolving in 100 cm3 of H2O
producing a 0.2M solution of 4 pH. Alkaline aqueous sulphide solutions are very unstable because the sulphide
ion is rapidly oxidized by exposure to the air, making it necessary to carry out immediate measurements for an
accurate quantitative analysis
In this study, we propose to improve the stability of sulphides in alkaline trapping solutions by the addition of
EDTA, so that the samples can be stored and analysed later at a convenient time. In this way, the original amount
of sulphides in the sample can still be determined by the analytical method employed.
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4o PDPETRO, Campinas, SP
21-24 de Outubro de 2007
2.3.0174-2 – 2
Figure 1 Methylene Blue Reaction Scheme
2. BACKGROUND
Methods most commonly used to measure hydrogen sulphide in samples, such as air, water, oil, sediment,
and sludge, can be grouped into three broad categories; titrimetric, spectroscopic and chromatographic. They
include gas chromatography with flame photometric detection (GC/FPD), Clay, Rogers and Jungers, (1977), gas
chromatography with electrochemical detection, (GC/ECD), Attari, Chao (1993), iodometric methods,
Konovalov and Romanov (1999), the methylene blue colorimetric or spectrophotometric method, Cline (1968)
the spot method using paper or tiles impregnated with lead acetate or mercuric chloride, Natusch, Sewell, and
Lanner, (1974) ion chromatography with conductivity, Bond, et.al. (1982) and potentiometric titration with a
sulphide ion-selective electrode, Hseu and Rechnitz (1968). Titrimetric methods, including the classical
iodimetric titration, are not suitable, for micro-amounts of sulphide according to Jeffery et.al. (1997). Accurate
measurements of hydrogen sulphide in water are usually complicated by the presence of other sulphide
compounds. A method of determining sulphide concentration in wastewater by first transforming to hydrogen
sulphide and then measuring the product using atomic absorption spectroscopy (AAS) has been published,
Parvinen and Lajunen, (1994).
Several reagents have been suggested for the trapping of hydrogen sulphide from aqueous liquids,
Balasubramianian and Kumar (1990), Wood and Marr (1988), Lindell et. al. (1988) and have been reviewed by
Singh et. al. (2000) who concluded that the trapping solutions had a low precision and the simple zinc acetate
solution used by Wood and Marr (1988) became acidic during the absorption of hydrogen sulphide due to the
production of acetic acid reducing the precision of the analysis method and recommended the use of an alkaline
medium or buffer. Singh proposed the use of an alkaline zinc acetate/EDTA (disodium salt) mixture with a final
pH 12 as described by Balasubramianian and Kumar (1990) who showed that the trapping system was stable for
three days measuring trace quantities of H2S down to 0.3 µg. They applied this trapping method for analyzing air
samples from a laboratory fume cupboard.
In this work we investigate the applicability of this method for trapping hydrogen sulphide from reservoir
fluids- liberated gas, formation water and residual oil. We measured the absorption efficiency of the trapping
solution, degradation of sulphides in the solution during storage and recoverability of sulphides from the solution
during analysis.
3. MATERIALS AND MEASUREMENT METHOD
In order to evaluate the proposed trapping mixture, the spectrophotometer was first calibrated, its precision
with successively diluted sulphide standards was checked and the absorption ability and stability of the proposed
trapping solution was investigated checking it against a standard 10% H2S methane mixture. Finally the
proposed trapping solution was used with an oil reservoir fluid sample testing its ability to recover a known
amount of H2S gas from the live oil-formation water sample which would be first separated into organic and
aqueous phases and each phase subsequently into liquid and gaseous phases producing four H2S sources,
namely, gas liberated from oil phase, residual oil, gas liberated from aqueous phase and the residual formation
water.
3.1 Preparation of the H2S trapping mixture.
All chemicals used were of analytical grade and their solutions were prepared using filtered doubly distilled
water. The proposed trapping mixture was made up from: 0.5% w/v zinc acetate-2-hydrate obtained from Vetec
Química Fina Ltda., Duque de Caxias, Rio de Janeiro, RJ. 0.5% w/v disodium ethylenediaminetetraacetate salt,
EDTA, from Merck KgaA, Damstadt, Germany. 0.8% w/v Sodium Hydroxide, obtained from Vetec Química
Fina Ltda., Duque de Caxias, Rio de Janeiro, RJ.
