Small glass electrode and process for preparation thereof.

0001The present invention relates to a small glass electrode and a process for the preparation thereof. More particularly, the present invention relates to a small glass electrode formed by utilizing a micro-machining technique and a process for the preparation thereof.

0002A glass electrode is easily usable as a sensor for determining the hydrogen ion (H⁺) concentration in an aqueous solution, and is widely and generally used.

0003The determination of the H⁺ concentration is required not only in ordinary chemical experiments but also in fermentation control and the medical field.

0004Furthermore, a biosensor fabricated by combining a glass electrode with either enzymes or microorganisms can be used for determining various chemical compounds.

0005For example, glucose reacts with dissolved oxygen with the aid of a catalyst called glucose oxidase and is oxidized to gluconolactone. Changes of the H⁺ concentration during this reaction are measured, and the glucose concentration can be determined from the change quantities. According to a similar principle, the urea concentration can be determined.

0006In the glass electrode, the H⁺ concentration is measured by utilizing the electroconductivity of glass. Namely, the phenomenon that when a glass film having a thickness of about 100 µm and an electric resistance of several hundred MΩ is placed in a solution, a voltage difference is produced according to the pH value of the solution is utilized.

0007Fig. 1 of the accompanying drawings illustrates the structure of a sensing portion of a conventional glass electrode, which comprises a reference electrode 2 composed of silver/silver chloride (Ag/AgCl), an internal solution 4 such as a potassium chloride (KCl) solution having a certain concentration, and a spherical sensing glass film 6 formed on the top end.

0008When this glass electrode is immersed in a solution containing H⁺, in response to the active quantity (ai) of H⁺, a potential is generated according to the Nernst equation:<maths id="math0001" num="(1)"><math display="block"><mrow><mtext>E = const + (RT/F)ℓnai</mtext></mrow></math><img file="EP0489601A2_D0001.tif" /></maths>

0009wherein E represents the potential of the glass electrode, R represents the gas constant, T represents the absolute temperature, and F is the Faraday constant. Accordingly, the H⁺ concentration is determined by the above equation.

0010However, the commercially available glass electrode has a size similar to that of a fountain pen, as shown in Fig. 1, and the glass electrode is formed by glazing and it is expensive.

0011An ion-sensitive electric field effect transistor (abbreviated to "ISFET") has been developed as a small H⁺ concentration sensor. Since a photo-lithographic technique of a semiconductor is utilized for the formation, the size can be reduced.

0012In a device immersed in an aqueous solution, such as ISFET, insulation of the substrate is important.

0013Accordingly, many elements formed on a silicon (Si) substrate are diced into chips and a silicon nitride (Si₃N₄) film is formed on the peripheries of the chips to effect insulation, or an SOS (silicon-on-sapphire) substrate is used. Alternatively, there is adopted a method in which a thin film transistor (TFT) is formed on a glass substrate. However, rise of the price cannot be avoided and therefore, the sensor cannot be manufactured at a low price.

0014The conventional glass electrode formed by glazing is large in size and is expensive. In ISFET formed by the photolithography of an Si substrate, insulation is indispensable and rise of the price is inevitably caused by the necessity of maintaining the insulation, and reduction of the price is not easy.

0015Under this background, development of another method of providing a practical glass electrode of small size and low price is desired.

0016It is an object of the present invention to provide a small glass electrode capable of overcoming the foregoing problems and a process for the preparation thereof.

0017In accordance with one aspect of the present invention, there is provided a glass electrode, which has a bonded structure comprising a reference electrode composed of silver/silver chloride, a glass substrate having a pad embedded therein, said pad being composed of gold or platinum and circuit-connected to the reference electrode, and a silicon substrate having a (100) plane selectively etched by the anisotroic etching technique and comprising a groove for injecting an electrolyte containing chlorine such as a potassium chloride buffer solution or hydrochloric acid solution, at least one hole for holding the electrolyte and a glass film formed in a portion corresponding to the reference electrode.

0018The glass film may have a multiple-layer (for example, two-layer) structure.

