JP2010060391A - Dissolved oxygen sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dissolved oxygen sensor for suppressing a change in pH of an interior liquid, and accurately measuring a concentration of dissolved oxygen in a sample solution. <P>SOLUTION: The dissolved oxygen sensor comprises an oxygen permeating membrane, the interior liquid containing Cl<SP>-</SP>in a chamber isolated from an exterior by the oxygen permeating membrane, and a working electrode and a counter electrode immersed in the interior liquid. The counter electrode contains silver. The interior liquid is formed by adding a buffering agent of pKa at 25°C (4.5≤pKa≤9.5). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a dissolved oxygen sensor that can suppress a change in pH of an internal solution and can measure a dissolved oxygen concentration in a sample solution with high accuracy.

  In general, the dissolved oxygen concentration in clean rivers has almost reached saturation, but when excess organic matter is discharged into water, a large amount of oxygen in the water is consumed as aerobic microorganisms oxidize and decompose organic matter. As a result, the dissolved oxygen concentration decreases. And when dissolved oxygen concentration falls, the oxidative decomposition | disassembly of the organic substance by an aerobic microorganism will be suppressed, the purification effect of a water area will fall, and water pollution will be caused. For this reason, the dissolved oxygen concentration is generally low in lakes and rivers where water quality has deteriorated. Therefore, dissolved oxygen concentration is regarded as an important index for evaluating water quality. In order to measure such a dissolved oxygen concentration, a polarographic type dissolved oxygen sensor is widely used.

  A polarographic-type dissolved oxygen sensor is generally composed of an internal liquid filled in a chamber separated from the outside by an oxygen permeable membrane, and a working electrode and a counter electrode immersed in the internal liquid. When a KCl solution is used as the counter electrode, silver or silver / silver chloride is used as the counter electrode, and a voltage is applied between the working electrode and the counter electrode, the following reactions occur at each electrode.

Working electrode (cathode): O ₂ + 2H ₂ O + 4e ⁻ → 4OH ⁻
Counter electrode (anode): 4Ag + 4Cl ⁻ → 4AgCl + 4e ⁻

A polarographic limit current proportional to the dissolved oxygen concentration flows to the external circuit, and the dissolved oxygen concentration can be measured from this current value. The current value I _lim is expressed by the following equation.
I _lim = −n × F × A × Do × Co × (1 / √πDot + 1 / r)
Here, each term in the formula is as follows.
n: number of electrons in electrode reaction F: Faraday constant A: electrode area Do: diffusion coefficient Co: oxidant concentration in bulk π Dot: diffusion layer thickness r: electrode radius

  As described above, since the current value changes depending on the thickness of the diffusion layer, keeping the thickness of the diffusion layer and the electrode area constant is important for performing highly accurate measurement. The diffusion layer is generally composed of a gap between the oxygen permeable membrane and the working electrode, and an oxygen permeable membrane.

  In the measurement using a polarographic type dissolved oxygen sensor, there is a so-called plateau region in which a current value depending on the dissolved oxygen concentration of the sample solution can be obtained even if the applied voltage with respect to the potential of the reference electrode varies somewhat. The dissolved oxygen concentration is measured at.

However, as the reaction at the counter electrode proceeds, the concentration of Cl ⁻ in the internal liquid decreases and the resistance of the internal liquid increases, and the potential of the reference electrode changes accordingly, the plateau region also fluctuates and dissolves in the plateau region. Since the oxygen concentration cannot be measured, KCl is added to the internal liquid for the purpose of reducing the resistance of the internal liquid in order to suppress fluctuations in the plateau region.

However, in a polarographic-type dissolved oxygen sensor using a high-concentration KCl solution as an internal solution and using a silver or silver / silver chloride as a counter electrode, Cl ⁻ generated by ionization of KCl was generated at the counter electrode. There is a problem that it reacts with AgCl, forms a complex such as AgCl ₂ ⁻ and is eluted from the counter electrode, dissolves in the internal solution, and silver adheres to the working electrode. There is a problem in that the distance between the permeable membrane and the working electrode varies, and as a result, the thickness of the diffusion layer changes and the measurement accuracy decreases. In addition, a measurement error is caused by a change in the surface area of the working electrode due to the adhesion of silver. For this reason, the KCl concentration of the internal liquid cannot be increased.

