HK1247277B - Electrochemical measuring cell for measuring the content of chlorine compounds in water - Google Patents

Description

Electrochemical measuring cell for measuring the content of chlorine compounds in water

Technical Field

The invention relates to an electrochemical measuring cell for measuring the chlorine compound content in water, comprising an electrolyte chamber, a measuring electrode delimiting the electrolyte chamber, a counter electrode, and a reference electrode.

Background Art

Such electrochemical measuring cells are described, inter alia, in EP 0740149 B1, EP 0563690 A1, DE 19515392 C2 and DE 10322894 A1.

These known measuring cells are so-called membrane-covered measuring cells, which have a hydrophilic, liquid-permeable membrane which delimits an electrolyte chamber or an electrolyte contained therein relative to the liquid to be tested, in particular water.

Known membrane-covered sensors have several disadvantages:

The compound to be measured must diffuse through the membrane into the electrolyte compartment. This is a slow process, resulting in a significant reduction in signal strength and a significantly increased response time compared to open electrode systems. Furthermore, salts dissolved in the electrolyte can diffuse in the opposite direction into the water being measured; this can significantly shorten the life of the measuring cell.

Because the diffusion process is slow, the membrane must be as thin as possible. This makes the membrane mechanically fragile. Because the electrolyte is depleted during measurement and storage, it must be replaced regularly. To do this, the membrane cap must be unscrewed, emptied, rinsed, filled, and screwed back on, while also providing pressure compensation. This process always carries the risk that the operator will damage the membrane during the process, rendering the sensor unusable.

Since material transport behind the membrane can still occur solely by diffusion, the distance between the membrane and the measuring electrode is crucial for the signal strength. However, this distance can change due to pressure fluctuations or changes in the inflow, causing the measurement signal to also reflect these pressure or flow changes.

Commercially available sensors have very small electrodes, corresponding to weak signals. This requires pre-amplification, i.e., in-sensor electronics; this has a negative impact on manufacturing costs. To counteract this, not the sensor itself, but only the membrane and electrolyte are considered consumable. In fact, some manufacturers offer to refurbish the sensor's electrodes for a fee (if desired).

Membrane sensors with a hydrophilic, liquid-permeable membrane comprise different plastic materials, adhesive connections, and seals that react partially sensitively to components of water, such as surfactants.

US 4707242 A discloses an electrochemical cell comprising two working electrodes 20, 22, a reference electrode 34, and a counter electrode 28, and is used to quantitatively measure harmful gases. The reference electrode 34 and the counter electrode attached to the porous membrane 30 are located in the electrolyte chamber 14 into which the gas to be tested is pumped. According to FIG3 , the working electrodes 20, 22 are attached as a first unit to a porous gas diffusion membrane 24 forming a second unit. The second working electrode 22 surrounds the first working electrode 20. The first working electrode 20 is produced by mixing a suitable catalyst 40 with a polytetrafluoroethylene dispersion. The catalyst 40 can be composed of platinum. The second working electrode 22 has a catalytic region 50 that can include platinum. This known electrochemical cell is not suitable for measuring chlorine compounds in water.

US Pat. No. 5,326,449 A discloses a sensor for the electrochemical analysis of catalytic reagents in solution. The sensor comprises a composite membrane 140 containing a catalytically immobilized protein, which is used to convert the reagent to be measured into an electrically measurable value. The composite membrane comprises a porous membrane 142 of a synthetic polymer material disposed in a thin flexible layer. At least one closed membrane 148 is partially embedded in the membrane comprising the protein and is positioned between the porous membrane and the analyte as a protective membrane. According to column 7, lines 15ff., the porous membrane element has a pore size in the range of approximately 0.01 micrometer to approximately 10 micrometers, preferably 0.1 micrometer to approximately 2 micrometers. This known sensor is not suitable for measuring the content of chlorine compounds in water.

US 4552013 A discloses a liquid chromatography device with a coulometric cell 23, which comprises at least one working electrode, at least one reference electrode and at least one counter electrode. The working electrode 54 comprises a material bag 68 of a platinum material in powder form with an average particle size between approximately 2 to 3 microns and approximately 400 microns. The material bag 68 is located between two porous membranes or sintering materials and is arranged in the flow channel 46 of the sample solution to be tested. The sample solution flows through the working electrode 54 and thus flows through the material bag 68 of the platinum material in powder form. The porous membrane or sintering materials are made of, for example, glass, glass fiber, polypropylene, porous Teflon, etc. The cell 23 does not have an electrolyte chamber. When manufacturing the cell 23, the dry platinum material in powder form is poured onto one of the two porous membranes (sintering materials) and then covered with a second film. The purpose of the known cell is to remove destructive components, such as oxygen, from the sample solution in the upstream of the chromatography column 28. In US 4552013, the known cell can also be used for measuring purposes when there is no more precise specification.