0019In accordance with another aspect of the present invention, there is provided a process for the preparation of a glass electrode, which comprises a step of selectively etching a glass substrate and, forming a reference electrode composed of silver/silver chloride and a pad composed of gold or platinum and circuit-connected to the reference electrode, on the etched portion, a step of subjecting a silicon substrate having a (100) plane as the substrate face to anisotropic etching to form an electrolyte-injecting groove and at least one electrolyte-holding hole, a step of forming a glass film acting as a sensing film by utilizing a silicon oxide film used as the mask of said anisotropic etching, and a step of bonding the formed glass substrate and silicon substrate to each other.

0020In accordance with still another aspect of the present invention, there is provided a process for the preparation of a glass electrode, which comprises a step of selectively etching a glass substrate and forming a reference electrode composed of silver/silver chloride and a pad composed of gold or platinum and circuit-connected to the reference electrode, on the etched portion, a step of subjecting a silicon substrate having a (100) plane as the substrate face to anisotropic etching to form an electrolyte-injecting groove and at least one electrolyte-holding hole by using a patterned silicon oxide as a mask, a step of removing the silicon oxide film left on the silicon substrate face and forming a glass film acting as a sensing film in a lower portion of the holding hole, and a step of bonding the formed glass substrate and silicon substrate to each other.

0021The process of the present invention may further comprise a step of injecting the electrolyte into the holding hole from the injecting groove according to need. Accordingly, a small glass electrode having an electrolyte injected in the holding hole is included within the scope of the present invention. The present invention enables a glass electrode having a small size to be provided.

0022For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- <ul id="ul0001" list-style="none"><li>Fig. 1 is a sectional view illustrating the conventional glass electrode;</li><li>Fig. 2(A) is a plane view showing a glass substrate in the glass electrode according to one embodiment of the present invention, Fig. 2(B) is a plane view showing an Si substrate in the glass electrode according to this embodiment, and Fig. 2(C) is a plane view showing the glass electrode of one embodiment of the present invention, formed by bonding the glass substrate and Si substrate shown in Figs. 2(A) and 2(B);</li><li>Fig. 3(A) is a diagram illustrating the section taken along line X-X′ in Fig. 2(C) and Fig. 3(B) is a sectional view showing a modification of the glass electrode according to one embodiment of the present invention;</li><li>Fig. 4 is a diagram illustrating steps of forming the glass electrode according to one embodiment of the present invention;</li><li>Fig. 5(A) is a plane view showing a glass substrate in the glass electrode according to another embodiment of the present invention, Fig. 5(B) is a plane view showing an Si substrate in the glass electrode according to this embodiment, and Fig. 5(C) is a plane view showing the glass electrode of another embodiment of the present invention, formed by bonding the glass substrate and Si substrate shown in Figs. 5(A) and 5(B);</li><li>Fig. 6 is a diagram illustrating the section taken along line X-X′ in Fig. 5(C);</li><li>Fig. 7 is a diagram illustrating steps of forming the glass electrode according to another embodiment of the present invention;</li><li>Fig. 8(A) is a plane view showing a glass substrate in the glass electrode according to still another embodiment of the present invention, Fig. 8(B) is a plane view showing an Si substrate in the glass electrode according to this embodiment, and Fig. 8(C) is a plane view showing the glass electrode of still another embodiment of the present invention, formed by bonding the glass substrate and Si substrate shown in Figs. 8(A) and 8(B);</li><li>Fig. 9 is a diagram illustrating the section taken along line X-X′ in Fig. 8(C);</li><li>Fig. 10 is a diagram illustrating steps of forming the glass electrode according to still another embodiment of the present invention;</li><li>Fig. 11 is a diagram illustrating a response curve of the glass electrode of the present invention; and</li><li>Fig. 12 is a curve illustrating the relation between the change of the H⁺ concentration and the change of the electrode potential in the glass electrode of the present invention.</li></ul>

0023The present inventors already succeeded in providing a practically utilizable small Clark oxygen electrode by applying the micro-machining technique to an Si substrate (USP 4975175).

0024This small oxygen electrode is often used for medical treatment. In this case, the oxygen concentration is measured relative to the H⁺ concentration, rather than the oxygen concentration alone.

0025Accordingly, practical utilization of a small glass electrode by using the micro-machining technique was tried.

