Sensor, device and method for analyzing electroactive components contained in an electrically non-conductive medium

Abstract

Sensor (1) for analyzing electroactive components contained in an electrically non-conductive medium (2), comprising: - a sensor housing (3), - an electrolyte chamber (4) arranged within the sensor housing (3), - an electrolyte (5) arranged in the electrolyte chamber (4) and immiscible with the medium (2), wherein the electrolyte (5) contains at least one carbonate selected from a group comprising ethylene carbonate, propylene carbonate and butylene carbonate and at least one conducting salt, - at least two electrodes (6) arranged in direct contact with the electrolyte (5) and - a separating device (7) which is permeable at least to the electroactive components contained in the electrically non-conductive medium (2), wherein the separating device (7) is arranged on the electrolyte chamber (4) so that the electrolyte chamber (4) is delimited, wherein the electrodes (6) are arranged opposite the separating device (7) and are immersed in the electrolyte (5).

Description

The invention relates to a sensor, a device and a method for the analysis of electroactive constituents contained in an electrically non-conductive medium.Oils as liquid lubricants fulfil various tasks. In addition to the main task of reducing friction and wear, they also serve for transmitting force, cooling, vibration damping, sealing and corrosion protection. In order to be able to fulfil these different tasks, a large number of different additives, such as antioxidants, corrosion inhibitors, detergents, dispersion auxiliaries, defoaming additives and wear inhibitors, are added to the base oil to be used according to the requirements of the specific application. Base oil and additives together form the oil in the sense of this description.During the useful life of the oil, both the base oil and the additives are subject to a wide variety of changes which can be attributed, inter alia, to an action of temperature, pressure, shear, contamination, (air) oxygen and / or moisture. For example, oxidative degradation may occur.The change in the oil due to the various chemical physical processes is also referred to as aging of the oil.Antioxidants can slow down the oxidative degradation of the base oil and thus extend the service life of the oil. The antioxidants are divided into three groups according to their mode of action.The first group are the radical scavengers which react with the free radicals of the decomposing base oil and form stable radicals. This counteracts further chain degradation of the carbon chain of the base oil. This group includes sterically hindered phenols or secondary aromatic amines.In order to counteract the formation of the peroxides formed, which are the actual initiators for the degradation of the chains of the base oil, peroxide decomposers (second group) are used. In most cases phosphites and thioethers are used.The third group are the passivators. These react with the free ions and form so-called chelates, whereby the reactivity of the metal ions is reduced.The loss of desired oil properties over the course of use due to this unavoidable aging of the oil leads to the need to replace the oil after a certain time. The time of the replacement may not take place too timely for environmental and cost reasons, but also not too late, since there is otherwise the risk of performance losses or even destruction of the lubricated machine.Therefore, there is a need to analyze the aging state of the oil, also referred to as an oil state below, in order to be able to select the replacement time of the oil according to the stated conditions.The prior art discloses the use of various parameters, such as acid number, viscosity, metal ion concentration and colour, for analysis of the oil state. For this purpose, an oil sample must be taken and examined in the laboratory. In situ measurement or online monitoring is not possible in this case.A further possibility for analyzing the oil condition is to examine the permittivity of the oil. This is therefore not an absolute method, since only a change in the permittivity is detected, from which the oil state is then deduced. It is also disadvantageous that a change in permittivity can only be detected after significant aging of the oil. At this time, however, it may already be too late for an oil exchange, so that the system lubricated with the oil could already have been damaged.Optical measurement principles are also known in which the turbidity of the oil is measured and taken into account in the analysis of the oil state. However, this results in difficulties provided that the oil already has a color and / or turbidity in the new state.In addition, oxidation of the oil in the laboratory is detectable by means of FTIR spectroscopy. In this case, the bands of specific compound classes (ketones, aldehydes or else carboxylic acids) are detected, which form as the aging of the oil progresses.A further possibility for analyzing the oil state is to investigate the antioxidants contained in the oil. This can be carried out by means of the so-called RULER test (Remarining use life evaluation routine-ASTM D-6971).An oil sample is taken and mixed with an extractant. The oil is coagulated by the addition of sand and falls to the bottom. The measuring probe is then immersed and a voltammogram recorded. The so-called RUBER number (proportional to the concentration of the antioxidants) is determined from the peak area.In order to be able to make a statement about the oil state, either the oil in the new state (new oil) must be measured or the concentration of antioxidants in the new oil must be known. On the basis of a comparison of the concentration of antioxidants in the new oil and the residual concentration of the antioxidants, it is then possible to infer the oil state.The RUBER test can be carried out in the laboratory in addition to the determination of acid number and viscosity. For a reliable monitoring of the oil state, samples must be taken and analyzed continuously.From the patent specification