CC BY
15DOI: 10.6060/ivkkt.20206306.6183
УДК: 541.11:536.7
ТЕРМОХИМИЧЕСКОЕ ИССЛЕДОВАНИЕ РЕАКЦИЙ КИСЛОТНО-ОСНОВНОГО ВЗАИМОДЕЙСТВИЯ В ВОДНОМ РАСТВОРЕ ПИРИДОКСИНА
А.И. Лыткин, О.Н. Крутова, Е.Ю. Тюнина, П.Д. Крутов, В.В. Дударь
Александр Иванович Лыткин, Ольга Николаевна Крутова *, Павел Дмитриевич Крутов, Виктория Викторовна Дударь
Кафедра аналитической химии, Ивановский государственный химико-технологический университет,
просп. Шереметевский, 7, Иваново, Российская Федерация, 153000
E-mail: [email protected] *, [email protected], [email protected], [email protected]
Елена Юрьевна Тюнина
Институт химии растворов им. Г.А. Крестова РАН, ул. Академическая, 1, Иваново, Российская Федерация, 153045
E-mail: [email protected]
Витамин В6 - один из самых важных витаминов, необходимых для жизнедеятельности живых организмов. Витамин В6 в организме представлен коферментными формами - пиридоксальфосфатом и пиридоксаминфосфатом, входит в состав более чем 100 ферментов. В качестве объекта исследования мы выбрали пиридоксин. Прямым калориметрическим методом измеряли тепловые эффекты взаимодействия раствора пиридокси-на с растворами азотной кислоты и гидроксида калия при 298,15 К и значениях ионной силы 0,25; 0,5 и 0,75, фоновый электролит нитрата калия. Измерения проводились в калориметре с изотермической оболочкой, оснащенной реакционным сосудом объемом 60 см3 и электрической калибровкой, при T = 298,15 ± 0,01 К и Р = 100,5± 0,7 кПа, а также автоматической записью кривой темпера-время. Относительная погрешность измерения для теплоты растворения стандартного вещества составляла 0,1-0,3%. Расчет равновесного состава системы с учетом процессов ступенчатой диссоциации и диссоциации воды проводился по программе KEV. Тепловые эффекты диссоциации пиридоксина в стандартном растворе были обнаружены путем экстраполяции теплоты ступенчатой диссоциации при фиксированных значениях ионной силы на нулевое значение ионной силы раствора. Поскольку тепловые эффекты разбавления измерялись в трех диапазонах концентраций KNO3, то число экспериментов в каждой серии было не менее трех. Для расчета доверительного интервала среднее значение критерия Стьюдента АН было взято при доверительном интервале 0,95. Рассчитаны стандартные термодинамические характеристики (AH°, ArG°, AS°) реакций кислотно-основного взаимодействия в водных растворах пири-доксина. С увеличением концентрации фоновой концентрации электролита увеличивается эндотермичность процессов ступенчатой ионизации пиридоксина.
Ключевые слова: термодинамика, растворы, химия, калориметр, пиридоксин
THERMOCHEMICAL STUDY OF ACID-BASE REACTIONS IN AQUEOUS SOLUTION
OF PYRIDOXINE
A.I. Lytkin, O.N. Krutova, E.Yu. Tyunina, P.D. Krutov, V.V. Dudar
Alexander I. Lytkin, Olga N. Krutova *, Pavel D. Krutov, Victoriya V. Dudar
Department of Analytical Chemistry, Ivanovo State University of Chemistry and Technology, Sheremetevskiy ave., 7, Ivanovo, 153000, Russia
E-mail: [email protected] *, [email protected], [email protected], [email protected] Elena Yu. Tyunina
G.A. Krestov Institute of Solution Chemistry of the RAS, Academicheskaya st., 1, Ivanovo, 153045, Russia E-mail: [email protected]
Vitamin B6 is one of the most important vitamins necessary for the life of living organisms. Vitamin B6, which in the body is represented by coenzyme forms - pyridoxal phosphate and pyridoxamine phosphate, is a part of more than 100 enzymes. As the object of study, we chose pyridoxine. The thermal effects of interaction of the solution ofpyridoxine with solutions of nitric acid and potassium hydroxide at 298.15K and ionic strength values of 0.25; 0.5 and 0.75 against the background of electrolyte-supporting potassium nitrate were measured with direct calorimet-ric method. The measurements were carried out in a calorimeter with an isothermal shell equipped with a 60 cm3 reaction vessel and electric calibration at T = 298.15 ± 0.01 К and P = 100.5 ± 0.7 kPa and automatic recording of the temperature-time curve. Relative measurement error for heats of dissolution of a standard substance was -0.1-0.3%. Calculation of the equilibrium composition of the system taking into account processes a step dissociation and water dissociation was carried out according to the KEV program. The thermal effects of dissociation of pyridoxine in a standard solution were found by extrapolating the heats of stepwise dissociation at fixed values of ionic strength to zero ionic strength. Since the thermal effects of dilution were measured in three concentration ranges of KNO3, the number of experiments in each series was at least three. To calculate the confidence interval of the average value of DН the Student's criterion was taken at a confidence interval of 0.95. Standard thermodynamic characteristics (DrH°, DrG°, DrS°) of reactions of acid-base interaction in aqueous solutions of pyridoxine were calculated. With an increase in the background electrolyte concentration, the endothermicity of stepwise pyridoxine ionization processes increases.
