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Determination of chloride in brazilian crude oils by ion chromatography after extraction induced by emulsion breaking
Journal of Chromatography A, 1458 (2016) 112–117
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Determination of chloride in brazilian crude oils by ion chromatography after extraction induced by emulsion breaking Nicolle F. Robaina a , Fernanda N. Feiteira a , Alessandra R. Cassella b , Ricardo J. Cassella a,∗ a b
Departamento de Química Analítica, Universidade Federal Fluminense, Outeiro de São João Batista s/n, Centro, Niterói/RJ, 24020-141, Brazil PETROBRAS, CENPES, PDEDS/QM, Av. Horácio Macedo 950, Ilha do Fundão, Rio de Janeiro/RJ 21941-915, Brazil
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Article history: Received 29 February 2016 Received in revised form 16 June 2016 Accepted 20 June 2016 Available online 21 June 2016 Keywords: Extraction induced by emulsion breaking Chloride Crude oil Ion chromatography
a b s t r a c t The present paper reports on the development of a novel extraction induced by emulsion breaking (EIEB) method for the determination of chloride in crude oils. The proposed method was based on the formation and breaking of oil-in-water emulsions with the samples and the consequential transference of the highly water-soluble chloride to the aqueous phase during emulsion breaking, which was achieved by centrifugation. The determination of chloride in the extracts was performed by ion chromatography (IC) with conductivity detection. Several parameters (oil phase:aqueous phase ratio, crude oil:mineral oil ratio, shaking time and type and concentration of surfactant) that could affect the performance of the method were evaluated. Total extraction of chloride from samples could be achieved when 1.0 g of oil phase (0.5 g of sample + 0.5 g of mineral oil) was emulsified in 5 mL of a 2.5% (m/v) solution of Triton X-114. The obtained emulsion was shaken for 60 min and broken by centrifugation for 5 min at 5000 rpm. The separated aqueous phase was collected, filtered and diluted before analysis by IC. Under these conditions, the limit of detection was 0.5 g g−1 NaCl and the limit of quantification was 1.6 g g−1 NaCl. We applied the method to the determination of chloride in six Brazilian crude oils and the results did not differ statistically from those obtained by the ASTM D6470 method when the paired Student-t-test, at 95% confidence level, was applied. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Petroleum is a complex mixture of hydrocarbons containing variable concentrations of organometallic and heteroatomic organic compounds, and inorganic substances. Liquid petroleum is usually called of crude oil, which is associated with water in the form of an emulsion; thus it contains dissolved inorganic salts. The association of petroleum with water primarily results from contact with the saline water naturally present in the geological formation of the reservoir where the oil is stored. Also, it is important to consider that seawater is used in the prospection operations carried out offshore. Therefore, chloride is considered one of the main contaminants of crude oil because it interferes with the refining process and is transferred to the final derived products [1,2]. The presence of chloride can lead to the formation of HCl during refining, which enhances corrosion of metallic parts of the processing unities. Additionally, chloride ions deactivate metallic catalysts (Ni,
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (R.J. Cassella). http://dx.doi.org/10.1016/j.chroma.2016.06.066 0021-9673/© 2016 Elsevier B.V. All rights reserved.
Cu or Pd-based) usually employed in the catalytic cracking process [3]. The determination of chloride in crude oils (expressed in terms of NaCl concentration) is commonly performed by the Mohr method (AgNO3 titration) or by a potentiometric titration. These methods are limited by their poor sensitivity and can be affected by the presence of other halides in the samples [4]. Some official methods can also be cited. Perhaps, the most important official method employed for chloride determination in crude is the ASTM (American Society for Testing and Materials) D6470 [5]. In this method, the samples are submitted to solvent extraction with a mixture of acetone, water and ethyl alcohol. The extracted chloride is titrated either with AgNO3 or potentiometrically. The main drawbacks of this method are the potential interference of other halide ions that are extracted along with chloride, and the large volume of solvent required to complete the procedure. The ASTM D3230 method [6] is recommended for the determination of crude oil salinity. However, in this method, no extraction is required and the samples are simply diluted with a mixture of xylene, ethyl alcohol and water. The conductivity of the final mixture is measured and related to the oil salinity.
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Alternative methods have been proposed to determine chloride in crude oils. Souza et al. [7] proposed an interesting way to estimate the chloride concentration in crude oil samples through determination of selected counter ions (Na(I), Ca(II), Ba(II), Sr(II) and Fe(III)) after their extraction with hot water. Hajian et al. [8] developed an apparatus for liquid-liquid extraction of chloride from crude oils. A mixture of solvents (toluene, ethanol and acetone) was used to extract chloride from the samples. The final extracts were analyzed using the Volhard method. Moraes et al. [9] developed a method for microwave-assisted extraction of chloride from heavy crude oils. The method required 60 min of microwave irradiation at 800 W to achieve total extraction of chloride from samples and 20 min for the cooling of the flasks. Chloride was determined in the aqueous phase by ICP OES. In 2010, our research group proposed a novel method for the extraction of metal cations from organic liquids, named extraction induced by emulsion breaking (EIEB). It is based on the formation and breaking of water-in-oil emulsions with the consequential transference of the metal cations to the acidic aqueous phase. This approach has been successfully employed for the determination of metals in different types of oil samples (vegetable oils [10–12], used lubricating oil [13,14], diesel oil [15–17], biodiesel [18,19] and mineral oil [20]). Since the chloride ion is highly soluble in water, we are now proposing to employ the extraction induced by emulsion breaking technique to chloride ion extraction from crude oil. It is important to note that, in the present work, no acid was added to the emulsions in order to avoid the introduction of very high concentrations of anions (typically nitrate, since nitric acid has been commonly used in EIEB) and, contrary to other EIEB systems, we decided to work with oil-in-water emulsions instead of typical water-in-oil emulsions in order to enhance the efficiency of the extraction process. 