U.S. Pat. No. 5,518,590 A, an electrochemical method and a corresponding sensor for analyzing electroactive constituents in a non-conductive medium are known. For this purpose, the electroactive substances, e.g. antioxidants in an oil, are extracted from the non-conductive medium by means of an electrolyte and are transferred into an electrically conductive phase of an electrochemical cell.The concentration of the electroactive constituents in the electrically conductive phase is determined by means of the electrochemical cell, from which the concentration of the electroactive constituents in the non-conductive medium can then be deduced. As a preferred electrolyte for the electrically conductive phase, acetonitrile with lithium perchlorate as the conducting salt is mentioned.Investigations carried out by the inventors of this method and of the corresponding sensor revealed that the method and the sensor can be used with the same sensor (online monitoring) to a continuous monitoring, if at all, of a concentration of the electroactive constituents in the non-conductive medium which decreases over time.Namely, it has been found that acetonitrile is extracting, i.e., the position of the distribution equilibrium for the electroactive components is greatly shifted in favor of acetonitrile. This is disadvantageous in particular when investigating small quantities of a non-conductive medium, since all electroactive constituents, i.e. antioxidants for example, can be removed from the non-conductive medium by the analysis, so that the non-conductive medium becomes unusable.In addition, acetonitrile can only be used at temperatures up to about 80° C., since the boiling temperature of acetonitrile is 82° C. This temperature is exceeded in many applications of non-conductive media, for example the use of lubricating oils. Online monitoring is not possible at higher temperatures.From JOSEPHSON, Jose; KIM, Hyun-Ok L.; OH, Seajin: In Situ Electrochemical Sensor for Measurement in Non-conductive Liquids. In: Journal of the Electrochemical Society, Vol. 140, 1993, No. 3, pp. L33-L34.-ISSN 0013-4651 discloses a sensor for the analysis of electroactive polar constituents of an oil. The electrolyte used is a conductive, polar and oil-immiscible solution, for example a solution of 0.1 M KCl in ethylene glycol.DE 42 25 904 A1 and U.S. Pat. No. 5,489,371 A disclose a sensor for electrochemical measurements. As possible electrolytes, mention is made of dimethyl sulphoxide and LiClO 4 and propylene carbonate and LiClO 4, LiCl or NaCl.US 4 662 996 A describes a method and an apparatus for detecting polar toxic species using a sensor electrode coated with a conductive polymer.It is therefore an object of the invention to specify a possibility for online monitoring of the concentration of electroactive constituents of a non-electrically conductive medium, in particular of an oil.It is also desirable to be able to predict an exchange time for an oil, so that, on the one hand, the oil can be used for as long as possible and, on the other hand, an exchange can take place before possible damage to a plant lubricated with the oil.In addition, a suitable sensor should be constructed as simply as possible and be produced economically and should be safe to use. Furthermore, the sensor should also be usable at higher temperatures, for example at temperatures above 100° C. or even at temperatures above 150° C. or 200° C.This object is achieved by the subject matters of the independent claims. The dependent claims contain embodiments of these solutions according to the invention.As used herein, the term "and / or," when used in a series of two or more elements, means that each of the listed elements may be used alone, or any combination of two or more of the listed elements may be used. For example, when a composition is described as containing components A, B, and / or C, the composition may contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination.The invention is based on the basic idea that the above-described disadvantages of the method and sensor known from the patent specification U.S. Pat. No. 5,518,590 A are based on the use of electrolytes which extract the electroactive constituents from the non-conductive medium. The distribution equilibrium of the electroactive constituents between the nonconductive medium and the electrically conductive phase, i.e. the electrolyte, is therefore greatly shifted in the direction of the electrically conductive phase.If the concentration of electroactive constituents in the non-conductive medium is initially high, a portion of electroactive constituents corresponding to this concentration is first extracted, and the determined concentration of electroactive constituents in the electrolyte correlates with the concentration of electroactive constituents in the non-conductive medium.As the time progresses, i.e., aging of the oil, the concentration of electroactive constituents in the non-conductive medium decreases. However, due to the highly shifted equilibrium, this decrease in concentration in the non-conductive medium does not result in a corresponding decrease in the concentration of electroactive constituents in the electrolyte. Rather, a persistently high concentration of electroactive constituents is determined in the electrolyte.In order to achieve a long-term stable sufficient correlation between the concentration of the electroactive constituents in the non-conductive medium and the electrolyte, the use of an electrolyte having at least one carbonate selected from a group comprising ethylene carbonate, propylene carbonate and butylene carbonate is proposed according to the invention. When such an electrolyte is used, there is no extraction, i.e. separation, of the electroactive constituents.The electrolyte according to the invention makes it possible to set a concentration ratio (distribution equilibrium) which is as balanced as possible between the concentration of the electroactive constituents in the non-conductive medium, for example a mineral oil, and the electrolyte over a long period of time, for example over the useful