Key words: thermodynamics, solutions, chemistry, calorimeter, pyridoxine
Для цитирования:
Лыткин А.И., Крутова О.Н., Тюнина Е.Ю., Крутов П.Д., Дударь В.В. Термохимическое исследование реакций кислотно-основного взаимодействия в водном растворе пиридоксина. Изв. вузов. Химия и хим. технология. 2020. Т. 63. Вып. 6. С. 25-29 For citation:
Lytkin A.I., Krutova O.N., Tyunina E.Yu., Krutov P.D., Dudar V.V. Thermochemical study of acid-base reactions in aqueous solution of pyridoxine. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. [Russ. J. Chem. & Chem. Tech.]. 2020. V. 63. N 6. P. 25-29
Vitamin B6 is one of the most important vitamins necessary for the life of living organisms [1-12]. The history of studies of the chemical characteristics of the group of compounds related to vitamin B6 is more than 30 years old, but only recently has a number of aspects of their electrochemistry been revised and new information has appeared that allows us to understand the nature of their catalytic activity. Vitamin B6, which in the body is represented by coen-zyme forms - pyridoxal phosphate and pyridoxamine phosphate, is part of more than 100 enzymes. As the object of study, we chose pyridoxine (C8H11NO3):
Fig. 1. Structural formula of pyridoxine molecule (HL± pH=6.9) Рис. 1. Структурная формула молекулы пиридоксина (HL± pH=6,9)
The dissociation of pyridoxine in an aqueous solution can be represented by the scheme:
H2L+ ~ Н+ + HL+ (1)
HL+ ~ Н+ + L- (2)
Reliable data on the pyridoxine ionization constants are available in the literature [13-17]. These works were performed at various values of ionic strength, against the background of supporting electrolytes differing in nature. In order to be able to compare the values of the stepwise dissociation constants of pyridoxine obtained by different authors, we recalculated the values of pK1 and pK2 to zero ionic strength. The thermodynamic constants of stepwise dissociation of pyridoxine were also determined graphically by the equation [18]:
pKc=pKo - ^Z2A—+ 07 (3) 1 + 1.6V7
After graphic processing of the literature data [13-17], the most probable values of the thermody-namic constants of pyridoxine dissociation can be taken at 298.15 K: pK,0 = 4.85 ± 0.02, pK20 = 8.96 ± 0.03. In Fig. 2 shows the equilibrium diagram in an aqueous pyridoxine solution constructed on the basis of calculations of the equilibrium composition of pyridoxine solutions at various pH values using the "KEV" program [19].
Data on the heats of the reactions of the acid-base interaction with the participation of pyridoxine are not available in the literature.
The aim of this work is to study the effect of the background electrolyte concentration on the thermal effects of pyridoxine dissociation by direct calo-rimetry, to calculate the standard thermodynamic characteristics of acid-base reactions in pyridoxine solutions and to discuss the results.
EXPERIMENTAL PART
The choice of concentration conditions for calorimetric experiments was carried out on the basis of the equilibrium diagram in an aqueous pyridoxine solution (Fig. 2).
