Measuring arrangement for measuring the total nitrogen bound in a measuring liquid

Disclosed is a measuring arrangement for measuring the total nitrogen bound (TN) in a measuring liquid, comprising: a radiation source emitting UV radiation; a radiation receiver configured to generate a signal that depends on the intensity of radiation impinging on the radiation receiver; a vessel having a first opening and a second opening opposite the first opening; a first window closing the first opening; and a second window closing the second opening. The first and second windows are transparent to the measuring radiation. The measuring radiation emitted by the radiation source propagates along a measuring path which extends from the radiation source through the first window, the vessel, and the second window to the radiation receiver. The measuring arrangement also includes a heating element in thermal contact with the vessel wall.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims the priority benefit of German Patent Application No. 10 2019 122 163.3, filed on Aug. 19, 2019, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a measuring arrangement for measuring the total nitrogen (TN) bound in a measuring liquid.

BACKGROUND

Wet-chemical analyzers are used, among other things, for determining ingredients of a measuring liquid in process measuring technology, in environment measuring technology and in the laboratory. These can be used to determine the concentration of individual substances, so-called analytes, in the liquid sample, e.g. ammonium, nitrate, phosphate. Such analyzers are often designed to add one or more reagents to the liquid sample in an automated manner, so that a chemical reaction with the analyte takes place in the liquid sample. The reagents are typically selected such that the chemical reaction is verifiable by physical methods, e.g., by optical measurements, using potentiometric or amperometric sensors, or by a conductivity measurement. The analyzer performs the corresponding measurement in the sample and ascertains the analyte concentration to be determined in the liquid sample based on the detected measurement signals. The chemical reaction may, for example, cause a coloring or a change of color which can be detected using optical means. In such cases, the intensity of the color is a measure of the measurand to be determined. The measurand may, for example, be ascertained photometrically by the analyzer by feeding electromagnetic radiation, such as visible light, from a radiation source into the liquid sample, and receiving it with a suitable receiver after transmission through the liquid sample. The receiver generates a measurement signal that depends on the intensity of the radiation received and which can be used to derive the measurand.

For monitoring the quality of water samples, so-called sum parameters are used, among other things, which are frequently detected as a measure of the nutrient content and/or the content of organically and inorganically bound carbon. Such parameters are, for example, the total carbon bound (TC), the total organic carbon (TOC), the chemical oxygen demand (COD), total nitrogen (TN or TNb) or total phosphorus (TP). Analyzers for determining these sum parameters frequently carry out a thermal or chemical disintegration of the liquid sample, with which the ingredients considered in the sum parameter are oxidized.

For example, an analyzer for determining the chemical oxygen demand of a liquid sample using a digestion reactor for chemically disintegrating a substance contained in a liquid sample is known from DE 10 2013 114 132 A1. The digestion reactor can be heated for digestion and is formed from a glass that is optically transparent to a photometric measurement following the digestion.

The total nitrogen (TN) is a sum parameter which is a measure of the proportion of nitrogen present in the sample, both in organic compounds, such as urea, peptides or proteins, and in inorganic compounds, such as ammonium, ammonia, nitrite or nitrate. Various methods exist for determining the total nitrogen bound in liquid samples. According to one of these methods, all nitrogen-containing compounds are oxidized to form nitrate, by means of a digestion using a strong oxidizing agent, for example peroxodisulfate, in alkaline solution at high temperatures, the solution is then cooled and neutralized, and optionally diluted, and the nitrate content of the solution is then photometrically determined. The wavelengths used for the photometric determination of the nitrate content are in the UVC range of the electromagnetic spectrum. This method is described, for example, in the Chinese Standard HJ 636-2012.

The digestion takes place at as high a temperature as possible in order to ensure that the bound nitrogen is completely converted into nitrate. Under excess pressure, temperatures of more than 100° C. can be reached. In order to ensure the optimal temperature control of the reaction, the digestion vessel should be made of an easily thermally conductive material and should be closable in a pressure-resistant manner.

Many materials commonly used in analyzers are not resistant to alkaline media at these temperatures. While the digestion reactor made of glass known from DE 10 2013 114 132 A1 is ideally suited for the digestion of a liquid sample with potassium dichromate in an acid solution described there in order to determine the chemical oxygen demand of the liquid sample, it is not possible to determine the total nitrogen according to the method described above using this reactor. While there are borosilicate or quartz glasses that are sufficiently resistant to hot alkaline solutions, they are not transparent to radiation in the UVC wavelength range. Other quartz glasses transparent to UVC radiation are not sufficiently resistant to hot alkaline solutions.

