JP5562179B2 - pH meter - Google Patents

Description

  The present invention relates to a pH meter, and more particularly to a pH meter having an ion sensitive field effect transistor.

  There are a glass electrode type and an ISFET type sensor for the pH meter for measuring the hydrogen ion exponent pH. First, the glass electrode type disclosed in Japanese Patent Application Laid-Open No. 2001-124722 will be described. FIG. 7 is an explanatory view showing a glass electrode type pH meter according to the prior art. In FIG. 7, 30 is a pH meter, 31 is a pH sensor, 32 is a pH responsive glass membrane, 33 is a pH responsive glass membrane support, 34 is an internal solution, 35 is a signal extraction electrode, 36 is a reference electrode internal solution, and 37 is a reference electrode. A reference electrode, 38 is a container, and 39 is a liquid junction.

  The pH sensor 31 of the pH meter 30 is provided with a portion constituting the pH sensor inside the container 38. When the liquid to be measured comes into contact with the pH responsive glass film 32 and the liquid junction 39, the pH responsive glass film 32 supported by the pH responsive glass film support 33 is polarized on both sides of the film and the pH of the internal liquid 34 and the liquid to be measured. An electromotive voltage depending on the pH of the is generated. The potential of the signal extraction electrode 35 is changed by the electromotive voltage. On the other hand, the reference electrode 37 in the reference electrode internal solution 36 generates a constant potential regardless of the pH of the solution to be measured. Therefore, a potential difference is generated between the signal extraction electrode 35 and the comparison electrode 37, and the electromotive voltage is generated. Is converted to pH by the calculation unit of the pH meter 30.

  By the way, in the case of such a glass electrode type, it is difficult to measure the pH value of a sample solution having a low pH buffering ability and a high purity due to the influence of high impedance. The glass electrode type can actually measure the pH value, for example, up to about 10 mS / m of tap water. In addition, even tap water whose pH value can be measured is susceptible to noise due to high impedance, and a phenomenon in which the potential at the time of measurement becomes unstable is observed. In addition, when measuring the pH value of a sample solution with low pH buffering capacity and low conductivity and high purity, the response of the glass electrode to the pH is slow and the potential response of the comparative electrode is extremely slow. Is substantially difficult and causes errors. At present, porous ceramics are often used as the material of the liquid junction 39. However, the porous ceramic member has a difficulty in that Ag + or AgCl2-complex dissolved from the reference electrode internal liquid 36, which is usually Ag / AgCl, is formed, and AgCl is easily eluted and clogged. As a result, leakage of the internal liquid is hindered and an inter-liquid potential difference is likely to occur, and an error has occurred between the measured value and the actual pH value.

  Therefore, the ISFET type having a fast pH response speed has attracted attention. The ISFET type disclosed in Japanese Unexamined Patent Publication No. 2000-338077 will be described below. FIG. 8 is an explanatory diagram showing an ISFET type pH meter according to the prior art. In FIG. 8, 40 is a sensor unit, 41 is an ISFET, 42 is a reference electrode unit, 43 is a seal unit, 44 is a case, 45 is a reference electrode internal liquid, 46 is a liquid junction, and 47 is an electrode wiring unit.

  Each part of the sensor unit 40 is housed in a case 44. A liquid junction 46 and a seal portion 43 that supports the liquid junction 46 and shields the opening are provided in one opening. Yes. Further, an ISFET 41 is disposed as a pH sensor at the tip of the case 44 on the side where the liquid junction 46 is provided. A reference electrode portion 42 is provided inside the case 44 and is further filled with a reference electrode internal liquid 45. Further, electrical connection to the outside to the reference electrode portion 42 is ensured by the electrode wiring portion 47.