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The sodium hydroxide prevents the zinc sulphide precipitate, which is produced on addition of H2S, forming
and the chelating agent forms attractions with the zinc ions increasing the stability of the zinc sulphide in
solution thus preventing the loss of S-2 ions by volatilization of H2S(g).
3.2 Calibration of the spectrophotometer.
Concentration microgram/litre (ppb)w
The instrument, HACH DR4000 UV-Visible spectrophotometer, purchased from Hach Company, Colorado,
USA, was calibrated originally by the manufacturer and recently by an authorized representative Hexis, Jundai,
SP, The calibration of the instrument was rechecked using standard solutions of sodium sulphide from two
different sources:
1 Using highly pure solid sodium sulphide nonohydrate, Na2S. 9H2O, donated by CENPES
2 and a Sulphide Anti-Oxidant Buffered (SAOB), 1000 ppm, standard S-2 solution, traceable to NIST
(National Institute of Standards and Technology) standards, acquired through Hexis in 14/11/2006
The spectrophotometer was first calibrated using a standard sodium sulphide solution. A single crystal of
sodium sulphide was carefully washed with distilled water and immediately dried with tissue paper. The crystal
was weighed to be 2.44g, and was dissolved in sufficient water to produce a total mass of 100g of solution. This
produced a theoretical sulphide ion concentration of 3254 ppm. Dilutions were made from this starting solution
to produce concentrations within the range of the spectrophotometer, 5-880 ppb. Solutions made from this
sodium sulphide crystal were not buffered in order to stabilize the solution. The solutions were used immediately
and then discarded.
The NIST standard solution was used as received and was diluted to the required concentration.
The results from the initial calibration using fresh unbuffered sodium sulphide solutions prepared in the
laboratory are shown in Figure 2.
800
y = 1.017x
700
779
2
R = 0.9912
600
559
500
400
300
Measured
252
200
Linear regression
100
0
0
200
400
600
800
-2
Calculated S ion concentration in ppb
1000
Figure 2 Calibration check using a simple sodium sulphide standard solution
The instrument was rechecked three months later against a S-2 standard solution, with a nominal
concentration of 1000 ppm traceable to NIST (National Institute of Standards and Technology) standards. The
NIST standard, although is stabilized using a SAOB the solution was received at an elevated temperature and the
concentration of the solution may have altered. On receiving the standard solution, a 1000 ppb nominal standard
solution was produced by diluting 1 cm3 of the NIST standard in 1 liter of double distilled water. A sample of
this standard was sent to CENPES for cross analysis. The results are shown in the Table 1 below:
Table 1 Initial Calibration using NIST traceable Standard Solution
This work
Cross Analysis (CENPES)
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Measured S-2 concentration in ppb
816
806
4o PDPETRO, Campinas, SP
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The percentage average absolute deviation was 4 and 3% respectively for the two methods carried out with a
three-month interval between the two calibration experiments. On both occasions linearity was observed,
R2 = 0.99, over the range of concentration observed. The instrument was considered to produce accuracy within
±5 ppb. The cross analysis performed by CENPES compared with the initial standard solution analysis to within
1% although the CENPES value is an average of two analyses, 842 and 770 ppm.
The NIST Standard was subsequently stored at 4 °C. The NIST standard solution with a measured concentration
of 816 ppb was checked over the working range of the spectrophotometer, 5 – 880 ppb, using the dilutions
shown in Table 2 The S-2 concentrations of these solutions were analyzed immediately on preparation and the
results are shown in
Table 2 Dilutions used to produce a calibration curve for the HACH spectrophotometer.
Volume of Standard
Estimated [S-2] in ppb
Experimental [S-2] in ppb
3
solution/liter, cm
after dilution
after dilution
0.85
694
630
0.71
579
568
0.43
351
353
0.18
147
145
800
Expimental concentration ppb
700
600
500
400
300
Experimental
200
Estimated concentration
100
0
0
200
400
600
800
Calculated concentration in ppb
Figure 3 Sulphide concentration difference between NIST standard and experimental.
3.3 Linearity of methylene blue method
To check the ability of the Methylene blue sulphide method to produce reliable results from developed
methylene blue concentrations successive dilutions of the dye were made and tested starting from a known H2S
concentration.
