ISSN 0965-5441, Petroleum Chemistry, 2017, Vol. 57, No. 12, pp. 1007–1011. © Pleiades Publishing, Ltd., 2017. Published in Russian in Neftekhimiya, 2017, Vol. 57, No. 6, pp. 628–631.
Simple Spectrophotometric Method for Determination of Iron in Crude Oil1 A. B. Shehataa, G. G. Mohamedb, and M. A. Gab-Allaha, * a
National Institute of Standards, Tersa St, Haram, Giza, 12211 Egypt Department, Faculty of Science, Cairo University, Giza, 12613 Egypt *e-mail: [email protected]
Received October 31, 2016
Abstract⎯In this research article, spectrophotometric method for the determination of iron with 1,10phenanthroline (phen) in two different crude oil samples from different oil fields in the Suez-Gulf region of Egypt has been proposed. The method is efficient, reliable and inexpensive where a cost-effective technique, along with commercially available spectrophotometric reagent, was utilized in this work. The method was based on decomposition of the organic matrix by combustion in a heating muffle furnace at 550°C. The inorganic residue was then dissolved in diluted nitric acid and the iron was reduced to the divalent state. The color was developed by the addition of 1,10-phenanthroline as chelating agent after adjusting the pH of the solution, then the absorbance of the solution was measured at approximately 510 nm after a short reaction period. The limit of detection (LOD) and limit of quantification (LOQ) obtained were found to be 0.017 and 0.051 μg/mL, respectively. The effect of interferences was studied and the accuracy of the method was evaluated by recovery experiment, analysis of oil reference material and by comparison of results with those obtained using flame atomic absorption spectrometer (FAAS) after dilution in an organic solvent for sample preparation. Keywords: spectrophotometry, crude oil, combustion, iron determination DOI: 10.1134/S096554411712012X
INTRODUCTION Petroleum is perhaps the most important substance consumed in modern society. It provides not only raw materials for the ubiquitous plastics and other products, but also fuel for energy, industry, heating, and transportation . Petroleum, as recovered from the reservoir, contains metallic constituents but also picks up metallic constituents during recovery, transportation, and storage [2, 3]. Some of the more abundant are sodium, calcium, iron, magnesium, aluminum, vanadium and nickel. The presence of metallic elements, above certain levels, in crude oil is undesirable due to their deleterious effects on refining processes, especially processes in which catalysts are used . Minute amounts of iron, nickel and vanadium in the charging stocks for catalytic cracking affect the activity of the catalyst and result in increased gas and coke formation and reduced yields of gasoline . Iron and chromium cause corrosion in furnaces and boilers during oil processing and be transported to distillation products, such as gasoline and diesel fuels [2, 3]; hence the level of metallic elements in fuel is an index of its corrosivity. Therefore, accurate determination of metals in crude oil samples is of a great 1 The article is published in the original.
importance and has always been a challenge in analytical chemistry. Sample pre-treatment can be performed by wet digestion, combustion, dilution in organic solvents, emulsion or microemulsion formation methods. Combustion method is recommended by American Society for Testing and Materials according to ASTM D5863 for the determination of Ni, V, Fe and Na by atomic absorption spectrometry (AAS) . It is also recommended according to ASTM D5708 for the determination of Ni, V and Fe by inductively coupled plasma-optical emission spectrometry (ICPOES) . Moreover, direct dilution of crude oil samples in organic solvent prior to measurements by ICPOES and FAAS has also been extensively used. It proved to be an attractive procedure because it is simple, rapid, accurate, and does not change the qualitative composition of crude oil samples . Trace metal contents of crude oils can be determined by various analytical techniques including AAS [3, 7–11], ICPOES [7, 12, 13], inductively coupled plasma-mass spectrometry (ICP-MS) [14–16], X-ray fluorescence (XRF) spectrometry [17–19], high performance liquid chromatography (HPLC) [20, 21] and neutron activation analyzer (NAA) [22–24]. However, UV–Visible spectrophotometry is one of the most frequently applied methods for the determination of iron in dif-
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ferent samples. It has been widely used in clinical chemistry, analytical chemistry, pharmaceutical industry, education and scientific research. The popularity of UV–Visible spectrophotometry technique stems from its ease of use, convenience of data acquisition, speed of analysis, high stability, wide analytical ranges and moderate to high selectivity. Therefore, the aim of the present study was directed to establish an accurate, precise and reliable method for the analysis of iron in crude oil samples by UV–Vis spectrophotometer using 1,10-phenanthroline as chelating agent after combustion procedure for sample preparation. EXPERIMENTAL Apparatus Iron was determined by dual beam UV–Vis spectrophotometer using a Specord 250 Plus (AnalytikJena, Jena, Germany) equipped