2. Experimental 2.1. Solutions and reagents The ultrapure water employed in this work was obtained with a Direct-Q 3 system from Millipore (Milford, MA, USA). The ultrapure water always presented resistivity of 18.2 M cm or higher. Working standard solutions of chloride ions were prepared daily by diluting the 1000 mg L−1 chloride stock standard solution supplied by Fluka (Buchs, Switzerland) with ultrapure water. The stock standard solution supplied by Fluka was prepared with NaCl. The 25% m/v Triton X-100 (Sigma-Aldrich, St. Louis, MO, USA) and Triton X-114 (Sigma-Aldrich) stock solutions were prepared by dissolving, separately, 12.5 g of each surfactant in approximately 40 mL of ultrapure water. After the surfactant was dissolved and the foam decreased, the obtained solution was transferred to a 50 mL volumetric flask. Then, the volume was completed to the mark with ultrapure water. The solutions employed in the emulsification of the samples were prepared, daily, by suitable dilution of the Triton X-100 and Triton X-114 stock solutions with ultrapure water. A spectroscopy-grade mineral oil (viscosity = 16.7 cps at 40 ◦ C, d = 0.838 g mL−1 at 25 ◦ C), supplied by Sigma-Aldrich, was employed in the dilution of the crude oil samples. 2.2. Apparatus and instruments The ion chromatography system employed in this work was a Dionex ICS 2100 system (Sunnyvale, CA, USA), equipped with an integrated eluent (potassium hydroxide) generator, model RFICEG (EGC III KOH cartridge) and an AERS 500 2 mm membrane conductivity suppressor. Conductivity signals were measured with a DS6 heated conductivity cell and the chromatograms were
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acquired using Chromeleon software, version 7.2, also supplied by Dionex. The separation of chloride ions was obtained with an IonPac AS15 analytical column (2 × 250 mm, 7.5 m particle size and 56.25 eq column−1 capacity) and an IonPac AG15 guard column (2 × 50 mm) was used to protect the analytical column. A sample volume of 20 L was injected in all experiments. Emulsions were prepared using a Fisatom magnetic stirrer (São Paulo, Brazil), model 754A. They were broken with the aid of a centrifuge, model 5804, furnished by Eppendorf (Hamburg, Germany). 2.3. General procedure for extraction induced by emulsion breaking of chloride The proposed procedure is based on the formation and breaking of oil-in-water emulsions in order to extract chloride from crude oil. The emulsions were formed by vigorously stirring a 0.5 g sample of crude oil with 0.5 g of mineral oil and 5 mL of the emulsifying solution (2.5% m/v Triton X-114) for 90 min. Afterwards, the emulsion was broken by centrifugation at 5000 rpm for 20 min (Fig. 1). Then, exactly 2 mL of the aqueous phase containing the extracted chloride was taken and diluted to 10 mL with ultrapure water in a volumetric flask. The final obtained solution was filtered through a cellulose acetate membrane with 0.20 m pore diameter and injected into the chromatographic system. 2.4. Chromatographic conditions The chromatographic run for separation of chloride was divided into three steps. In the first step (0–6 min), a KOH concentration of 38 mM was employed. The chloride peak appeared at 5.6 ± 0.1 min. In the second step (6–15 min), the KOH concentration was increased to 50 mM in order to provide a convenient cleaning of the column. Finally, in the third step (15–20 min), the KOH concentration was returned to 38 mM to prepare the column for a new separation cycle. The current in the suppressor system was set to 41 mA, the mobile phase flow-rate was 0.33 mL min−1 during the entire chromatographic run and the injection volume was always 20 L. Typical chromatograms of the chloride standard solution (0.1 g mL−1 ) and for an actual sample of crude oil treated by the proposed method are presented in Fig. 2. 2.5. Samples Crude oil samples analyzed in this work were kindly supplied by Petrobras (Rio de Janeiro, Brazil). They were stored in sealed, dark, glass flasks, at ambient laboratory temperature. Sample S2 was employed in the optimization experiments. 3. Results and discussion The development of the proposed extraction methodology was performed in two steps: (i) evaluation/optimization of the extraction conditions and (ii) application to real samples of crude oil, including a comparison of the obtained results with those obtained using the ASTM D6470 standard method. The methodology was optimized by studying the effect of the following variables: (i) type and concentration of surfactant, (ii) the ratio of oil (crude oil + mineral oil):aqueous phases, (iii) the amount of mineral oil added to the crude oil before emulsification and (iv) emulsion shaking time. 3.1. Effect of oil phase:aqueous phase ratio One of the most important parameters to be studied in the proposed extraction method was the ratio of oil phase/aqueous phase. This parameter controlled the amount of sample employed in the
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Fig. 1. Emulsion (A) before and (B) after centrifugation for the emulsion breaking.
Fig. 2. Typical chromatograms of a mineral oil extract (black line), a sample extract diluted 1:5 (gray line) and a 0.25 g mL−1 chloride standard solution (dashed line).
procedure and could have an important impact on the sensitivity of the method and on the extraction capacity, since the aqueous phase is responsible for the transference of chloride ions from the oil to the aqueous extract. Some conditions were established randomly for the initial experiments. A 5% (m/v) solution of Triton X-100 was employed as the extractant solution and 0.5 g of crude oil was mixed with 0.5 g of mineral oil to form the oil phase for the extraction procedure. The mixing of crude and mineral oils prior to the application of EIEB was a fundamental aspect of the extraction procedure, since crude oils of different origins presented very different physical and chemical characteristics that affected the emulsification process. Mixing of crude oil samples with mineral oil minimized these differences and allowed all crude oils analyzed in this work to be emulsified easily by simple shaking of the mixtures. The influence of the ratio of the oil phase:aqueous phase was evaluated by varying the mass of the oil phase (crude oil:mineral oil, 1:1) and maintaining a constant volume of the aqueous phase. The experiments were performed by extracting chloride from 0.5 to 3.0 g of oil phase with 5 mL of 5% (m/v) Triton X-100 solution. The emulsions were broken just after emulsification by centrifugation for 20 min at 5000 rpm.
