Activity Coefficients Of Components Research Articles

Innovative vacuum distillation technology for preparing refined Al from Al–Mg alloy

This study presents an advanced approach utilizing vacuum distillation (VD) to purify Al from Al–Mg alloys. The activity coefficients of Al-Mg binary alloy components were meticulously determined by applying the molecular interaction volume model (MIVM), non-random two-liquid (NRTL) model, and Wilson equation. Comparative analysis between experimental data and calculated activity coefficients enabled the selection of an optimal model exhibiting high computational accuracy and extensive applicability. The NRTL model, chosen for its precision, was then employed to compute the activity coefficients of Al-Mg alloy across various temperatures, leading to the development of a vapor-liquid equilibrium (VLE) diagram. This VLE diagram provides a robust quantitative foundation for assessing the purification limits of Al within the temperature range of 973–1373 K in the distillation process. Theoretical analyses underscore that VD offers a rapid and efficient method for Al purification. Experimental verification at a kilogram scale, conducted at 1373 K for 120 min, achieved an Al purity level of 99.9989% with a direct recovery efficiency of 97.98%. Notably, this VD process aligns with environmental standards for sustainable metallurgical practices, as it prevents the release of gaseous emissions or effluents, thereby meeting the industry’s growing demand for eco-friendly Al recycling methods.

Comparison of methods for calculating the enthalpy of vaporization of binary azeotropic mixtures

Objectives. To calculate the molar enthalpy of vaporization of binary homogeneous mixtures based on isothermal and isobaric vapor–liquid equilibrium data, and to compare the results of calculation of molar enthalpy of vaporization by different methods with experimental data.Methods. Simulation of the vapor–liquid equilibrium of binary systems according to the Non-Random Two Liquid “local compositions” equation and thermodynamic calculations of molar vaporization enthalpies of binary mixtures at different conditions of vapor–liquid equilibrium were used.Results. Arrays of calculated data were obtained with regard to molar enthalpies of vaporization for 25 compositions of binary azeotropes (isothermal, isobaric conditions of phase equilibrium), and the full range of compositions of the benzene–ethanol system at atmospheric pressure.Conclusions. The accuracy of thermodynamic methods for calculating the vaporization enthalpy of binary azeotropic mixtures according to vapor–liquid equilibrium data is higher in 85% of cases for isothermal, and in 75% of cases for isobaric conditions. By taking into account the influence of temperature on the activity coefficients of components in the liquid phase, the values of excess molar enthalpy both for azeotrope compositions and for the full concentration range of the benzene–ethanol system under isobaric conditions of liquid–vapor phase equilibrium can be accurately reproduced.

РАСЧЁТ ПАРОЖИДКОСТНОГО РАВНОВЕСИЯ НА ОСНОВЕ ТЕРМОДИНАМИЧЕСКИХ МОДЕЛЕЙ КОЭФФИЦИЕНТОВ АКТИВНОСТИ

The main thermodynamic models for calculating the activity coefficients of components of non-ideal solutions are considered. The need for an accurate assessment of vapor-liquid equilibrium when modeling mass transfer processes is shown

Rates of evaporation of Bismuth and Selenium from Bi-Se alloy under vacuum condition

Vacuum distillation has obvious advantages, including a short flow, short production cycle, high direct metal, and low cost. The evaporation rates of bismuth and selenium from Bi-Se alloy were investigated at 953, 963, 973, 983, 993 K, and 1 Pa. The element's evaporation rate is required for selecting the best experimental conditions in vacuum distillation. SEM and EDS were used to describe and analyze the residue and volatile. The activity coefficients of components for the Bi-Se system were determined using the Non-random two liquid (NRTL) equation and agree with the literature. The activities of components bismuth and selenium for the Bi-Se system are proportional to their evaporation rates during vacuum distillation. The influence of compound Bi2Se3 formation on the evaporation rate of elements in Bi-Se alloy at 973 K was discussed in detail. The activation energies of pure bismuth and Bi in Bi-Se alloy were also presented.