A starting solution, using 0.43 cm3 of the original NIST standard solution and diluting to 1000 cm3 with
freshly distilled water was produced giving an adjusted theoretical concentration of 351 ppb. During the make up
of the solution 2 drops of 2M NaOH was added to ensure solution stability. Successive dilutions were made from
the original 25 cm3 aliquot used in the Methylene blue sulphide method. The concentration of the methylene blue
solution produced by the method was successively halved with distilled water to a calculated concentration of
11 ppb and the concentration found using the spectrophotometer is shown in Figure 4
These results show that the methylene blue colour produced by the Hach spectrophotometer sulphide method
to be stable upon successive dilutions and the Hach method to be linear. This shows that it is possible to simply
dilute the developed methylene blue colour to produce a sulphide ion concentration within the working
capability of the Hach instrument. This is assuming that the initial S-2 concentration was not so high that the
methylene blue colour becomes unstable as mentioned in the manufacturers’ manual.
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4o PDPETRO, Campinas, SP
21-24 de Outubro de 2007
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-2
Adjusted Theoretical [S ] in ppb
400
350
y = 0.9642x
300
2
R = 0.9954
250
200
150
100
50
0
0
100
200
300
400
Measured H2S concentration in ppb
Figure 4 Successive dilutions of methylene blue solution produced from a NIST Standard solution at an
initial adjusted calculated concentration of 351 ppb S-2
4. RESULTS
4.1 Absorption and storage ability of the trapping solution
The ability of the method to effectively absorb H2S was evaluated by passing a known quantity of methane
gas, known to contain 10% v/v of H2S through two the types of trapping solutions, a simple 1% w/v zinc acetate
solution and a 1% w/v zinc acetate/1% w/v EDTA/0.8% w/v NaOH mixture. 100 cm3 of this standard gas,
measured using a calibrated valve releasing 0.3 liters/min for 20 seconds was passed through a series of two
wash bottles each containing 151g of the simple zinc acetate trapping solution. In a separate identical experiment
the use of the chelating alkaline solution was investigated.
The results of the two identical experiments comparing the proposed trapping mixture of alkaline zinc
acetate/EDTA solution with a simple zinc acetate solution are shown in Table 3. H2S was not detected in the
second wash bottle in either situation, showing that the proposed trapping solution behaves exactly like simple
zinc acetate trapping solution and was as effective, in absorption ability, as the traditional zinc acetate solution
under the conditions of the present work..
Table 3 Absorption and Stability of the Trapping Solutions.
Zinc acetate trapping
solution
alkaline zinc
acetate/EDTA mixture
* 24 hours at 4◦C
Concentration of H2S
in 1st wash bottle ppm
80.4
58.7
Concentration of H2S in H2S concentration in 1st wash
2nd wash bottle ppb
bottle after storage* ppm
<8.0
10.2
<8.0
60.9
The stability of the proposed trapping solution was evaluated by storing the absorb H2S in their respective
trapping solutions for a 24 hour period at a temperature of 4 °C. It can be seen from Table 3, that the simple zinc
acetate trapping solution was unable to hold the H2S gas during this period, whereas the alkaline zinc
acetate/EDTA mixture proved to be very stable during this period.
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4.2. H2S recovery as S-2 from diverse sources using the modified trapping solution.
The ability of the proposed alkaline zinc acetate/EDTA trapping solution to retain sulphides from different
sources was tested by a mass balance exercise, measuring the concentrations of hydrogen sulphide in all the
products obtained by flashing a high pressure reservoir fluid system contaminated with a known amount of H2S.
For this two different live oil samples were obtained from the PETROBRAS Research Center (CENPES) were
used.
A known volume of a live oil sample was mixed with a known volume of formation water containing 3.8%
NaCl and a known amount of a standard, 90%methane/10%H2S gas mixture obtained from White Martins
Praxair Inc., Osasco, São Paulo was added. The multi phase system was maintained in equilibrium at sufficiently
high pressure and temperature to dissolve all the gases, with the aid of a PVT System, Ruska Instrument
Corporation, model 2370, Houston USA. This equipment permitted the manipulation and separation of phases of
the live oil system near reservoir temperatures and pressures. On flashing separately the oil and aqueous phases
to ambient pressure and temperature, four products obtained namely, gas liberated from oil phase, residual oil,
gas liberated from aqueous phase and residual formation water. Each of the products were analyzed for H2S
using a series of two wash bottles each containing the proposed trapping solution. The total amount of sulphides
present in the products is then compared with the amount of sulphide in the originally added hydrogen
sulphide/methane gas mixture.