  The ISFET as shown in FIG. 8 not only functions as a high-speed response pH sensor at the time of measurement, but also the ISFET itself functions as an impedance converter to lower the input impedance, so the pH value of the low buffer capacity sample solution It is expected to be used in the measurement of However, even if the response is dramatically increased and the input impedance can be reduced by adopting the ISFET, a delay in potential response at the reference electrode has been a problem. That is, as in the RE electrode 45, a comparative electrode having a structure in which a silver-silver chloride electrode wire is provided and liquid junction liquid is leached from the liquid junction portion has various problems in the liquid junction portion in contact with the sample solution. Has begun to be pointed out. A particularly important problem among them is contamination of the liquid junction. Every time the pH value is measured, the components of the sample solution are adsorbed to the liquid junction, but the adsorbed material cannot be easily detached and becomes a contaminant. For this reason, the liquid-liquid potential between the sample solution and the liquid junction increases, and the response speed decreases, and noise increases due to adsorption and desorption of contaminants. Therefore, even if ISFET is used, a flow potential is likely to be generated, and in a sample having a low pH buffering capacity, there is a problem that it is difficult to obtain an accurate pH value every time the pH value is measured.

JP 2001-124722 A JP 2000-338077 A

  In order to solve the above-mentioned problems, the present invention provides a pH meter having an ion-sensitive field-effect transistor, which has a low pH buffering capacity and a low conductivity and a high purity, even if the measurement of the pH value is repeated. An object of the present invention is to provide a pH meter having a structure capable of accurately measuring a value.

  The invention according to claim 1 is a pH meter having a sensor electrode and a comparison electrode, wherein the sensor electrode includes an ion-sensitive field effect transistor, and the comparison electrode includes a tubular portion formed in a substantially cylindrical shape. And a silver-silver chloride electrode portion provided inside the tubular portion, and the internal liquid inside the tubular portion is led out to the outside, and the equivalent circle diameter in a cross section perpendicular to the axial direction of the tubular portion is 2 mm. It is a pH meter characterized by having a liquid junction part formed so that it may exceed.

  A second aspect of the present invention is the pH meter according to the first aspect of the present invention, wherein the liquid junction portion is made of porous ceramic or porous glass.

  A third aspect of the present invention is the pH meter according to the first or second aspect of the present invention, wherein the reference electrode has a liquid junction part made of an organic polymer porous body.

  According to the first aspect of the present invention, the equivalent circle diameter in the cross section orthogonal to the axial direction of the tubular portion of the liquid junction exceeds 2 mm. By increasing the area, a certain number or more of pores to which molecules contained in the sample liquid are not attached as contaminants are present. As a result, the leaching of the internal liquid of the comparison electrode can be ensured to the extent necessary for accurate measurement, so that the potential response of the comparison electrode is greatly improved and a high-speed response during measurement can be realized. Further, since the pH meter is resistant to contamination, it is difficult to cause an error between the measured value and the actual pH value even if the measurement is repeated. As a result, the reproducibility in the measurement of the pH value is also improved, and even when measuring the pH of an aqueous solution having an electrical conductivity of 10 mS / m or less, a measurement result of a pH value with good reproducibility can be easily obtained.

  According to the second aspect of the present invention, it becomes difficult for a pigment such as fruit juice to adhere to the liquid junction part, and the deterioration of the appearance of the comparison electrode during long-term use can be reduced.

  According to the third aspect of the present invention, even if a sample solution container or the like hits the tip of the comparison electrode, the comparison electrode is less likely to be chipped or cracked.

It is sectional drawing which shows the liquid junction part which concerns on embodiment of this invention, and its periphery part. It is sectional drawing of the pH meter to which the liquid junction part which concerns on embodiment of this invention is applied. It is explanatory drawing of the principle of operation in an ISFET type pH meter. It is the graph which compared the response state of the glass electrode system and ISFET system at the time of measurement of pH value. It is the graph which computed the relationship between the cross-sectional area of a liquid junction part, and resistance. It is a graph which shows the Example which compared the response state at the time of pH measurement of the reference electrode which concerns on this invention. It is explanatory drawing which shows the glass electrode-type pH meter which concerns on a prior art. It is explanatory drawing which shows the ISFET type pH meter which concerns on a prior art.