According to the current state of the art, analyzers for the total nitrogen determination therefore comprise different containers for the digestion and the photometric measurement. The digestion vessel is formed from a special, chemically resistant material, while the measuring cell is formed from a material optimized for photometric measurement. After the digestion has been carried out, and the subsequent cooling and neutralization, the liquid sample is transported from the digestion vessel into the measuring cell for the photometric measurement.

A disadvantage of such a solution is the larger number of components of the analyzer required as a result. This can lead to a more complex design and a higher maintenance effort compared to an analyzer comprising a single receptacle for digestion and measurement. The measurement duration is also prolonged by the transport of the liquid sample after digestion into another vessel for the measurement. In order to avoid entrainment effects, longer purge phases have to be factored in between the individual measurements when using separate vessels for the digestion and the photometric measurement, which additionally increases the overall measurement duration. Both the higher maintenance effort and the increase in the measuring duration ultimately lead to higher operating costs.

SUMMARY

The object of the present disclosure is therefore to provide a measuring arrangement for an automatic analysis device for determining the total nitrogen content of a liquid sample which avoids these disadvantages. In particular, an integration of an alkaline digestion and a subsequent photometric measurement in a single vessel is desirable.

The object is achieved according to the present disclosure by the measuring arrangement according to claim1. Further advantageous embodiments are provided in the dependent claims.

The measuring arrangement according to the present disclosure for measuring the total nitrogen bound (TN) in a measuring liquid comprises:a radiation source, which is designed to emit at least measuring radiation of a wavelength or wavelength range in the UV wavelength range, in particular the UVC wavelength range;a radiation receiver, which is configured to generate a signal that depends on the intensity of measuring radiation impinging on the radiation receiver;a vessel comprising a vessel wall having at least one first opening and at least one second opening located opposite the first opening, a first window closing the first opening, in particular in a pressure-tight manner, and a second window closing the second opening, in particular in a pressure-tight manner, wherein the first and second windows are transparent to the measuring radiation;

wherein the radiation source and the radiation receiver are arranged with respect to the vessel such that at least a portion of the measuring radiation emitted by the radiation source propagates along a measuring path which extends from the radiation source through the first window, the vessel and the second window to the radiation receiver; anda heating element that is in thermal contact with the vessel wall.

Since the vessel comprises a vessel wall that has openings closed by windows, it can serve both as a reactor for a digestion of the nitrogen-containing components of the liquid sample in alkaline solution at high temperatures and as a measuring cell for the photometric determination of the concentration of nitrate formed by the digestion. More expensive materials, which are more difficult to process compared to conventional silicate glasses and which are transparent to UVC radiation, for example crystalline materials such as sapphire or magnesium fluoride, can be selected for the windows. Since the material for the vessel wall in the measuring arrangement according to the present disclosure does not have to be transparent to UVC radiation, a selection of suitable materials that are resistant to hot alkaline media and easily thermally conductive is obtained.

The measuring radiation in the UVC wavelength range may encompass one or more wavelengths between 200 nm and 300 nm. The measuring radiation for the determination of the total nitrogen bound preferably encompasses at least the wavelengths 220 nm and 275 nm.

In an advantageous embodiment, the vessel can comprise, on the outer side thereof, a plurality of cooling elements, for example cooling ribs, cooling vanes or cooling fins. For this purpose, the vessel wall can, on the outer side thereof, comprise a plurality of cooling elements or be in thermally conducting contact with a plurality of cooling elements. The cooling elements may be part of a heat sink that is in thermally conductive contact with the vessel wall.

This embodiment can advantageously be used for analysis methods such as the method for total nitrogen determination, with which a reaction mixture is produced from a liquid sample to be analyzed and an added reagent, the reaction mixture being heated to accelerate the chemical reaction and subsequently being cooled to a defined temperature for a measurement, for example an absorption measurement, by means of the radiation source and the radiation receiver.

The use of the cooling elements significantly reduces the time required to cool the reaction mixture.

The cooling elements and the vessel wall may be formed from the same material. In one possible embodiment, the cooling elements can be formed integrally with the vessel wall. In another embodiment, the cooling elements may be formed from a different material than the vessel wall. In this case, the vessel wall can be formed from a material that is optimized with respect to the chemical resistance thereof, while the cooling elements are formed from a material having optimized thermal conductivity and thermal capacity. The cooling elements can be connected to the vessel wall by means of fastening means, for example screws or rivets or a thermally conductive adhesive. In order to ensure good heat transfer between the vessel wall and the cooling elements, a heat-conducting layer, for example made of a heat conductive paste, can be arranged between the vessel wall and the cooling elements.