Fig. 2. The equilibrium diagram in an aqueous solution of pyridoxine at 298.15K and ionic strength of I = 0.0 Рис. 2. Диаграмма равновесий в водном растворе пиридоксина при 298,15К и ионной силе I = 0,0
Fractional distribution of particles H2L+, HL1 and L- at various pH values of the medium (Fig. 2) indicates the possibility of independent determination of the heats of dissociation of pyridoxine. The heats of mixing pyridoxine solutions with HNO3 and KOH solutions were measured in various pH ranges using isoperibol calorimeter with automatic recording of the temperature - time curve [20, 21]. This type of calorimeter has successfully proven itself in many calori-metric studies of biologically active compounds both
in aqueous and non-aqueous media [22-24]. The operation of the calorimetric installation was checked according to the generally accepted calorimetric standard - the heat of dissolution of crystalline potassium chloride in water. The KCl preparation was purified by double recrystallization of the "Kh.Ch." grade reagent from bidistillate. Before taking the samples, potassium chloride was dried in an oven at 393.15 K to constant weight. Coordination of the experimentally obtained enthalpies of the dissolution of KCl(k) in water AsolH(xH20) = 17.25 ± 0.06 kJ/mol with the most reliable literature data indicates the absence of a noticeable systematic error in the operation of the calo-rimetric setup [25]. Weighed portions of solutions were weighed on a VLR-200 balance with accuracy of 210-4 g.
Pyridoxine (manufactured by Sigma-Aldrich USA, C8HnNO3) used without preliminary purification. The purity of the reagents declared by the manufacturer was more than 98 wt. %. Solutions of the drug were prepared by dissolving weighed portions in freshly prepared bidistillate immediately before the experiment. The carbon-free KOH solution and the HNO3 solution were prepared from the chemical grade reagent according to the usual procedure [26].
When determining the thermal effect of process (1), 0.02 M pyridoxine solution (pH = 5.5) was used as a calorimetric liquid, and an exact weighed portion of the HNO3 solution (with a concentration of 0.8814 mol/kg solution) was respectively placed in the ampoule. After mixing the solutions, the pH was close to 4.3. The experimental data on the heats of mixing and dilution are given in Table 1.
When determining the thermal effects of process (2), the heats of interaction of an aqueous pyri-doxine solution (solution concentration 0.8827 mol/kg solution) with 0.1M KOH solution, as well as the heat of dilution of the pyridoxine solution in background electrolyte solutions (KNO3) were measured. The obtained values of the heats of mixing and dilution of pyridoxine are presented in Table 2.
Table 1
Thermal effects (J/mol) of the interaction of a 0.02M solution of pyridoxine with HNO3 solution, Т = 298.15К Таблица 1. Тепловые эффекты (Дж/моль) взаимодействия 0,02М раствора пиридоксина с раствором Н NO3,
Т = 298,15К_
I m, g. AmxH, J/mol m, g. AdilH, J/mol a AdsH(H2L+), J/mol
0.5021 -16066 0.5001 -1052 0.8238
0.25 0.5021 -16052 0.5005 -1089 0.8237 -18175+300
0.5021 -16069 0.5006 -1076 0.8238
0.5028 -16754 0.5006 -890 0.8279
0.5 0.5027 -16722 0.5002 -896 0.8278 -19110+300
0.5028 -16785 0.5001 -894 0.8279
0.5026 -16307 0.5002 -394 0.7963
0.75 0.5025 -16300 0.5003 -383 0.7962 -20105+300
0.5026 -16310 0.5006 -362 0.7963
Таble 2
Thermal effects (J/mol) of solution interaction of pyridoxine with 0.02M solutions of КОН at Т=298.15 К Таблица 2. Тепловые эффекты (Дж/моль) взаимодействия раствора пиридоксина с 0,02М растворами КОН __ при Т = 298,15К __
I m, g -AmixH, J/mol -AdllH', J/mol -A„eutH, J/mol -AdisH(HL±), J/mol
0.25 0.5002 0.4993 0.5003 31102 31210 31298 2258 2232 2296 28845 28836 28841 28160±340
AmxH = 31203 ± 260 AdilH = 2262 ± 150 AneutH. = 28840 ± 260
0.5 0.5002 0.4999 0.4998 31148 31256 31146 3804 3771 3815 27342 27351 27360 29562 ± 300
AmxH = 31183 ± 210 AdilH = 3797 ± 150 AneutH. = 27351 ± 220
0.75 0.5003 0.5000 0.4995 29744 29719 29745 4256 4225 4143 25531 25536 25543 31195±310
AmxH = 29736 ± 200 AdilH = 4208 ± 120 AneutH = 25536 ± 220
Each series of experiments included from 3 to 5 measurements. To calculate the confidence interval, the average student's criterion was taken at a confidence level of 0.95. The error was calculated by the formula: S = tp,f x[EA2/n(n-1)]1/2, where tp,f is the Student criterion with a confidence level of 0.95 and the number of degrees of freedom f = 5.