with a 15-sample tray in the wavelength range from 190 to 1100 nm with 10 mm matched quartz cells. The absorbance was measured at 510 nm. A Metrohm digital pH meter (Switzerland) with a combined glass electrode was used to measure pH values. A Model ZEEnit 700 atomic absorption spectrometer (Analytik Jena, Jena, Germany) equipped with a transversally heated graphite atomizer and a transverse Zeeman-effect background correction system was used for iron analysis. A continuum source (deuterium lamp) background corrector was used for flame mode. The optimal instrumental settings to produce the highest sensitivity for iron measurements were performed. Hollow cathode lamp of Fe was used as the radiation source with a lamp current of 6.0 mA. The absorbance was measured at 248.3 nm with a spectral band pass of 0.2 nm. Air-acetylene flame was used with 10 mm burner height and 65 NL/h fuel flow. Sample combustion was performed using a high temperature furnace (Barnstead international, USA) capable of maintaining constant temperatures in the range of 20–1800°C. A model ME235S microelectronic balance, precision ±0.0001 g was employed. Vortex mixer was used for homogenization purposes. Sample dilution was directly performed in polyethylene autosampler cups. Chemicals, Reagents and Samples All chemicals and reagents used in this work were of analytical reagent (AR) grade and ultrapure water (0.054 μS/cm) was used throughout. 1,10-phenanthroline (≥99%) and sodium acetate trihydrate (99%) were supplied by Sigma–Aldrich, Germany. Hydroxylamine hydrochloride (99%) and organometallic standard solution of 1000 ± 10 μg/g iron were purchased from Alfa Aesar, Germany. Concentrated nitric acid was purchased from Merck, Germany. Concentrated sulfuric acid was obtained from Fluke, Germany. Aqueous certified reference material of
1000 ± 10 μg/mL iron for the calibration process along with natural-matrix oil reference material (NIS CRM 045) were obtained from National Institute of Standards (NIS), Egypt. Crude oil samples were obtained from different oil fields in Suez-Gulf region of Egypt. A 0.3% (w/v) solution of 1,10-phenanthroline was prepared by dissolving 303 mg of the substance in deionized water with gentle heating followed by dilution up to 100 mL in a volumetric flask. Ten percentage (w/v) solution of hydroxylamine hydrochloride was prepared by dissolving 10.101 g in deionized water and diluting up to 100 mL in a volumetric flask. Acetate buffer solution was prepared by dissolving 8.30 g of sodium acetate in deionized water with addition of 12 mL glacial acetic acid (Analytical Rasayan, India) and diluting up to 100 mL in a volumetric flask. Before use, all glassware, plastics and autosampler cups were soaked overnight in 10% (v/v) HNO3 then rinsed with high-purity water at least three times before use and air-dried, avoiding any contact with metallic surfaces and dust contamination. Contamination was always checked by a strict blank control. Procedure Proposed Crude oil samples and reference material analyzed in this work were previously heated up to 40°C with shaking in closed vessels during 2 h with the aid of water bath shaker. After cooling, a mass of homogenized crude oil sample (5 g) was accurately weighed into 150 mL Pyrex beaker. Sulfuric acid (3 mL/g oil) was used as ashing agent. The sample along with sulfuric acid was cocked on a hot plate with constant mixing. The temperature was progressively elevated to 300°C and maintained until dry and no more smoke appeared. The remaining coke was burned off in a heating muffle furnace at 550°C for about 4.5 h. After cooling, the ash in the beaker was digested on a hot plate using 10 mL (2 : 10 v/v) nitric acid. The obtained solution was transferred quantitatively to 50 mL volumetric flask and diluted to the mark with deionized water. A reagent blank was prepared similar to samples. Afterwards, 10 mL aliquot of the prepared ash solution was pipetted into an iron-free Pyrex test tube and 10% (w/v) solution of hydroxylamine hydrochloride (2 mL) was added. The solution was shaken and left for 10 min then acetate buffer solution (6 mL) was added. Finally, 0.3% (w/v) solution of 1,10-phenanthroline (2 mL) was added and then a permanent orange-red complex [(C12H8N2)3Fe]2+ was developed and the solution was kept for 1 h, then the absorbance was measured at 510 nm using 10 mm cells against the reagent blank. Calibration graph was constructed by using aqueous standard stock solution (1000 μg/g) of iron. This standard was diluted in deionized water with nitric acid to give standards in the range 0.25–4 μg/mL. 10 mL aliquots of the freshly prepared standards were pipetted into iron-free Pyrex test tubes, PETROLEUM CHEMISTRY
SIMPLE SPECTROPHOTOMETRIC METHOD FOR DETERMINATION
following by preparation using the same procedure as for samples. The accuracy was investigated by means of recovery test with standard addition. In this test, a quantity (100 mg) of oil based standard solution (1000 μg/g) of Fe was added to the crude oil sample and analyzed by the proposed method. Accuracy was also evaluated by the analysis of oil reference material as well as by comparison with an independent method using flame atomic absorption spectrometry (FAAS) after direct dilution with organic solvent.