The amount of oil phase employed in the extraction significantly affected both the amount of chloride extracted and the precision of the experiments (Fig. 3). A higher extraction efficiency was observed when 0.5 g of oil phase was employed. However, in this condition, the repeatability of the measurements was poor, probably due to the use of a small and non-representative amount of crude oil to prepare the oil phase. When greater masses of oil phase were used, the precision improved from 16% to approximately 7% in terms of RSD, but with a consequential decrease in the amount of chloride extracted, despite the fact that there was no statistical difference between the extraction efficiency at 0.5 and 1.0 g of oil phase. Therefore, we decided to establish that 1.0 g of oil phase always would be used for the extraction of chloride. 3.2. Effect of the crude oil:mineral oil ratio As mentioned previously, crude oils can have different physical and chemical characteristics, which significantly influence the emulsification process. In order to minimize the influence of these characteristics on the preparation of emulsions, all crude oil samples were first mixed with mineral oil before application of EIEB. In the previous experiment, we chose to always employ 1.0 g of organic phase. So, in this experiment we varied the amount of crude
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Normalized chloride signal
Normalized chloride signal
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1.0 0.8 0.6 0.4
Triton X-100 Triton X-114
1.0 0.8 0.6 0.4 0.2
0.2 0.0
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0.0 0.0
0.5
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Mass of oil phase (g) Fig. 3. Influence of the mass of oil phase in relation to aqueous phase on the chloride extraction. The volume of aqueous phase was always 5 mL (5% (m/v) Triton X-100 solution) and the oil phase was prepared by mixing 0.5 g sample with 0.5 g mineral oil. Error bars represent the standard deviation obtained from three independent determinations.
and mineral oils, but always maintained a final mass of 1.0 g. Four ratios were tested: (i) 100% crude oil, (ii) 75% crude oil, (iii) 50% crude oil and (iv) 25% crude oil. The results demonstrated that this parameter had no noticeable influence on the extraction efficiency of chloride ions. Nevertheless, again, the repeatability of the measurements was strongly affected by the variation of this parameter. The repeatability was much better (1.1% RSD) at 50% crude oil (0.5 g crude oil + 0.5 g mineral oil), which was exactly the same ratio utilized in the previous experiment and provided convenient repeatability. Therefore, we decided to keep this condition for all further experiments.
3.3. Effect of shaking time The application of the EIEB is based on the intense contact between the continuous and dispersed phases. In general, when the emulsions are shaken after their preparation, the movement of the droplets (dispersed phase) through the continuous phase enhances the extraction efficiency [17,18]. Classically, the EIEB has been based on the formation and breaking of water-in-oil emulsions, which are more viscous than the oil-in-water emulsions obtained in the present work. Theoretically, in oil-in-water emulsions, the mixing between the phases is facilitated, which also enhances the extraction efficiency. In order to evaluate the effect of shaking on the extraction efficiency of chloride, we tested the influence of shaking time before emulsion breaking. For this purpose, the emulsions were shaken in the range of 0 (emulsion breaking just after its formation) to 120 min. The other experimental conditions were those optimized in the previous experiments and the shaking of the flasks was carried out on an horizontal mixer operated at 90 rpm. Only a small increase in the extraction efficiency was observed with an increase in the shaking time, revealing that the transference of the greater part of chloride from the oil to the aqueous phase occurred just after the preparation of the emulsions. Despite this fact, maintaining the emulsion under shaking for at least 60 min increased the extraction efficiency by 20%. Thus, we set a shaking time of 60 min for the method.
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Surfactant concentraon (% m/v) Fig. 4. Influence of type and concentration of surfactant on the chloride extraction. The extractions were performed using 1.0 g of oil phase (0.5 g sample + 0.5 g mineral oil) and 60 min shaking time. Error bars represent the standard deviation obtained from three independent determinations.
3.4. Effect of the type and concentration of surfactant As EIEB depends on the formation of emulsions, the type and concentration of the surfactant employed is of fundamental importance. The surfactant contributes to the dispersion of oil through the water, making possible the preparation of a homogeneous system. EIEB has been performed with non-ionic surfactants, such as Triton X-100 and Triton X-114. They were selected for the extraction of chloride from crude oil using the EIEB approach in the form of oil-in-water emulsions. The concentration of both Triton X-100 and Triton X-114 was varied from 1 to 10% (m/v). It was not possible to prepare emulsions without the addition of surfactant. The volume of aqueous solution was 5 mL, the mass of organic phase was 1.0 g (0.5 g crude oil + 0.5 g mineral oil) and the emulsions were shaken for 60 min prior to their breaking. The results (Fig. 4) demonstrated that the type of surfactant plays a more important role than its concentration. The extraction efficiency obtained with Triton X-114 was 70% higher than the extraction efficiency obtained with Triton X-100. We believe that this phenomenon is associated with the emulsion breaking process, which is more easily performed when Triton X-114 was employed in the emulsification. In the presence of Triton X-114, the separation between the phases was complete and quantitative recovery of chloride could be observed. The use of Triton X-100 provided more stable emulsions that were more difficult to break. In this situation, it is possible that some water remained in the oil phase and, consequently, so did chloride ions. The effect of the surfactant concentration was less pronounced than the effect of the type of surfactant. In both cases (Triton X-100 and X-114), an increase in surfactant concentration decreased the extraction efficiency. Again, this phenomenon can be explained by the emulsion stability, which increased with higher concentrations of surfactant. In this context, we decided to use a 2.5% (m/v) Triton X-114 solution for emulsification, since, under these conditions, we observed a satisfactory compromise between extraction efficiency and measurement precision.
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Table 1 Results obtained in the determination of chloride in Brazilian crude oil samples by the proposed and reference (ASTM D6470) methods. The results are expressed as mean ± standard deviation (n = 3). Sample
ASTM D6470 (g g−1 NaCl)
Proposed Method (g g−1 NaCl)
S1 S2 S3 S4 S5 S6
2.0 12.3 5.9 2.0 2.0 2.7
2.4 11.1 5.3 1.8 0.91 2.9
a b
± ± ± ± ± ±
0.4 1.7 0.1 0.2 0.20 0.6
Recoveryb (%) nda nda nda 87 ± 4 92 ± 1 97 ± 4
nd = not determined. Samples were fortified with 5 g g−1 NaCl.