In a further embodiment, at least a section of the vessel is surrounded by thermal insulation.

The thermal insulation can be formed by a thermally insulating attachment, which surrounds a section of the vessel comprising cooling elements, in such a way that the cooling elements project into a space arranged between the vessel wall of the vessel and a wall of the attachment, wherein an opening is formed in the wall of the attachment, or between the wall of the attachment and the vessel, and leads into a lower region of the space surrounding the cooling elements.

The attachment may, for example, have the shape of a housing surrounding the vessel. The housing can be designed as a hood having one side open to the surrounding area. Such a hood may, for example, be placed on the vessel from above so that the open side thereof faces downwardly and surrounds at least a section of the vessel. The open side of the hood forms the aforementioned opening leading into the space surrounding the cooling elements.

In an advantageous embodiment, the vessel is arranged completely within the insulating attachment, in particular in such a way that the attachment protrudes markedly beyond the vessel toward the bottom. In all embodiments described here, the thermal insulation counteracts heat dissipation from the vessel during heating of the vessel and advantageously prevents, in particular hot air from escaping toward the top. The use of the terms “top” and “bottom” here and below refer to the orientation of the vessel and of the attachment intended during operation of the measuring arrangement.

As already mentioned, the cooling elements increase the efficiency of heat dissipation from the vessel during cooling, so that the reaction mixture present in the vessel can be cooled more quickly to a target temperature. On the other hand, the cooling elements increase the thermal mass of the overall device formed from the vessel and cooling elements and thus effect an extension of the heating time required for heating the reaction mixture to an elevated temperature. This can be counteracted by a suitable control of the heating power of the heating element. In addition, the thermally insulating attachment impedes heat transport by convection and thus contributes to the acceleration of the temperature increase of the vessel in heating phases.

In an advantageous refinement of this embodiment, the measuring arrangement can include a ventilation system comprising one or more fans, which is configured to generate a flow of air flowing around the cooling elements. For example, the one or more fans may be arranged at the opening leading into the space surrounding the cooling elements so as to transport air from the surrounding area into the space. This results in a flow of air flowing around the cooling elements in the attachment, by way of which heat is dissipated from the cooling elements. The ventilation system therefore makes it possible, even if the thermally insulating attachment is present during cooling phases, to effectively dissipate the heat from the cooling elements, and thereby accelerate the process of reaching a target temperature of the vessel during cooling phases.

In an advantageous embodiment, the wall of the attachment and the vessel wall of the vessel enclose an essentially U-shaped channel, which extends around the vessel and includes a first section extending upwardly parallel to a side wall of the vessel, a second section extending around an upper end of the vessel, and a third section extending downwardly parallel to a side wall of the vessel,

wherein the opening forms an inlet opening at one end of the channel, and wherein the channel has an outlet opening at the other end thereof. In this embodiment, the ventilation system is arranged at the inlet opening. During operation of the ventilation system, in this embodiment a gas flow forms from the inlet opening through the channel in the direction of the outlet opening, which flows around the cooling elements and thus dissipates heat from the cooling elements.

The inlet opening and the outlet opening are advantageously arranged in the lower region of the attachment, ideally at the lowest point of the attachment. Advantageously, the outlet opening and the inlet opening are arranged essentially at the same height in order to avoid stack effects.

The measuring device can comprise an electronic control unit, in particular for carrying out analyses in an automated manner. The ventilation system and the heating element may be connected to the control unit. The control unit may be configured to control the heating element and the ventilation system in accordance with a predefined operating program. Advantageously, the control unit is designed to switch off the ventilation system during heating phases, in which the heating element is operated for heating the vessel, and to switch on the ventilation system during cooling phases, in which the heating element is shut off. In this way, both heating times and cooling times of the vessel can be optimally reduced.

The vessel can include at least one liquid inlet, which can be fluidically connected to a sample receiver containing the measuring liquid. A liquid sample, i.e. a predefined volume of the measurement liquid to be analyzed, can be introduced into the vessel via the liquid inlet. In addition to the liquid inlet, the vessel may include at least one pressure equalization opening. This enables pressure equalization when liquid is introduced into the vessel or when liquid is discharged from the vessel via the at least one liquid inlet.