Table 1. Optimization of sulfuric acid ratio using sample 1 Acid ratio, acid, mL/oil, g
Iron concentration, μg/g
0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0
9.08 11.50 13.80 14.64 15.71 16.40 16.38 16.40
RESULTS AND DISCUSSION The present proposed method was based on dry ashing-acid dissolution for sample preparation. The sample was coked, charred and then muffled inside a heating muffle furnace until complete ashing (carbon removal). The lowest muffling temperature that yielded carbon-free ash was 550°C and this was achieved in the shortest time period of 4.5 h. The ashes obtained were completely soluble in diluted nitric acid and a clear solution was obtained after 10 min of digestion. Therefore, there was no need for performing additional filtration step. A factor reputed to cause systematic error in the analytical results is the loss of iron by volatilization during the ashing procedure. Sulfuric acid was effective as ashing agent to strengthen the release of iron in the ashing stage and increase its recovery. Table 1 shows the effect of varying ratio of sulfuric acid to oil. Iron concentration increased gradually with increasing sulfuric acid until an optimum value was obtained. The peak concentration was attained with 3 mL of sulfuric acid per gram of oil which was the minimum volume required to give the maximum release for iron. An aliquot of the obtained ash solution was treated with hydroxylamine hydrochloride as reducing agent; it rapidly reduces Fe (III) to Fe (II), and hence determination of total iron. The pH was adjusted to 3–4 by acetate buffer solution; this pH range permitted maximum color development. The 1,10-phenanthroline was used as Fe complexing agent to chelate Fe2+ as PETROLEUM CHEMISTRY
[(C12H8N2)3Fe]2+. The color was then developed and after a short reaction period (1 h), the absorbance of the solution was measured at approximately 510 nm. The analytical curve was constructed using aqueous standard solutions and the linearity of the curve was verified by estimation of the residual errors (Fig. 1). From the plot, it can be seen that residuals are randomly distributed on both sides of the zero-axis which indicate that the calibration function was assumed to be linear . The analytical parameters obtained from the calibration curve are summarized in Table 2. Sensitivity of the method can be determined through the limit of detection (LOD) and limit of quantification (LOQ). The LOD and LOQ were calculated according to the IUPAC definition (LOD = 3.3 s/m, LOQ = 10 s/m; where m is the slope of the respective calibration curve and s is the standard deviation of ten blank replicates) and their values are recorded in Table 2. The absorption spectra of samples (1 and 2) are depicted, respectively, in Figs. 2 and 3. The absorbance of the solution, once the color is developed, is stable for at least several months . The iron concentration results are shown in Table 3. Of the few ions present in significant quantities in crude oil ashes, the ones that were found to interfere are nickel and copper. In the present study, it was found that nickel gave no interference when added in quantities up to three parts per million. Meanwhile, copper gave no interference when added in quantities up to ten parts per million and that was achieved when 0.004 Residuals
This method was applied to the determination of iron in crude oil sample. For preparation of the sample, crude oil was first warmed up to 40°C followed by vigorous shaking for homogenization. The homogenized oil was then diluted ten-fold with xylene in dark glass-stoppered bottles. The calibration was carried out using organometallic standard stock solution (1000 μg/g) of the target element; this was diluted with xylene to give standards in the range of 0.5–4 μg/g. After constructing the calibration curves, the diluted sample was then analyzed by FAAS against a blank sample containing the same amount of the diluent added to the samples.
0.002 1E–17 0.002 0.004 Fig. 1. Residual plot of the calibration graph of UV-Vis spectrophotometer for Fe.