In contrast, Patent Document 1, the sulfate is an ion conductor which the aqueous solution is excellent Cl ^- concentration was added to the low internal fluid, Cl ^- concentration is high internal fluid equivalent ionic conductivity by preparing an electrolytic solution having, to maintain the current required for the reaction between the working electrode and the counter electrode, and sufficient Cl to oxidation reaction at the counter electrode ^- also while maintaining the concentration, Cl internal solution ^- the concentration A method of lowering and suppressing elution of silver chloride from the counter electrode is disclosed.

However, even if a sulfate such as sodium sulfate is added to the internal liquid, the OH ⁻ generated at the working electrode makes the internal liquid alkaline, and when silver hydroxide (silver oxide) is generated, the oxygen permeable membrane and the working electrode There arises a problem that the thickness of the diffusion layer changes due to penetration.

  Further, when the internal liquid becomes alkaline, there is a problem that safety at the time of exchanging the internal liquid cannot be ensured, and it is necessary to neutralize the replaced discarded internal liquid.

Furthermore, as described above, in the measurement using a polarographic type dissolved oxygen sensor, there is a so-called plateau region in which a current value depending on the dissolved oxygen concentration of the sample solution can be obtained even if the applied voltage with respect to the potential of the reference electrode varies slightly. In this plateau region, the dissolved oxygen concentration is measured. At this time, the following reaction occurs at the reference electrode.
AgCl + e−⇔Ag + Cl−

The potential of the reference electrode is obtained from the following Nernst equation.
E = E ⁰ + (RT / F) (lna _Cl− )
Here, each term in the formula is as follows.
E: Reference electrode potential R: Gas constant T: Temperature a: Activity F: Faraday constant Thus, the potential of the reference electrode depends on the concentration of Cl ⁻ .

On the other hand, as described above, OH ⁻ is generated at the working electrode, and as a result, the concentration of OH ^{− in} the internal liquid increases, and when the concentration of Cl ⁻ decreases along with the reaction at the counter electrode, A competitive reaction occurs as shown.
4Ag + 4Cl ⁻ → 4AgCl + 4e ⁻
4Ag + 4OH ⁻ → 4AgOH + 4e ⁻

However, when a competitive reaction as described above occurs, the potential of the reference electrode changes and the plateau region also fluctuates as is apparent from the Nicholsky equation, so that measurement within the plateau region cannot be performed.
US Pat. No. 5,632,882

  Therefore, the present invention aims to provide a dissolved oxygen sensor capable of suppressing the pH change of the internal solution and measuring the dissolved oxygen concentration in the sample solution with high accuracy.

That is, the dissolved oxygen sensor according to the present invention comprises an oxygen permeable membrane, an internal liquid containing Cl ⁻ in a chamber separated from the outside by the oxygen permeable film, and a working electrode and a counter electrode immersed in the internal liquid. The counter electrode contains silver, and the internal solution is characterized by adding a buffer having a pKa at 25 ° C. of 4.5 ≦ pKa ≦ 9.5. Here, the buffering agent refers to a reagent that buffers a predetermined pH region, and the generally controllable pH region is in the range of about pKa ± 1 of the buffering agent.

