CC BY
2Journal of Siberian Federal University. Chemistry 2023 16(3): 360-368
EDN: YDAUFM УДК 543.544.5.068.7
Optimization of Sample Preparation Conditions for Ionochromatographic Determination of Chloride Ions That are Part of High-Molecular Organic Compounds of Oil
Maria A. Polosukhina*a, Olga P. Kalyakinaa, Sergei N. Kalyakina b and Sergei V. Kachina
aSiberian Federal University Krasnoyarsk, Russian Federation bInstitute of Chemistry and Chemical Technology of the SB RAS
Krasnoyarsk, Russian Federation
Received 31.01.2023, received in revised form 30.06.2023, accepted 05.07.2023
Abstract. A procedure of sample preparation of oil samples for the determination of chloride ions, which are part of high-molecular organic compounds, has been developed. The procedure consists in the extraction of chloride ions from oil into the aqueous phase with a solution of sodium nitrate, followed by ionochromatographic detection. The work was performed on a high-performance LC-20 Prominence liquid chromatograph with LC Solution software (Shimadzu, Japan), equipped with a conductometric detector (CDD-10 Avp/10Asp), a 120x5 mm KanK-ASt14 ^m separating column (GEOHI RAS, Russia) and a 200x6 mm SPS-SAC 50 suppression column ^m (LLC PC «Aquilon», Russia). A carbonate buffer solution (2.5 mM Na2CO3 + 3.0 mM NaHCO3) was used as an eluent. The volume of the injected sample is 100 ц1. The eluent flow rate was 2.0 ml/min. The temperature of the column thermostat is 33 °C. Under these conditions, satisfactory separation of fluoride, chloride, nitrate, and sulfate ions is achieved, and hydrogen sulfide does not have ionic forms and does not manifest ionochromatographically. Using the method of full factorial experiment, the main parameters determining the efficiency of extraction of chloride ions from the organic phase were optimized: extraction of 1 mM with an aqueous solution of NaNO3 at a temperature of 90 ± 2 °C, the volume ratio of oil and extractant 1:10, extraction time 20 min. The developed procedure of sample preparation has been tested on model solutions and real oil samples. In comparison with the known methods, the combination of extraction isolation of organic chlorides with ionochromatographic detection makes it possible to significantly simplify the procedure for determining the analyte due to the high selectivity of the method without loss in sensitivity and measurement accuracy.
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This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). Corresponding author E-mail address: mariua584@gmail.com
Keywords: oil, organic chlorides, ion chromatography, determination.
Citation: Polosukhina M. A., Kalyakina O. P., Kalyakin S. N., Kachin S. V. Optimization of sample preparation conditions for ionochromatographic determination of chloride ions that are part of high-molecular organic compounds of oil. J. Sib. Fed. Univ. Chem., 2023, 16(3), 360-368. EDN: YDAUFM
Оптимизация условий пробоподготовки для ионохроматографического определения хлорид-ионов, входящих в состав высокомолекулярных органических соединений нефти
М. А. Полосухина3, О. П. Калякинаа, С. Н. Калякина, б, С. В. Качина
аСибирский федеральный университет Российская Федерация, Красноярск бИнститут химии и химической технологии СО РАН ФИЦ «Красноярский научный центр СО РАН» Российская Федерация, Красноярск
Аннотация. Разработан способ пробоподготовки образцов нефти для определения хлорид-ионов, входящих в состав высокомолекулярных органических соединений. Способ заключается в экстракционном выделении хлорид-ионов из нефти в водную фазу раствором нитрата натрия с их последующим ионохроматографическим детектированием. Работу выполняли на высокоэффективном жидкостном хроматографе LC-20 Prominence с программным обеспечением LC Solution (Shimadzu, Япония), укомплектованным кондуктометрическим детектором (CDD-10 Avp/10Asp), разделяющей колонкой 120^5 мм КанК-АСт14 мкм (ГЕОХИ РАН, Россия) и подавительной колонкой 200*6 мм CnC-SAC 50 мкм (ООО ПК «Аквилон», Россия). В качестве элюента применяли карбонатный буферный раствор (2,5 мМ Na2CO3 + 3,0 мМ NaHCO3). Объем вводимой пробы 100 мкл. Скорость потока элюента составляла 2,0 мл/мин. Температура термостата колонки 33 °C. В данных условиях достигается удовлетворительное разделение фторид-, хлорид-, нитрат-, сульфат- ионов, а сероводород не имеет ионных форм и ионохроматографически не проявляется. С использованием метода полного факторного эксперимента оптимизированы основные параметры, определяющие эффективность извлечения хлорид-ионов из органической фазы: экстракция 1мМ водным раствором NaNO3 при температуре 90±2 °C, объемное соотношение нефти и экстрагента 1:10, время экстрагирования 20 мин. Разработанный способ пробоподготовки апробирован на модельных растворах и реальных образцах нефти. По сравнению с известными методами сочетание экстракционного выделения органических хлоридов с ионохроматографическим детектированием позволяет существенно упростить процедуру определения аналита за счет высокой селективности метода без потери в чувствительности и точности измерений.