  Below, the pH meter which concerns on embodiment of this invention is demonstrated based on drawing. In addition, although the following description demonstrates based on the form applied to the pH meter which formed the sensor in pencil shape, this invention is applicable to the sensor with various external shapes. That is, the present invention can be preferably applied not only to the configuration in which the liquid junction portion and the silver-silver chloride electrode portion are accommodated in the cylinder, but also to the configuration disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-265727.

  FIG. 1 is a cross-sectional view showing a liquid junction part and its peripheral part according to an embodiment of the present invention. In FIG. 1, 12 is a liquid junction part, 13 is a tubular part, 14 is a rubber plug part. Moreover, FIG. 2 is sectional drawing of the pH meter to which the liquid junction part which concerns on embodiment of this invention is applied. In FIG. 2, 10 is a pH meter, 11 is a reference electrode, 15 is a silver-silver chloride electrode wire, 16 is a reference electrode internal solution, 17 is an external connection terminal, 18 is a body insertion portion, 19 is an O-ring, and 20 is a body. Part, 21 is an ISFET, and 22 is a sample solution. FIG. 3 is an explanatory diagram of the operating principle of an ISFET type pH meter.

  First, the operating principle of the ISFET type pH meter will be described with reference to FIG. The ISFET 21 serving as a pH sensor is an N channel type ISFET. ISFET is an abbreviation for the English name Ion-Sensitive Field-Effect Transistor, and is an application of the structure of an FET, which is a unipolar transistor, to a sensor. The gate insulating film is sensitive to ions. In ISFET, a high-performance pH sensor is realized by actively utilizing this property of the insulating film. The gate electrode of the ISFET has a structure in which an electrolyte solution is in contact with a thin insulating film formed on the surface of a P-type silicon substrate, and the reference electrode 11 is in contact with this solution. The device operation of the ISFET utilizes a charge channel formed under this insulating film. That is, two upper and lower N-type semiconductor layers are formed, and each is used as a drain electrode and a source electrode, and a voltage is applied between them. In this system, when a voltage is applied to form a current path called a channel, that is, an electron flow between the N-type semiconductor layers, a conduction current flows.

  In this structure, when the hydrogen ion concentration in the solution 16 is increased, holes which are main carriers of the P-type semiconductor are repelled and moved away from the gate insulating film. The electrons that exist as a minority are drawn directly under the gate. As a result, a current path called a channel is formed between the drain electrode and the source electrode. In this way, the carrier path through which electrons accumulated along the insulating film flow in the vertical direction in the figure can be controlled by the potential of the gate electrode applied to the insulating film. That is, the potential response type sensor can control the current between the drain electrode and the source electrode by the potential of the gate electrode. Therefore, the higher the hydrogen ion concentration, the thicker the N channel layer and the more current flows, so that the ion concentration can be detected from this current value. FIG. 4 is a graph comparing the response states of the glass electrode system and the ISFET system when measuring the pH value. As shown in FIG. 4, the ISFET method has an advantage that the response speed at the time of pH measurement is faster than the glass electrode method.

  FIG. 2 shows an application of this configuration to a pencil-type pH meter. That is, the pH meter 10 allows maintenance of the reference electrode 11 to be replaced or separated by making the main body insertion portion 18 detachable from the main body portion 20 formed in a pencil shape. The main body insertion portion 18 is provided with an O-ring 19 on the outer peripheral portion thereof to prevent the sample solution and the cleaning solution from infiltrating to the site of the external connection terminal 17. The reference electrode 11 is provided with a silver-silver chloride electrode wire 15 and filled with the reference electrode internal solution 16. The internal liquid 16 of the reference electrode exudes from the liquid junction portion 12 provided in the state inserted in the tubular portion 13. An ISFET 21 is embedded at the tip of the main body 20, and the surface of the ISFET 21 is covered with an insulating film (not shown). In addition, the vicinity of the portion of the main body 20 where the insulating film is provided is formed flat so that the sample solution 22 can be easily held.