VLE measurement of binary systems 1-butyl-3-methylimidazolium-based ionic liquids and ethanol or water

Experimental vapor–liquid equilibrium (VLE) data for binary systems of ethanol or water with four ionic liquids (ILs) comprising 1-butyl-3-methylimidazolium cation and different anions were measured at atmospheric pressure within a broad region of IL concentrations, IL mole fraction ranging between 0 and 0.7 or even 0.87. Siwoloboff procedure of vapor–liquid phase transition temperature measurement was applied to estimate VLE of the binary mixtures. Obtained t–x data were fitted assuming ideal vapor and non-ideal liquid phase behavior, using the original NRTL equation to calculate component activity coefficients. VLE description results are presented in form of equilibrium diagrams and as the variation of activity coefficients with mixture composition. Quality of the VLE data fitting was assessed based on root mean square deviations and mean deviations in temperature. Some experimental binary t–x data sets were compared to those available in literature (measured in much narrower concentration range) or predicted from ternary VLE data. The selection of ILs in this study was related to the separation of ethanol−water mixture by extractive distillation. Principal IL selection criterium was its ability to affect the ethanol–water relative volatility. Thermodynamic description of the VLE data of IL-containing systems within a wide concentration range is an essential building block of the mathematical model of separation devices, especially in case of regeneration equipment where concentrated IL is present.

Estimating the nonideality of eutectic systems containing thermally unstable substances.

Eutectic systems design requires an in-depth understanding of their solid-liquid equilibria (SLE). Modeling SLE in eutectic systems has as prerequisites, the melting properties and activity coefficients of components in the liquid phase. Thus, due to the unavailable melting properties of thermally unstable substances, it is impossible to estimate their activity coefficients from experimental SLE data and model the SLE phase diagram of their eutectic systems. Here, we evaluate the activity coefficients of thermally unstable constituents in the liquid phase, which were calculated independent of their melting properties by correlating the SLE data of their cocrystals. Differential scanning calorimetry and powder x-ray diffraction were employed to obtain the SLE phase diagram of three eutectic systems, i.e., tetramethylammonium chloride/catechol, tetraethylammonium chloride/catechol, and betaine/catechol systems, and identify the formation of nine cocrystals. The non-random, two-liquid equation was used to calculate the activity coefficients of the components in the liquid phase. The substantial negative deviation from ideality in the three studied systems indicated strong hydrogen bonding interactions in the liquid solution. Furthermore, modeling ion-ion interactions in eutectic systems containing ionic constituents is of utmost importance for understanding their nonideality.

Experimental study on separation of isopropanol-water azeotrope by packed capillary distillation and the prediction model of artificial neural network

ABSTRACT Capillary distillation is an azeotropic distillation technology, which combines the mechanism of surface adsorption, capillary action, as well as the distillation process. Different variations of activity coefficient and saturated vapor pressure of azeotropic components occur in the capillary pore structure, which leads to the separation. In this paper, 5A, 4A, and 3A molecular sieve capillary porous media were selected as capillary packing to study the separation effect on azeotropic mixture of isopropanol-water solution experimentally. It was found that 4A molecular sieve worked better, the best reflux ratio is 6, and the best packing height is 0.4 m. Based on the abovementioned experimental results, a back propagation (BP) neural network is developed and utilized in the separation process. The effects of parameters such as the composition of feedstock solution, reflux ratio, and packing height on the concentration of isopropyl alcohol at the top of packed capillary distillation column were predicted, and the accuracy reached 96.7%. The proposed prediction method is helpful for developing the technology of separating azeotropic solution by capillary distillation with packing suitable for industrial application.

Micelle formation and physico-chemical variables for the sodium dodecyl sulfate/cetylpyridinium chloride and their mixture in aqueous propranolol hydrochloride drug solvent: Conductivity and theoretical analysis