3.5. Evaluation of the calibration strategy and analytical figures of merit After optimization of the extraction conditions, we evaluated the influence of the calibration strategy in order to set the correct quantification approach for the method and ensure that no matrix interferences were present in the determination of chloride in the final extracts. In this study, analytical curves were compared with standard addition curves prepared in the final extract medium. The standard addition and analytical curves were prepared on three different days in the range of 0.025–1 g mL−1 chloride. The slopes of the curves were practically the constant (variation from 0.895 to 0.925), independently of the day of the measurements. This result demonstrated that no matrix interferences were present in the determination of chloride in the extracts and that analytical curves could be employed for the quantification of chloride in the extracts. Even so, the curves were statistically evaluated by Student’s t-test and the significance of the regression was determined. Each of the curves presented statistical significance (95% confidence level), with F values varying in the range of 6476–26160. As the critical value of F is 10.13, the linear models were considered statistically satisfactory. Also, the paired Student’s t-test was applied to compare the difference between the analytical and standard addition curves. No statistical differences were observed at the 95% confidence level, since the calculated value of t was 0.210 and the critical value is 4.30 (two degrees of freedom) [21]. The methodological limits of detection and quantification were estimated using an analytical curve constructed with chloride standard solutions prepared in ultrapure water and taking into account the extraction/dilution of samples. The estimated limits of detection (3s) and quantification (10s) were expressed in terms of NaCl concentration, which is the parameter regularly employed to express the salinity of crude oils. The limit of detection of the developed method was 0.5 g g−1 NaCl and the limit of quantification was 1.6 g g−1 . The limit of detection of the proposed method is four times better than the limit of detection reported for the ASTM D6470 standard method, which is 2 g g−1 NaCl [5]. The typical analytical curve employed in the quantification of chloride in the extracts presented the following equation: S = 0.9193 [Cl− ] + 0.0020 (r2 = 0.999), where S is the chloride peak area and [Cl− ] is the chloride concentration in g mL−1 . The precision of the method was estimated by analyzing five independent aliquots of the sample S2 . In this case, a relative standard deviation of 15% was obtained. The intermediary precision of the method was also evaluated. For this purpose, the sample S2 was analyzed in three different days and the relative standard deviation of the mean value was taken as intermediary precision. An intermediary precision of 17.1% was observed. 3.6. Analysis of brazilian crude oil samples In order to test the applicability of the proposed method, we employed it for the determination of the chloride concentration
in six Brazilian crude oils from different origins. The results were compared with those obtained by application of the ASTM D6470 method [5], which was used as a reference method. Additionally, a recovery test was performed. Three samples were fortified with 5 g g−1 of NaCl and analyzed using the proposed method. All results are displayed in Table 1. As shown in Table 1, there was a good agreement between the results obtained by the proposed method and the reference ASTM D6470 method. Also, the recovery percentages were in the range of 87–97%, indicating that the quantitative extraction of chloride can be achieved using the emulsion breaking procedure optimized in this work. We also evaluated the statistical significance of the results. For this purpose, a paired Student’s t-test was applied. A t value of 1.148 was observed for the paired samples. As the critical value of t for five degrees of freedom is 2.571 (95% confidence level), the null hypothesis was maintained, which indicated that there were no statistical differences between the results provided by the two methods (proposed and reference) in the determination of chloride in the analyzed samples. 4. Conclusions The method proposed in the present work was shown to be suitable for the total extraction of chloride from crude oils, making possible its quantification in the samples by ion chromatography. The results obtained with the proposed method were not statistically different from those obtained by the ASTM D6470 reference method at a 95% confidence level, which demonstrated the accuracy of this method. The application of the EIEB procedure permitted the total separation of chloride from the complex organic matrix, through its transference to an aqueous phase that was easily analyzed by ion chromatography with conductivity detection. Among the studied parameters, the type of surfactant employed for emulsion preparation presented a remarkable effect on the extraction efficiency. In this case, the use of Triton X-114 resulted in higher extraction efficiency than the use of Triton X-100. The optimized methodology was successfully applied to the determination of chloride in six samples of Brazilian crude oil supplied by Petrobras. Therefore, we believe that the method can be considered an excellent alternative to other methods that require the use of large volumes of solvent or necessitated the use of an expensive apparatus for chloride extraction from samples. We believe that the method can be expanded to include the determination of other water-soluble substances present in crude oils. Acknowledgments The authors are grateful to PETROBRAS (Petróleo Brasileiro S.A.) for the financial support and to CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnológico) and FAPERJ (Fundac¸ão Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro)
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for providing grants and fellowships. The authors are also grateful to CAPES (Coordenac¸ão de Aperfeic¸oamento de Pessoal de Nível Superior) for the scholarship granted to N.F.R. References [1] M.K. Morigaki, R.Q.F. Chimin, C.M.S. Sad, P.R. Filgueiras, E.V.R. de Castro, J.C.M. Dias, Salinity of crude oil: optimization of methodology and new method of extraction of salt in petroleum, Quím. Nova 33 (2010) 607–612. [2] J.S.F. Pereira, P.A. Mello, D.P. Moraes, F.A. Duarte, V.L. Dressler, G. Knapp, E.M.M. Flores, Spectrochim. Acta Part B 64 (2009) 554–558. [3] A.R. Cassella, E.F.R. Rocha, M.L. Duarte, Technical Communication CT QM 02/2007, CENPES, Petrobras, 2007. [4] A. Doyle, A. Saavedra, M.L.B. Tristão, L.A.N. LAN, R.Q. Aucélio, Spectrometric methods for the determination of chlorine in crude oil and petroleum derivates, Spectrochim. Acta Part B 86 (2013) 102–107. [5] American Society for Testing and Materials (ASTM), ASTM D6470 − Standard Test Methods for Salt in Crude Oil (potentiometric Method), ASTM International, West Conshohocken, PA, USA, 2010. [6] American Society for Testing and Materials (ASTM), ASTM D3230 − Standard Test Methods for Salt in Crude Oil (electrometric Method), ASTM International, West Conshohocken, PA, USA, 2010. [7] M.O. Souza, M.A. Ribeiro, M.T.W.D. Carneiro, G.P.B. Athayde, E.V.R. de Castro, F.L.F. da Silva, W.O. Matos, R.Q. Ferreira, Evaluation and determination of chloride in crude oil based on the counterions Na, Ca Mg, Sr and Fe, quantified via ICP-OES in the crude oil aqueous extract, Fuel 154 (2015) 181–187. [8] R. Hajian, A.H. Oroojloo, S.S. Mousavian, N. Shams, Determination of sodium chloride in crude oil of gachsaran county oil wells, Asian J. Chem. 23 (2011) 4223–4224. [9] D.P. de Moraes, F.G. Antes, J.S.F. Pereira, M.F.P. dos Santos, R.C.L. Guimarães, J.S. Barin, M.F. Mesko, J.N.G. Paniz, E.M.M. Flores, Microwave-assisted procedure for salinity evaluation of heavy crude oil emulsions, Energy Fuels 24 (2010) 2227–2232. [10] N.F. Robaina, D.M. Brum, R.J. Cassella, Application of the extraction induced by emulsion breaking for the determination of chromium and manganese in edible oils by electrothermal atomic absorption spectrometry, Talanta 99 (2012) 104–112.