Ключевые слова: нефть, органические хлориды, ионная хроматография, определение.
Цитирование: Полосухина М. А., Калякина О. П., Калякин С. Н., Качин С. В. Оптимизация условий пробоподготовки для ионохроматографического определения хлорид-ионов, входящих в состав высокомолекулярных органических соединений нефти. Журн. Сиб. федер. ун-та. Химия, 2023, 16(3). С. 360-368. EDN: YDAUFM
Introduction
The chemical composition of oil is very diverse and is a complex mixture of substances formed by basic (C, H, N, S, O) and impurity (10-7-10-2 %) elements [1]. Among the latter, the chlorine content can reach 10-2 % [2]. At the same time, chlorine is included in the composition of oil both in the form of inorganic chlorides (IC) and in the form of organochlorine compounds (COC) of various nature [3]. IC mainly sodium, magnesium and calcium enter the oil together with reservoir water. Their content can reach 2-3 g/ dm3 and be 0.3-0.4 % by weight. The presence of IC causes corrosion of pipes. In addition, during oil refining, IC can form solid deposits, and under certain conditions, hydrogen chloride with high corrosive activity is isolated [4, 5]. Therefore, oil is thoroughly desalinated during preparation and transportation, and hydrogen chloride neutralizers - alkalis and amines - are added during distillation.
There are mainly two types of COC in crude oil: natural and chlorine-containing chemical reagents [6]. Natural COC are found in almost all oils and are mainly represented by organic salts and chlorine-containing ligands in metal complexes that are part of high-molecular components (asphaltenes). Their content depends on the nature of the oil and varies widely: from trace amounts to 97 mg/dm3 [7]. Natural COC also tend to turn into hydrogen chloride and ammonium chloride, which contributes to the failure of equipment at oil refineries [8]. Traditional technologies cannot provide complete removal of COC, therefore, for some oils their content is more than an order of magnitude higher than that for IC remaining in the oil after desalination.
Chlorine-containing chemical reagents (chloroform, carbon tetrachloride, dichloroethane and the like) are added to oil to increase oil recovery and remove paraffin deposits. In their pure form, these compounds are very stable, but during the distillation of oil they can also release hydrogen chloride [9].
Based on the above, it is obvious that analytical control of the content of chloride compounds in oil by known methods, as well as the development of new effective approaches to their determination remain urgent tasks. The purpose of this study is to optimize the sample preparation conditions for ionochromatographic determination of chloride ions that are part of high-molecular organic compounds of oil.
Experimental
The following materials were used in the work: oil samples; GSO 7262-96 composition of the chloride ion solution, GSO 7258-96 composition of the nitrate ion solution (JSC Ural Chemical Reagents Plant, Russia); carbonate, sodium bicarbonate (Sigma-Aldrich, USA); hexane, p.ch. for chromatography (JSC "ECOS-1", Russia); nitric acid, p.ch. (LLC "Polychem", Russia); deionized water (Yonglin installation, Korea).
The work was performed on a high-performance LC-20 Prominence liquid chromatograph with LC Solution software (Shimadzu, Japan), equipped with a conductometric detector (CDD-10
Avp/10Asp), a 120x5 mm KanK-ASt14 pm separating column (GEOHI RAS, Russia) [10] and a 200x6 mm ATP suppression column-SAC 50 pm (LLC PC «Aquilon», Russia). A carbonate buffer solution (2.5 mM Na2CO3 + 3.0 mM NaHCO3) was used as an eluent. The volume of the injected sample is 100 pi. The eluent flow rate was 2.0 ml/min. The temperature of the column thermostat is 33 °C. The device was controlled and chromatograms were processed using the LCsolution program on a personal computer.