The measuring arrangement can furthermore include a liquid container containing a digestion reagent having a pH of at least 12, wherein the liquid container can be fluidically connected to the liquid inlet. The liquid container can be connected to the liquid inlet of the vessel via a fluid line. A valve can be arranged in the fluid line, which selectively blocks or allows a transport of digestion reagent from the liquid container into the vessel. The digestion reagent can be a solution comprising one or more substances that chemically react with the nitrogen-containing compounds in the liquid sample. In the case of the above-described method according to the standard HJ 636-2012, the digestion reagent contains a strong oxidizing agent, such as peroxodisulfate, which oxidizes nitrogen-containing compounds present in the liquid sample in alkaline solution to form nitrate. The measuring arrangement may comprise further liquid containers, which can be fluidically connected to the liquid inlet of the vessel. These can contain further reagents required for the digestion or the detection of the nitrate formed by the digestion, standard solutions for calibration measurements, diluents or cleaning liquids.

The vessel wall can comprise a metal, a metal alloy, a ceramic or a high-performance plastic. Advantageously, the metal, the metal alloy, the ceramic or the high-performance plastic is not chemically attacked by the digestion reagent, which can have a pH value between 12 and 14, at a temperature of up to 130° C., in particular between 0 and 130° C. Machining processes, which ensure high accuracy and reproducibility, can be used for the processing of metals or metal alloys. High-performance plastics are, for example, thermoplastics, which have improved chemical resistance and temperature resistance compared to standard engineering plastics. It is also possible for the vessel wall to be made of a combination of the aforementioned materials, for example of a metal-coated ceramic or a plastic-coated or ceramic-coated metal, or a metal or a metal alloy comprising one or more metal and/or ceramic and/or plastic coatings.

If the vessel wall comprises a metal or a metal alloy, it may comprise, for example, titanium, gold, stainless steel (for example V4A steel) or Hastelloy. As mentioned, the vessel wall can be formed from several materials, for example by comprising a base material and a coating applied to the base material or several coatings applied to the base material. The vessel wall can also be processed by way of a surface treatment, for example by mechanically generated polishing, electropolishing or anodization.

In an advantageous embodiment, the first and the second windows can include, at least in a region of the windows facing the interior of the vessel, a material that is transparent to the measuring radiation, in particular in the UVC wavelength range, and that is chemically stable with respect to hot liquids up to 130° C. that have an alkaline pH value, in particular a pH value between 12 and 14. This material may be one of the materials sapphire, diamond, magnesium fluoride, calcium fluoride or barium fluoride. The windows can consist entirely of one of these materials. Alternatively, the windows can comprise a base body, which is made of a material that is transparent to UVC radiation, but is not stable with respect to hot alkaline media up to 130° C., and which, at least on the surface thereof facing the interior of the vessel, includes a coating made of a material that is stable against such media and, at the same time, is transparent to UVC radiation. This coating can, for example, be made of the aforementioned materials of sapphire, diamond, magnesium fluoride, calcium fluoride or barium fluoride. In this case, the base body can be formed from quartz glass.

The first and the second windows can each have two plane-parallel surfaces extending essentially perpendicularly to the radiation path.

Alternatively, the first and/or the second windows can each have at least one curved surface, in particular an optically effective surface for the beam shaping of the measuring radiation. The curved surface may be formed as a convex or concave, spherical or aspherical surface.

The measuring arrangement can furthermore comprise:a first liquid path extending from the sample receiver to the at least one liquid inlet;a second liquid path extending from the liquid container to the at least one liquid inlet;at least one metering unit, which comprises at least one pump and/or at least one valve arranged in the first and/or second liquid paths and is configured to transport a predefined amount of liquid along the first liquid path into the vessel, and is furthermore configured to transport a predefined amount of liquid from the liquid container along the second liquid path into the vessel; andan electronic control unit, which is configured to control the metering unit to transport measuring liquid and digestion reagent into the vessel, to control the heating element for controlling a temperature of liquid present in the vessel, and to excite the radiation source to emit the measuring radiation and detect and process signals of the radiation receiver.

Advantageously, the measuring arrangement can form an automatic analyzer or be an integral part of an automatic analyzer. The analyzer can comprise a housing designed as a cabinet, in which the vessel serving as a digestion reactor and measuring cell is arranged. In addition to the digestion reactor, other parts of the measuring arrangement, for example liquid vessels, fluid lines, pumps, valves and the electronic control unit, can also be arranged in the housing. An input unit, which can, for example, comprise keys and a display or a touch screen, can be used to operate the electronic control unit.

The electronic control unit can comprise an evaluation program, which it can execute to determine values of the total nitrogen bound of the measuring liquid from the signals of the radiation receiver.

