EP0307424A1 - Process for cleaning the indicator electrode of a titrator for the determination of water content by the k. fischer method and device for carrying out the process - Google Patents

Process for cleaning the indicator electrode of a titrator for the determination of water content by the k. fischer method and device for carrying out the process

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Publication number
EP0307424A1
EP0307424A1 EP19880902117 EP88902117A EP0307424A1 EP 0307424 A1 EP0307424 A1 EP 0307424A1 EP 19880902117 EP19880902117 EP 19880902117 EP 88902117 A EP88902117 A EP 88902117A EP 0307424 A1 EP0307424 A1 EP 0307424A1
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EP
European Patent Office
Prior art keywords
electrode
voltage
cleaning method
indicator electrode
cleaning
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19880902117
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German (de)
French (fr)
Inventor
Heinrich Maurer
Peter Wuhrmann
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Tecan Schweiz AG
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Tecan AG
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Filing date
Publication date
Application filed by Tecan AG filed Critical Tecan AG
Publication of EP0307424A1 publication Critical patent/EP0307424A1/en
Withdrawn legal-status Critical Current

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/28—Electrolytic cell components
    • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
    • G01N27/38—Cleaning of electrodes

Definitions

  • the invention relates to a cleaning method for the indicator electrode of a titrator for water determination according to K. Fischer and a device for carrying it out.
  • a mechanical one Cleaning is often unavoidable, but should be carried out as rarely as possible, since the measuring cell must be opened for this purpose and air humidity automatically penetrates, which must be titrated out before the next determination.
  • the protruding platinum pins of the electrode can be bent or broken off. While the breakage of a platinum pin renders the electrode unusable, bending the platinum pin creates changed electrode parameters, which are reflected in a changed or unfavorable end point indication. As a result, the precision and correctness of the titration may deteriorate.
  • the invention seeks to remedy this.
  • the invention has for its object to provide a cleaning method and an apparatus for performing the same, which enables regular self-cleaning of the platinum electrodes.
  • the invention achieves the stated object with a cleaning method which has the features of claim 1 and a device for carrying out the cleaning method which has the features of claim 7.
  • a further improvement of the invention consists in the selection of a suitable geometry of the electrode surface, which further reduces the adhesion of dirt and which greatly facilitates the purely mechanical cleaning. Due to the extreme possibility of amplification of even the smallest measuring currents, the platinum pins of the electrode can be cut off flush with the surface of the glass tube enclosing the electrode; a protrusion of the free wires of up to 1 cm, as is the case with known devices for generating a sufficiently large measuring current, is unnecessary. This not only results in significantly reduced contamination, but also the possibility of creating eddies in the measurement sample due to the angular design of the glass surface, preferably in the vicinity of the emerging platinum pins, which results in an increased mechanical cleaning effect.
  • the bipolar platinum electrode is preferably ground and polished flat at its lower end that dips into the measurement sample. A sticking of fibers or threads from appropriate measurement samples that change or interfere with the electrode parameters is thus made impossible. In addition, mechanical cleaning is completely unproblematic because the electrode has a high mechanical stability.
  • the method according to the invention can of course be initiated automatically.
  • the electrolytic cleaning is carried out regularly, e.g. carried out after each titration, it is also possible to work with samples that are known to be difficult over a month with constant sensitivity without having to mechanically clean the electrodes.
  • Another advantage is that the reaction speeds vary only slightly between different measurement samples, which greatly simplifies the creation of work instructions.
  • FIG. 1 shows a longitudinal section through the indicator electrode as it is used to carry out the cleaning method according to the invention.
  • FIG. 2 shows a basic circuit for carrying out the cleaning method according to the invention.
  • FIG. 3 shows a basic circuit of the measuring amplifier according to FIG. 2.
  • FIG. 4 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, the relay position being set to measurement.
  • FIG. 5 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, the relay position being set to cleaning.
  • FIG. 6 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, with an additional auxiliary electrode.
  • FIG. 7 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, with manual actuation.
  • FIG. 8 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, with reversal of the polarity.
  • Figure 9 shows the diagram of the voltage curve at the indicator electrode during the cleaning process.
  • the indicator electrode 11 used in the inventive device for performing the Karl Fischer titration essentially consists of a glass tube 3 containing the two platinum pins 5, the immersion end of which has a smoothly polished, angled surface 4.
  • the two pins 5 made of pure platinum (99.99% Pt) with a diameter of 0.8 mm and a length of 10 mm, approximately 110 mm long silver wires 6 are welded, which are taken up by the connecting cable 1, which is attached to the glass tube 3 by means of the screw connection 2.
  • the platinum pins 5 are melted in the lead glass tube 3 with an outside diameter of 4.7 mm and a wall thickness of 0.7 mm parallel to the axis and flush.
  • the distance between the two platinum pins 5 is approximately 2 mm.
  • the immersion end of the indicator electrode 11 is essentially planar, but the two platinum pins 5 protrude by a few hundredths of a millimeter on the polished glass surface 4, which is not exactly planar, but is slightly spherical.
  • This shape which differs slightly from the theoretical flat surface, has great practical advantages over electrodes of conventional design.
  • the electrode 11 can be pulled vertically over a flat cloth or cleaning paper and is thereby mechanically cleaned excellently.
  • the exposed platinum surfaces, which are important for the electrode function will be wiped off with their very slightly protruding shape, while the continuous seamless platinum-glass transition completely avoids the attachment or tearing of fibers.
  • An electrode constructed in this way shows excellent operating behavior in a device according to the invention.
  • the basic circuit for the electrode cleaner according to the invention for the Karl Fischer titrator can be seen from FIG.
  • the measuring electrode 11 can be connected via the relay 10 either to the measuring amplifier 9 (measuring process) or to the rectangular generator 8 (cleaning process).
  • the basic circuit of the measuring amplifier 9 for the Karl Fischer titrator is shown in detail in FIG. 3 and consists of the constant current source (20 ⁇ A, 25 Hz), the impedance converter 13, the rectifier 14, the analog-digital converter 15 and the digital display and Calculator 16.
  • FIGS. 4 and 5 The functioning of the device according to the invention for electrolytic cleaning of the indicator electrode 11 immersed in the titration vessel 19 with the solution 20 is shown schematically in FIGS. 4 and 5.
  • the controller 7 of the relay 18 permits the alternative connection of the AC constant current source and measuring electronics 12 (position M as shown in FIG. 4) and the AC generator 17 for the cleaning process (position R as shown in FIG. 5).
  • the electrolysis products 21 are formed during the cleaning process (FIG. 5).
  • the low-frequency AC voltage can also be applied between the two individual electrodes 5 and the additional auxiliary electrode 22 instead of between the two individual electrodes 5 of the indicator electrode 11, as illustrated in FIG. 6.
  • electrolytic cleaning with polarity change is also possible without a relay and control by manually flipping switch 26, as shown in FIG.
  • the switch 26 either the DC generator 23 or the measuring arrangement 24 can be connected to the indicator electrode 11. If the switch 26 is in the R position, the cleaning process takes place, the measuring process in the M position and the cleaning process in the P position with the opposite polarity.
  • FIG. 8 shows a preferred embodiment in which the "AC voltage generator” consists of a DC voltage source 23 which is applied once in one polarity and then in the reverse polarity by means of the relay 18B.
  • This alternating voltage generator produces only one positive (R +) and one negative (R-) half-wave per cleaning cycle, that is to say an entire period (FIG. 9).
  • a purely electronic oscillator unit solid state
  • the measuring arrangement 24 (measuring process M) or the voltage source 23 (cleaning process R + / R-) is alternatively connected to the indicator electrode 11 via the relay 18A.
  • Relay control 7 and measuring arrangement 24 are connected to the computer 25.
  • the voltage curve at the indicator electrode 11 during the The cleaning process is shown diagrammatically in FIG. 9 with the associated positions P (X or Y) of the relays 18A and 18B.
  • the measuring cell 19 is emptied automatically