0026The following plan describes said utilization by the present inventors: <ul id="ul0002" list-style="none"><li>(1) The size of the glass electrode now used as the H⁺ concentration sensor and found to operate well, is reduced.</li><li>(2) At least one holding hole formed by anisotropic etching of an Si substrate is used as a vessel for storing an electrolyte.</li><li>(3) A glass film formed by heat oxidation or sputtering is used as a sensing film (sensing element) responding to the H⁺ concentration.</li><li>(4) A reference electrode is formed on a glass substrate, and the glass substrate and Si substrate are bonded and integrated by the anodic bonding method.</li><li>(5) A reference electrode composed of Ag/AgCl, a lead-in line and a pad are embedded in the glass substrate.</li></ul>

0027The glass substrate to be bonded to the Si substrate is required to adhere tightly to the Si substrate not only during the cooling course from the anode bonding treatment temperature, that is, about 250°C, to normal temperature but also in the application atmosphere. For this purpose, it is required that (1) the glass substrate should have a thermal expansion coefficient approximate to that of Si, (2) the glass substrate should be composed of a glass having a low softening point, and (3) the glass substrate should have a high resistance to thermal stress. In view of the foregoing, Pyrex glass or lead glass is preferably used. This glass alone can be used as the substrate, or this glass can be used in a state bonded to other glass substrates or Si substrate.

0028Under this background, a glass electrode is formed by using the micro-machining technique according to the present invention.

0029In the glass electrode of the present invention, a thin glass film acting as the sensing film is formed by utilizing at least a part of an SiO₂ film obtained by wet oxidation of a wafer.

0030However, when a glass film having an increased strength is desired, the above-mentioned embodiment of the process for preparing a glass electrode is adopted in the present invention.

0031The thickness of the SiO₂ film formed by the wet oxidation of the silicon wafer is about 1 µm at greatest, and it is impossible to increase the film thickness. Accordingly, in the process of the above-mentioned embodiment, a glass substrate is etched in advance to form a glass film having a desired thickness, a silicon oxide film left on the surface of the silicon substrate is removed, and then, the glass film is bonded to the bottom portion of the holding hole.

0032In the so-prepared glass electrode of the second embodiment of the present invention, the strength of the glass film acting as the sensing film is increased. Therefore, the yield is improved and the glass electrode can be used without breaking.

0033In the glass electrodes obtained according to the above-mentioned two embodiments of the present invention, the sensitivity to the H⁺ concentration is in practice sufficiently high. However, when a glass electrode having a further enhanced sensitivity is desired, a glass electrode having a structure described below can be adopted.

0034The voltage generated at the glass electrode is generally represented by the following Nernst equation:<maths id="math0002" num="(1)"><math display="block"><mrow><mtext>E = constant - 0.059pH</mtext></mrow></math><img file="EP0489601A2_D0002.tif" /></maths> However, this equation is a theoretical formula, and it is not easy to construct the glass electrode so that the potential in accordance with this equation is obtained.

0035When a sensing film is formed by using a Pyrex glass having a thickness of about 50 µm, the gradient coefficient -0.059 (-59 mV) of the Nernst equation is about -0.03 (-30 mV).

0036Therefore, the present invention improves the glass material.

0037Lithium (Li) glass, sodium-calcium (Na-Ca) glass and the like are known as the glass for a glass electrode. However, in view of the heat resistance and strength, these materials cannot be used as the sensing film for a small glass electrode. The reasons are as follows. <ul id="ul0003" list-style="none"><li>(1) A heat treatment at about 800°C is necessary for bonding to the Si substrate, and the heat resistance is insufficient.</li><li>(2) A considerable strain is left after heat bonding, but the glass cannot resist this strain.</li></ul>

0038According to the present invention, a double-layer film obtained by forming a film of a glass having excellent characteristics, such as a lithium glass or a sodium-calcium glass, by sputtering or vacuum deposition on a film of a glass insufficient in characteristics required for the sensing film, such as a Pyrex glass, is used as the sensing film, whereby a glass electrode satisfying substantially the requirement of the Nernst equation can be obtained.

0039In the present invention, it is sufficient if at least one holding hole is formed. However, if a plurality of holding holes are formed and glass films are bonded thereto, the risk of breaking the glass film is effectively reduced.

0040In the process of the present invention, bonding of the glass substrate and the silicon substrate is accomplished, for example, by a method of heating and bonding both substrates, a method using an adhesive or an anodic bonding method. From a practical viewpoint, the anode bonding method is preferably adopted.

0041The present invention will now be described in detail with reference to the following examples that by no means limits the scope of the invention.