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Determination of chloride in brazilian crude oils by ion chromatography after extraction induced by emulsion breaking Nicolle F. Robaina a , Fernanda N. Feiteira a , Alessandra R. Cassella b , Ricardo J. Cassella a,∗ a b
Departamento de Química Analítica, Universidade Federal Fluminense, Outeiro de São João Batista s/n, Centro, Niterói/RJ, 24020-141, Brazil PETROBRAS, CENPES, PDEDS/QM, Av. Horácio Macedo 950, Ilha do Fundão, Rio de Janeiro/RJ 21941-915, Brazil
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Article history: Received 29 February 2016 Received in revised form 16 June 2016 Accepted 20 June 2016 Available online 21 June 2016 Keywords: Extraction induced by emulsion breaking Chloride Crude oil Ion chromatography
a b s t r a c t The present paper reports on the development of a novel extraction induced by emulsion breaking (EIEB) method for the determination of chloride in crude oils. The proposed method was based on the formation and breaking of oil-in-water emulsions with the samples and the consequential transference of the highly water-soluble chloride to the aqueous phase during emulsion breaking, which was achieved by centrifugation. The determination of chloride in the extracts was performed by ion chromatography (IC) with conductivity detection. Several parameters (oil phase:aqueous phase ratio, crude oil:mineral oil ratio, shaking time and type and concentration of surfactant) that could affect the performance of the method were evaluated. Total extraction of chloride from samples could be achieved when 1.0 g of oil phase (0.5 g of sample + 0.5 g of mineral oil) was emulsified in 5 mL of a 2.5% (m/v) solution of Triton X-114. The obtained emulsion was shaken for 60 min and broken by centrifugation for 5 min at 5000 rpm. The separated aqueous phase was collected, filtered and diluted before analysis by IC. Under these conditions, the limit of detection was 0.5 g g−1 NaCl and the limit of quantification was 1.6 g g−1 NaCl. We applied the method to the determination of chloride in six Brazilian crude oils and the results did not differ statistically from those obtained by the ASTM D6470 method when the paired Student-t-test, at 95% confidence level, was applied. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Petroleum is a complex mixture of hydrocarbons containing variable concentrations of organometallic and heteroatomic organic compounds, and inorganic substances. Liquid petroleum is usually called of crude oil, which is associated with water in the form of an emulsion; thus it contains dissolved inorganic salts. The association of petroleum with water primarily results from contact with the saline water naturally present in the geological formation of the reservoir where the oil is stored. Also, it is important to consider that seawater is used in the prospection operations carried out offshore. Therefore, chloride is considered one of the main contaminants of crude oil because it interferes with the refining process and is transferred to the final derived products [1,2]. The presence of chloride can lead to the formation of HCl during refining, which enhances corrosion of metallic parts of the processing unities. Additionally, chloride ions deactivate metallic catalysts (Ni,
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (R.J. Cassella). http://dx.doi.org/10.1016/j.chroma.2016.06.066 0021-9673/© 2016 Elsevier B.V. All rights reserved.
Cu or Pd-based) usually employed in the catalytic cracking process [3]. The determination of chloride in crude oils (expressed in terms of NaCl concentration) is commonly performed by the Mohr method (AgNO3 titration) or by a potentiometric titration. These methods are limited by their poor sensitivity and can be affected by the presence of other halides in the samples [4]. Some official methods can also be cited. Perhaps, the most important official method employed for chloride determination in crude is the ASTM (American Society for Testing and Materials) D6470 [5]. In this method, the samples are submitted to solvent extraction with a mixture of acetone, water and ethyl alcohol. The extracted chloride is titrated either with AgNO3 or potentiometrically. The main drawbacks of this method are the potential interference of other halide ions that are extracted along with chloride, and the large volume of solvent required to complete the procedure. The ASTM D3230 method [6] is recommended for the determination of crude oil salinity. However, in this method, no extraction is required and the samples are simply diluted with a mixture of xylene, ethyl alcohol and water. The conductivity of the final mixture is measured and related to the oil salinity.
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Alternative methods have been proposed to determine chloride in crude oils. Souza et al. [7] proposed an interesting way to estimate the chloride concentration in crude oil samples through determination of selected counter ions (Na(I), Ca(II), Ba(II), Sr(II) and Fe(III)) after their extraction with hot water. Hajian et al. [8] developed an apparatus for liquid-liquid extraction of chloride from crude oils. A mixture of solvents (toluene, ethanol and acetone) was used to extract chloride from the samples. The final extracts were analyzed using the Volhard method. Moraes et al. [9] developed a method for microwave-assisted extraction of chloride from heavy crude oils. The method required 60 min of microwave irradiation at 800 W to achieve total extraction of chloride from samples and 20 min for the cooling of the flasks. Chloride was determined in the aqueous phase by ICP OES. In 2010, our research group proposed a novel method for the extraction of metal cations from organic liquids, named extraction induced by emulsion breaking (EIEB). It is based on the formation and breaking of water-in-oil emulsions with the consequential transference of the metal cations to the acidic aqueous phase. This approach has been successfully employed for the determination of metals in different types of oil samples (vegetable oils [10–12], used lubricating oil [13,14], diesel oil [15–17], biodiesel [18,19] and mineral oil [20]). Since the chloride ion is highly soluble in water, we are now proposing to employ the extraction induced by emulsion breaking technique to chloride ion extraction from crude oil. It is important to note that, in the present work, no acid was added to the emulsions in order to avoid the introduction of very high concentrations of anions (typically nitrate, since nitric acid has been commonly used in EIEB) and, contrary to other EIEB systems, we decided to work with oil-in-water emulsions instead of typical water-in-oil emulsions in order to enhance the efficiency of the extraction process. 