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  • Chemical Kinetics & Catalysis (AREA)
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  • Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
  • Investigating Or Analyzing Non-Biological Materials By The Use Of Chemical Means (AREA)

Abstract

Dans ce procédé de nettoyage de l'électrode de l'indicateur (11) d'un appareil de titrage pour la détermination de la teneur en eau selon la méthode K. Fischer, une tension alternative (17) de basse fréquence est appliquée aux deux électrodes individuelles (5) de l'indicateur. Cette tension doit être suffisamment élevée pour qu'une électrolyse (21) se produise, par laquelle il se forme de l'hydrogène à l'une des électrodes et de l'iode à l'autre électrode. Tant l'effet oxydant de l'iode que la formation de bulles lors du dégagement d'hydrogène engendrent un très fort effet de surface. Le nettoyage est (encore davantage) augmenté en changeant une ou plusieurs fois la polarité du courant dans les électrodes de telle manière que de l'iode et de l'hydrogène se forment à chaque électrode (5). Le procédé selon l'invention permet d'éviter dans une large mesure l'adhérence d'impuretés à la surface des électrodes.In this method of cleaning the electrode of the indicator (11) of a titration apparatus for determining the water content according to the K. Fischer method, an alternating voltage (17) of low frequency is applied to both individual electrodes (5) of the indicator. This voltage must be high enough for electrolysis (21) to occur, whereby hydrogen is formed at one of the electrodes and iodine at the other electrode. Both the oxidizing effect of iodine and the formation of bubbles during the evolution of hydrogen generate a very strong surface effect. The cleaning is (even more) increased by changing the polarity of the current in the electrodes one or more times so that iodine and hydrogen are formed at each electrode (5). The method according to the invention makes it possible to largely avoid the adhesion of impurities to the surface of the electrodes.