DETAILED DESCRIPTION

FIG.1schematically shows, in a longitudinal sectional view, a measuring arrangement1for determining the total nitrogen content of a liquid sample. The measuring arrangement1comprises a vessel2, which in the present example is essentially cylindrical and into which a fluid line3and a pressure equalization line4lead. The fluid line3and the pressure equalization line4each comprise a valve5by means of which they can be selectively blocked or released. If both lines are blocked by the valves5, the vessel2is closed with respect to the surrounding atmosphere in a pressure-tight manner.

The vessel2comprises a housing wall6, which encloses an interior and, in the present example, is formed from a metal or a metal alloy, for example of titanium, gold or stainless steel. The housing wall6has a first opening7and a second opening8located opposite the first opening7. A first window9is inserted into the first opening7, and a second window10is inserted into the second opening8. The windows9and10are made of a material that is transparent to radiation in the UVC wavelength range, in particular to radiation of the wavelengths between 220 nm and 280 nm. In the present example, the windows9and10are made of sapphire. The windows9and10are sealed in a pressure-tight manner with respect to the housing wall6, for example by means of O-ring seals, so that even in the event that the liquid level, as shown inFIG.1, is above the windows9,10, or an overpressure is present in the interior of the vessel2, no liquid escapes from the vessel2to the outside. In the present example, the windows9,10each have two mutually opposing plane-parallel surfaces. In an alternative embodiment, they can also have curved surfaces, for example they can be designed as lenses.

The measuring arrangement1furthermore comprises a radiation source11and a radiation receiver12which are arranged opposite one another in relation to the openings7and8in the housing wall6of the vessel2in such a way that measuring radiation emitted by the radiation source11propagates along a measuring path extending between the radiation source11and the radiation receiver12. The measuring path extends through the first window9, the vessel inner and the second window10in the process. Measuring radiation propagating along the measuring path thus interacts with the liquid present in the vessel2and is absorbed by the analyte, which may be present in the liquid. In the present example, a UV flash lamp serves as the radiation source11. One or more Si photodiodes for detecting UV radiation are used as the radiation receiver12. In the present example, the radiation receiver12is configured to detect radiation of individual wavelengths, for example 220 nm and 275 nm. For this purpose, a filter and/or beam splitter device can be provided in a manner known to the person skilled in the art, which makes it possible to detect the radiation of selected wavelengths, or selected wavelength ranges, using individual photodiodes or other suitable detection elements.

For setting a temperature in the vessel2, the measuring arrangement1comprises a heating element13, which in the present example comprises a heating wire that is electrically insulated with respect to the metallic housing wall6. The heating wire extends helically around the housing wall6. In the present example, a temperature control system is provided, which comprises a temperature sensor14detecting the temperature in the interior of the vessel2and a controller15that is configured, based on the signals of the temperature sensor14, to set a heating power of the heating element13in such a way that a predefined target temperature of the interior of the vessel or of the liquid present in the vessel2is reached. The introduced heating power can additionally be controlled in such a way that the target temperature is reached at a predefined point in time.

FIG.2schematically shows an alternative embodiment of the vessel20and the heating element13. Components designed identically to the corresponding components of the measuring arrangement1shown inFIG.1are denoted by the same reference numerals as inFIG.1. The vessel20comprises a housing wall6in which windows9and10are arranged in openings. The windows9,10are made of a material that is transparent to UVC radiation, for example MgF2. A fluid line3, which can be blocked by means of a valve5, and a pressure equalization line4, which can also be blocked by means of a valve5, lead into the vessel20. In the present example, the housing wall6is made of a metal alloy, for example Hastelloy. In contrast to the exemplary embodiment illustrated inFIG.1, a heating resistor16attached to the outer side of the housing wall6serves as the heating element for controlling the temperature of a liquid present in the interior of the vessel20in the present exemplary embodiment. This heating resistor can be connected to a temperature regulator (not shown inFIG.2). The temperature regulator can furthermore be connected to a temperature sensor, which is likewise not shown inFIG.2and which detects measured temperature values representative of the temperature of the liquid present in the vessel20and outputs it to the temperature regulator. The temperature regulator can be designed to adjust a heating power of the heating resistor16based on the measured temperature values in such a way that a desired target temperature of the liquid is reached and maintained for a predefinable period of time.

FIG.3schematically shows a measuring arrangement100for measuring a total nitrogen content of a sample. The measuring arrangement100forms an analyzer operating in a completely automated manner.