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Table 2. Performance characteristics of the proposed analytical method [(C12H8N2)3Fe]2+
Parameter Maximum wavelength (λmax)
Correlation coefficient (r2) Intercept (a)* Slope (b)* RSD, % (n = 10) Detection limit, DL, μg/mL Quantitation limit, QL, μg/mL
0.9999 0.0025 0.0977 Less than 1% 0.017 0.051
* Y = a + bx, where x is the concentration, mg/mL.
a larger excess of the reagent was used. It is worthwhile to point out that Cu can rarely be found in crude oil ash solution at this high concentration level especially for light and medium crude oil . It was also found that the following ions gave no interferences: chloride, acetate, nitrate, aluminum, calcium, magnesium, potassium and sodium. The precision, expressed in terms of relative standard deviation (% RSD) was found to be less than 1%. Accuracy of the methodology was assessed by comparison of the results with those obtained by other analytical method. The method was based on direct dilution of oil sample with xylene prior to the analysis by FAAS after constructing the calibration curve using oil based standards also diluted in xylene. The iron concentration results obtained for this method (mean ± SD) in the samples (n = 10) are recorded in Table 3. A paired Student’s t-test indicated that there was no significant difference between the results obtained by both methods at the 95% confidence level which means that the results obtained by both these methods are in close agreement with each other. Verification of accuracy was also performed by means of recovery test by adding Fe standard to the sample before any chemical or heat treatment and then performing the proposed procedure. Finally, the concentration was determined and the amount of Fe
Table 3. Results of iron concentration of both methodsa Iron Concentration, μg/g Sample code
proposed method comparative (UV-Vis spectrophotometer) method (FAAS)
16.4 ± 0.121
17.2 ± 0.430
8.03 ± 0.056
8.4 ± 0.195
a Each value represents the mean concentration ± standard deviation (n = 10).
standard added appeared as an additional value to that of the unknown in a sample to which no standard had been added which was analyzed under the same conditions. Average recovery value was then calculated and found to be 97%. Although the recovery test showed a good accuracy of the methodology, it was also assured through the analysis of oil certified reference material with similar composition (NIS CRM 045). The results are shown in Table 4 where it can be seen that they are in a good agreement (better than 95%) with the certified or reported value which ensures the well suitability of the proposed method. CONCLUSIONS An accurate, reproducible, simple, and hence reliable method is reported for iron determination in crude oil samples. The analysis was based on UV–Vis spectrophotometry with sample preparation by ashing procedure. It was possible to calibrate by using inorganic standard solution of iron, avoiding the use of organic standard solutions which are more expensive and less available. Accuracy and precision data showed percentage recovery value of 97% and percentage relative standard deviation to be less than 1%. Agreement with certified value and FAAS results was better 0.10 0.08 Absorbance
0.20 0.15 0.10
0.05 0 380
480 580 Wavelength, nm
Fig. 2. Absorption spectrum of [(C12H8N2)3Fe]2+ in S1.
480 580 Wavelength, nm
Fig. 3. Absorption spectrum of [(C12H8N2)3Fe]2+ in S2. PETROLEUM CHEMISTRY
SIMPLE SPECTROPHOTOMETRIC METHOD FOR DETERMINATION Table 4. Results of NIS CRM 045 certified reference material by proposed method Iron Concentration, μg/g CRM NIS CRM 045
9.21 ± 0.2
8.8 ± 0.13
than 95%. This study shows that the methodology can be successfully applied to these types of samples hence being appropriated for routine determination. REFERENCES 1. J. G. Speight, The Chemistry and Technology of Petroleum, 4th ed., Taylor & Francis Group, 2006. 2. J. G. Speight, Handbook of Petroleum Product Analysis, New Jersey: John Wiley & Sons, 2002. 3. I. M. Dittert, J. S. A. Silva, R. G. O. Araujo, A. J. Curtius, B. Welz, and H. Becker-Ross, Spectrochim. Acta Pt. B, 64, 537 (2009). 4. ASTM D5863–00a: Standard Test Methods for Determination of Nickel, Vanadium, Iron, and Sodium in Crude Oils and Residual Fuels by Flame Atomic Absorption Spectrometry (Reapproved 2005), 2000. 5. ASTM D5708–05: Standard Test Methods for Determination of Nickel, Vanadium, and Iron in Crude Oils and Residual Fuels by Inductively Coupled Plasma (ICP) Atomic Emission Spectrometry, 2005. 6. T. A. Maryutina and N. S. Musina, J. Anal. Chem. 67, 862 (2012). 7. E. M. Sedykh, L. N. Bannykh, G. S. Korobeinik, and N. P. Starshinova, Inorg. Mater. 47, 1539 (2011). 8. G. Brandão, R. C. de Campos, E. V. R. de Castro, and H. C. de Jesus, Spectrochim. Acta Pt. B 62, 962 (2007). 9. C. Duyck, N. Miekeleyda, C. L. P. Silveira, R. Q. Aucélio, R. C. Campos, P. Grinberg, and G. P. Brandão, Spectrochim. Acta Pt. B 62, 939 (2007).
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