Description

Reinigungsverfahren für die Indikatorelektrode eines Titrators zur Wasserbestimmung nach K.Fischer und Vorrichtung zu dessen Durchführung Cleaning process for the indicator electrode of a titrator for water determination according to K. Fischer and device for carrying it out
Die Erfindung bezieht sich auf ein Reinigungsverfahren für die Indikatorelektrode eines Titrators zur Wasserbestimmung nach K.Fischer und eine Vorrichtung zu dessen Durchführung.The invention relates to a cleaning method for the indicator electrode of a titrator for water determination according to K. Fischer and a device for carrying it out.
Bei der Wasserbestimmung nach K.Fischer handelt es sich um ein bewährtes analytisches Verfahren, das beispielsweise in der Monographie "Wasserbestimmung durch Karl-Fischer-Titration" Theorie und Praxis, von G.Wieland erschienen bei GIT VERLAG GMBH, Darmstadt beschrieben ist.Water determination according to K. Fischer is a proven analytical method which is described, for example, in the monograph "Water determination by Karl Fischer titration" theory and practice, published by G. Wieland at GIT VERLAG GMBH, Darmstadt.
Sowohl bei der bivoltametrischen (potentiometrische Titration bei konstanter Stromstärke mit zwei Indikatorelektroden) als auch bei der biamperometrisehen (Amperometrie mit zwei Indikatorelektroden) Endpunktsbestimmung der Karl Fischer Titration fliessen kleine Ströme zwischen den zwei Platinelektroden. Dies führt bekanntermassen zur Ablagerung geladener Teilchen an den Elektrodenoberflächen und nachfolgend zur Bildung schwerlöslicher, katalytischer Reaktionsprodukte. Bei sehr vielen Proben, insbesondere bei der Wasserbestimmung in Lebensmitteln liegen Substanzen in der Titrationslösung vor die ebenfalls zur Verunreinigung der Elektroden beitragen, wie Proteine, Fette, Zucker, Partikel, Fasern u.s.w.Both in bivoltametric (potentiometric titration at constant current with two indicator electrodes) and in biamperometric (amperometry with two indicator electrodes) end point determination of Karl Fischer titration, small currents flow between the two platinum electrodes. As is known, this leads to the deposition of charged particles on the electrode surfaces and subsequently to the formation of poorly soluble, catalytic reaction products. In many samples, especially when determining water in food, there are substances in the titration solution that also contribute to the contamination of the electrodes, such as proteins, fats, sugars, particles, fibers, etc.
Beide Arten von Verunreinigungen führen zu verfälschten Resultaten, sei es durch Beeinflussung der Empfindlichkeit oder der Ansprechgeschwindigkeit der Elektrode. Eine mechanische Reinigung ist zwar oft nicht zu vermeiden, sollte aber möglichst selten vorgenommen werden, da hierzu die Messzelle geöffnet werden muss und dabei automatisch Luftfeuchtigkeit eindringt, welche vor der nächsten Bestimmung austitriert werden muss.Both types of contamination lead to falsified results, whether by influencing the sensitivity or the response speed of the electrode. A mechanical one Cleaning is often unavoidable, but should be carried out as rarely as possible, since the measuring cell must be opened for this purpose and air humidity automatically penetrates, which must be titrated out before the next determination.
Bei der Reinigung können die vorstehenden Platinstifte der Elektrode verbogen oder abgebrochen werden. Während der Bruch eines Platinstiftes die Elektrode unbenutzbar macht, erzeugt ein Verbiegen des Platinstiftes veränderte Elektrodenparameter, welche sich in veränderter oder ungünstiger Endpunktsindikation niederschlagen. Dadurch können sich Präzision und Richtigkeit der Titration verschlechtern.When cleaning, the protruding platinum pins of the electrode can be bent or broken off. While the breakage of a platinum pin renders the electrode unusable, bending the platinum pin creates changed electrode parameters, which are reflected in a changed or unfavorable end point indication. As a result, the precision and correctness of the titration may deteriorate.
Hier will die Erfindung Abhilfe schaffen. Der Erfindung liegt die Aufgabe zugrunde, ein Reinigungsverfahren und ein Gerät zur Durchführung desselben zu schaffen, welches eine regelmässige Selbstreinigung der Platinelektroden ermöglicht.The invention seeks to remedy this. The invention has for its object to provide a cleaning method and an apparatus for performing the same, which enables regular self-cleaning of the platinum electrodes.