Nine experiments were performed using two different live oil samples. The nominal amount (Mnominal) of H2S
injected into the live oil–formation water equilibrium systems were determined by the injection time via a
constant flow value connected to the H2S cylinder. The amount of H2S, calculated from the measured
concentrations, of the four products, residual oil, (Ores), residual formation water, (FWres), the gas liberated from
the oil, (GLO), and the gas liberated from the aqueous phase, (GLA), for each of the experiments is shown in
Table 3 showing the effectiveness of the proposed trapping solution for sulphides from diverse sources using
measurements taken after several hours, at a convenient time during the progress of the experiment. In the
original method, the loss of H2S would have made H2S undetectable after several hours.
Table 3 Hydrogen Sulphide Recoveries from Live Oil Samples
1
2
3
4
5
6
7
8
9
Mnominal
mg
34.5
84.0
84.9
85.4
84.7
84.1
84.6
84.6
85.0
Ores,
mg
4.4
12.0
15.1
19.0
6.5
19.0
18.3
12.6
25.1
FWres,
mg
11.7
23.3
12.6
23.6
27.7
19.0
11.2
27.0
20.6
GLO, GLA, Total amount of
mg
mg H2S recovered, mg
14.3 3.0
33.4
25.3 3.0
63.6
23.9 2.8
54.4
13.0 3.1
58.6
15.8 4.0
54.0
27.8 2.5
68.2
20.5 2.6
52.6
19.6 2.4
61.7
24.5 1.0
71.2
5. CONCLUSION
This work demonstrates that the alkaline zinc acetate/EDTA trapping solution is as effective as alkaline zinc
acetate solution, in absorbing hydrogen sulphide. In the experiments, the traditional simple zinc acetate trapping
solution proved to be unstable in retaining sulphides even for a 24-hour storage period maintained under
refrigeration, at a temperature of 4 °C. The proposed trapping solution, proved to be stable over the same storage
period in the same conditions, in preserving zinc sulphide precipitates produced by the absorption of hydrogen
sulphide.
The present work shows that the proposed zinc acetate/EDTA trapping solution is equally effective and
stable for trapping hydrogen sulphide from diverse sources, different phases- aqueous, organic and gaseous,
composed of substances of different chemical natures. This characteristic makes the present trapping method
attractive for determining equilibrium concentrations of different phases using a single method of measurement,
for example the methylene blue method, with comparable experimental error for all the phases.
6. ACKNOWLEDGEMENTS
The authors acknowledge the financial support of MCT/FINEP/CTPETRO/PETROBRAS under the project
number: FBR 2528-06 entitled, “Distribuição do Gás Sulfídrico entre Óleos Pesados, Gás Liberado e a Água de
Formação nas Condições de Reservatório”. The authors thank CNPq for the scholarship “ bolsa de produtividade
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2.3.0174-2 – 7
to Krishnaswamy Rajagopal and FINEP/CNPq for DTI schlorships to Ian Hovell and Luis Augusto Medeiros
Rutledge.
7. REFERENCES
ATTARI, A. and CHAO, S., “Sampling and analysis of natural gas trace constituents”, The First International
Oil, Gas & Petrochemical Congress, Institute of gas technology, Iran, 1993
BALASUBRAMIANIAN N. and KUMAR, B. S. M., “Extraction-spectrophotometric determination of hydrogen
sulfide”, 115(6), 859-863, Analyst, 1990.
BOND, A..M., HERITAGE, I.D., WALLACE, G..G., MCCORMICK, M.J., “Simultaneous determination of
free sulfide and cyanide by ion chromatography with electrochemical detection”, Analytical Chemistry,
54(3), 582-585, 1982.
CLAY, D.A. ROGERS, C.H. JUNGERS, R.H., “Determination of total sulfur in gasoline by gas
chromatography with a flame photometric detector”, 49(1), 126-128, Analytical Chemistry, 1977.