Herein, study of both single and mixed micelle development of an anionic surfactant sodium dodecyl sulfate (SDS) and a cationic surfactant cetylpyridinium chloride (CPC) in propranolol hydrochloride (PPH) drug medium has been performed using a conductometric technique at different temperatures and compositions. The critical micelle concentration (CMC) along with degree of ionization (g) were assessed from the specific conductivity (κ) versus [surfactant] plots. Within the concentration of surfactant studied, a single CMC was achieved in the situations of pure and mixed surfactant micellization. The CMC and g values have been noticed reliant on the employed drug concentrations, variations of mole fractions of the mixture of surfactants and experimental temperature. For the micellization of single SDS/ CPC surfactants in aq. PPH drug medium, PPH drug causes delayed micelle development which has been detected as the upsurge of CMC values of SDS/CPC in PPH drug media. For the association of SDS/ CPC surfactants in aq. PPH drug medium, the CMC values experienced a lessening with the growth of study temperature. At a fixed temperature, the experimental CMC values, of the SDS + CPC mixture, endure a decline through the upsurge of mole fractions of CPC (α1), attain a minimum value and the CMC values tend to enhance at and above α1= 16.2 × 10-5. The ideal critical micelle concentrations (CMCid) are detected always as higher values than experimental CMC up to the α1= 10.8 × 10-5. The -β values and |β| > |ln(CMC1/ CMC2)| for the present study reveal that the interaction among the amphiphiles under analysis is synergistic. The negative free energy (-ΔGmO) of micellization were obtained throughout the study. The enthalpy (ΔHmO) and entropy (ΔSmO) of micellization reveal the presence of hydrophobic interaction as the principal type of interaction among the studied constituents. The negative excess free energy (ΔGex) values inferred that stable mixed micelles are formed for the mixture of SDS + CPC in aq. PPH drug medium. The micellar mole fractions (both experimental and ideal values), activity coefficients of components in mixed micelles as well as enthalpy–entropy compensation parameters were also measured and illustrated. It is possible that the knowledge gained from the current analysis may be used to enhance pharmaceutical formulations, drug delivery carriers, along with drug act proficiency.

Development of a Kinetic Model for Ti‐Bearing Steel‐Slag Reaction

A kinetic model based on the double‐film theory is developed to predict the composition‐change behavior of Ti‐bearing steel and slag during the slag‐liquid steel reaction process and validated by experimental results. The chemical reactions between the slag‐metal, slag‐crucible refractory, and metal‐crucible refractory are considered. Specifically, a coupled reaction model based on the CaO–MgO–CaF2–SiO2–Al2O3–TiO2–FeO–MnO and Fe–Si–Al–Ti–Ca–Mg–Mn–S–O systems is applied to describe the reaction between liquid steel and slag. Furthermore, the activity coefficients of slag components are calculated using the ion and molecular coexistence theory model. The reaction between the slag and refractory is described using an empirical equation. The reaction between the liquid steel and refractory is described using a coupled reaction model, wherein the reaction between [Al] and MgO is considered. Finally, the effect of (TiO2) on the [Ti] is discussed using this model. The results indicate that the [Ti] decreases, the [Si] increases, and the [Al] increases initially and subsequently decreases with reaction time. An increase in (TiO2) can decrease the oxidation of [Ti] by (SiO2) and (Al2O3). When the (TiO2) increases, the mass ratio of CaO to SiO2 should be further increased to decrease the oxidation of [Ti].

Effects of sulfur and potassium on rhenium retention in simplified low-activity waste glass

Four consecutive sets of simplified glass feed compositions were designed and tested both with and without rhenium (Re) additions. The objectives of this study concerning the effects of sulfur (S, expressed as SO3 in glass) and potassium (K, K2O in glass) on Re retention during melting of low-activity glass feeds were twofold: (i) design a simplified system that reproduces the Re behavior observed in fully simulated feeds, the system will be used to investigate mechanisms related to Re incorporation into glass and (ii) Provide experimental data that support or reject the mechanisms that have been proposed to explain the previous findings from scaled melter and crucible tests. The simplified system of alkali borosilicate glass feeds in this study successfully reproduced the published results from the melter tests with fully simulated feeds on the effects of S and K on Re behavior. The present study also provides experimental evidence on the combined effect of S and K. The findings from this study regarding the effects of S and K on Re retention also are explained by the previously proposed mechanism on the effect of sulfate on Re retention and by the effect of component concentrations and activity coefficients of volatile species on its vapor pressure, also discussed in a recent study.