In such a case, a buffer having a pKa at 25 ° C. of 4.5 ≦ pKa ≦ 9.5 exhibits a buffering action in a pH range near neutrality, so that the internal solution becomes alkaline and hydroxylated. The situation in which the thickness of the diffusion layer changes due to the adhesion of silver to the working electrode is suppressed, and the thickness of the diffusion layer is kept constant and the dissolved oxygen concentration can be measured with high accuracy. Moreover, since the competitive reaction at the counter electrode is suppressed by keeping the internal liquid neutral, the plateau region can be kept constant, and measurement within the plateau region can be ensured. Further, since the buffer agent is ionized in the internal liquid and functions as an ionic conductor, the resistance of the internal liquid is reduced even if the Cl ⁻ concentration of the internal liquid is lowered in order to suppress elution of silver from the counter electrode. Sufficient ionic conductivity to be maintained can be ensured, and also from this point, the plateau region is kept constant, and measurement in the plateau region can be ensured.

  Further, according to the present invention, since the internal liquid is kept neutral, safety at the time of replacement of the internal liquid can be secured, and there is no need to neutralize the replaced waste internal liquid.

Furthermore, by reducing the Cl ⁻ concentration of the internal liquid, it is possible to prevent silver from eluting from the counter electrode and dissolved in the internal liquid and adhere to the working electrode, so that the surface area of the working electrode and the diffusion layer The thickness can be kept constant, and from this point, the dissolved oxygen concentration can be measured with high accuracy.

As the buffer, for example, MH ₂ PO ₄ and M ₂ HPO ₄ (M is a group 1 element. Each M may be the same or different) are preferably used. It is done. MH ₂ PO ₄ dissociates in water and takes an equilibrium state as shown in the following formula.
MH ₂ PO ₄ → H ₂ PO ₄ ⁻ + M ^{+
_{H 2 PO 4 - ⇔HPO 4 2-}} + H +
HPO ₄ ²⁻ ⇔PO ₄ ³⁻ + H ⁺

On the other hand, M ₂ HPO ₄ is completely ionized in water and takes an equilibrium state as shown in the following formula.
M ₂ HPO ₄ → HPO ₄ ²⁻ + 2M ^{+
_{HPO 4 2- ⇔PO 4 3- + H}} +

For this reason, ionic species such as H ₂ PO ₄ ⁻ , HPO ₄ ²⁻ , and PO ₄ ³⁻ are generated in the internal liquid, and the resistance of the internal liquid is kept low. Moreover, since HPO ₄ ^2- which is a divalent anion has high ionic strength, it exhibits an excellent buffering action.

M is not particularly limited as long as it is a Group 1 element, and examples thereof include Na and K. In addition, M of MH ₂ PO ₄ and M ₂ HPO ₄ may be the same or different.

  The dissolved oxygen sensor according to the present invention preferably includes a reference electrode when measurement with higher accuracy and sensitivity is required. Measuring with three electrodes, counter electrode, working electrode, and reference electrode, can control the absolute value of the voltage applied between the working electrode and the counter electrode, so that more accurate and sensitive measurement is performed. It is possible.

  When measuring various indicators regarding water quality in a multifaceted manner, it is preferable to use a composite type water quality analyzer equipped with other types of sensors in addition to the dissolved oxygen sensor according to the present invention. Such a water quality analyzer is also one aspect of the present invention.

  As described above, according to the present invention, the thickness of the diffusion layer is kept constant, the fluctuation of the working electrode surface area is prevented, and the measurement within the plateau region is ensured, so that the dissolved oxygen concentration in the sample solution is highly accurate. Can be measured.

  An embodiment of the present invention will be described below with reference to the drawings.

  The water quality analyzer 1 according to the present embodiment continuously measures measurement items such as pH, conductivity, dissolved oxygen concentration, turbidity, water temperature, etc., as shown in FIG. As shown in the figure, an immersion type sensor unit 2 having a plurality of measurement sensors for measuring water quality, and an instrument body 3 electrically connected to the sensor unit 2 via a waterproof electric cable CA are provided. ing. For example, when water quality analysis of seawater is performed, the measurement is performed with the electric cable CA, the sensor unit 2 suspended in the seawater, and the sensor unit 2 immersed in the seawater.