Chloride ions were extracted with an aqueous solution of 1 mM sodium nitrate. Extraction conditions were optimized by a full factorial experiment. To do this, 1 cm3 of crude oil was placed in a dividing funnel, 10 cm3 of heated extractant was poured, extraction was carried out for 20 min. Before chromatography to remove oil emulsion residues, the obtained aqueous extracts (5 cm3) were placed in a 50 cm3 separating funnel, 2,5 cm3 hexane was added and extracted for 10-15 min. The aqueous phase was filtered through a PTFE 13 syringe filter (Millipore, Germany) and subjected to tests. In the methods of additives and «added-found» as a model high-molecular COC a solution of trioctylamine chloride ((C8Hn)3NH+Cl-) in the dean was used.
The experiment was carried out at the Center for Collective Use "Science-intensive methods of research and analysis of new materials, nanomaterials and mineral raw materials" of the Federal State Autonomous Educational Institution of Higher Education "Siberian Federal University".
Results and their discussion
In accordance with GOST R 52247-2021 «Oil. Methods for the determination of organochlorine compounds» the content of chloride ions in the COC of oil is determined by various methods [11]. Method A is diphenyl reduction followed by potentiometric titration. Method B - combustion in an inert gas and oxygen environment with subsequent microculometric titration. Method C is polychromatic wave dispersion X-ray fluorescence spectrometry using an internal standard. Method D - monochromatic wave- or energy-dispersive X-ray fluorescence spectrometry, as well as polychromatic energy-dispersive X-ray fluorescence spectrometry without using an internal standard. These methods make it possible to determine the mass fraction of organic chlorides in oil from 1 ppm. Along with the obvious advantages of the methods, it should be noted the complex and lengthy sample preparation, as well as the insufficient selectivity of analytical procedures. Thus, the initial stage of each method is the distillation of oil samples in order to isolate a fraction boiling to a temperature of 204 °C, and the removal of hydrogen sulfide in the resulting fraction by washing with a solution of potassium hydroxide. In addition, it is necessary to take into account the interfering effect of salts of hydrochloric acids (methods A, B), sulfur (method C, D), as well as matrix effects (method D).
Earlier [12] we published the results of studies on the ionochromatographic determination of IC in oil. The combination of deinonized water extraction with ionochromatographic detection makes it possible to selectively determine inorganic chlorides with a detection limit of 0.01 mg/dm3. At the same time, satisfactory separation of fluoride, chloride, nitrate, and sulfate ions is achieved, and hydrogen sulfide does not manifest itself ionochromatographically under the conditions of a two-column ion chromatography with a carbonate-bicarbonate eluent [13]. The figure 1 shows a typical chromatogram of a model mixture of a number of anions obtained by us during the experiment.
According to the literature data [1] in asphaltenes of petroleum, COC are composed of nitrogen atoms with basic properties in which chlorine is in the ionic state. When these salts interact, in
Fig. 1. Chromatogram of the model mixture of anions (mg/dm3): F- (1.5), Cl- (4.0), NO3- (3.0), SO42- (3.5), S2- (5)
particular, high-molecular heterocyclic nitrogen-containing compounds with organic acid, the ion exchange displacement of chloride ions into the aqueous phase occurs:
[R-NH3]+ Cl-org + RC 6H4 SO3Haq = [R-NH3]+ RC 6H4 SO3 -OTg + HCIaq
There are known series of ion exchange of single-charged anions in similar systems in which the nitrate ion has a greater affinity for the organic matrix than the chloride ion [14]. Therefore, ion exchange extraction with an aqueous solution of nitrate ions is proposed for the effective extraction of chloride ions that are part of the high-molecular organic bases of oil. Using the example of a reaction with nitrogenous base chlorides [15], the basic heterogeneous equation can be represented as follows:
[R-NH3]+ Cl-0rg + NO3-aq = [R-NH3]+ NO3- org + Cl-aq
To optimize the extraction conditions, the method of a complete factorial experiment was used (Table 1) [16]. The following parameters are selected: xj - volume ratio of oil and extractant (1:51:10); x2 - extraction time, min (10-20); x3 - the temperature of the extractant, oC (60-90); y - the concentration of chloride ions, mg/dm3.
The maximum values of the variation parameters are chosen for reasons of expediency. Regression coefficients were calculated using the Yates method and known equations [17]. The significance of regression coefficients is estimated, a linear model is constructed and its adequacy is verified. The final regression equation looks like this:
yu= 8,26 + 0,08x; + 0,03x2 + 0,05x3.