Summary of the Invention

The invention has the object of providing an electrochemical measuring cell for measuring the chlorine compound content in water, in which the thinnest possible flexible hydrophilic membrane can be eliminated without adversely affecting future measurement results in order to avoid the risks or disadvantages described above when handling and cleaning the measuring cell.

The measuring electrode is a rigid, porous platinum membrane with a pore size of 0.15 to 0.25 μm, creating contact with the electrolyte and water. The porous platinum membrane, which forms the measuring electrode, prevents the water being tested from entering the electrolyte chamber containing the electrolyte, allowing reactions to occur within the pores of the platinum membrane and directly measuring the resulting reaction products as they are produced.

The water to be tested flows through the measuring cell and comes into contact with the electrolyte only in the electrode pores and membrane pores.

The measuring cell according to the invention is a measuring cell for amperometric, potentiostatic measurements of chlorine compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrochemical measuring cell according to the invention.

DETAILED DESCRIPTION

In the measuring cell according to the invention, the membrane and measuring electrodes are combined in a single component. The microporous platinum membrane separates the electrolyte chamber from the water to be measured. This allows the active components of the electrolyte and the active components of the water containing the substance to be measured to meet in the pores of the measuring electrodes, react there, and the resulting reaction products are detected.

Combining both functions (membrane and measuring electrode) in one component eliminates existing issues with pressure and flow influences and optimizes diffusion. Because it provides a fixed installation and does not require replacement of rigid components, the new measuring cell is mechanically robust and does not pose a risk to users of jeopardizing measurements due to incorrect replacement of consumable parts.

Because the electrolyte contacts the water only through the rigid platinum membrane, washing out the electrolyte is essentially eliminated.

In porous membranes, the electrolyte/water contact surface area is many times greater than in conventional membrane sensors, resulting in a strong signal and no need for pre-amplification. Consequently, the measuring cell can be constructed using the same 12 mm glass sensor format as conventional water analysis cells and can be installed using the standard PG13.5 thread in conventional pH or redox instruments.

The measuring cell is made of glass and is therefore insensitive to major components of water such as, for example, surfactants.

The combined embodiment of the membrane and the porous platinum form of the measuring electrode has the advantage that the measuring cell remains clean and effective over a long period of time. The measuring cell according to the invention can be used without problems for a long period of time in applications without the presence of disinfectants.

Platinum electrodes are used, for example, in fuel cells. In this context, only surface enlargement is used to reduce material costs. Known platinum electrodes are not yet used specifically for amperometric measurements, but only for mass conversion.

Porous platinum electrodes for measurement, where substances pass through the membrane pores, are found, for example, in lambda sensors, which are used to regulate the air supply in exhaust catalysis. However, in this context, this does not involve amperometric measurements, but rather redox reaction measurements.

The measuring cell, as shown in the figure, comprises a 12 mm glass housing 1, to which is attached at its bottom end a measuring electrode 3 in the form of a rigid microporous platinum membrane with a pore size of 0.15 μm to 0.25 μm, preferably 0.2 μm. Inside the housing 1 are located a reference electrode 5 and an electrolyte chamber 2. A platinum ring serving as a counter electrode 4 is attached to the shaft-shaped housing 1. At the top end of the housing is a connection head 6 for connecting to a measured value evaluation unit.

The porous platinum membrane of the measuring electrode 3 ensures intimate contact between the electrolyte and the substance to be measured within the pores of the platinum membrane 3. The resulting product is detected at the measuring electrode 3 and reduced against this background. This results in a current proportional to the concentration of the substance to be measured.

Claims (5)

1. An electrochemical measuring cell for measuring the content of chlorine compounds in water, the electrochemical measuring cell comprising: Electrolyte chamber (2), which contains electrolytes; Measuring electrode (3) defines the electrolyte chamber (2); A reference electrode (5) is disposed within the electrolyte chamber (2); and Counter electrode (4), The electrochemical measuring cell is characterized by: The measuring electrode (3) is a rigid porous platinum membrane with a pore size of 0.15 μm to 0.25 μm, so that the physical and electrical contact between the water and the electrolyte to be tested occurs only through the rigid porous platinum membrane. The counter electrode is disposed outside the electrolyte chamber (2). 2. The electrochemical measuring cell according to claim 1, wherein the pore size is 0.2 μm. 3. The electrochemical measuring cell according to claim 1 or 2, wherein the counter electrode (4) comprises platinum. 4. The electrochemical measuring cell according to claim 1 or 2, characterized in that the counter electrode (4) is a platinum ring surrounding the electrolyte chamber (2). 5. The electrochemical measuring cell according to claim 3, wherein the counter electrode (4) is a platinum ring surrounding the electrolyte chamber (2).