  The sensor unit 2 includes an immersion type sensor unit main body 21 having a plurality of types of measurement sensors 4 and a sensor protection unit 22 attached to the sensor unit main body 21 to protect the measurement sensor 4 from the outside. .

  As shown in FIG. 2, the sensor unit main body 21 includes a power unit, a calculation unit having a memory function unit, a watertight case 211 having a pressure-resistant structure including a data logger for recording time-series measurement data of the calculated water quality, and the like. A pH sensor, a conductivity sensor, a turbidity sensor, a dissolved oxygen sensor (hereinafter referred to as a DO sensor) composed of, for example, a glass pH electrode for pH measurement and a comparative electrode attached to the lower end portion 211A of the watertight case 211. 41, and a plurality of sample measurement sensors 4 such as temperature sensors. The glass pH electrode, the comparison electrode, and the DO sensor 41 are of a cartridge type considering that they are generally deteriorated or unexpectedly damaged with use, and can be easily replaced. The DO sensor 41 will be described in detail later.

  The sensor protection unit 22 is attached to the sensor unit main body 21 and protects the measurement sensor 4 from the outside while guiding a liquid (for example, seawater) that is an external measurement target to the inside of the sensor unit 2.

  The meter body 3 includes a display unit for displaying measurement data from the sensor unit 2, a power key, a function key, a measurement start / end key, a calibration key, a select key, an up / down key, and the like. When the sensor unit 2 is submerged by operating the electric cable CA, measurement data based on the output from each measurement sensor 4 is recorded in the memory function unit, and the measurement value is displayed on the display unit.

  As shown in FIG. 3, the DO sensor 41 includes a hollow casing 42, an oxygen permeable film 43 provided at the upper end opening of the casing, and a room formed in the casing 42 by the oxygen permeable film 43. A filled internal liquid 44, a working electrode 45 immersed in the internal liquid 44, a counter electrode 46, and a reference electrode 47 are provided, and the dissolved oxygen concentration is measured by a three-electrode system. Lead wires L are connected to the working electrode 45, the counter electrode 46, and the reference electrode 47, respectively, and these lead wires are connected to a voltage application device (DC power supply) PS and an ammeter AM.

  The casing 42 is screwed into a base end opening of the casing main body 421 having a cylindrical shape in which an oxygen permeable film 43 is stretched at a front end opening, and the opening is liquid-tightly closed. And a base member 422.

  The base member 422 is made of a resin having excellent insulating properties such as polyphenylene sulfide (PPS), and is formed integrally with the working electrode 45, the counter electrode 46, and the reference electrode 47.

  The base member 422 includes a columnar member 422a extending inside the housing body 421 and a columnar member 422b provided on the outer periphery of the base end of the columnar member 422a. The base member 422 is provided on the outer peripheral surface of the columnar member 422b. The formed screw groove is configured to be screwed into the base end opening of the housing body 421 and press the seal member O. A columnar working electrode 45 passes through the center of the columnar member 422a so as to match the central axis, and a cylindrical counter electrode 46 is fitted outside the columnar member 422a. Further, the reference electrode 47 passes through the peripheral edge of the cylindrical member 422b.

  Of these three electrodes 45, 46, 47, the working electrode 45 is made of gold, platinum, silver, or the like, and is provided to face the oxygen permeable film 43. The counter electrode 46 is made of silver, silver / silver chloride or the like, and the reference electrode 47 is made of silver / silver chloride. Examples of the combination of these three electrodes include a combination of a working electrode 45 made of gold, a counter electrode 46 made of silver, and a reference electrode 47 made of silver / silver chloride.

  By the way, the three electrodes 45, 46 and 47 penetrate the base member 422 in a liquid-tight manner without interposing a seal member or an adhesive. Therefore, the base member 422 and the three electrodes 45, 46, 47 are formed by an injection molding method in which the three electrodes 45, 46, 47 are arranged in the mold in advance and the resin is injected into the gap to form the base member 422. It is integrally formed.