Components that can be designed identically to the corresponding components of the measuring arrangement1shown inFIG.1or of the vessel20shown inFIG.2are denoted by identical reference numerals as inFIGS.1and2.

The measuring arrangement100comprises a vessel2into which a fluid line3and a pressure equalization line4lead. The vessel2includes a housing wall6made of a metal alloy, in which two mutually opposing windows made of sapphire are inserted in a pressure-tight manner (not shown inFIG.3). A heating wire extending spirally around the outer wall of the vessel2is used for controlling the temperature of a liquid accommodated in the interior of the vessel2. A radiation source11and a radiation receiver12are arranged outside the vessel2. The radiation source11is configured to emit measuring radiation of one or more predefined wavelengths in the UVC range of the electromagnetic spectrum. For example, the radiation source11may be a flash lamp. The radiation receiver12is configured to receive the measuring radiation and convert it into an electrical measurement signal. For example, it may comprise one or more photodiodes. The radiation source11and the radiation receiver12are arranged with respect to the windows in the housing wall6of the vessel2in such a way that measuring radiation enters the interior of the vessel2through one of the windows, passes through the interior of the vessel, and thus also through a liquid accommodated in the vessel, and exits through the other window and impinges on the radiation receiver12.

The fluid line3leading into the vessel2is fluidically connected via a valve assembly17to a sample receiver18and a plurality of liquid containers19,20,21,22,23,24. The sample receiver18may be a vessel containing a larger quantity of a sample liquid taken from a body of water, a basin, or a process container, such as a reactor or a liquid line of a process plant. A liquid sample of a certain volume can be taken from the sample receiver18for analysis. It is also possible for the measuring arrangement100to be configured to take the liquid sample directly from a body of water, a basin or a process container.

The liquid container19contains a digestion reagent, which is to be added to the liquid sample in order to convert all nitrogen that is present in the sample and bound in chemical compounds into nitrate. The digestion reagent can, for example, be an alkaline solution of a strong oxidizing agent, for example peroxodisulfate.

The liquid container20contains another reagent to be added to the liquid sample after digestion, for example an acid used to neutralize the mixture of the sample liquid and the digestion reagent.

The liquid containers21and22contain a standard solution for calibration measurements. The standard solutions may be zero standards, i. e. solutions free of nitrogen-containing compounds, and/or solutions containing a particular predefined proportion of nitrogen bound in compounds.

The liquid container23contains a diluting solution, i.e. a solution which is free of nitrogen-containing compounds. This solution can optionally be added to the liquid sample.

The liquid container24serves as a collection container for consumed liquids.

In the present example, the measuring arrangement100comprises a peristaltic pump25for transporting liquid from the sample receiver18or the liquid containers19to24into the vessel2. The peristaltic pump25is arranged in a fluid line connecting the fluid line3leading into the vessel2to the valve assembly17. Via the valve assembly17and various fluid lines, each connected to one of the liquid containers19to24and the sample receiver18, the peristaltic pump15and the vessel2can be connected to the liquid containers19to24and to the sample receiver18so as to meter liquids into the vessel2and/or to discharge liquid from the vessel2into the collection container24. The peristaltic pump25, the fluid lines, the valve assembly17and the valves5form a metering unit of the measuring arrangement100which is used to transport and meter the liquids to be used for the measurement and for calibration measurements.

In the present example, a combination of a single peristaltic pump with multiple valves and a valve assembly is used to transport and meter the fluids. A plurality of variants are possible, which achieve the same purpose. For example, multiple pumps can be provided, which are used to transport different liquids in each case. Accordingly, the number of valves is reduced. Instead of one or more peristaltic pumps, other pumps, for example, diaphragm pumps or piston pumps, can be used.

In order to operate the measuring arrangement100in a completely automated manner for determining measured values of the total nitrogen content, the measuring arrangement comprises an electronic control unit26, which is designed as a computer, as a measurement transmitter, as a memory-programmable logic controller or as another data processing device that can be used for data processing and process control. The control unit26is connected to the heating wire, a fan27and a temperature sensor (not shown inFIG.3) arranged in the vessel2in order to automatically regulate the temperature of a liquid present in the vessel2based on a predefined operating program. The control unit26is also connected to the radiation source11and the radiation receiver12, in order to control the radiation source for the emission of measuring radiation, and to receive and further process signals of the radiation receiver12according to an evaluation program executable by the control unit26, so as to ascertain measured values of the total nitrogen content of the liquid sample based on the signals of the radiation receiver12.