Example 1

0042Fig. 2(A) is a plane view showing a glass substrate 10 in the glass electrode according to one embodiment of the present invention, Fig. 2(B) is a plane view showing an Si substrate 20 in the glass electrode according to this embodiment, and Fig. 2(C) is a plane view showing the glass electrode of one embodiment of the present invention, formed by reversing the glass substrate onto the Si substrate 20 and bonding them.

0043A reference electrode 12 composed of Ag/AgCl, a lead-in line 14 composed of Au and a pad 16 composed of Au are embedded in the glass substrate 10.

0044The substrate face of the Si substrate 20 is the (100) plane, and the Si substrate 20 is subjected to anisotropic etching, whereby a groove 22 for injecting an electrolyte, an electrolyte-holding hole 24 and a glass film 26 acting as the sensing film on at least a part of the hole 24 are formed. Incidentally, the groove only can also be made by the separate anisotropic etching.

0045In Fig. 2(C), the broken line indicates an interior glass electrode-forming region, and the pad 16 and glass film 26 appear on the side of the Si substrate 20.

0046Fig. 3(A) is a view showing the section taken along X-X′ in Fig. 2(C). As is seen from Fig. 3(A), the sensing film 26 is formed in the bottom portion of the etched Si substrate 20.

0047Fig. 3(B) is a sectional view showing a modification of this Example 1, where the Si substrate is etched also from the bottom so that the sensing film 26 is formed slightly on the inner side of the Si substrate.

0048Fig. 4 is a diagram illustrating the steps of preparing the glass electrode of the present invention. One embodiment of the preparation process will now be described with reference to Fig. 2 and Fig. 4.

Formation of Glass Substrate:

0049A negative photoresist is spin-coated on the surface of a Pyrex glass substrate (Iwaki 7740) having a diameter of 2 inches and a thickness of 500 µm and is heated and dried at 150°C for 30 minutes. Regions for formation of many reference electrodes 12, lead-in lines 14 and pads 16 are windowed and exposed by the photolithography, and the same resist is similarly coated and dried on the back surface.

0050Then, the glass substrate is immersed in a mixed solution comprising 50% hydrofluoric acid (HF), concentrated nitric acid (HNO₃) and ammonium fluoride [(NH₄)F] at a ratio of 1/1/8 for 80 minutes to etch the glass substrate in a depth of 3 µm. Then, the resist is peeled by using a mixed solution comprising sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) at a ratio of 2/1 [see Fig. 4(A)].

0051Then, the glass substrate 10 is sufficiently washed with a mixed solution of H₂O₂ and ammonia (NH₄OH) and pure water, and is then dried.

0052Then, an Au film is vacuum-deposited on the glass substrate 10. Since Au adheres very poorly to glass, a very thin chromium (Cr) film is vacuum-deposited on the glass substrate in advance to improve the adhesiveness.

0053The thickness of the Cr film is 400 Å and the thickness of the Au film is 4000 Å.

0054Then, a positive resist film (DFRP-5000 supplied by Tokyo Oka) is spin-coated, and the resist is coated on regions for forming reference electrodes 12, lead-in lines 14 and pads 16 by the photolithography. Then, the Au film and Cr film are selectively etched to form a reference electrode pattern comprising reference electrodes 12, lead-in lines 14 and pads 16.

0055The Au-etching solution is one formed by dissolving 4 g of KI and 1 g of I₂ in 40 ml of water, and the Cr-etching solution is one formed by dissolving 0.5 g of NaOH and 1 g of K₃Fe(CN)₆ in 4 ml of water.

0056Then, silver (Ag) is vacuum-deposited on the portion for forming the reference electrode 12, and in the same manner as described above, coating of a positive resist, heat-drying, light exposure and development are carried out to coat the resist only on the reference electrode-forming portion. Then, Ag etching is conducted and the resist is dissolved and removed, whereby a silver film is formed on the reference electrode-forming portion.

0057The Ag-etching solution is one comprising 29% NH₄OH, 31% H₂O₂ and pure water at a ratio of 1/1/20.

0058Then, the entire substrate is sufficiently washed with pure water and immersed in a 0.1 M solution of FeCl₃ for 10 minutes to form a thin AgCl layer on the surface of Ag.

0059Then, the entire substrate is sufficiently washed with pure water to complete reference electrodes 12, lead-in lines 14 and pads 16 [Fig. 4(B)].

