Classifications

• • 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 & Material Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Health & Medical Sciences (AREA)
• Physics & Mathematics (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Molecular Biology (AREA)
• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
• Investigating Or Analyzing Non-Biological Materials By The Use Of Chemical Means (AREA)

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Citations (4)

• 1987
  • 1987-03-14 CH CH94587A patent/CH672845A5/de not_active IP Right Cessation
• 1988
  • 1988-03-10 EP EP19880902117 patent/EP0307424A1/de not_active Withdrawn
  • 1988-03-10 JP JP63501953A patent/JPH01502695A/ja active Pending
  • 1988-03-10 WO PCT/CH1988/000058 patent/WO1988007194A1/de not_active Application Discontinuation

Patent Citations (4)

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