  In addition, the cylindrical member 422a and the tip surface 451 of the working electrode 45 are formed in a partial spherical shape, and when the housing body 421 is screwed to the base member 422, the oxygen permeable membrane 43 has a certain degree of contact with the tip surface 451. It is configured to stick with tension. With this configuration, the internal liquid 44 penetrates between the oxygen permeable membrane 43 and the tip surface 451 by capillary action or the like, and the gap between the inner surface of the oxygen permeable membrane 43 and the tip surface 451 becomes a constant value of about 10 μm or less. The effect is maintained. The peripheral edge of the tip surface 451 is subjected to R processing so that no edge is formed, so that a capillary phenomenon occurs smoothly with the oxygen permeable film 43.

  The oxygen permeable film 43 is a film that transmits oxygen but does not transmit liquid. For example, the oxygen permeable film 43 includes a polyethylene film or a fluororesin film such as a tetrafluoroethylene-hexafluoropropylene copolymer (FEP). The film thickness is about 25 to 50 μm.

The internal liquid 44 contains Cl ⁻ and is added with a buffer having a pKa at 25 ° C. of 4.5 ≦ pKa ≦ 9.5. The formed chamber is filled.

Examples of the Cl ⁻ include those derived from chloride salts such as KCl and NaCl. As such an internal solution containing Cl ⁻ , for example, a KCl solution is preferably used. In the present embodiment, it is preferable that the Cl ⁻ concentration of the internal solution is low in order to prevent silver chloride from eluting from the counter electrode 46 and adhering to the working electrode 45. For this reason, it is preferable that the density | concentration of chloride salts, such as KCl, in an internal liquid is 0.1-1M. If it is less than 0.1M, it will be difficult to ensure sufficient ionic conductivity to keep the resistance of the internal liquid 44 low. On the other hand, when it exceeds 1M, silver chloride elutes from the counter electrode 46 and adheres to the working electrode 45, the surface area of the working electrode 45 increases, and the thickness of the diffusion layer also fluctuates. Becomes difficult.

  Examples of the buffer having a pKa at 25 ° C. of 4.5 ≦ pKa ≦ 9.5 include, for example, citric acid (pKa2 = 4.76, pKa3 = 6.40), phosphoric acid (pKa2 = 7.20), Tris (hydroxymethyl) aminomethane (Tris) (tris conjugate acid pKa = 8.06), boric acid (pKa = 9.23) and the like. Note that a plurality of types of these buffering agents may be used in combination.

Among these various buffering agents, MH ₂ PO ₄ and M ₂ HPO ₄ (M represents a group 1 element. Each M may be the same or different) are preferably used. It is done. MH ₂ PO ₄ and _M 2 HPO _4, in the internal solution, so ionized to ionic strength resulting in _HPO ^{4 2-a} high divalent anions exhibits excellent cushioning, the pH of the internal solution It can be stably kept near neutrality. Moreover, since MH ₂ PO ₄ and M ₂ HPO ₄ are ionized in the internal liquid to generate ionic species such as H ₂ PO ₄ ⁻ , HPO ₄ ²⁻ , PO ₄ ³⁻ , and also function as an ionic conductor, Even when the Cl ⁻ concentration in the internal liquid is low, the resistance of the internal liquid can be kept low, the plateau region can be kept constant, and the measurement in the plateau region can be ensured.

Further, by using MH ₂ PO ₄ and M ₂ HPO ₄ as buffers, it is possible to suppress a pH change with respect to a temperature change, and MH ₂ PO ₄ and M ₂ HPO ₄ have high solubility. , The buffer capacity can be increased.

The concentration (total) of MH ₂ PO ₄ and M ₂ HPO ₄ in the internal liquid 44 is preferably about 0.1 to 1M. If it is less than 0.1 M, the buffering action is not sufficient, and sufficient ionic conductivity cannot be imparted to keep the resistance of the internal solution 44 having a low Cl ⁻ concentration low. On the other hand, when it exceeds 1M, it precipitates at a low temperature, and the thickness of the diffusion layer may change due to the precipitate.