The interaction of the surface active ionic liquid with nonionic surfactants (Triton X-100 and Triton X-405) in aqueous solution by using tensiometry method

The synergistic interactions of the surface active ionic liquid (1-dodecyl-3-methylimidazolium bromide, IL) with non-ionic surfactants TritonX-100 (TX-100) and TritonX-405 (TX-405) were investigated in aqueous solution at various mole fractions by using the surface tension technique at 298.15 K. In this work, the pendant drop method was used for surface tension measurement of solutions. The surfactant properties such as critical micelle concentration (CMC), micellar mole fractions (X1Rub), interaction parameter (βm), and activity coefficients of components in the mixed micelle (f1Rub and f2Rub) were calculated by theoretical Clint and Rubingh models. Also, surface and adsorption parameters, maximum surface excess values (Γmax), minimum area per molecule (Amin), surface tension at CMC (σCMC), and surface pressure at the CMC (πCMC) values have been estimated in (TX-405/TX-100 + IL + water) systems. The synergistic effects operating in these amphiphile mixtures were confirmed by the negative values of the interaction parameter. Generally, the CMC value of surfactant solution increases with the addition of ionic liquid in the mixture. It was demonstrated that IL can be judiciously used at different mole fractions for modifying the physicochemical properties of (TX-100 and TX-405). Different thermodynamics parameters were computed for mixtures and were also discussed in detail. The Gibbs free energy change of micellization (ΔGmo), the standard Gibbs energy of adsorption (ΔGado) and the excess free energy of micellization (ΔGexRub) values have been estimated at different compositions. The calculation of thermodynamic parameters shows that the micellization process is spontaneous and becomes more favorable with addition of the non-ionic component in mixture.

Thermodynamic analysis of oligomeric blends by applying the Kirkwood-Buff theory of solutions

The accurate prediction of the thermodynamic properties of oligomeric blends and, in general, binary liquid mixtures from atomistic simulations is a challenging task. In this work we develop a methodology for the full thermodynamic analysis of oligomeric blends and the extraction of the Flory-Huggins interaction parameter from the Gibbs energy of mixing, combining Flory-Huggins thermodynamics with Kirkwood-Buff theory of solutions. We perform a series of Molecular Dynamics (MD) simulations of 2-methylpentane/n-heptane mixtures, at various mole fractions. Firstly we validate the forcefield we apply in our MD simulations, comparing the density and excess volume we obtain against the corresponding experimental estimates found in the literature. Then we calculate the Kirkwood-Buff integrals in the isothermal-isobaric (NpT) ensemble, applying the particle fluctuations method, and we extract the component activity coefficients, the excess Gibbs energy, the excess enthalpy, and the excess entropy of mixing as functions of the mole fraction. Finally we calculate the Flory-Huggins interaction parameter χ by interpreting the Gibbs energy of mixing in the framework of Flory-Huggins theory, and explore its dependence on composition. All results are compared against experimental measurements in order to evaluate our methodology. Agreement is found to be very good.

Mixed micelle formation between a surface active ionic liquid and tricyclic antidepressant drugs (imipramine hydrochloride and amitriptyline hydrochloride)

Mixed micelle formation between surface active ionic liquid (1-dodecyl-3-methylimidazolium bromide, IL) and two amphiphilic drugs consisting of imipramine hydrochloride (IMP) and amitriptyline hydrochloride (AMT) were investigated in different mole fractions at 298 K using electrical conductivity measurements. The experimental results of mixed antidepressant drugs (IMP and AMT) with ionic liquid in aqueous solution indicated that CMC values decreased with the addition ionic liquid in mixture. Also, the CMC values in AMT/IL system are comparatively lower than those found in IMP/IL system, which indicates that the formation of mixed micelle is more favorable in presence of AMT. The different theoretical models (Clint, Rubingh, Motomura and Rodenas) were used to evaluate the micellar mole fractions (Xi), interaction parameter (βm) and activity coefficients of components (f1 and f2) in the mixed micelle. In both studied systems, the values of interaction parameter suggest the synergistic effects in mixed state. Activity coefficients are less than unity which means non-ideal mixing. Also, the excess free energy of micellization (ΔGex) values have been estimated using activity coefficients at different compositions. This parameter not only confirm that mixed micellization is spontaneous in nature but also is extra stable than individual micelle.