Die Erfindung löst die gestellte Aufgabe mit einem Reinigungsverfahren, welches die Merkmale des Anspruchs 1 aufweist, sowie einer Vorrichtung zur Durchführung des Reinigungsverfahrens, welches die Merkmale des Anspruchs 7 aufweist.The invention achieves the stated object with a cleaning method which has the features of claim 1 and a device for carrying out the cleaning method which has the features of claim 7.
Die durch die Erfindung erreichten Vorteile sind im wesentlichen darin zu sehen, dass dank des erfindungsgemässen Verfahrens das Haften von Schmutz an der Elektrodenoberfläche weitgehend vermieden werden kann. Durch Anlegen einer Spannung wird elektrolytisch an der einen Elektrode Wasserstoff, respektive Jod an der anderen Elektrode erzeugt. Sowohl die oxidierende Wirkung des Jodes als auch die Blasenbildung beim Entstehen des Wasserstoffes haben einen stark oberflächenaktiven Effekt. Die Reinigungswirkung wird (noch zusätzlich) erhöht, wenn die Polarität des Elektrodenstromes ein- oder mehrmals gewechselt wird, so dass an jeder Einzelelektrode sowohl Jod als auch Wasserstoff gebildet wird.The advantages achieved by the invention can essentially be seen in the fact that, thanks to the method according to the invention, the adherence of dirt to the electrode surface can be largely avoided. By applying a voltage, hydrogen or iodine is generated electrolytically on one electrode or on the other electrode. Both the oxidizing effect of iodine and the formation of bubbles when hydrogen is formed have a highly surface-active effect. The cleaning effect is (additionally) increased if the polarity of the electrode current is changed one or more times, so that both iodine and hydrogen are formed on each individual electrode.
Eine weitere Verbesserung der Erfindung besteht in der Wahl einer geeigneten Geometrie der Elektrodenoberfläche, welche das Haften von Schmutz weiter verringert und welche die rein mechanische Reinigung stark erleichtert. Durch die heute mögliche, extreme Verstärkungsmöglichkeit auch kleinster Messströme, können die Platinstifte der Elektrode mit der Oberfläche des die Elektrode umschliessenden Glasrohres bündig abgeschnitten werden; ein Vorstehen der freien Drähte von bis zu 1 cm, wie dies bei bekannten Geräten zur Erzeugung eines genügend grossen Messstromes der Fall ist erübrigt sich. Damit ergibt sich nicht nur eine erheblich verringerte Verschmutzung, sondern auch die Möglichkeit durch kantige Ausgestaltung der Glasoberfläche, vorzugsweise in der Nachbarschaft der austretenden Platinstifte, starke Wirbel in der Messprobe zu erzeugen, welche einen erhöhten mechanischen Reinigungseffekt zur Folge haben. Zu diesem Zwecke wird die bipolare Platinelektrode an ihrem unteren in die Messprobe eintauchenden Ende vorzugsweise plan geschliffen und poliert. Ein Hängenbleiben von Fasern oder Fäden aus entsprechenden Messproben, welche die Elektrodenparameter verändern oder stören, wird damit verunmöglicht. Ausserdem ist die mechanische Reinigung völlig problemlos, da die Elektrode eine hohe mechanische Stabilität besitzt.A further improvement of the invention consists in the selection of a suitable geometry of the electrode surface, which further reduces the adhesion of dirt and which greatly facilitates the purely mechanical cleaning. Due to the extreme possibility of amplification of even the smallest measuring currents, the platinum pins of the electrode can be cut off flush with the surface of the glass tube enclosing the electrode; a protrusion of the free wires of up to 1 cm, as is the case with known devices for generating a sufficiently large measuring current, is unnecessary. This not only results in significantly reduced contamination, but also the possibility of creating eddies in the measurement sample due to the angular design of the glass surface, preferably in the vicinity of the emerging platinum pins, which results in an increased mechanical cleaning effect. For this purpose, the bipolar platinum electrode is preferably ground and polished flat at its lower end that dips into the measurement sample. A sticking of fibers or threads from appropriate measurement samples that change or interfere with the electrode parameters is thus made impossible. In addition, mechanical cleaning is completely unproblematic because the electrode has a high mechanical stability.
Erfolgt die Titration rechner-gesteuert, so kann das erfindungsgemässe Verfahren selbstverständlich automatisch eingeleitet werden.If the titration is computer-controlled, the method according to the invention can of course be initiated automatically.