When the internal solution 44 is a KCl solution, MH ₂ PO ₄ and M ₂ HPO ₄ are not particularly limited. For example, NaH ₂ PO ₄ and KH ₂ PO ₄ , Na ₂ HPO ₄ and K ₂ HPO ₄ Can be used in appropriate combinations.

  In order to measure the dissolved oxygen concentration in the sample solution using the DO sensor 41, first, when the DO sensor 41 is immersed in the sample solution, the oxygen dissolved in the sample solution permeates the oxygen permeable film 43 and the working electrode 45. Dissolved in the internal liquid 44 existing in the gap between the oxygen permeable membrane 43 and the oxygen permeable membrane 43. When a voltage is applied between the working electrode 45 and the counter electrode 46, the following reaction occurs at each electrode.

Working electrode 45 (cathode): O ₂ + 2H ₂ O + 4e ⁻ → 4OH ⁻
Counter electrode 46 (anode): 4Ag + 4Cl ⁻ → 4AgCl + 4e ⁻

  The current value of the current that flows when oxygen is reduced on the surface of the working electrode 45 is measured by the ammeter AM, and the calculation unit that receives the output signal indicating the current value performs a predetermined calculation process. Thus, the dissolved oxygen concentration in the sample solution is calculated.

Therefore, according to the water quality analyzer 1 according to the present embodiment configured as described above, the buffer added to the internal liquid 44 of the DO sensor 41 has a pKa at 25 ° C. of 4.5 ≦ pKa ≦ 9.5. Since the agent exhibits a buffering action in a pH range near neutrality, the situation in which the internal liquid 44 becomes alkaline and silver hydroxide adheres to the working electrode 45 and the thickness of the diffusion layer changes is suppressed. Therefore, it is possible to measure the dissolved oxygen concentration with high accuracy. Moreover, since the competitive reaction in the counter electrode 46 is suppressed by keeping the internal liquid 44 neutral, the plateau region is kept constant, and measurement within the plateau region can be ensured. Further, since the buffering agent is ionized in the internal liquid 44 and functions as an ionic conductor, even if the Cl ⁻ concentration of the internal liquid 44 is lowered to suppress elution of silver chloride from the counter electrode 46, the internal liquid The ion conductivity sufficient to keep the resistance low can be ensured. Also from this point, the plateau region is kept constant, and the measurement in the plateau region can be ensured.

  Further, according to this embodiment, since the internal liquid 44 is kept neutral, it is possible to ensure safety when replacing the internal liquid 44, and there is no need to neutralize the replaced internal waste liquid 44. .

Further, by lowering the Cl ⁻ concentration of the internal liquid 44, it is possible to prevent silver chloride from eluting from the counter electrode 46 and dissolved in the internal liquid 44, thereby preventing silver from adhering to the working electrode 45. The surface area and the thickness of the diffusion layer can be kept constant, and from this point, the dissolved oxygen concentration can be measured with high accuracy.

  The present invention is not limited to the above embodiment.

  For example, the DO sensor 41 in the embodiment performs measurement by the three-electrode method including the working electrode 45, the counter electrode 46, and the reference electrode 47, but the DO sensor according to the present invention includes the working electrode 45 and the counter electrode. It may be based on the two-electrode method provided with only. In the three-electrode method, the absolute value of the voltage applied between the working electrode 45 and the counter electrode 46 can be controlled, so that measurement with higher accuracy and sensitivity can be performed. According to this, since the electrodes to be used are two electrodes of the working electrode 45 and the counter electrode 46, the structure of the DO sensor 41 can be simplified and miniaturized.