2. Experimental 2.1. Solutions and reagents The ultrapure water employed in this work was obtained with a Direct-Q 3 system from Millipore (Milford, MA, USA). The ultrapure water always presented resistivity of 18.2 M cm or higher. Working standard solutions of chloride ions were prepared daily by diluting the 1000 mg L−1 chloride stock standard solution supplied by Fluka (Buchs, Switzerland) with ultrapure water. The stock standard solution supplied by Fluka was prepared with NaCl. The 25% m/v Triton X-100 (Sigma-Aldrich, St. Louis, MO, USA) and Triton X-114 (Sigma-Aldrich) stock solutions were prepared by dissolving, separately, 12.5 g of each surfactant in approximately 40 mL of ultrapure water. After the surfactant was dissolved and the foam decreased, the obtained solution was transferred to a 50 mL volumetric flask. Then, the volume was completed to the mark with ultrapure water. The solutions employed in the emulsification of the samples were prepared, daily, by suitable dilution of the Triton X-100 and Triton X-114 stock solutions with ultrapure water. A spectroscopy-grade mineral oil (viscosity = 16.7 cps at 40 ◦ C, d = 0.838 g mL−1 at 25 ◦ C), supplied by Sigma-Aldrich, was employed in the dilution of the crude oil samples. 2.2. Apparatus and instruments The ion chromatography system employed in this work was a Dionex ICS 2100 system (Sunnyvale, CA, USA), equipped with an integrated eluent (potassium hydroxide) generator, model RFICEG (EGC III KOH cartridge) and an AERS 500 2 mm membrane conductivity suppressor. Conductivity signals were measured with a DS6 heated conductivity cell and the chromatograms were
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acquired using Chromeleon software, version 7.2, also supplied by Dionex. The separation of chloride ions was obtained with an IonPac AS15 analytical column (2 × 250 mm, 7.5 m particle size and 56.25 eq column−1 capacity) and an IonPac AG15 guard column (2 × 50 mm) was used to protect the analytical column. A sample volume of 20 L was injected in all experiments. Emulsions were prepared using a Fisatom magnetic stirrer (São Paulo, Brazil), model 754A. They were broken with the aid of a centrifuge, model 5804, furnished by Eppendorf (Hamburg, Germany). 2.3. General procedure for extraction induced by emulsion breaking of chloride The proposed procedure is based on the formation and breaking of oil-in-water emulsions in order to extract chloride from crude oil. The emulsions were formed by vigorously stirring a 0.5 g sample of crude oil with 0.5 g of mineral oil and 5 mL of the emulsifying solution (2.5% m/v Triton X-114) for 90 min. Afterwards, the emulsion was broken by centrifugation at 5000 rpm for 20 min (Fig. 1). Then, exactly 2 mL of the aqueous phase containing the extracted chloride was taken and diluted to 10 mL with ultrapure water in a volumetric flask. The final obtained solution was filtered through a cellulose acetate membrane with 0.20 m pore diameter and injected into the chromatographic system. 2.4. Chromatographic conditions The chromatographic run for separation of chloride was divided into three steps. In the first step (0–6 min), a KOH concentration of 38 mM was employed. The chloride peak appeared at 5.6 ± 0.1 min. In the second step (6–15 min), the KOH concentration was increased to 50 mM in order to provide a convenient cleaning of the column. Finally, in the third step (15–20 min), the KOH concentration was returned to 38 mM to prepare the column for a new separation cycle. The current in the suppressor system was set to 41 mA, the mobile phase flow-rate was 0.33 mL min−1 during the entire chromatographic run and the injection volume was always 20 L. Typical chromatograms of the chloride standard solution (0.1 g mL−1 ) and for an actual sample of crude oil treated by the proposed method are presented in Fig. 2. 2.5. Samples Crude oil samples analyzed in this work were kindly supplied by Petrobras (Rio de Janeiro, Brazil). They were stored in sealed, dark, glass flasks, at ambient laboratory temperature. Sample S2 was employed in the optimization experiments. 3. Results and discussion The development of the proposed extraction methodology was performed in two steps: (i) evaluation/optimization of the extraction conditions and (ii) application to real samples of crude oil, including a comparison of the obtained results with those obtained using the ASTM D6470 standard method. The methodology was optimized by studying the effect of the following variables: (i) type and concentration of surfactant, (ii) the ratio of oil (crude oil + mineral oil):aqueous phases, (iii) the amount of mineral oil added to the crude oil before emulsification and (iv) emulsion shaking time. 3.1. Effect of oil phase:aqueous phase ratio One of the most important parameters to be studied in the proposed extraction method was the ratio of oil phase/aqueous phase. This parameter controlled the amount of sample employed in the
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Fig. 1. Emulsion (A) before and (B) after centrifugation for the emulsion breaking.
Fig. 2. Typical chromatograms of a mineral oil extract (black line), a sample extract diluted 1:5 (gray line) and a 0.25 g mL−1 chloride standard solution (dashed line).
procedure and could have an important impact on the sensitivity of the method and on the extraction capacity, since the aqueous phase is responsible for the transference of chloride ions from the oil to the aqueous extract. Some conditions were established randomly for the initial experiments. A 5% (m/v) solution of Triton X-100 was employed as the extractant solution and 0.5 g of crude oil was mixed with 0.5 g of mineral oil to form the oil phase for the extraction procedure. The mixing of crude and mineral oils prior to the application of EIEB was a fundamental aspect of the extraction procedure, since crude oils of different origins presented very different physical and chemical characteristics that affected the emulsification process. Mixing of crude oil samples with mineral oil minimized these differences and allowed all crude oils analyzed in this work to be emulsified easily by simple shaking of the mixtures. The influence of the ratio of the oil phase:aqueous phase was evaluated by varying the mass of the oil phase (crude oil:mineral oil, 1:1) and maintaining a constant volume of the aqueous phase. The experiments were performed by extracting chloride from 0.5 to 3.0 g of oil phase with 5 mL of 5% (m/v) Triton X-100 solution. The emulsions were broken just after emulsification by centrifugation for 20 min at 5000 rpm.