Wird die elektrolytische Reinigung nach dem erfindungsgemässen Verfahren regelmässig, z.B. nach jeder Titration durchgeführt, kann auch mit bekannterweise schwierigen Proben über einen Monat bei gleich bleibender Empfindlichkeit gearbeitet werden ohne die Elektroden mechanisch reinigen zu müssen. Ein weiterer Vortoil liegt darin, dass die Reaktionsgeschwindigkeiten zwischen verschiedenen Messproben nur wenig variieren, was die Erstellung von Arbeitsvorschriften stark vereinfacht.If the electrolytic cleaning is carried out regularly, e.g. carried out after each titration, it is also possible to work with samples that are known to be difficult over a month with constant sensitivity without having to mechanically clean the electrodes. Another advantage is that the reaction speeds vary only slightly between different measurement samples, which greatly simplifies the creation of work instructions.
Ein Ausführungsbeispiel der Erfindung, welches zugleich das Funktionsprinzip erläutert, ist in der Zeichnung dargestellt und wird im folgenden näher beschrieben.An embodiment of the invention, which also explains the principle of operation, is shown in the drawing and is described in more detail below.
Figur 1 stellt einen Längsschnitt durch die Indikatorelektrode dar, wie sie zur Durchführung des erfindungsgemässen Reinigungsverfahrens verwendet wird.FIG. 1 shows a longitudinal section through the indicator electrode as it is used to carry out the cleaning method according to the invention.
Figur 2 stellt eine Prinzipschaltung zur Durchführung des erfindungsgemässen Reinigungsverfahrens dar.FIG. 2 shows a basic circuit for carrying out the cleaning method according to the invention.
Figur 3 stellt eine Prinzipschaltung des Messverstärkers gemäss Figur 2 dar. Figur 4 stellt das Schaltbild einer Vorrichtung zur Durchführung des erfindungsgemässen Reinigungsverfahrens dar, wobei die Relaisposition auf Messung steht.FIG. 3 shows a basic circuit of the measuring amplifier according to FIG. 2. FIG. 4 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, the relay position being set to measurement.
Figur 5 stellt das Schaltbild einer Vorrichtung zur Durchführung des erfindungsgemässen Reinigungsverfahrens dar, wobei die Relaisposition auf Reinigung steht.FIG. 5 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, the relay position being set to cleaning.
Figur 6 stellt das Schaltbild einer Vorrichtung zur Durchführung des erfindungsgemässen Reinigungsverfahrens dar, mit einer zusätzlichen Hilfselektrode.FIG. 6 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, with an additional auxiliary electrode.
Figur 7 stellt das Schaltbild einer Vorrichtung zur Durchführung des erfindungsgemässen Reinigungsverfahrens dar, mit manueller Betätigung.FIG. 7 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, with manual actuation.
Figur 8 stellt das Schaltbild einer Vorrichtung zur Durchführung des erfindungsgemässen Reinigungsverfahrens dar, mit Umkehrung der Polarität.FIG. 8 shows the circuit diagram of a device for carrying out the cleaning method according to the invention, with reversal of the polarity.
Figur 9 stellt im Diagramm den Spannungsverlauf an der Indikatorelektrode während des Reinigungsvorgangs dar.Figure 9 shows the diagram of the voltage curve at the indicator electrode during the cleaning process.
Wie in Figur 1 dargestellt besteht die in erfindungsgemässen Vorrichtung zur Durchführung der Karl Fischer Titration verwendete Indikatorelektrode 11 im wesentlichen aus einem die beiden Platinstifte 5 enthaltenden Glasrohr 3, dessen Eintauchende eine glatt polierte, abgewinkelte Oberfläche 4 aufweist. An die beiden aus Reinplatin (99,99 % Pt ) bestehenden Stifte 5 mit einem Durchmesser von 0,8 mm und einer Länge von 10 mm sind ca. 110 mm lange Silberdrähte 6 angeschweisst, welche vom Anschlusskabel 1 aufgenommen werden, das mittels der Verschraubung 2 am Glasrohr 3 befestigt ist. Die Platinstifte 5 sind im Bleiglasrohr 3 vom Aussendurchmesser 4,7mm und der Wandstärke 0,7 mm achsenparallel und bündig eingeschmolzen. Der Abstand der beiden Platinstifte 5 beträgt ca. 2 mm. Das Eintauchende der Indikatorelektrode 11 ist wie bereits erwähnt im wesentlichen planar, doch stehen die beiden Platinstifte 5 um einige Hunderstelmillimeter auf der polierten Glasfläche 4 vor, welche nicht exakt planar, sondern leicht sphärisch ist. Diese von der theoretischen planen Fläche leicht abweichenden Form weist gegenüber Elektroden üblicher Bauform grosse praktische Vorteile auf. Die Elektrode 11 kann senkrecht über ein flach liegendes Tuch oder Reinigungspapier gezogen werden und wird dadurch vorzüglich mechanisch gereinigt. Die für die Elektrodenfunktion