life of an oil used as lubricant. Extraction as with acetonitrile is avoided.As a result, only small amounts of the electroactive constituents are taken from the non-conductive medium for the analysis, which, on the one hand, make a sufficiently precise analysis possible and, on the other hand, do not cause any significant change in the non-conductive medium to be analyzed.In addition, by means of a distribution equilibrium which is as balanced as possible, it can be prevented that the change in concentration of the electroactive constituents, for example on account of aging of the oil, runs faster than the establishment of the distribution equilibrium, and the distribution equilibrium is also actually established during the measurement duration. Erroneous measurement results due to insufficient actual adjustment of the distribution balance can thus be avoided. Information about the oil state can be derived on the basis of the concentration and / or the concentration profile of the electroactive constituents, for example the concentration of antioxidants, for example antioxidants of the first group mentioned in the introduction.For example, a prediction of the future oil state may be possible on the basis of the current concentration of antioxidants of the electroactive constituents, in that it is possible on the basis of the current concentration and, if appropriate, of the previous concentration profile of the antioxidants to predict at which point in time the antioxidants will have been used up and an increased oxidative degradation of the base oil is to be expected.In contrast, the acid number and viscosity of the oil used increase only after the antioxidants have been consumed. Thus, monitoring the concentration of the antioxidants makes it possible to react more quickly than in the complicated laboratory measurement of acid number and viscosity.The invention thus enables online monitoring of the non-conductive medium. As a result, a service technician or plant operator can inform himself at any time about the state of the non-conductive medium, i.e. for example about the oil state, without a separate sampling being necessary and, if appropriate, react in good time to the determined state, i.e. plan for example an oil change.In combination with a data transmission, e.g. of the measured values or information obtained by evaluating the measured values, an action on site can be dispensed with. This is advantageous in particular in the case of poorly accessible installations having a non-conductive medium to be analyzed, for example in the case of offshore wind power installations or hermetically or hydraulically closed systems in which, for example, mineral oil such as turbine oil can be used as lubricant.Furthermore, the invention can also be used for non-conductive media which have only very short service times, such as hydraulic oils with service times of a few 100 h. Penetration of water and other impurities can be ascertained quickly, so that suitable measures, for example an oil change, can be taken promptly and the service life of such installations can be increased.A sensor according to the invention for the analysis of electroactive constituents contained in an electrically non-conductive medium has a sensor housing, an electrolyte chamber arranged within the sensor housing, a liquid electrolyte arranged in an electrolyte chamber and immiscible with the medium, and at least two electrodes arranged in direct contact with the electrolyte. The electrolyte chamber may be bounded by the sensor housing. In other words, the sensor housing can form a chamber wall of the electrolyte chamber.An electrically non-conductive medium is understood to mean a composition which is electrically non-conductive, i.e. has an electrical conductivity of less than 10 -8 S / m, for example. With such a low electrical conductivity, a direct electrochemical concentration determination of the electroactive constituents is not possible. The non-conductive medium can be, for example, a gaseous or liquid medium.Specific examples of non-conductive media are non-conductive oils, such as base oils for engine or transmission lubricant oils, for hydraulic oils or for turbine oils. Oils of this type may, as described at the outset, comprise additives, for example antioxidants or corrosion inhibitors. They are usually subject to an aging process which is associated with a change in concentration of at least some additives.Electroactive constituents are to be understood as meaning electrochemically active species, e.g. ions or molecules, i.e. species which are oxidized or reduced when a particular electrical voltage is applied to an electrode, resulting in a maximum or minimum in the current flow which can be detected.Examples of electroactive constituents are ions or molecules of antioxidants, for example antioxidants of the first, second and / or third group mentioned in the introduction. For example, para-substituted, e.g., alkylated, or unsubstituted BHT (2,6-bis(1,1-dimethylethyl)-4-methylphenol) as well as substituted, e.g., alkylated, or unsubstituted PANA (N-phenyl-1-naphthylamine) may be electroactive constituents.The proportion of electroactive constituents in the non-conductive medium may be, for example, between 0.0001 and 20% by weight, for example between 0.01 and 15% by weight or between 5 and 10% by weight, or else only a few ppm (parts per million, based on the mass), for example 1 to 100 ppm.An electrolyte is understood here to mean a medium which contains one or more chemical compounds which, in the solid, liquid or dissolved state, are dissociated into ions which can move in a directed manner under the influence of an electric field.In the present case, immiscible is understood to mean that the electrically non-conductive medium and the electrolyte do not form a common mixed phase under application conditions.The electrolyte is disposed in an electrolyte chamber. For example, an electrolyte chamber may be filled with the electrolyte.The electrolyte chamber is partially open at least in a state of use, i.e. during the intended use of the sensor for the