  In addition, it is needless to say that some or all of the above-described embodiments and modified embodiments may be appropriately combined, and various modifications can be made without departing from the scope of the invention.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Using a plurality of DO sensors having the following configuration, various tests were performed by applying a voltage of 650 mV between the working electrode and the counter electrode.
Internal solution (composition): 0.8 M KCl, 0.5 M phosphate buffer (KH ₂ PO ₄ , Na ₂ HPO ₄ ) (pH 6.5)
Oxygen permeable membrane: FEP membrane (thickness 25 μm)
Working electrode: Gold Counter electrode: Silver Reference electrode: Silver / silver chloride Base member: PPS
Diffusion layer thickness: about 10 μm

  As a comparison object, a conventional polarographic DO sensor was used.

<Linearity>
Using a plurality of sample solutions having different dissolved oxygen concentrations (each dissolved oxygen concentration is 0, 20, 40, 60, 80, 100%), each DO sensor is used to measure the dissolved oxygen concentration, and the measured value (display) And the correlation with the indicated value displayed in the part). The results are shown in the graph of FIG.

  From the graph of FIG. 4, when regression analysis was performed on the measurement results obtained for each DO sensor, the determination coefficient R2 of the product of the present invention was 0.9992, and it was confirmed that the measurement accuracy was high.

<Reproducibility>
Each DO sensor was immersed in saturated dissolved oxygen water and then immersed in sodium sulfite water, and the dissolved oxygen concentration was measured to evaluate the variation in results. The results are shown in the graph of FIG.

  From the graph of FIG. 5, as a result of repeating the measurement three times, it was confirmed that the coefficient of variation CV of the product of the present invention was 1.2%, and there was little variation in the measurement result and the measurement accuracy was high.

<Response>
Each DO sensor was immersed in saturated dissolved oxygen water and then immersed in sodium sulfite water, and the response speed was evaluated. The results are shown in the graph of FIG.

  From the graph of FIG. 6, the product of the present invention has a 90% response time of 15 seconds and a 95% response time of 18 seconds, whereas the conventional product has a 90% response time of 16 seconds and a 95% response. The time was 20 seconds, and it was confirmed that the product of the present invention had a faster response speed.

<PH change of internal liquid>
Continuous measurement was continued for 30 days using each DO sensor, and the pH change of the internal liquid during that period was recorded. The results are shown in the graph of FIG.

  From the graph of FIG. 7, in the product of the present invention, the pH of the internal liquid changed only slightly and the liquidity was kept neutral, whereas in the conventional product, the pH of the internal liquid changed greatly and the liquidity Became alkaline.

The perspective view of the water quality analyzer concerning one embodiment of the present invention. Sectional drawing of the sensor part main body in the embodiment. The end view of the DO sensor in the same embodiment. The graph which shows the result of having investigated the linearity of the measured value of DO sensor which concerns on this invention. The graph which shows the result of having investigated the reproducibility of DO sensor concerning the present invention. The graph which shows the result of having investigated the response speed of the DO sensor which concerns on this invention. The graph which shows the result of having investigated the pH change of the internal liquid of the DO sensor which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Water quality analyzer 2 ... Sensor part 21 ... Sensor part main body 22 ... Sensor protection part 3 ... Instrument main body 4 ... Measurement sensor 41 ... Dissolved oxygen sensor 43 ... Oxygen permeable membrane 44 ... inner liquid 45 ... working electrode 46 ... counter electrode

Claims (4)

Comprising an internal solution containing a working electrode and a counter electrode were immersed in the internal solution, the, ^- the oxygen-permeable membrane, in a room that is separated from the outside by the oxygen-permeable membrane, Cl
The counter electrode contains silver;
The dissolved oxygen sensor is characterized in that the internal liquid is obtained by adding a buffer having a pKa at 25 ° C. of 4.5 ≦ pKa ≦ 9.5. The internal liquid is formed by adding MH ₂ PO ₄ and M ₂ HPO ₄ (M represents an element of Group 1; each M may be the same or different). The dissolved oxygen sensor according to 1.   The dissolved oxygen sensor according to claim 1, further comprising a reference electrode immersed in the internal liquid.   A water quality analyzer comprising the dissolved oxygen sensor according to claim 1, and at least one other sensor.