wichtigen exponierten Platinoberflächen werden durch ihre ganz leicht vorstehende Form mit Sicherheit abgewischt, während der kontinuierliche nahtlose Platin-Glas-Üebergang das Anhängen oder Losreissen von Fasern völlig vermeidet. Eine solchermassen konstruierte Elektrode zeigt in einer erfindungsgemässen Vorrichtung ein vorzügliches Betriebsverhalten. Es konnten mehr als 500 Karl-Fischer-Titrationen mit "schwierigen" Proben bestehend aus Naturstoffgemischen (z.B. Schokolade) ausgeführt werden, ohne dass sich eine merkliche Veränderung in der Elektrodencharakteristik und somit der Endpunktsindikation ergab. Aus Figur 2 ist die Prinzipschaltung für den erfindungsgemässen Elektrodenreiniger für den Karl-Fischer- Titrator ersichtlich. Durch die Relaissteuerung 7 kann die Messelektrode 11 über das Relais 10 entweder an den Messverstärker 9 (Messvorgang) oder an den Rechteckgenerator 8 (Reinigungsvorgang) angeschlossen werden.As shown in FIG. 1, the indicator electrode 11 used in the inventive device for performing the Karl Fischer titration essentially consists of a glass tube 3 containing the two platinum pins 5, the immersion end of which has a smoothly polished, angled surface 4. To the two pins 5 made of pure platinum (99.99% Pt) with a diameter of 0.8 mm and a length of 10 mm, approximately 110 mm long silver wires 6 are welded, which are taken up by the connecting cable 1, which is attached to the glass tube 3 by means of the screw connection 2. The platinum pins 5 are melted in the lead glass tube 3 with an outside diameter of 4.7 mm and a wall thickness of 0.7 mm parallel to the axis and flush. The distance between the two platinum pins 5 is approximately 2 mm. As already mentioned, the immersion end of the indicator electrode 11 is essentially planar, but the two platinum pins 5 protrude by a few hundredths of a millimeter on the polished glass surface 4, which is not exactly planar, but is slightly spherical. This shape, which differs slightly from the theoretical flat surface, has great practical advantages over electrodes of conventional design. The electrode 11 can be pulled vertically over a flat cloth or cleaning paper and is thereby mechanically cleaned excellently. The exposed platinum surfaces, which are important for the electrode function, will be wiped off with their very slightly protruding shape, while the continuous seamless platinum-glass transition completely avoids the attachment or tearing of fibers. An electrode constructed in this way shows excellent operating behavior in a device according to the invention. More than 500 Karl Fischer titrations with "difficult" samples consisting of natural substance mixtures (eg chocolate) could be carried out without a noticeable change in the electrode characteristics and thus in the end point indication. The basic circuit for the electrode cleaner according to the invention for the Karl Fischer titrator can be seen from FIG. By means of the relay control 7, the measuring electrode 11 can be connected via the relay 10 either to the measuring amplifier 9 (measuring process) or to the rectangular generator 8 (cleaning process).
Die Prinzipschaltung des Messverstärkers 9 für den Karl-Fischer Titrator ist in Figur 3 im Detail dargestellt und besteht aus der Konstantstromquelle (20 μA, 25 Hz), dem Impedanzwandler 13, dem Gleichrichter 14, dem Analog-Digital-Wandler 15 und der Digitalanzeige und Rechner 16.The basic circuit of the measuring amplifier 9 for the Karl Fischer titrator is shown in detail in FIG. 3 and consists of the constant current source (20 μA, 25 Hz), the impedance converter 13, the rectifier 14, the analog-digital converter 15 and the digital display and Calculator 16.
Die Funktionsweise der erfindungsgemässen Vorrichtung zur elektrolytischen Reinigung der in das Titrationsgefäss 19 mit der Lösung 20 eintauchenden Indikatorelektrode 11 ist in den Figuren 4 und 5 schematisch dargestellt. Die Steuerung 7 des Relais 18 erlaubt den alternativen Anschluss der AC-Konstantstromquelle und Messelektronik 12 (Stellung M wie in Figur 4 dargestellt) und dem AC-Generator 17 für den Reinigungsvorgang (Stellung R wie in Figur 5 dargestellt). Beim Reinigungsvorgang (Figur 5) entstehen die Elektrolysenprodukte 21 (Jod und Wasserstoff).The functioning of the device according to the invention for electrolytic cleaning of the indicator electrode 11 immersed in the titration vessel 19 with the solution 20 is shown schematically in FIGS. 4 and 5. The controller 7 of the relay 18 permits the alternative connection of the AC constant current source and measuring electronics 12 (position M as shown in FIG. 4) and the AC generator 17 for the cleaning process (position R as shown in FIG. 5). The electrolysis products 21 (iodine and hydrogen) are formed during the cleaning process (FIG. 5).