Сорбция гистидина ионообменниками различной природы

С целью сравнения сорбции полифункциональных веществ на ионообменниках различной природы исследовано поглощение гистидина гидрохлорида из водных растворов анионообменником АН-251 в хлоридной ионной форме, катионообменником КБ-4П-2 в натриевой ионной форме и полиам-фолите АНКБ-2 в солевой NaCl-форме. Установлено, что в первом случае протекает необменная сорб-ция за счет ион-дипольных взаимодействий между сорбционным центром «функциональная группа - противоион» и заряженной группой аминокислоты с участием молекул воды. На катионообменнике и полиамфолите протекает ионный обмен, поскольку в системе присутствует два сорта катионов Na+ и His+, величины которого, однако, невелики. В этих же системах протекает необменное поглощение за счет ион-дипольных взаимодействий противоположно заряженных структурных элементов с участием молекул расторителя, которые, могут протекать с противоионом любой природы, возникающим в си-стеме за счет ионного обмена.Установлено, что независимо от природы сорбента количество поглощенного гистидина при-близительно одинаково из-за дополнительных взаимодействий, протекающих в системе с участием сорбционных центров (в КБ-4П-2 и АНКБ-2) и малого набухания ионита (АН-251).Проведено термодинамическое описание исследуемых систем с использованием подходов, ос-нованных на представлении о стехиометричности закрепления вещества в сорбенте. Рассчитаны вели-чины коэффициентов равновесия, исправленных коэффициентов равновесия, коэффициентов активно-сти компонентов фазы сорбента, и термодинамических констант ионообменного и необменного равно-весия, а также дифференциальные энергии Гиббса, относящиеся к ионообменнику определенного со-става. В качестве состояния сравнения для обоих случаев выбирается состояние равновесия ионооб-менника в моноионной форме с водой.Установлено, что в процессе сорбции отклонение систем от состояния сравнения незначитель-ное, что связано с малым количеством поглощенного вещества по сравнению с количеством сорбцион-ных центров, термодинамические контанты равновесия близки к единице. Совместное протекание ион-ного обмена и необменной сорбции оказывает большее влияние на состояние сорбента по сравнению с необменной сорбцией в силу изменения ионной формы сорбента с минеральной на органическую

DETERMINATION OF SELECTIVITY AND MASS TRANSFER IN LIQUID-PHASE EXTRACTION FOR BUTYL GLYCOL-WATER-ISOPROPYL ETHER SYSTEM

The problems of phase equilibrium, the equilibrium distribution of a component between phases are considered, and the distribution coefficient is determined in two versions. A formula for determining the selectivity coefficient is obtained, equations relating the compositions of coexisting phases, by equating the activities in these phases, Margules constants are found. Based on the given content of the components, the numerical values of (Margules constant for A component in solvent S) and (Margules constant for S component in solvent A) were found Using the Margules equation for ternary systems, the activity coefficients of components A and B are determined in two phases. The experimentally obtained values of the molar fractions of each of the three components, responsible to different points of the binodal curve, the corresponding activity coefficients of component B, and also calculated on these coefficients of the activity values

Estimating activity coefficients of target components in poorly specified mixtures with NMR spectroscopy and COSMO-RS

Poorly specified mixtures are common in process engineering, especially in bioprocess engineering. The properties of such mixtures of unknown composition cannot be described using conventional thermodynamic models. The NEAT method, which has recently been developed in our group, enables the calculation of activity coefficients of known target components in such poorly specified mixtures. In NEAT, the group composition of the mixture is determined by NMR spectroscopy and a thermodynamic group contribution method is used for calculating the activity coefficients. In all previous studies with NEAT, the UNIFAC group contribution method was used. In the present work, we demonstrate that NEAT can also be applied with another important method for predicting activity coefficients: COSMO-RS. COSMO-RS (OL) developed in Oldenburg together with its group contribution version GC-COSMO-RS (OL) is used here. The new version of NEAT was successfully tested. For a variety of aqueous mixtures excellent agreement of the NEAT predictions, for which only information on the target component was used, with results that were obtained using the full knowledge on the composition of the mixture was found. The results demonstrate the generic nature of the idea of NEAT and the broad applicability of the method.

Construction of Gallium–Tin Nonequilibrium State Diagram and Its Analysis

Overcooling of gallium–tin alloys under normal conditions has been studied by thermal analysis. The following samples have been analyzed: Ga (I); two hypoeutectic alloys: 95% Ga + 5% Sn (II), 90% Ga + 10% Sn (III); eutectic alloy: 96.3% Ga + 13.7% Sn (IV); and five hypereutectic alloys with Sn content of 20% (V), 35% (VI), 50% (VII), 80% (VIII), and pure tin (IX). A nonequilibrium state diagram of this system is constructed. Herewith, the eutectic composition does not vary and the eutectic temperature decreases to 5.5°C, that is, 26°C below that of three-phase eutectic equilibrium. The eutectic temperature does not actually vary upon a variation of cooling rates of eutectic alloy from 0.06 to 60°C/min. It has been detected that a slight decrease in overcooling is expected in the hypoeutectic region, whereas in the hypereutectic region overcooling increases while the alloy composition approaches eutectic. Activities and activity coefficients of components on the lines of equilibrium and nonequilibrium liquidus have been calculated. It is demonstrated that the activities on the lines of both equilibrium and nonequilibrium liquidus decrease in a predictable manner, and the activity coefficients increase while the composition approaches eutectic. Concentration paths of equilibrium and nonequilibrium crystallization are shown in the state diagrams.

