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
  • G01N27/416—Systems
  • G01N27/42—Measuring deposition or liberation of materials from an electrolyte; Coulometry, i.e. measuring coulomb-equivalent of material in an electrolyte
  • G01N27/423—Coulometry
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/18—Water
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/18—Water
  • G01N33/1813—Specific cations in water, e.g. heavy metals

Definitions

• the present invention relates to a process for tine coulometric determination of the content of components dissolved in water, such as gases, e.g. oxygen gas in sea-water, and ions, e.g. iron ions, and a measuring cell for carrying out the process.
• gases e.g. oxygen gas in sea-water
• ions e.g. iron ions
• Various titrimetric methods can be used for the determination of the content of components dissolved in water.
• a number of electrochemical measuring devices are available for measurements in situ. These devices are often based upon a galvanometric or polarographic principle. This means that the measuring signal is proportional to the flow of the component towards an electrode surface. At constant diffusion conditions said flow is proportional to the area of the electrode and to the chemical potential of the dissolved component. The area of the electrode will vary due to precipitation on and contamination of the electrode surface. As a result thereof this type of instrument exhibits an unsatisfactory precision and long term stability.
• salinity and temperature must be determined in addition to the partial pressure of the gas if the concentration is to be calculated.
• the invention relates to a process for the coulometric determination of the content of a component dissolved in water, in which process a definite constant voltage is applied across the electrodes of a coulometer cell filled with a water sample, said constant voltage being maintained for a period of time sufficient for a complete reaction of the dissolved component in a desired electrode reaction, the quantity of electricity passing through the cell being measured by integration, and the content of the dissolved component being calculated on the basis of said integration.
• the process is characterized by employing an elongated three-part coulometer cell composed of a central measuring cell and two auxiliary cells which are directly connected to each end of the measuring cell, and by perform ing electrolysis simultaneously in the measuring cell and the auxiliary cells under such conditions that the concentration of the dissolved component will decrease at the same rate in all three cell elements, two such electrolyses being performed with the same integration time in order to correct for temperature and salt content dependence and for charging of the electric double layer.
• the coulometer cell used in this process is characterized in that it consists of an elongated central measuring cell which has both of its ends directly connected each to one of two elongated auxiliary cells, each of the measuring and auxiliary cells being provided with an anode and a cathode in the form of electrically conducting wires, said wires being parallel inter se and extending in the longitudinal direction of the respective cell elements.
• a specific embodiment of the invention is described below, viz. the determination of the content of oxygen gas dissolved in sea-water.
• the invention is not limit ed to said embodiment but can be used also for the determination of other reactive gases dissolved in water, such as hydrogen sulphide and sulphur dioxide, and ions, such as iron and manganese ions, etc.
• the electrode materials and the voltage applied are chosen in such a manner that the component to be analyzed will participate in a definite electrode reaction which shall be the dominating reaction at the electrode in question.
• the principle utilized in the determination of oxygen in sea-water is based upon the fact that in sea-water the reduction of oxygen is the electrode reaction of quantitative importance which has the lowest activation potential at a platinum cathode.
• the reduction of oxygen is the electrode reaction of quantitative importance which has the lowest activation potential at a platinum cathode.
• anode and potential difference of the cell essentially all current through the cell is a function of the reduction of oxygen and is limited only by the supply of oxygen for the cathode reaction.
• the total number of electrons which can pass through the cell is quantitatively determined by the amount of dissolved oxygen gas in the volume according to Faraday's law.
• a silver-plated platinum wire is suitably used as the anode, the anode reaction being the following:
• the dimentions of the electrolysis cell are determined i.a. by the conditions that the electrolysis time should be moderate (normally maximum 15 minutes) and that the current through the cell should be measurable with a reasonable precision and simplicity. Said conditions lead to a very long and narrow cell in the form of a tube with the electrodes extending in the axial direction. Suitable cell dimensions are the following: diameter 0.3-0.4 mm; length 100-200 mm.
• the solution according to the invention is to use a measuring cell open in both ends but connected to identical electrolysis cells (auxiliary cells) which are switched in at the same time as the measuring cell.
• auxiliary cells identical electrolysis cells
• the liquid column through the measuring and auxiliary cells is kept immovable during the measurement by using a pump not permitting any passage of liquid. Small liquid movements due to a moderate leakage or due to temperature expansion do not affect the measuring result since the liquid entering into the cell has the same oxygen concentration as the liquid being replaced.