  Further, the configuration of the liquid junction 12 and its peripheral part will be described in detail. As shown in FIG. 1, the liquid junction 12 is supported on the tubular portion 13 by the rubber stopper 14 as described above. The tubular portion 13 forms part of the case of the reference electrode, and is arranged to face the ISFET 21 so that the sample solution 22 and the liquid junction portion 12 in FIG. doing. Note that the outer peripheral side of the tubular portion 13 may not be formed in a substantially cylindrical shape as long as a substantially cylindrical inner space is formed. Further, the rubber plug portion 14 supports the liquid junction portion 12 with respect to the tubular portion 13, and also has a function of preventing the reference electrode internal liquid 16 from leaking from the periphery of the liquid junction portion 12. The liquid junction part 12 is formed of a porous material so that the reference electrode internal liquid 16 oozes out from the inside. As the porous material, porous ceramic or porous glass (Vycor glass) is preferable as the hard material, and an organic polymer porous body is preferable as the soft material. When using porous ceramic or porous glass, pigments such as fruit juice are less likely to adhere to the liquid junction, so the color of the sample liquid will adhere to the liquid junction during a long period of use and give an unclean impression. Giving can be reduced. When the organic polymer porous body is used, there is an advantage that the comparative electrode is less likely to be chipped or cracked even if a hard object such as a sample solution container hits the tip of the comparative electrode or the pH meter is dropped on the floor.

  By the way, in this invention, it forms so that the circle equivalent diameter in the cross section orthogonal to the axial direction of the tubular part 13 of the liquid junction part 12 may become 3 mm or more. In order to accurately measure the pH value of a sample solution having a low pH buffering capacity and a low conductivity and a high purity, a silver-silver chloride electrode and a liquid junction part of a reference electrode containing a liquid junction liquid are used. According to the inventor's knowledge that the cross-sectional area in the direction orthogonal to the axial direction of the tubular portion supporting the liquid junction is made as large as possible, so that the infiltration of the liquid inside the reference electrode through the liquid junction is stabilized. Is based. Assuming that the liquid junction is cylindrical and its diameter is d (mm), the resistance R of the liquid junction with the cross-sectional diameter d is R = k * 4 / (πd2). Here, when the resistance in the case of d = 1 mmΦ is R0, R0 = k * 4 / π. Therefore, when normalized with the resistance R0 in the case of d = 1 mmΦ, the normalized resistance Rn becomes Rn = K0 / d2. As will be described later, this diameter is a circle-equivalent diameter, and the cross-sectional shape is not limited to a circle.

  FIG. 5 is a graph in which the relationship between the cross-sectional area of the liquid junction and the resistance is calculated. FIG. 5 is a graph of the above calculation results. As can be seen from FIG. 5, the junction resistance decreases sharply with the square of the junction diameter. Based on this graph, the inventor manufactured and compared the examples described below. FIG. 6 is a graph showing an example in which the response state at the time of pH measurement of the reference electrode according to the present invention is compared. As an example of the pH meter, the inventor manufactured a cylindrical liquid junction portion having diameters of 1 mmΦ and 3 mmΦ, and a portion other than the liquid junction portion configured as shown in FIGS. 1 and 2. With respect to these examples, each measurement characteristic was examined using rainwater as a pH value measurement sample. In general, pH measurement of rainwater is considered to be difficult, but in an example in which the cross-sectional diameter of the liquid junction is 1 mmΦ, the pH measurement value was not stable even after 5 minutes or more. On the other hand, in the case of 3 mmΦ, as shown in FIG. 6, it quickly responded and settled to a stable measurement value in about 1 minute. The inventor examined the cleaning effect after using these liquid junctions many times, and manufactured a cylindrical liquid junction with a diameter of 2 mm to 12 mm in increments of 1 mm and 2.5 mm. The actual resistance value and the like were examined. From these results, the inventors obtained the following knowledge.