Die niederfrequente Wechselspannung kann statt zwischen den beiden Einzelelektroden 5 der Indikatorelektrode 11 auch zwischen den beiden Einzelelektroden 5 und der zusätzlichen Hilfselektrode 22 angelegt werden, wie in Figur 6 illustriert. Bei einfachen Titrations-Geräten ist die elektrolytische Reinigung mit Polaritätswechsel auch ohne Relais und Steuerung durch manuelles Umlegen des Schalters 26 möglich, wie in Figur 7 dargestellt. Durch Betätigung des Schalters 26 kann wahlweise der DC-Generator 23 oder die Messanordnung 24 an die Indikatorelektrode 11 angeschlossen werden. Befindet sich der Schalter 26 in der Stellung R so erfolgt der Reinigungsvorgang, in Stellung M der Messvorgang und in Stellung P der Reinigungsvorgang mit umgekehrter Polarität.The low-frequency AC voltage can also be applied between the two individual electrodes 5 and the additional auxiliary electrode 22 instead of between the two individual electrodes 5 of the indicator electrode 11, as illustrated in FIG. 6. In simple titration devices, electrolytic cleaning with polarity change is also possible without a relay and control by manually flipping switch 26, as shown in FIG. By actuating the switch 26, either the DC generator 23 or the measuring arrangement 24 can be connected to the indicator electrode 11. If the switch 26 is in the R position, the cleaning process takes place, the measuring process in the M position and the cleaning process in the P position with the opposite polarity.
In Figur 8 ist eine bevorzugte Ausführungsform dargestellt bei welcher der "Wechselspannungsgenerator" aus einer Gleichspannungsquelle 23 besteht, die mittels des Relais 18B einmal in der einen und dann in der umgekehrten Polarität angelegt wird. Pro Reinigungszyklus produziert dieser Wechselspannungsgenerator nur eine positive (R+) und eine negative (R-) Halbwelle, zusammen also eine ganze Periode (Figur 9). Anstelle des "Wechselspannungsgenerators" bestehend aus Gleichspannungsquelle 23 und Relais 18B kann auch eine rein elektronische Oszillatoreinheit (solid state) verwendet werden. Ueber das Relais 18A wird wie in den vorhergehend beschriebenen Ausführungsformen alternativ die Messanordnung 24 (Messvorgang M) oder die Spannungsquelle 23 (Reinigungsvorgang R+/R-) an die Indikatorelektrode 11 angeschlossen. Relaissteuerung 7 und Messanordnung 24 sind mit dem Rechner 25 verbunden. Der Spannungsverlauf an der Indikatorelektrode 11 während des Reinigungsvorgangs ist in Figur 9 diagrammatisch mit den dazugehörenden Positionen P (X oder Y) der Relais 18A und 18B dargestellt.FIG. 8 shows a preferred embodiment in which the "AC voltage generator" consists of a DC voltage source 23 which is applied once in one polarity and then in the reverse polarity by means of the relay 18B. This alternating voltage generator produces only one positive (R +) and one negative (R-) half-wave per cleaning cycle, that is to say an entire period (FIG. 9). Instead of the "AC voltage generator" consisting of DC voltage source 23 and relay 18B, a purely electronic oscillator unit (solid state) can also be used. As in the previously described embodiments, the measuring arrangement 24 (measuring process M) or the voltage source 23 (cleaning process R + / R-) is alternatively connected to the indicator electrode 11 via the relay 18A. Relay control 7 and measuring arrangement 24 are connected to the computer 25. The voltage curve at the indicator electrode 11 during the The cleaning process is shown diagrammatically in FIG. 9 with the associated positions P (X or Y) of the relays 18A and 18B.
Die Durchführung der Karl-Fischer-Titration mit der erfindungsgemässen Vorrichtung kann weitgehend automatisiert werden und verläuft nach folgendem Schema:Carrying out the Karl Fischer titration with the device according to the invention can be largely automated and proceeds according to the following scheme:
- Probe und Lösungsmittel 20 werden automatisch in die Messzelle 19 gegeben- Sample and solvent 20 are automatically placed in the measuring cell 19
- die Titration wird gestartet und dauert ca. 1 bis 3 Minuten- The titration is started and takes about 1 to 3 minutes
- ein Reinigungszyklus von 2 Sekunden wird gestartet- A cleaning cycle of 2 seconds is started
- die Messzelle 19 wird automatisch entleert- The measuring cell 19 is emptied automatically
- der Ablauf kann von neuem beginnen. - The process can start again.