The amount of oil phase employed in the extraction significantly affected both the amount of chloride extracted and the precision of the experiments (Fig. 3). A higher extraction efficiency was observed when 0.5 g of oil phase was employed. However, in this condition, the repeatability of the measurements was poor, probably due to the use of a small and non-representative amount of crude oil to prepare the oil phase. When greater masses of oil phase were used, the precision improved from 16% to approximately 7% in terms of RSD, but with a consequential decrease in the amount of chloride extracted, despite the fact that there was no statistical difference between the extraction efficiency at 0.5 and 1.0 g of oil phase. Therefore, we decided to establish that 1.0 g of oil phase always would be used for the extraction of chloride. 3.2. Effect of the crude oil:mineral oil ratio As mentioned previously, crude oils can have different physical and chemical characteristics, which significantly influence the emulsification process. In order to minimize the influence of these characteristics on the preparation of emulsions, all crude oil samples were first mixed with mineral oil before application of EIEB. In the previous experiment, we chose to always employ 1.0 g of organic phase. So, in this experiment we varied the amount of crude
1.4
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Normalized chloride signal
Normalized chloride signal
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1.0 0.8 0.6 0.4
Triton X-100 Triton X-114
1.0 0.8 0.6 0.4 0.2
0.2 0.0
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0.0 0.0
0.5
1.0
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Mass of oil phase (g) Fig. 3. Influence of the mass of oil phase in relation to aqueous phase on the chloride extraction. The volume of aqueous phase was always 5 mL (5% (m/v) Triton X-100 solution) and the oil phase was prepared by mixing 0.5 g sample with 0.5 g mineral oil. Error bars represent the standard deviation obtained from three independent determinations.
and mineral oils, but always maintained a final mass of 1.0 g. Four ratios were tested: (i) 100% crude oil, (ii) 75% crude oil, (iii) 50% crude oil and (iv) 25% crude oil. The results demonstrated that this parameter had no noticeable influence on the extraction efficiency of chloride ions. Nevertheless, again, the repeatability of the measurements was strongly affected by the variation of this parameter. The repeatability was much better (1.1% RSD) at 50% crude oil (0.5 g crude oil + 0.5 g mineral oil), which was exactly the same ratio utilized in the previous experiment and provided convenient repeatability. Therefore, we decided to keep this condition for all further experiments.
3.3. Effect of shaking time The application of the EIEB is based on the intense contact between the continuous and dispersed phases. In general, when the emulsions are shaken after their preparation, the movement of the droplets (dispersed phase) through the continuous phase enhances the extraction efficiency [17,18]. Classically, the EIEB has been based on the formation and breaking of water-in-oil emulsions, which are more viscous than the oil-in-water emulsions obtained in the present work. Theoretically, in oil-in-water emulsions, the mixing between the phases is facilitated, which also enhances the extraction efficiency. In order to evaluate the effect of shaking on the extraction efficiency of chloride, we tested the influence of shaking time before emulsion breaking. For this purpose, the emulsions were shaken in the range of 0 (emulsion breaking just after its formation) to 120 min. The other experimental conditions were those optimized in the previous experiments and the shaking of the flasks was carried out on an horizontal mixer operated at 90 rpm. Only a small increase in the extraction efficiency was observed with an increase in the shaking time, revealing that the transference of the greater part of chloride from the oil to the aqueous phase occurred just after the preparation of the emulsions. Despite this fact, maintaining the emulsion under shaking for at least 60 min increased the extraction efficiency by 20%. Thus, we set a shaking time of 60 min for the method.
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Surfactant concentraon (% m/v) Fig. 4. Influence of type and concentration of surfactant on the chloride extraction. The extractions were performed using 1.0 g of oil phase (0.5 g sample + 0.5 g mineral oil) and 60 min shaking time. Error bars represent the standard deviation obtained from three independent determinations.
3.4. Effect of the type and concentration of surfactant As EIEB depends on the formation of emulsions, the type and concentration of the surfactant employed is of fundamental importance. The surfactant contributes to the dispersion of oil through the water, making possible the preparation of a homogeneous system. EIEB has been performed with non-ionic surfactants, such as Triton X-100 and Triton X-114. They were selected for the extraction of chloride from crude oil using the EIEB approach in the form of oil-in-water emulsions. The concentration of both Triton X-100 and Triton X-114 was varied from 1 to 10% (m/v). It was not possible to prepare emulsions without the addition of surfactant. The volume of aqueous solution was 5 mL, the mass of organic phase was 1.0 g (0.5 g crude oil + 0.5 g mineral oil) and the emulsions were shaken for 60 min prior to their breaking. The results (Fig. 4) demonstrated that the type of surfactant plays a more important role than its concentration. The extraction efficiency obtained with Triton X-114 was 70% higher than the extraction efficiency obtained with Triton X-100. We believe that this phenomenon is associated with the emulsion breaking process, which is more easily performed when Triton X-114 was employed in the emulsification. In the presence of Triton X-114, the separation between the phases was complete and quantitative recovery of chloride could be observed. The use of Triton X-100 provided more stable emulsions that were more difficult to break. In this situation, it is possible that some water remained in the oil phase and, consequently, so did chloride ions. The effect of the surfactant concentration was less pronounced than the effect of the type of surfactant. In both cases (Triton X-100 and X-114), an increase in surfactant concentration decreased the extraction efficiency. Again, this phenomenon can be explained by the emulsion stability, which increased with higher concentrations of surfactant. In this context, we decided to use a 2.5% (m/v) Triton X-114 solution for emulsification, since, under these conditions, we observed a satisfactory compromise between extraction efficiency and measurement precision.
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Table 1 Results obtained in the determination of chloride in Brazilian crude oil samples by the proposed and reference (ASTM D6470) methods. The results are expressed as mean ± standard deviation (n = 3). Sample
ASTM D6470 (g g−1 NaCl)
Proposed Method (g g−1 NaCl)
S1 S2 S3 S4 S5 S6
2.0 12.3 5.9 2.0 2.0 2.7
2.4 11.1 5.3 1.8 0.91 2.9
a b
± ± ± ± ± ±
0.4 1.7 0.1 0.2 0.20 0.6
Recoveryb (%) nda nda nda 87 ± 4 92 ± 1 97 ± 4
nd = not determined. Samples were fortified with 5 g g−1 NaCl.