The adequacy of the regression equation was checked by the Fisher criterion. Based on the above calculations, it was concluded that the optimum was reached at ymax = 8.41 mg/dm3. Thus, optimal conditions for the extraction of chloride ions, which are part of the high-molecular organic bases of oil,
are achieved at a volume ratio of oil and extractant 1:10, the temperature of the extractant 90 °C and the extraction time 20 min.
The effectiveness of the proposed approach was evaluated on model solutions of trioctylamine chloride in decane by the «added-found» method (Table 2) and on real samples of prepared oil by the additive method (Table 3).
As can be seen from Tables 2, 3, satisfactory data for ionochromatographic determination of the chloride ion content have been obtained. The relative standard deviation in the determination of chloride ions in model solutions does not exceed 6 %, and the error of determination in oil by the additive method is 2 %.
Table 1. Matrix for planning a complete factorial experiment 23
Experience No. Xl X2 X3 yi y2 y3 ?
1 - - - 8,10 8,09 8,11 8,10
2 + - - 8,25 8,26 8,24 8,25
3 - + - 8,17 8,16 8,17 8,17
4 + + - 8,33 8,33 8,32 8,33
5 - - + 8,22 8,21 8,23 8,22
6 + - + 8,37 8,38 8,36 8,37
7 - + + 8,23 8,22 8,23 8,23
8 + + + 8,41 8,41 8,40 8,41
Table 2. Results of ionochromatographic determination of chloride ions in model solutions (n=3, P=0.95)
Concentration of chloride ions, mg/dm3 S Sr, %
Added Found
1,5 1,6±0,3 0,1 6
9,5 10,0±1,2 0,5 5
35 36±3 1 3
Table 3. Results of ionochromatographic determination of chloride ion content in oil by addition method (n=3; P=0.95)
Experience No. The value of the chloride ion additive, ppm Found chloride ion quantity, ppm The found quantity of the chloride ion additive, ppm A, %
Without additives With an additive
1 5,0 8,4 ±0,1 13,3±0,1 4,9 2
2 10,0 8,4 ±0,1 18,6±0,2 10,2 2
3 20,0 8,4 ±0,1 28,5±0,3 20,4 2
The developed approach has been tested in the analysis of oil samples from the fields of the Krasnoyarsk Territory. To ensure the correctness of the data, the quantity of IC in the oil samples was preliminarily determined before the tests according to the method [12]. Organic chlorides were determined by the difference between the contents of chloride ions during the extraction of oil samples with sodium nitrate solution and deinonized water (Table 4).
Table 4. Results of ionochromatographic determination of chloride ions in oil samples (n=3, P=0.95)
Concentration of chloride ions in the extract, mg/dm3 The quantity of chloride ions in oil, ppm
extractant
Н2О 1 мМ NaNO3 Inorganic salts Total chloride quantity Organic salts (ionexchange chlorine)
3,4±0,3 3,7±0,3 40±3 43±4 3
5,0±0,6 5,0±0,7 59±4 59±5 0
17,0±2,0 17,7±2,1 200±15 208±16 8
1,6±0,1 2,9±0,2 19±2 34±3 15
4,9±3,8 45,8±3,8 58±5 538±43 480
0,4±0,1 4,6±0,4 5±0,3 54±4 49
It follows from the data in Table 4 that the content of organic as well as inorganic chlorides in oil varies in a fairly wide range, which seems very important, for example, for screening samples in order to select the appropriate algorithm for preparing oil for transportation and subsequent processing.
Conclusion
In this paper, an approach for ionochromatographic determination of organic chlorides with preliminary extraction extraction of the latter with a solution of sodium nitrate is proposed and implemented on real oil samples. In comparison with the known methods [11], the combination of extraction isolation of organic chlorides with ionochromatographic detection makes it possible to significantly simplify the procedure for determining the analyte due to the high selectivity of the method without loss in sensitivity and measurement accuracy. However, it would not be entirely correct to assert that the nature of chlorine in the chemical composition of oil is limited only to high-molecular heterocyclic nitrogen-containing compounds. There is evidence of a variety of other condensed cyclic structures. To date, more than 5,000 halogen-containing organic compounds of natural origin have been described in the literature [18]. In addition, COC can undergo structural changes as a result of the influence of various factors.
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