analysis of electroactive constituents contained in an electrically non-conductive medium. In this context, partially open is understood to mean that at least the electroactive constituents contained in the electrically nonconductive medium can pass into the electrolyte chamber, for example by means of diffusion. In other words, contact between the electrolyte and the electrically non-conductive medium may be enabled during use of the sensor.According to the invention, at least two electrodes are arranged in direct electrically conductive contact with the electrolyte, for example, in that the electrodes are arranged at least partially surrounded by the electrolyte, for example, in that the electrodes are immersed in the electrolyte.The electrodes can be at least one working electrode and at least one counter electrode. In addition, one or more reference electrodes may be provided. If no separate reference electrode is present, the counter electrode can simultaneously act as a reference electrode.One or more electrodes can comprise gold, platinum, vitreous carbon (vitreous carbon), stainless steel or carbon, for example, or consist of the materials mentioned, wherein working electrode and counter electrode can comprise the same material. The electrodes can be applied, for example, to printed circuit board material, e.g. FR-4.According to the invention, the electrolyte contains at least one carbonate and at least one conducting salt. For example, the electrolyte may consist of a carbonate and a conducting salt.According to the invention, the carbonate is selected from a group comprising ethylene carbonate (CAS number 96-49-1), propylene carbonate (CAS number 108-32-7) and butylene carbonate (CAS number 4437-85-8). The electrolyte may also contain a mixture of the carbonates mentioned.Advantages of these carbonates are their high boiling temperatures (ethylene carbonate 248° C., propylene carbonate 242° C., 1,2-butylene carbonate 240° C.), as a result of which the use temperatures can be significantly increased. For example, the sensor can be used at temperatures of up to 200° C., optionally even above this.In addition, the carbonates used according to the invention are largely harmless compared to acetonitrile, since they cannot be readily ignited in contrast to acetonitrile. This is especially important in the analysis of electroactive constituents in media located near ignition sources.Furthermore, the carbonates mentioned have a comparatively high density (propylene carbonate 1.21 g / cm 3(20 ° C.), ethylene carbonate 1.32 g / cm 3( at 40° C.), butylene carbonate 1.14 g / cm 3(20 ° C.)). This enables the sensor to be arranged with respect to the electrically non-conductive medium to be analyzed, which usually has a lower density, in such a way that even in the event of a leak there is no risk that the medium is contaminated with the electrolyte.In addition, the carbonates mentioned are distinguished by a reaction inertia, i.e. they do not react with the electrically non-conductive medium at least under application conditions. As a result, in the event of a leak in the sensor, i.e. the undesired penetration of the electrolyte into the electrically non-conductive medium, the occurrence of a chemical reaction can be prevented. This increases the safety when using the sensor according to the invention.In the present case, a conducting salt is understood to mean a chemical compound which is present in the electrolyte in a state at least partially dissociated into ions and increases the electrical conductivity of the electrolyte without reacting itself at one of the electrodes. The concentration of the conductive salt in the electrolyte may be in the range of 10 -4 to 10 -1 mol / l, for example in the range of 10 -3 to 10 -2 mol / l.The conducting salt can be, for example, a lithium conducting salt, sodium conducting salt or potassium conducting salt or a mixture of these conducting salts.The conducting salt may be selected from a group comprising KClO 4, NaClO 4, LiClO 4, LiPF 6, LiAsF 6, LiMnF 6, LiBiF6, LiF 4, LiPtCl 6, Li 2 SnO 6, LiCF3SO3, LiC4F9SO3, Li[C(SO 2 CF 3)2], Li[N(SO 2 CF 3)2], Li[C(SO 2 CF 3)2( SO 2 C 4 F 9), Li[C(SO 2 CF3)4(S2O4C3F6)], Li[B(CeH3FO2)2], and mixtures thereof.For example, the electrolyte may contain propylene carbonate and potassium perchlorate and / or lithium perchlorate as the conducting salt or consist of these constituents.The sensor can preferably have an electrolyte which is liquid or solid under application conditions, particularly preferably a liquid electrolyte. For example, the electrolyte may contain a liquid component and a further component at least partially dissolved therein, wherein the further component is present at least partially dissociated into ions.Application conditions are to be understood as meaning the specific temperature and pressure conditions under which the sensor is used. For example, the temperature may be in a temperature range of at least +5 °C to +80 °C, e.g. in a temperature range of -10 °C to +100 °C or of -30 °C to +200 °C, at a pressure (absolute pressure) in a range of at least 5×10 4 Pa to 3×10 5 Pa, e.g. in a range of 10 4 Pa to 10 6 Pa or of 10 3 Pa to 10 8 Pa.According to the invention, the sensor further comprises a separator permeable at least to the electroactive constituents contained in the electrically non-conductive medium, which separator is arranged so as to delimit the electrolyte chamber. The electrodes are disposed opposite the separator and are immersed in the electrolyte.In other words, at least one boundary side of the electrolyte chamber can be formed by the separating device. During use, the sensor may be arranged in or on the electrically non-conductive medium such that the separating device is at least partially in direct contact with the electrically non-conductive medium. As a result, the electroactive constituents can be diffused from the electrically non-conductive medium through the separating device into the electrolyte in the electrolyte chamber.The separating device can be, but need not be, selectively permeable to the electroactive constituents