Effects of alkyl chain length on synergetic interaction and micelle formation between a homologous series of n-alkyltrimethylammonium bromides and amphiphilic drug propranolol hydrochloride

The aggregation behavior of a homologous series of n-alkyltrimethylammonium bromides (CnH2n+1TAB; n = 12, 14, and 16) in the presence of amphiphilic drug propranolol hydrochloride (PPL) in aqueous solutions were studied using conductometric technique. The critical micelle concentrations (CMCs) were determined for pure PPL and surfactant as well as mixed micelles [surfactant(1)/PPL(2)] using the Carpena method. Various physicochemical parameters such as ideal CMC (CMCid), degree of counter-ion binding (α), micellar mole fractions of surfactant (X1m and X1id), interaction parameter (βm), the activity coefficients of components (f1m and f2m), standard Gibbs free energy of micellization (ΔGmic0) and the excess free energy of micellization (GmicE) values have been calculated and compared in detail using Clint's and Rubingh's theories. The activity coefficients of surfactant and PPL are less than 1 (f1m < 1 and f2m < 1) and the values of βm are negative in all mixtures, which indicates the synergistic interaction between surfactant and PPL. Also, the results show that the synergistic effects of mixed micelles strongly depend on the chain length of cationic surfactant, and the values of interaction parameter increase with increasing the alkyl chain length of surfactant. Of the several interactive interactions, it seems hydrophobic interaction is the most dominant interaction between the molecules of the mixed systems among all observed interactive interactions in this study.

Thermodynamic properties of iron, aluminum, boron and phosphorus in dilute silicon solutions by molecular interaction volume model

The thermodynamic properties of impurity components in silicon solutions play an important role in the chemical removal process to the metallurgical route. In this paper, the component activity coefficients and interaction parameters of dilute silicon solutions were estimated by the molecular interaction volume model (MIVM). The activity coefficients (?i) of component i in dilute binary Si-i and ternary Si-i-j solutions at 1687-1873 K were firstly calculated. The concentration dependences of the interaction parameter and activity coefficient were also obtained. The self-interaction parameters (?i,i) for Si-i system were obtained as ?B,B= -2.728-362.031/T, ?Al,Al = 2.157-1876.776/T, ?Fe,Fe= -4.842+14445.926/T and ?P,P = -2.543+13767.036/T. At the same time, the interaction parameters among components B, Al, and Fe in dilute ternary Sii- j solutions were also derived as ?Fe,B=-1.2758-2946.306/T, ?Al,B = 0.7467-9765.9298/T and ?Fe,Al= -1.39677+3319.6803/T. Most important of all, the temperature dependences of the interaction parameters and activity coefficients in dilute Si-i and Si-i-j solutions with a certain i or j concentration were deduced. The results show that the predicted self-interaction parameters of B, Al, Fe, and P in binary silicon solutions reasonably agree with the experimental data. This further shows that MIVM is of reliability and can be expanded to a multi-component dilute silicon solution.

Application of NEAT for determining the composition dependence of activity coefficients in poorly specified mixtures

Poorly specified mixtures, of which the composition is only partially known, are important in many fields. In a recent work of our group, the method NEAT for estimating activity coefficients of target components in such mixtures was introduced. In the present work, it is shown that based on a single NMR analysis of a poorly specified mixture not only the activity coefficient of the target component in that mixture but also its composition dependence can be predicted with NEAT. Hence, based on a single NMR analysis, the activity coefficient of the target component in many mixtures can be predicted with NEAT. This is possible, if the unknown part of the mixture remains unchanged, which is the case e.g. when the target component is selectively removed or when a known solvent is added or removed. The approach is successfully tested using several aqueous test mixtures.