RESULTS AND DISCUSSION
Thermal effect of particle dissociation H2L+ (AdisHH2L+) was calculated by the equation:
AdlsHH2i+ = -(AmixH - AdllH)/a! (4)
where AmixH - is heat of interaction 0.1M solution HNO3 with a solution of pyridoxine, AdilH - is heat of dilution of a solution of a mineral acid in a solution of a background electrolyte, a1 - is completeness of the protonation of the particle HL1 (a = 0.83). The calculation results are presented in Table 1.
The process of neutralizing an aqueous solution of pyridoxine can be represented by the equation: HL1 + OH- ^ L- + H2O (5)
Thermal effect of dissociation (AdisHHL±) calculated by the equation:
AneutH = (AmixH1 - AdilH1)/a2 (6)
AdisHHL1 = AneutH - AHW (7)
where AmixH1 - is thermal effect of mixing a pyridox-ine solution with a solution KOH; AdilHJ - is the thermal effect of diluting a pyridoxine solution in a background electrolyte solution; a2 - is process completeness (5) 0.99; AHw - is the heat of neutralization of a strong acid with a strong base against the background of a supporting electrolyte (potassium nitrate).
The found thermal effects of pyridoxine dissociation are given in Table 2. The error is defined as the standard deviation of the average value from three to four parallel experiments.
The thermal effects of dissociation of pyri-doxine in a standard solution were found by extrapolating the heats of stepwise dissociation at fixed val-
ues of ionic strength to zero ionic strength according to the equation proposed in [18]:
ArHi - Az2^(I) = A rHi0 + bl (8)
where, ArHi, ArH0 - is change in enthalpy at a finite value of ionic strength and at I = 0, respectively, W(l) -is theoretically calculated ionic strength function, Az2 -is the difference between the squared charges of the reaction products and the starting components, b- is empirical coefficient.
Points in coordinates (AH-AZ2V(l) as a function of l), satisfactorily fit on a straight line. The corresponding graph is shown in Fig. 3. With an increase in the background electrolyte concentration, the endo-thermicity of stepwise pyridoxine ionization processes increases.
Fig. 3. Determination of standard thermal effects of HL± particle
dissociation at 298.15 K Рис. 3. Определение стандартных тепловых эффектов диссоциации частицы HL± при 298,15 К
When processing by MNC, pyridoxine dissociation enthalpies at zero ionic strength were obtained: AdlsH(H2L+) = -17.25 ± 0.30 kJ/mol and AdisH(HL±) = -26.17 ± 0.30 kJ/mol.
The standard thermodynamic characteristics of the processes of stepwise dissociation of pyridoxine, obtained by us for the first time, are presented in Table 3.
Table 3
Standard thermodynamic characteristics of the processes of stepwise dissociation of pyridoxine Таблица 3. Стандартные термодинамические характеристики процессов ступенчатой диссоциации пи-ридоксина_
Т
рК0
AG0 kJ/mol
ArH0, kJ/mol
-AS0, J/molK
H2L+ = HL± + H+
298.15 |4.85 ± 0.05 |27.68 ± 0.6 | -17.25 ± 0.30 |150.7 ±09
HL± = L- + H+
298.15 18.96 ± 0.05 |51.14 ± 0.6 | -26.17 ± 0.30 |259.3 ±09
This work was carried out as part of the Research Institute of Thermodynamics and Kinetics of Chemical Processes of the Ivanovo State University of Chemical and Technology with financial support from the Russian Federal Property Fund and the Government of the Ivanovo Region as part of research project No. 18-43-370018.
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Поступила в редакцию (Received) 18.12.2019 Принята к опубликованию (Accepted) 05.03.2020