to be analyzed.The separating device can serve for the additional separation of electrolyte and electrically non-conductive medium. Such an additional separation is not necessarily necessary, however, since electrolyte and non-conductive medium separate from one another already because of their inblendability.Furthermore, the separating device can be used for separating dirt particles possibly contained in the electrically non-conductive medium, for example dirt particles contained in an engine, transmission or hydraulic oil due to abrasion. As a result, contamination of the electrolyte and thus a reduction in the functionality of the sensor can be largely avoided.By means of the separating device, it is also possible to avoid swirling of the electrolyte with the non-conductive medium or leakage of the electrolyte, since direct contact between the electrolyte and the electrically non-conductive medium is avoided. In various embodiments, the separating device can have a metal, ceramic and / or plastic filter, for example a stainless steel filter or bronze filter. The filter can be designed, for example, as a frit or membrane.In various embodiments, the separating device may have a pore size in the range between 1 μm and 50 μm, preferably between 5 μm and 20 μm, for example between 10 μm and 15 μm. For example, a stainless steel filter disk having a pore size in the range from 5 μm to 20 μm can be used as the separating device.For example, filters of filter class 03, 05, 07, 10, 15 or 20, preferably filters of filter class 05, 07 or 10, can be used (specification of the filter classes according to the definition of the manufacturer Amtag-Alfred Merchelbach Technologies AG, catalogue of the standard supply programme, 1st edition, page 6, 2007).The described separating devices can be produced easily and economically with regard to material and pore size. A complicated production of selective separating devices for the electroactive constituents to be analyzed can be avoided.According to various embodiments, the sensor may further comprise a voltage source configured to provide an electrical potential at at least one of the electrodes. For example, the voltage source can be designed as a battery or accumulator. This can enable autonomous operation of the sensor, e.g. independently of a power supply network.For example, the voltage source can be designed to provide a potential, such as is suitable for carrying out voltammetry, for example cyclovoltammetry. The voltage source can be designed as a direct voltage source, e.g. as a potentiostat, or as an alternating voltage source.According to various embodiments, the sensor may further comprise a measurement unit configured to record current and / or voltage values at at least one of the electrodes. For example, the measuring unit can be designed to record current and / or voltage curves or time current and / or voltage profiles.For example, the measuring unit can be designed to record current and / or voltage values when electrochemical analysis methods are carried out, for example voltammetry methods such as cyclovoltammetry, differential pulse voltammetry, normal pulse voltammetry, square wave voltammetry.According to various embodiments, the sensor may further comprise an evaluation unit configured to determine information about the electroactive species by means of recorded current and / or voltage values at at least one of the electrodes, e.g. on the basis of current and / or voltage values recorded by means of the described measurement unit.For example, the evaluation unit can be designed to determine the concentration of the electroactive species in the electrolyte and / or the non-conductive medium by means of recorded current and / or voltage values at at least one of the electrodes. In addition, the evaluation unit can be designed to identify the electroactive species on the basis of the recorded current and / or voltage values, e.g. to assign a chemical composition and / or chemical structure to them. For this purpose, for example, the positions of the peak maxima of a cyclovoltammogram, e.g. the potential which is to be assigned or assigned to a maximum current intensity and provided at the electrode, can be used.According to various embodiments, the sensor may further comprise a transmission unit configured to transmit data, e.g. by means of radio transmission. For example, the transmission device can be designed for transmitting current and / or voltage values to at least one of the electrodes and / or information about the electroactive constituents. Transmitting includes receiving and / or transmitting.The transmission unit can be designed for wired or wireless data transmission, e.g. for data transmission by means of a local area network (LAN) or a wireless local area network (WLAN).Transmission is advantageous in particular when the sensor is to be used on poorly accessible installations for the analysis of the electroactive constituents, for example in offshore wind turbines.According to further embodiment variants, the voltage source and / or the evaluation unit and / or the measuring unit and / or the transmission unit can be arranged on the sensor housing. For example, the voltage source and / or the measuring unit and / or the evaluation unit and / or the transmission unit can form a module together with the sensor housing.This enables a compact sensor design, i.e. the embodiment of the sensor as a compact component, and a space-saving arrangement of the sensor with respect to the non-conductive medium to be analyzed.By arranging the measuring unit and / or the evaluation unit and / or the transmission unit, influence on these units by electric and / or magnetic fields can be minimized.A device according to the invention for the analysis of electroactive components contained in an electrically non-conductive medium has a chamber for receiving the electrically non-conductive medium (medium chamber) and a sensor as described above, wherein the separating device with its associated features and the specification of the arrangement of the electrodes are optionally arranged opposite