The control unit26is moreover connected to the valves5and the valve device7as well as to the pump25so as to carry out a digestion of the liquid sample as well as, if necessary, a subsequent neutralization and/or dilution of the solution formed as a result of the digestion, intermittent calibration measurements and possibly rinsing steps in order to avoid entrainment between individual analysis cycles, according to a sequence predefined by the operating program.

A determination of the total nitrogen content of a liquid by means of the measuring arrangements illustrated inFIGS.1to3can take place in the following way: In a first step, a certain volume of a liquid is transported as a liquid sample from the sample receiver18into the vessel2. In a second step, depending on the measuring range, the sample is diluted with a defined quantity of diluting liquid from the container23, or left undiluted in vessel2. In a third step, a predefined amount of the digestion reagent, an alkaline solution of peroxodisulfate in the present example, is transported from the liquid container19into the vessel2, and the reaction mixture thus formed in the vessel2is heated by means of the heating element13with the vessel2closed in a pressure-tight manner (valves5closed). Under pressure, temperatures up to 120° C. can be achieved in the vessel2. This temperature is maintained for a period of 20 minutes to one hour. Ideally, the total nitrogen bound in the liquid sample is converted into nitrate by the alkaline digestion. Thereafter, the reaction mixture is diluted in the vessel2, and neutralized, by adding the acid from container20.

The solution thus obtained is cooled to a target temperature, and a photometric measurement for ascertaining the nitrate content is carried out at the target temperature. The photometric measurement comprises irradiating measuring radiation of wavelengths 220 nm and 275 nm into the reaction mixture, and detecting the measuring radiation after passing through the reaction mixture by means of the radiation receiver12. Radiation of the wavelength 220 nm is absorbed by nitrate, so that the transmission or absorption of radiation of this wavelength is a measure of the nitrate content of the liquid sample. The second wavelength 275 nm is used to correct influences of interfering substances and the turbidity of the liquid sample.

The nitrate content correlates with the total nitrogen content of the liquid sample, so that a value of the parameter TN can be ascertained from the measurement signals of the photometric measurement based on an assignment rule (e.g. table or calibration function) ascertainable by calibration. Based on this relationship, the electronic control unit25ascertains a value for the total nitrogen content of the liquid sample from the measurement signals of the radiation receiver12.

All these steps are carried out completely automatically by means of the electronic control unit26.

FIGS.4,5and6schematically show an arrangement comprising a vessel2and an attachment31for use in a measuring arrangement according to a third exemplary embodiment, which can otherwise be designed identically to the measuring arrangement100described based onFIG.3.FIG.5shows the vessel2in a view from above, andFIG.4shows it in a sectional view along the sectional plane A-A shown inFIG.5.FIG.6shows the vessel2in a side view in a direction perpendicular to the viewing direction shown inFIG.5.

In the exemplary embodiment shown inFIGS.4,5and6, the vessel2has an essentially cuboid or cubic shape. The vessel may be formed from a metal or a metal alloy, in the present example the vessel2is made of titanium. The vessel wall can also consist of multiple components, wherein a surface intended for contact with a reaction mixture accommodated in the vessel2is formed from a chemically stable material, for example titanium, gold, a ceramic or a high-performance plastic. Fluid lines3and4lead into the vessel2, which, as in the exemplary embodiments described above, are used to introduce fluids into the vessel2and to discharge fluids from the vessel2. Openings9are provided on two mutually opposing sides of the vessel, in which mutually opposing windows made of sapphire are inserted in a pressure-tight manner.

The vessel2furthermore comprises a heating element, for example a heating resistor or a heating wire, which is not shown inFIGS.4to6for the sake of improved clarity. Cooling elements30are arranged on two further mutually opposing sides of the vessel2. In the present example, the cooling elements30are designed in the form of individual cooling ribs and are formed from a different material than the vessel2, namely aluminum or another metal having the best possible thermal conductivity and the lowest possible thermal capacity. They may be formed from a combination of multiple materials. The heat sinks30are connected to the vessel wall of the vessel2via connecting means, for example screws. To improve the heat-conducting contact between the vessel wall and the cooling elements30, a heat-conductive paste applied at the joint may be used.

In the exemplary embodiment shown inFIGS.4,5and6, the vessel2is surrounded by an attachment31, which is open on one side. InFIG.5, the attachment31is only hinted at by dashed lines. The attachment31has an open side, which serves as an opening35leading into the space34surrounding the cooling elements30for supplying air from the surrounding area to the cooling elements30. To this end, a fan32is arranged at the opening35of the attachment31and is configured to generate an air flow flowing around the cooling elements30so as to dissipate heat from the cooling elements30. The air flow through the attachment31when the fan32is running is illustrated inFIG.4by arrows.