Claims

Patentansprüche Claims
1. Reinigungsverfahren für die Indikatorelektrode (11) eines Titrators zur Wasserbestimmung nach K.Fischer, dadurch gekennzeichnet, dass eine niederfrequente Wechselspannung an die beiden Einzelelektroden (5) der in die Messlösung (20) tauchenden Indikatorelektrode (11) angelegt wird, welche mindestens so hoch ist, dass eine Elektrolyse an den beiden Einzelelektroden (5) auftritt.1. Cleaning method for the indicator electrode (11) of a titrator for water determination according to K. Fischer, characterized in that a low-frequency AC voltage is applied to the two individual electrodes (5) of the indicator electrode (11) immersed in the measurement solution (20), which is at least as follows What is high is that electrolysis occurs at the two individual electrodes (5).
2. Reinigungsverfahren nach Anspruch 1, dadurch gekennzeichnet, dass die niederfrequente Wechselspannung eine beliebige Wellenform der Frequenz 0,01 bis 100 Hz, vorzugsweise von 1 bis 10 Hz aufweist.2. Cleaning method according to claim 1, characterized in that the low-frequency AC voltage has any waveform of the frequency 0.01 to 100 Hz, preferably from 1 to 10 Hz.
3. Reinigungsverfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, dαss die niederfrequente Wechselspannung eine Rechteckspannung ist.3. Cleaning method according to claim 1 or 2, characterized in that the low-frequency AC voltage is a square wave voltage.
4. Reinigungsverfahren nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass die niederfrequente Wechselspannung eine Spannung im Bereich von 5 bis 30 V, vorzugsweise im Bereich von 8 bis 12 V ist. 4. Cleaning method according to one of claims 1 to 3, characterized in that the low-frequency AC voltage is a voltage in the range from 5 to 30 V, preferably in the range from 8 to 12 V.
5. Reinigungsverfahren nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, dass in unmittelbarer Nähe der Indikatorelektrodenoberfläche (4) Wirbelströmungen in der Messprobe (20) erzeugt werden.5. Cleaning method according to one of claims 1 to 4, characterized in that eddy currents are generated in the measurement sample (20) in the immediate vicinity of the indicator electrode surface (4).
6. Reinigungsverfahren nach einem der Ansprüche 1 bis 5, dadurch gekennzeichnet, dass die niederfrequente Wechselspannung zwischen den beiden Einzelelektroden (5) der Indikatorelektrode (11) einerseits und mindestens einer zusätzlichen Hilfselektrode (22) anderseits angelegt wird.6. Cleaning method according to one of claims 1 to 5, characterized in that the low-frequency alternating voltage between the two individual electrodes (5) of the indicator electrode (11) on the one hand and at least one additional auxiliary electrode (22) is applied on the other.
7. Vorrichtung zur Durchführung des Reinigungsverfahrens nach einem der Ansprüche 1 bis 6, gekennzeichnet durch einen, vorzugsweise über ein Relais (10; 18), an die zur Bestimmung des Endpunktes der Titration verwendete Indikatorelektrode (11) anschliessbaren Spannungsgenerators { 8 ; 17 ; 23 ) .7. Device for carrying out the cleaning method according to one of claims 1 to 6, characterized by a, preferably via a relay (10; 18), to the indicator electrode (11) which can be connected to determine the end point of the titration, voltage generator {8; 17; 23).
8. Vorrichtung nach Anspruch 7, dadurch gekennzeichnet, dass die Indikatorelektrode (11) derart ausgebildet ist, dass die beiden Platinstifte (5) direkt an der Oberfläche des die Elektrode (11) umschliessenden Glasrchrs (3) enden und vorzugsweise mit dieser eine im wesentlichen plan geschliffene Fläche (4) bilden. 8. The device according to claim 7, characterized in that the indicator electrode (11) is designed such that the two platinum pins (5) end directly on the surface of the glass (3) enclosing the electrode (11) and preferably with one essentially Form a flat surface (4).
9. Vorrichtung nach Anspruch 7 oder 8, dadurch gekennzeichnet, dass der in die Messprobe (20) eintauchende Teil der Indikatorelektrode (11) scharfe Kanten aufweist, vorzugsweise in der Nachbarschaft der nach aussen austretenden Einzelelektroden (5).9. The device according to claim 7 or 8, characterized in that the part of the indicator electrode (11) immersed in the measurement sample (20) has sharp edges, preferably in the vicinity of the individual electrodes (5) emerging to the outside.
10. Vorrichtung nach einem der Ansprüche 7 bis 9, dadurch gekennzeichnet, dass die polierte Oberfläche (4) leicht sphärisch ausgebildet ist und die Einzelelektroden (5) geringfügig über die Oberfläche (4) hervorragen. 10. Device according to one of claims 7 to 9, characterized in that the polished surface (4) is slightly spherical and the individual electrodes (5) protrude slightly above the surface (4).
EP19880902117 1987-03-14 1988-03-10 Process for cleaning the indicator electrode of a titrator for the determination of water content by the k. fischer method and device for carrying out the process Withdrawn EP0307424A1 (en)

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CH94587A CH672845A5 (en) 1987-03-14 1987-03-14
CH945/87 1987-03-14

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US5210496A (en) * 1988-04-13 1993-05-11 Endress & Hauser Flowtec Ag Method and circuit arrangement for intermittent application of a voltage to an electrode
DE10309022A1 (en) * 2003-03-01 2004-09-09 Dr. A. Kuntze Gmbh Process for cleaning electrode surfaces and device for carrying out the process
JP2004333413A (en) * 2003-05-12 2004-11-25 Dia Instr:Kk Moisture measuring method and moisture measuring device
CN102213690A (en) * 2011-05-20 2011-10-12 长沙瑞翔科技有限公司 Automatic cleaning device for electrolytic cell electrode
DE102011120819A1 (en) 2011-12-13 2013-06-13 Dinotec Gmbh Wassertechnologie Und Schwimmbadtechnik Method for cleaning electrode surfaces
DE102018004450A1 (en) 2018-06-05 2019-12-05 Ecm Gmbh Method for activating and reactivating electrochemical sensors and a device for carrying out the method
DE102018113640A1 (en) 2018-06-07 2019-12-12 Prominent Gmbh Method for cleaning, conditioning, calibration and / or adjustment of an amperometric sensor
DE102021116153A1 (en) 2021-06-22 2022-12-22 Prominent Gmbh Process for the optical activation of the sensor surface, especially for zero chlorine sensors

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