the separating device and are immersed in the electrolyte.The medium chamber can have various geometric shapes. It serves as a receptacle for receiving the electrically non-conductive medium to be analyzed. For example, the medium chamber can be designed as a pipeline.The medium chamber can have a wall with a passage into which the sensor can be inserted or is inserted. In order to ensure that no electrically non-conductive medium escapes from the medium chamber in the region of the feedthrough, the sensor can have a seal.The electrolyte chamber of the sensor is arranged below and / or next to, preferably below, the medium chamber. An arrangement below includes that the sensor can project at least partially from below into the medium chamber. The details "next to" and "below" relate to the specific installation situation of the medium chamber and sensor.It can thus be achieved that undesirable mixing of medium and electrolyte cannot occur, since the electrolyte usually has a higher density than the medium. The density of the medium may be, for example, less than 1 g / cm 3 e.g. if the medium is an oil composition.Contamination of the medium by the electrolyte can thereby be prevented. This is of importance in particular in installations in which a rapid reaction in the event of a leak is made more difficult, for example in offshore wind turbines.A method according to the invention for the analysis of electroactive constituents contained in an electrically non-conductive medium has the following steps: providing an electrically non-conductive medium containing electroactive constituents, transferring a portion of the electroactive constituents into an electrolyte immiscible with the medium, oxidizing and / or reducing the transferred electroactive constituents, recording current and / or voltage values associated with the oxidation and / or reduction, and ascertaining information about the electroactive constituents on the basis of the recorded current and / or voltage values.According to the invention, the method is carried out using the above-described sensor according to the invention. Alternatively, the method can be carried out by means of the apparatus according to the invention explained above.In this respect, the above statements also serve to describe the method according to the invention.The advantages of the method according to the invention correspond to those of the sensor according to the invention and its corresponding embodiment variants and to those of the device according to the invention.The electrically non-conductive medium can be provided, for example, in a medium chamber, while the electrolyte can be arranged in an electrolyte chamber. The electrolyte chamber can be arranged, for example, below and / or next to the medium chamber.The transfer of a portion of the electroactive constituents takes place by forming a distribution equilibrium.The oxidation and / or reduction of the transferred electroactive constituents can be effected by applying a voltage to an inert surface, i.e. a surface which is not reactive with the electrolyte or the electroactive constituents to be analyzed and is in contact with the electrolyte. For example, voltammetry may be performed.The recording of current and / or voltage values associated with the oxidation and / or reduction can comprise the recording of current-voltage curves and their temporal profile.As information, for example, the concentration of the electroactive species in the electrically non-conductive medium can be determined, e.g. from the peak area of the oxidation and / or reduction peaks in the current-voltage curves. For this purpose, calibration curves which have been produced beforehand and which indicate the dependence of the peak area on the concentration of the electroactive species can be used. For the exact determination of the concentration, the type of electroactive constituent to be analyzed, e.g., the antioxidant, is important, since the exact calibration curves that also contain the cell constant can be stored in the electronic measurement system only for calibrated substances.It is also possible to identify the electroactive species on the basis of the position of oxidation and / or reduction peaks in the current-voltage curves, i.e. to assign a chemical structure to them.Electroactive constituents may be constituents of antioxidants, for example antioxidants of the 1st group described above, or these antioxidants themselves, for example.The concentration of the electroactive species can be used to infer an ageing state of an oil, for example of a mineral oil such as turbine oil, with the result that, for example, an oil change can be planned and carried out in advance before, for example, oxidative degradation of the oil itself occurs.According to various variant embodiments, the proportion of electroactive constituents can be transferred by bringing the medium and the electrolyte into contact with one another by means of a separating device which is permeable at least to the electroactive constituents in such a way that a distribution equilibrium is established for the electroactive constituents.According to further embodiment variants, the electroactive constituents transferred can be oxidized by means of differential pulse voltammetry. The electroactive species information may then be determined from the recorded oxidation peaks.Advantageously, a better resolution of the measurement signal, i.e. of the oxidation peak, can be achieved with the differential pulse voltammetry, since a higher potential is present for a short time and a switch is subsequently made again to a lower potential. The resulting curves can therefore be evaluated more accurately, for example more accurately than would be possible with cyclovoltammetry.The invention will be explained in more detail below with reference to exemplary embodiments, reference being made to the appended figures.In the examples explained below, reference is made to the accompanying drawings, which form a part of the examples and in which are shown by way of illustration specific embodiments in which the invention may be practiced. The figures are not necessarily drawn to detail and scale, and may be exaggerated or minimized to provide a better overview. Therefore, functional details disclosed herein are not to be interpreted as limiting, but merely as an illustrative basis that will provide guidance to those skilled in the art to variously employ the present invention.It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. It should be understood that the features of the various exemplary embodiments described herein may be combined with each other, unless specifically stated otherwise. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. In the figures, identical or similar elements are provided with identical reference numerals, as appropriate.The accompanying drawings show: FIG. 1 shows a schematic sectional illustration of a device according to the invention, FIG. 2 a is a schematic external view of a sensor according to the invention, FIG. 2 b shows a schematic external view of a further sensor according to the invention, FIG. 3 shows a current-voltage curve of a method according to the invention, FIG. 4 shows the dependence of the peak area of an oxidation peak obtained by means of a method according to the invention on the concentration of BHT.FIG. 1 shows an exemplary embodiment of a device according to the invention having a sensor 1 with a sensor housing 3 made of stainless steel for the analysis of electroactive constituents contained in an electrically non-conductive medium 2. The sensor 1 has an electrolyte chamber 4, in which an electrolyte 5 that is immiscible with the medium 2 is located. In the exemplary embodiment, this electrolyte 5 consists of propylene carbonate and 0.1 M KClO 4 as conducting salt. Alternatively, the electrolyte can also consist of ethylene carbonate or butylene carbonate or a mixture of the three carbonates. Other conducting salts, such as LiClO 4, for example likewise with a concentration of 0.1 M, can also be used.The sensor 1 also has three electrodes 6 (working, reference and counter electrodes), wherein the reference electrode can optionally be omitted. In the exemplary embodiment, the electrodes 6 consist of gold.The electrodes 6 are applied to printed circuit board material (FR-4). This is the structure copper (35 μm)-nickel-gold. Nickel merely serves as an adhesion promoter between the gold and copper. Gold is applied in a layer thickness of 1 μm as standard. By means of a subsequent golding, which is arranged later in time, the stability of the electrodes 6 can be increased.The sensor 1 of the exemplary embodiment has a separating device 7 which is permeable to the electroactive constituents contained in the electrically non-conductive medium 2. The separator 7 is arranged on the electrolyte chamber 4 so as to be confined. The separating device 7 serves for separating the electrolyte 5 located in the electrolyte chamber 4 from the electrically non-conductive medium 2 located in the medium chamber 11.The device according to the invention of the exemplary embodiment further comprises a medium chamber 11 in which the electrically non-conductive medium 2 to be analyzed is located. In the exemplary embodiment, the medium 2 is a mineral oil, as is used, for example, as lubricant for turbines.The sensor 1 protrudes into the medium chamber 11 from below, so that the electrolyte chamber 4 of the sensor 1 is arranged below the medium chamber 11. Since the electrolyte 5 has a higher density than the medium 2, it is possible to prevent the electrolyte 5 from being mixed inadvertently with the medium 2. Even in the event of a leak in the electrolyte chamber 4, no electrolyte 5 can enter the medium 2.The separating device 7 has a metal filter with a filter disk with an absolute filter threshold, i.e. pore size of 14 μm (filter class 07). For example, an AMPOR-P IC 021.20.07 filter may be used. Alternatively, an AMPOR-P IC 010.20.05 filter (filter class 05) or an AMPOR-P IC 010.20.10 filter (filter class (10) are provided.The sensor 1 further comprises a measuring unit 8 configured to record current and voltage values at at least one of the electrodes 6 in the form of current-voltage curves, an evaluation unit 9 configured to determine information about the electroactive species by means of the current-voltage curves recorded by the measuring unit 8, and a transmission unit 10 configured to transmit data. The measuring unit 8, the evaluation unit 9 and the transmission unit 10 are arranged on the sensor housing 3, so that the sensor 1 is designed as a compact component.FIGS. 2 aand 2 b show external views of sensors 1 according to the invention. The sensors 1 according to FIGS. 2 aand 2 bcan be fastened to the medium chamber 11 by means of a thread 12, for example in a passage through the wall of the medium chamber 11.Whereas the sensor 1 according to FIG. 2a has an external thread G3 / 4", and is equipped with an AMPOR-P IC 021.20.07 filter, the sensor 1 according to FIG. 2b is equipped with an external thread G1 / 2", and has an AMPOR-P IC 010.20.05 or AMPOR-P IC 010.20.10 filter. The sensor 1 according to FIG. 2 bis therefore also suitable for smaller installations.In order to seal the passage of the medium chamber 11, i.e. of the sensor 1 with respect to the medium chamber 11, the sensors 1 each have a seal 13, which in the exemplary embodiment consists of teflon.To produce the sensor 1, the sensor housing 3 is first welded to the separating device 7. The electrolyte 5 is then filled into the electrolyte chamber 4 from the side of the sensor housing 3 opposite the separating device 7. The electrolyte chamber 4 is then closed with a plug which has a passage for the electrodes 6. Undesired escape of the electrolyte 5 from the electrolyte chamber 4 can be avoided due to the external air pressure.Both sensors 1 were tested on the basis of the analysis of BHT (2,6-bis(1,1-dimethylethyl)-4-methylphenol), an antioxidant of the 1st group described above, as electroactive constituent in a base oil of a mineral oil (Fuchs Alkylbenzene 100, Europe Schmiber GmbH), which would be usable, for example, as turbine oil, as non-conductive