3.5. Evaluation of the calibration strategy and analytical figures of merit After optimization of the extraction conditions, we evaluated the influence of the calibration strategy in order to set the correct quantification approach for the method and ensure that no matrix interferences were present in the determination of chloride in the final extracts. In this study, analytical curves were compared with standard addition curves prepared in the final extract medium. The standard addition and analytical curves were prepared on three different days in the range of 0.025–1 g mL−1 chloride. The slopes of the curves were practically the constant (variation from 0.895 to 0.925), independently of the day of the measurements. This result demonstrated that no matrix interferences were present in the determination of chloride in the extracts and that analytical curves could be employed for the quantification of chloride in the extracts. Even so, the curves were statistically evaluated by Student’s t-test and the significance of the regression was determined. Each of the curves presented statistical significance (95% confidence level), with F values varying in the range of 6476–26160. As the critical value of F is 10.13, the linear models were considered statistically satisfactory. Also, the paired Student’s t-test was applied to compare the difference between the analytical and standard addition curves. No statistical differences were observed at the 95% confidence level, since the calculated value of t was 0.210 and the critical value is 4.30 (two degrees of freedom) [21]. The methodological limits of detection and quantification were estimated using an analytical curve constructed with chloride standard solutions prepared in ultrapure water and taking into account the extraction/dilution of samples. The estimated limits of detection (3s) and quantification (10s) were expressed in terms of NaCl concentration, which is the parameter regularly employed to express the salinity of crude oils. The limit of detection of the developed method was 0.5 g g−1 NaCl and the limit of quantification was 1.6 g g−1 . The limit of detection of the proposed method is four times better than the limit of detection reported for the ASTM D6470 standard method, which is 2 g g−1 NaCl [5]. The typical analytical curve employed in the quantification of chloride in the extracts presented the following equation: S = 0.9193 [Cl− ] + 0.0020 (r2 = 0.999), where S is the chloride peak area and [Cl− ] is the chloride concentration in g mL−1 . The precision of the method was estimated by analyzing five independent aliquots of the sample S2 . In this case, a relative standard deviation of 15% was obtained. The intermediary precision of the method was also evaluated. For this purpose, the sample S2 was analyzed in three different days and the relative standard deviation of the mean value was taken as intermediary precision. An intermediary precision of 17.1% was observed. 3.6. Analysis of brazilian crude oil samples In order to test the applicability of the proposed method, we employed it for the determination of the chloride concentration
in six Brazilian crude oils from different origins. The results were compared with those obtained by application of the ASTM D6470 method [5], which was used as a reference method. Additionally, a recovery test was performed. Three samples were fortified with 5 g g−1 of NaCl and analyzed using the proposed method. All results are displayed in Table 1. As shown in Table 1, there was a good agreement between the results obtained by the proposed method and the reference ASTM D6470 method. Also, the recovery percentages were in the range of 87–97%, indicating that the quantitative extraction of chloride can be achieved using the emulsion breaking procedure optimized in this work. We also evaluated the statistical significance of the results. For this purpose, a paired Student’s t-test was applied. A t value of 1.148 was observed for the paired samples. As the critical value of t for five degrees of freedom is 2.571 (95% confidence level), the null hypothesis was maintained, which indicated that there were no statistical differences between the results provided by the two methods (proposed and reference) in the determination of chloride in the analyzed samples. 4. Conclusions The method proposed in the present work was shown to be suitable for the total extraction of chloride from crude oils, making possible its quantification in the samples by ion chromatography. The results obtained with the proposed method were not statistically different from those obtained by the ASTM D6470 reference method at a 95% confidence level, which demonstrated the accuracy of this method. The application of the EIEB procedure permitted the total separation of chloride from the complex organic matrix, through its transference to an aqueous phase that was easily analyzed by ion chromatography with conductivity detection. Among the studied parameters, the type of surfactant employed for emulsion preparation presented a remarkable effect on the extraction efficiency. In this case, the use of Triton X-114 resulted in higher extraction efficiency than the use of Triton X-100. The optimized methodology was successfully applied to the determination of chloride in six samples of Brazilian crude oil supplied by Petrobras. Therefore, we believe that the method can be considered an excellent alternative to other methods that require the use of large volumes of solvent or necessitated the use of an expensive apparatus for chloride extraction from samples. We believe that the method can be expanded to include the determination of other water-soluble substances present in crude oils. Acknowledgments The authors are grateful to PETROBRAS (Petróleo Brasileiro S.A.) for the financial support and to CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnológico) and FAPERJ (Fundac¸ão Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro)
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for providing grants and fellowships. The authors are also grateful to CAPES (Coordenac¸ão de Aperfeic¸oamento de Pessoal de Nível Superior) for the scholarship granted to N.F.R. References [1] M.K. Morigaki, R.Q.F. Chimin, C.M.S. Sad, P.R. Filgueiras, E.V.R. de Castro, J.C.M. Dias, Salinity of crude oil: optimization of methodology and new method of extraction of salt in petroleum, Quím. Nova 33 (2010) 607–612. [2] J.S.F. Pereira, P.A. Mello, D.P. Moraes, F.A. Duarte, V.L. Dressler, G. Knapp, E.M.M. Flores, Spectrochim. Acta Part B 64 (2009) 554–558. [3] A.R. Cassella, E.F.R. Rocha, M.L. Duarte, Technical Communication CT QM 02/2007, CENPES, Petrobras, 2007. [4] A. Doyle, A. Saavedra, M.L.B. Tristão, L.A.N. LAN, R.Q. Aucélio, Spectrometric methods for the determination of chlorine in crude oil and petroleum derivates, Spectrochim. Acta Part B 86 (2013) 102–107. [5] American Society for Testing and Materials (ASTM), ASTM D6470 − Standard Test Methods for Salt in Crude Oil (potentiometric Method), ASTM International, West Conshohocken, PA, USA, 2010. [6] American Society for Testing and Materials (ASTM), ASTM D3230 − Standard Test Methods for Salt in Crude Oil (electrometric Method), ASTM International, West Conshohocken, PA, USA, 2010. [7] M.O. Souza, M.A. Ribeiro, M.T.W.D. Carneiro, G.P.B. Athayde, E.V.R. de Castro, F.L.F. da Silva, W.O. Matos, R.Q. Ferreira, Evaluation and determination of chloride in crude oil based on the counterions Na, Ca Mg, Sr and Fe, quantified via ICP-OES in the crude oil aqueous extract, Fuel 154 (2015) 181–187. [8] R. Hajian, A.H. Oroojloo, S.S. Mousavian, N. Shams, Determination of sodium chloride in crude oil of gachsaran county oil wells, Asian J. Chem. 23 (2011) 4223–4224. [9] D.P. de Moraes, F.G. Antes, J.S.F. Pereira, M.F.P. dos Santos, R.C.L. Guimarães, J.S. Barin, M.F. Mesko, J.N.G. Paniz, E.M.M. Flores, Microwave-assisted procedure for salinity evaluation of heavy crude oil emulsions, Energy Fuels 24 (2010) 2227–2232. [10] N.F. Robaina, D.M. Brum, R.J. Cassella, Application of the extraction induced by emulsion breaking for the determination of chromium and manganese in edible oils by electrothermal atomic absorption spectrometry, Talanta 99 (2012) 104–112.
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