On two mutually opposing sides, the attachment31has a recess33(FIG.6) into which housings of the light source11and of the photoreceiver12of the measuring arrangement fit. The attachment31can thus be placed over the vessel2in the manner of a hood. Tubes3and4, via which fluids can be transported into the vessel2and out of the vessel2, can be led out of the open side of the attachment31, as shown inFIG.6.

The heat dissipation from the vessel2during cooling of the reaction mixture can be carried out more efficiently by means of the heat sinks30, which accelerates the cooling of a reaction mixture present in the vessel2to a target temperature. When the target temperature is reached more quickly, the above-described photometric measurement can be performed earlier, and thus the time required for a measuring cycle can be shortened. On the other hand, although the cooling elements30increase the thermal mass of the overall device to be heated, an acceptable heating time for the reaction mixture can be achieved, despite the additional thermal mass, by suitable control of the heating power, even if no additional measures are taken.

The attachment31surrounding the vessel2in the exemplary embodiment shown here is used to minimize the required heating power by retaining warm air in the upper, closed region of the attachment31during heating phases when the fan32is switched off. Heat loss via the heat sinks30during the heating phase is thus counteracted. So as to amplify this effect, the attachment31is advantageously made of a thermally insulating material, for example of a plastic. Additionally or alternatively, the attachment31may comprise an insulating material, for example a thermally insulating foam plastic.

Using the very simple measures described here, the measuring cycle time, in particular that for heating and cooling the reaction mixture made of the sample and the reagents, can be effectively shortened, without the need for complex active cooling measures, e.g. the use of fluid cooling, heat exchangers or Peltier elements. This can be applied particularly advantageously in the above-described measuring arrangement100for measuring the total nitrogen bound in a measuring liquid, which is configured to carry out the digestion of the liquid sample, by adding an oxidizing agent and heating the reaction mixture thus formed, and the subsequent photometric measurement at a defined target temperature in the range of room temperature in one and the same vessel.

If the arrangement shown inFIGS.4to6is used in a measuring arrangement such as the measuring arrangement100illustrated inFIG.3for carrying out the above-described method for determining the total nitrogen content of a liquid, the heating of the reaction mixture by way of the heating element comprises controlling the heating power of the heating element when the fan32is switched off, so as to set a rapid heating rate and thereafter a constant temperature of the reaction mixture for digestion of the nitrogen-containing compounds. The cooling comprises switching on the fan32when the heating element is switched off so as to reach the target temperature for the photometric measurement with a rapid cooling rate. This can be carried out in an automated manner by the control unit26of the measuring arrangement100.

FIGS.7and8schematically show an arrangement comprising a vessel2and an attachment41for use in a measuring arrangement according to a fourth exemplary embodiment, which can otherwise be designed identically to the measuring arrangement100described based onFIG.3.FIG.8shows the vessel2in a view from above, andFIG.7shows it in a sectional view along the sectional plane A-A illustrated inFIG.8.

The vessel2comprising cooling elements30has essentially the same design as the vessel2of the third exemplary embodiment illustrated inFIGS.4to6. Identically configured parts are denoted by identical reference numerals. The attachment41, which accommodates the regions of the vessel2provided with cooling elements30, is placed over the vessel2in the exemplary embodiment shown here. A space44surrounding the cooling elements30is formed between the wall48of the attachment41and the vessel wall47of the vessel2. This space44has the shape of a U-shaped channel having an inlet opening45and an outlet opening46. A fan32is arranged at the inlet opening45, which is configured to generate a gas flow through the channel from the inlet opening45to the outlet opening46(direction of arrow inFIG.7). In the flow direction, proceeding from the inlet opening45, the channel includes a first section49extending upwardly parallel to a first side wall of the vessel2. Furthermore, downstream of the first section, the channel includes a second section50extending around the vessel2at the upper end of the vessel2. Downstream, a third section51adjoins the second section50and extends downwardly to the outlet opening46, parallel to a side wall located opposite the first side wall of the housing2. Air or gas transported through the U-shaped channel thus flows along the cooling elements30around the vessel2. The arrangement according to this fourth exemplary embodiment otherwise has the same functions and advantages as the arrangement according to the third exemplary embodiment described based onFIGS.4to6. It can also be used in a measuring arrangement such as the measuring arrangement100described in connection withFIG.3in order to optimize heating and cooling times.