Formation of Si Substrate:

0060An Si substrate 20 having a (100) plane as the substrate face and having a thickness of 350 µm and a diameter of 2 inches is prepared, sufficiently washed with a mixed solution of H₂O₂ and NH₄OH and pure water, and dried. The Si substrate 20 is subjected to wet oxidation at 1050°C for 200 minutes to form an SiO₂ film 28 having a thickness of 1 µm on the entire surface.

0061A negative resist (OMR-83 supplied by Tokyo Oka) having a viscosity of 60 cP is coated on the surface of the Si substrate, and light exposure, development and rinsing are carried out to form a resist pattern on the substrate. Then, the Si substrate 20 is immersed in a mixed solution comprising 50% HF and 40% NH₄F at a ratio of 1/6 and the exposed portion of SiO₂ is etched and removed to form a holding hole 24 [see Fig. 4(C)].

0062Then, the resist film is peeled in a mixed solution comprising sulfuric acid and hydrogen peroxide at a ratio of 2/1. Then, the Si substrate 20 is immersed in 35% KOH at 80°C and anisotropic etching of silicon is carried out to form an electrolyte-holding hole 24 in the reference electrode portion.

0063A glass film 26 acting as the sensing film is formed in the bottom portion of the holding hole 24 by utilizing an SiO₂ film 28 having a thickness of 1 µm, formed by wet oxidation.

0064If the SiO₂ used as the mask is left on the surface of the Si substrate 20, a higher temperature is necessary for the anodic bonding. Therefore, SiO₂ other than the glass film 26 is completely removed by the photolithography using a mixed solution comprising 50%HF and 40% NH₄F at a ratio of 1/6. Thus, a vessel portion for storing an electrolyte is completed [see Fig. 4(D)].

Bonding of Glass Substrate and Si Substrate:

0065The so-prepared glass substrate 10 and Si substrate 20 are immersed in pure water, sufficiently washed under ultrasonic vibrations and dried, and both the substrates are bonded and a voltage of 1200 V is applied at a temperature of 250°C between the substrates so that the Si substrate 20 is located on the positive side and the glass substrate 10 is located on the negative side, whereby the anodic bonding of the glass substrate and the silicon substrate is effected [see Fig. 4(E)].

0066Many glass electrode elements formed on the substrate are cut out into chips by using a dicing saw to obtain a small glass electrode.

0067When the so-obtained small glass electrode is used, an electrolyte is introduced into the interior of the electrode according to the following method.

0068A beaker is charged with 0.1 M hydrochloric acid aqueous solution or an aqueous potassium chloride buffer solution contining a phosphoric acid (electrolyte), and the glass electrode is entirely immersed in the electrolyte and the entire system including the beaker is placed in a sealed vessel. Then, deaeration is carried out by a vacuum pump.

0069After bubbles have not come out of the hole 22 for injecting the electrolyte 30, air is introduced into the vessel. The hole can be filled with an epoxy regin.

0070By the above operation, the electrolyte is introduced into the inner space of the electrode, whereby a small glass electrode is obtained [see Fig. 4(F)].

0071According to this embodiment, a fine glass electrode can be prepared by using the micro-machining technique while maintaining a wafer-like shape. Accordingly, reduction of the cost is possible.

0072Furthermore, since the glass electrode can be stored in a dry state, long-period storage is possible.

Example 2

0073Another embodiment of the process for preparing the glass electrode of the present invention will now be described.

0074Fig. 5(A) is a plane view showing a glass substrate in the glass electrode according to another embodiment of the present invention, Fig. 5(B) is a plane view showing an Si substrate in the glass electrode according to this embodiment, and Fig. 5(C) is a plane view showing the glass electrode of another embodiment of the present invention, formed by bonding the glass substrate and Si substrate shown in Figs. 5(A) and 5(B).

0075Fig. 6 is a diagram illustrating the section taken along line X-X′ in Fig. 5(C).

0076Fig. 7 is a diagram illustrating steps of forming the glass electrode according to another embodiment of the present invention.

0077In Figs. 5 through 7, reference numerals represent the same members as in Figs. 1 through 4 unless otherwise indicated.

0078The glass substrate-forming step is the same as described in Example 1. Accordingly, the description is omitted.