  That is, in any material of porous ceramic, porous glass, and organic polymer porous body, among the pores, the effective porosity that effectively exudes the liquid inside the reference electrode to the outside is the adhesion of the sample solution. Considering the strength of the material and the contamination by the exuded reference electrode internal solution, it is generally 50% or less. In such a situation, even if it is attempted to control the outflow amount of the liquid inside the reference electrode by changing the porosity, the control range is limited due to problems such as excessive leaching of the liquid inside the reference electrode, so a dramatic improvement is expected. Can not. On the other hand, if the diameter of the liquid junction exceeds 2 mm, the effective number of effective pores increases, so that the influence of adsorbed molecules at the liquid junction can be reduced to a considerable extent or negligible. . In addition, when the diameter of the liquid junction part was 2 mm, it was not as stable as the case of 3 mm, but rather was close to the case of 1 mm. In the case of 2.5 mm, stability closer to 3 mm than 1 mm was obtained, and thus it was found that the diameter at which the stability was increased exceeded 2 mm. This greatly improves the potential response of the reference electrode and ensures a high-speed response when measuring the pH value, even if the sample solution adheres to the liquid junction, is affected by the strength of the material, or contamination by the exuded reference electrode internal solution. become able to. Moreover, even if the shape of the liquid junction portion is not cylindrical, for example, it is a rectangular tube, there is no difference in resistance value. Therefore, the cross-sectional shape of the liquid junction is not particularly limited. In addition, it can be said that the larger the diameter of the liquid junction part, the better. However, the difference in resistance value is less than 1% when the diameter is 11 mm and 12 mm, compared with the case where the diameter is 10 mm. Is expected to improve little. If the diameter of the liquid junction increases, the manufacturing cost increases and the pH meter increases in size. Therefore, it can be said that the diameter of the liquid junction is preferably 10 mm or less in consideration of this point. After all, when the equivalent circle diameter in the cross section perpendicular to the axial direction of the tubular portion 13 of the liquid junction portion 12 is more than 2 mm and 10 mm or less, the potential response of the reference electrode is greatly improved and a high-speed response at the time of measurement is obtained. It was found that the potential response of the reference electrode was improved so as to be dramatic when the distance was 3 mm or more and 10 mm or less.

  As described above, the present invention is a pH meter having an ion-sensitive field effect transistor (ISFET). Even if the measurement of the pH value is repeated, the pH of the sample liquid having a low pH buffering capacity and a low conductivity and a high purity. The value can be measured accurately, that is, the pH value can be easily measured with good reproducibility for rainwater or the like. Therefore, the pH meter embodying the present invention makes it possible to measure pH values in fields that are difficult to measure with conventional pH meters, such as environmental water such as rainwater, rivers and well water, and cooling water for boilers. It can be said that it is very big. In addition, the present invention is not limited to the contents described above, for example, about the shape of the liquid junction part or the tubular part, the internal structure and outer shape of the reference electrode, whether or not it can be inserted into and removed from the pH meter body, etc. Various modifications can be made without departing from the scope of the claims.

10 pH meter 11 Reference electrode 12 Liquid junction part 13 Tubular part 14 Rubber plug part 15 Silver-silver chloride electrode wire 16 Reference electrode internal liquid 17 External connection terminal 18 Main body insertion part 19 O-ring 20 Main body part 21 ISFET
22 Sample solution 30 pH meter 31 pH sensor 32 pH responsive glass membrane 33 pH responsive glass membrane support 34 internal liquid 35 signal extraction electrode 36 comparative electrode internal liquid 37 comparative electrode 38 container 39 liquid junction 40 sensor unit 41 ISFET
Reference electrode part 43 Seal part 44 Case 45 Reference electrode internal liquid 46 Liquid junction part 47 Electrode wiring part

Claims (3)

In a pH meter having a sensor electrode and a reference electrode,
The sensor electrode comprises an ion sensitive field effect transistor,
The comparative electrode includes a tubular portion formed in a substantially cylindrical shape, a silver-silver chloride electrode portion provided inside the tubular portion, and the internal liquid inside the tubular portion is led out to the outside, and the tubular portion And a liquid junction part having a circle equivalent diameter of 3 mm or more and 10 mm or less in a cross section perpendicular to the axial direction of the part.   The pH meter according to claim 1, wherein the liquid junction part of the reference electrode is made of porous ceramic or porous glass.   The pH meter according to claim 1, wherein the liquid junction part of the reference electrode is made of an organic polymer porous body.

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