medium 2 at 23° C.For this purpose, calibration curves were first recorded, for which BHT in a concentration range of 0-4000 ppm in 0.1M KClO 4 was dissolved in propylene carbonate. The corresponding curves were recorded with the aid of differential pulse voltammetry (VersaSTAT, Ametek). The following settings were selected:Pulse height: 40 mV (tested in the range 20-200 mV)Pulse width: 0.05 s (tested in the range 0.025-0.4 s)Step height: 10 mV (tested in the range 5-50 mV)Step Size 0.5 s (tested in the range 0.06-4 s)Start (initial) potential: -0.2 VFinal potential: 1.7 V (damage to the working electrode is possible at higher final potentials)The current intensity must be recorded in the range of the voltage of the oxidation peak, in the present case in a range between 0.8 and 1.7 V.FIG. 3 shows the measurement curves obtained (current-voltage curves) for different BHT concentrations in the oil. By plotting the respective peak area of each measurement curve from FIG. 3 over the BHT concentration, the calibration curve shown in FIG. 4 can be obtained.On the basis of this calibration curve, an oil with an unknown BHT content can now be analyzed by means of the sensor 1 by recording a corresponding current-voltage curve and assigning a BHT concentration to the peak area on the basis of the calibration curve.List of reference characters1 Sensor 2 electrically non-conductive medium 3 sensor housing 4 electrolyte chamber 5 electrolyte 6 electrode 7 separating device 8 measuring unit 9 evaluation unit 10 transmission unit 11 medium chamber 12 thread 13 seal

Claims

Sensor (1) for the analysis of electroactive constituents contained in an electrically non-conductive medium (2), having: - a sensor housing (3), - an electrolyte chamber (4) arranged within the sensor housing (3), - an electrolyte (5) arranged in the electrolyte chamber (4) and immiscible with the medium (2), wherein the electrolyte (5) contains at least one carbonate selected from a group comprising ethylene carbonate, propylene carbonate and butylene carbonate and at least one conducting salt, - at least two electrodes (6) arranged in direct contact with the electrolyte (5), and - a separating device (7) permeable at least to the electroactive constituents contained in the electrically non-conductive medium (2), wherein the separating device (7) is arranged on the electrolyte chamber (4) such that the electrolyte chamber (4) is delimited, wherein the electrodes (6) are arranged opposite the separating device (7) and are immersed in the electrolyte (5).Sensor (1) according to claim 1, wherein the separating device (7) comprises a metal, ceramic and / or plastic filter.The sensor (1) according to claim 1 or 2, wherein a pore size of the separation device (7) is in the range between 1 and 50 μm.Sensor (1) according to one of the preceding claims, further comprising: - a voltage source configured to provide an electrical potential at at least one of the electrodes (6).Sensor (1) according to one of the preceding claims, further comprising: - a measuring unit (8) configured to record current and / or voltage values at at least one of the electrodes (6).Sensor (1) according to one of the preceding claims, further comprising: - an evaluation unit (9) configured to determine information about the electroactive constituents by means of recorded current and / or voltage values at at least one of the electrodes (6).Sensor (1) according to one of the preceding claims, further comprising: - a transmission unit (10) configured to transmit data.Sensor (1) according to at least one of Claims 4 to 7, wherein the voltage source and / or the measuring unit (8) and / or the evaluation unit (9) and / or the transmission unit (10) are arranged on the sensor housing (3).Device for the analysis of electroactive constituents contained in an electrically non-conductive medium (2), having: - a chamber for receiving the electrically non-conductive medium (2), referred to below as medium chamber (11), and - a sensor (1) having a sensor housing (3), an electrolyte chamber (4) arranged within the sensor housing (3), an electrolyte (5) arranged in the electrolyte chamber (4) and immiscible with the medium (2), and at least two electrodes (6) arranged in direct contact with the electrolyte (5), wherein the electrolyte (5) contains at least one carbonate selected from a group comprising ethylene carbonate, propylene carbonate and butylene carbonate and at least one conducting salt, wherein the electrolyte chamber (4) of the sensor (1) is arranged below and / or next to the medium chamber (11).Method for the analysis of electroactive constituents contained in an electrically non-conductive medium (2) by means of a sensor (1) according to one of Claims 1 to 8, having: - provision of the electrically non-conductive medium (2) containing electroactive constituents, - transfer of a proportion of the electroactive constituents into the electrolyte (5) which is immiscible with the medium, - oxidation and / or reduction of the transferred electroactive constituents, - recording current and / or voltage values associated with the oxidation and / or reduction, - determination of information about the electroactive constituents on the basis of the recorded current and / or voltage values.Method for the analysis of electroactive constituents contained in an electrically non-conductive medium (2) by means of a device according to Claim 9, having: - provision of the electrically non-conductive medium (2) containing electroactive constituents, - transfer of a fraction of the electroactive constituents into the electrolyte (5) immiscible with the medium, - oxidation and / or reduction of the transferred electroactive constituents, - recording current and / or voltage values associated with the oxidation and / or reduction, - determination of information about the electroactive constituents on the basis of the recorded current and / or voltage values.The method of claim 10 or 11, wherein the transferred electroactive constituents are oxidized by differential pulse voltammetry.

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