0079Furthermore, the Si substrate is prepared substantially in the same manner as described in Example 1, except that a part of the SiO₂ film is not utilized as the glass film acting as the sensing film, but the SiO₂ film is entirely removed.

Bonding of Glass Film to Si Substrate:

0080A Pyrex glass (Iwaki 7740) is etched in a mixed solution comprising 50% HF and concentrated HNO₃ at a ratio of 2/1 to obtain a film having a thickness of about 50 µm, and the film is sufficiently washed to obtain a glass film 26 acting as the sensing film 26A.

0081The glass film 26A is placed on the surface of the Si substrate 20 formed by perforating the Si substrate by the anisotropic etching and heated at 800°C to effect bonding [see Fig. 7(D)].

0082Bonding of the so-prepared glass substrate and Si substrate [Fig. 7(E)] and injection of the electrolyte [Fig. 7(F)] are carried out in the same manner as described in Example 1.

0083According to this embodiment of the present invention, the strength of the glass film of the formed glass electrode is increased and hence, the yield is increased, and the glass electrode is advantageous over the conventional glass electrode in that no breaking is caused during use.

Example 3

0084Still another embodiment of the process for preparing the glass electrode of the present invention will now be described.

0085Fig. 8(A) is a plane view showing a glass substrate in the glass electrode according to still another embodiment of the present invention, Fig. 8(B) is a plane view showing an Si substrate in the glass electrode according to this embodiment, and Fig. 8(C) is a plane view showing the glass electrode of still another embodiment of the present invention, formed by bonding the glass substrate and Si substrate shown in Figs. 8(A) and 8(B).

0086Fig. 9 is a diagram illustrating the section taken along line X-X′ in Fig. 8(C).

0087Fig. 10 is a diagram illustrating steps of forming the glass electrode according to still another embodiment of the present invention.

0088In Figs. 8 through 10, reference numerals represent the same members as in Figs. 1 through 4 unless otherwise indicated.

0089The glass substrate-forming step is the same as described in Example 1. Accordingly, the description is omitted.

0090Furthermore, the Si substrate is prepared substantially in the same manner as described in Example 1, except that a part of the SiO₂ film is not utilized as the glass film acting as the sensing film, but the SiO₂ film is entirely removed.

Bonding of Glass Film to Si substrate:

0091A Pyrex glass (Iwaki 7740) is etched in a mixed solution comprising 50% HF and concentrated HNO₃ at a ratio of 2/1 to obtain a film having a thickness of, for example, 20-150 µm, in this case 50 µm, and the film is sufficiently washed to obtain a first glass film 29A constituting a film 29A acting as the sensing film.

0092The first glass film 29A is placed on the back surface of an Si substrate 20 having a perforated or piercing holding hole 24 formed by anisotropic etching of an Si substrate and heated at 750°C to effect bonding of the first glass film 29A to the Si substrate 20. Then, the Si substrate having the first glass film formed thereon is set at a sputtering device and an sodium-calcium Na-Ca glass is sputtered in a thickness of 500 nm to form a second glass film 29B on the first glass film 29A, whereby a glass film 26B acting as the sensing film is obtained [see Fig. 10(D)].

0093Bonding of the so-prepared glass substrate and Si substrate [Fig. 10(E)] and injection of an electrolyte [Fig. 10(F)] are carried out in the same manner as described in Example 1, whereby a small glass electrode is obtained.

0094The characteristics of the small glass electrode obtained in Example 3 are evaluated according to the following method.

0095The evaluation is performed by monitoring changes of the potential of the glass electrode relative to the potential of an external reference electrode of silver/silver chloride by using an electrometer. For examining changes of the potential relative to changes of the pH value, the sensing portion of the electrode is immersed in a 10 mM TRIS (Trimethyl aminomethane) solution, and HCl is added to reduce the pH value. At each pH value, the potential of the glass electrode is examined. The experiment is conducted at 25°C.

0096The response curve obtained when the pH value of the external buffer solution is changed is shown in Fig. 11.

0097As is seen from Fig. 11, a very clear response curve is obtained. When the pH value is changed, the small glass electrode immediately shows a change and a 90% response time is 10 seconds. A change of the electrode potential observed when the pH value is changed is shown in Fig. 12. It is seen that a linear relation is established between them over a broad pH value range of from 2 to 10. The gradient of the linear relation is a value approximate to -59 mV/pH of the theoretical value of the Nernst equation at 25°C.