CLINE, J. D. “Spectrophotometric detection of hydrogen sulfide in natural water”, 14, 454–458, Limnology and
Oceanography, 1968.
HSEU, T.M., RECHNITZ, G. A., “Analytical study of a sulfide ion-selective membrane electrode in alkaline
solution”, 40(7), 1054-1060, Analytical Chemistry, 1968.
JEFFERY, G. H., BASSETT, J., MENDHAM J. and DENNEY, R. C. in “Vogel’s Text Book of Quantitative
Chemical Analysis”, ELBS-Longman, Essex, 5th. Edn., 398-399, 1997.
JOHNSON, C. M. and NISHITA, H.,” Microestimation of Sulfur in Plant Materials, Soils, and Irrigation
Waters” , 24, 736, Analytical. Chemistry,1952.
KONOVALOV S.K. and ROMANOV, A.S., “Spectrophotometric and iodometric raethods for the detection of
hydrogen sulfide”, 10(4), 365-377, Journal of Physics Oceanography, 1999.
NATUSCH, D. F. S. SEWELL, J. R. and LANNER, R. L., “Determination of Hydrogen Sulfide in Air-An
Assessment of Impregnated Paper Tape Methods”, 46(3), 410-415, Analytical Chemistry, 1974.
PARVINEN, P., LAJUNEN, L.H.J., “Determination of sulfide as hydrogen sulfide in water and sludge
samples by gas phase molecular AS”, 15, 83–86, Atomic Spectroscopy, 1994.
SINGH, V., GOSAIN, S., MISHRA S., JAIN A. and VERMA K. K., “Solid-phase extreaction and
spectrophotometric determination of hydrogen sulfide in air and water utilizing ethylene blue formation”,
125, 1185-1188, Analyst, 2000.
O USO DE EDTA EM SOLUÇÃO ALCALINA DE ACETATO DE ZINCO COMO
UMA MISTURA PARA ABSORVER GÁS SULFÍDRICO
O gás sulfidrico (H2S) é extremamente tóxico e encontrado de forma crescente nos reservatórios off-shore,
sendo, portanto de extrema importância encontrar procedimentos analíticos confiáveis e apurados. Normalmente
o procedimento padrão USEPA 376.2 é usado para determinação de H2S nas amostras de água de formação e de
óleo vivo. Neste método o H2S é absorvido em solução aquosa alcalina, sendo assim os sulfetos analisados pelo
método de azul de metileno (MB). Este procedimento, porém, requer a imediata analise da amostra, já que os
precipitados são instáveis. Acetato de zinco produz ácido acético durante a absorção de H2S desta maneira
reduzindo a eficácia da solução sequestrante, o que torna importante o controle inicial e final do valor de pH
desta solução. O precipitado formado adquire uma massa gelatinosa irregular, que adere à parede do vidro do
borbulhador e às partes do vidro sinterizado da garrafa do gás de lavagem, sendo necessária a solvatação ácida
do precipitado para liberar o H2S para análise, resultando em perdas devido à redução da solubilidade do gás em
meio ácido. A proposta é estabilizar a amostra com uma solução alcalina de um agente quelante, o sal de disódio
etilenoldiaminotetracetato (EDTA). Desta forma o H2S liberado dos reservatórios de petróleo por redução de
pressão seria capturado numa solução alcalina aquosa de acetato de zinco e EDTA. O precipitado resultante de
sulfeto de zinco é analisado utilizando um espectrofotômetro de UV-Visível da HACH Inc., seguindo o método
MB. A calibragem do espectrômetro foi feita utilizando soluções-padrão de sulfeto de sódio. O precipitado de
sulfeto na solução sequestrante original se decompõe e se dispersa em algumas horas, enquanto que com o
agente quilante proposto, o precipitado se mantém estável e em suspensão por vários dias. Esse método de
estabilização temporário pode ser facilmente adaptado em métodos padrões utilizados para a análise da
quantidade de H2S encontrado em fluidos de reservatórios e outros processos.
Palavras chave: Absorção do gás sulfídrico, Analise Quantitativa,
espectrofotometria, EDTA,
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Os autores são os únicos responsáveis pelo conteúdo deste artigo.
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