• the advantage of the design according to the invention is that the measuring volume is well defined since longitudinal advection and diffusion of oxygen have been eliminated.
• Fig. 1 shows the fundamental construction of the coulometer cell
• Figs. 2 and 3 show the extension of the electrode wires
• Fig. 4 illustrates the joining of a measuring cell with an auxiliary cell.
• the fundamental construction of the cell is shown in Fig. 1.
• the cell consists of a central measuring cell 1 and two auxiliary cells 2 on each side of the measuring cell.
• the measuring cell as well as the auxiliary cells consist of open tube segments (capillary tubes) interconnected to each other.
• the capillary tubes are normally glass tubes, e.g. of Pyrex glass. If the length of the measuring cell is 100 mm, the length of each auxiliary cell may be e.g. 50 mm.
• the auxiliary cells 2 are connected to outer tube segments 3.
• the tube segment 3 in the intake end is connected to a particle filter 4, possibly via a plastics hose.
• the tube segment 3 in the other end is, via a plastics hose, connected to a pump 5, preferably a peristaltic pump.
• Figs. 2 and 3 show how threadlike electrodes 6 are arranged in the longitudinal direction of a measuring cell or auxiliary cell and, in the ends of the capillary tube, are located in grooves 7 between the inner surface and the outer surface of the capillary tube.
• the cathode consists of a fine platinum wire
• the anode consists of a similar platinum wire coated with a silver layer. If the inner diameter of the capillary tube is e.g. 0.4 mm the diameter of the platinum wires may be 0.05 mm. The thickness of the anode silver layer may be 0.01 mm.
• the wires are applied in the following way. Each wire is passed through the capillary tube, one end of the wire is folded over one end of the tube in said groove, and the wire end is fixed on the outer side of the tube, e.g. by means of an adhesive and shrinkable plastics hose 8 (see Fig, 4). The wire is stretched by hand along the inner side of the tube, and the other wire end is fixed in the same way as the first end. The two wires in each capillary tube are applied diametrically opposite to each other.
• Fig. 4 illustrates the joining of a measuring cell 1 and an auxiliary cell 2.
• the auxiliary cell is turned 90° in relation to the measuring cell so that the electrodes 6 o.f the two cells will not contact each other.
• Some silicon rubber is applied to the ends of the tube segments, the ends are then pressed against each other and shrinkable plastics hose 9 is applied to hold said ends together.
• Each auxiliary cell 2 is joined with an outer tube segment 3 in the same way.
• each electrode wire 6 is connected in any suitable way to a connection wire.
• the various connection wires are brought together in a screened cable.
• the two auxiliary cells are connected electrically.
• the cell assembly is applied in a plexiglass housing and all joints and solders are casted into an adhesive, e.g. of epoxy type.
• the flow velocity is controlled in such a manner that the depression in the m easuring cell will not be so great that gas bubbles are foar med .
• the rinsing of the cell must be performed fo r a period of time sufficient for the replacement of the w hole liquid volume of the measuring cell (thus , also the liquid nearest the cell wall) by new test liquid . After stopping the pump the electrolysis can be started .
• the voltage applied is main tained constant during the whole electrolysis time and this can be accomplished by means of two operational amplifiers , one for the measuring cell and one for the two auxiliary cel ls .
• the reference voltage is obtained via a voltage divider from the stabilize d supply voltage .
• the oxygen concentration in the auxiliary sells will be equal to the oxygen concentration in the measur ing cell .
• V the cell volume
• A the area of the working electrode
• E' the electrode potential in relation to electrocapillary maximum.
• the charge Q F is a measure of the oxygen concentration to be determined.
• Q D and TK o are probably salt and tem perature dependent and proportional to the effective electrode area. According to the invention said terms are determined by a second electrolysis of the same sample with the same integration time. The charge will then be:
• the integration is carried out in the following way
• the anode of the measuring cell is connected to an operational amplifier which functions as a current-to-voltage converter and is provided with a booster amplifier so that the current surge passing through the cell and the current-to-voltage converter initially shall not affect the input parameters of the converter.
• the voltage is converted by means of a voltage-to-frequency converter to a pulse train with voltage proportional frequency which is counted in an up/ down counter. In the first integration the frequency is counted up, and in the second integration the frequency is counted down from the first value.

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• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
• Investigating Or Analysing Biological Materials (AREA)
• Investigating Or Analysing Materials By The Use Of Chemical Reactions (AREA)

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Citations (2)

• 1977
  • 1977-09-14 SE SE7710308A patent/SE407983B/sv unknown
• 1978
  • 1978-09-14 GB GB7911769A patent/GB2023842B/en not_active Expired
  • 1978-09-14 WO PCT/SE1978/000043 patent/WO1979000146A1/en unknown
• 1979
  • 1979-03-27 EP EP78900118A patent/EP0006910A1/de not_active Withdrawn
• 1980
  • 1980-01-21 FR FR8001480A patent/FR2455280A1/fr not_active Withdrawn
• 1982
  • 1982-11-17 SG SG589/82A patent/SG58982G/en unknown
• 1984
  • 1984-03-29 HK HK281/84A patent/HK28184A/xx unknown

Patent Citations (2)

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