Patent Publication Number: US-2009236224-A1

Title: Electrochemical sensor and method for manufacturing the same

Description:
TECHNICAL FIELD 
     The present invention relates to an electrochemical sensor such as a pH electrode comprising a support tube to which a sensitive glass membrane or ceramics is welded, and a method for manufacturing the same. 
     BACKGROUND ART 
     Conventionally, in a glass electrode such as a pH electrode, a sensitive glass membrane (pH-sensitive membrane) serving as a sensitive part is generally provided, by welding, to an extremity of an electrode supporting member formed of glass. As a material for the sensitive glass membrane, Li-based glass containing SiO 2 , BaO, Li 2 O, La 2 O 3 , Cs 2 CO 3 , TiO 2 , etc. (which may further contain Ta 2 O 5 , Pr 2 O 3 , Cr 2 O 3 , etc.) is used, for example. Many kinds of useful Li-based glass have a coefficient of thermal expansion (“CTE”) (coefficient of linear thermal expansion; same applies to others hereafter) of 80 to 120×10 −7 /° C. (at 30 to 380° C.; same applies to others hereafter). 
     On the other hand, in a reference electrode, a liquid junction is provided to establish electric conduction with a sample liquid. The reference electrode may be used together with a pH electrode, etc., or formed integrally with a pH electrode, etc. as a combined electrode. Ceramics, or more specifically, porous ceramics is widely used as the junction. The junction is usually sealed in a wall of an electrode supporting member formed of glass. Many kinds of useful ceramics for the junction have a CTE of 80 to 110×10 −7 /° C. 
     In order to conduct good welding, sealing or inclusion (which may be comprehensively referred to as “welding”) between the sensitive glass membrane or ceramics and the electrode supporting member, and to prevent cracks caused by temperature change at the welding portion, it is necessary to bring the CTE of the electrode supporting member to that of the sensitive part or ceramics. 
     As a material for the electrode supporting member responding to the above-mentioned needs, a glass composition containing lead, or more specifically, PbO (generally referred to as “lead glass”) has generally been used heretofore. Such a glass composition contains, for example, BaO, Al 2 O 3 , Na 2 O, K 2 O, etc. as well as SiO 2  and PbO. 
     A typical lead glass has a CTE of 94×10 −7 /° C. which is close to that of the sensitive glass membrane or ceramics, and has a good welding property. Also, lead glass has a low-temperature softening property, and is good in processability and workability. Further, lead glass is good in transparency, water resistance and weather resistance. 
     However, from the point of view of environment-friendliness, there is recently an increasing demand for replacing lead glass with a glass composition not containing lead (hereinafter referred to as “lead-free glass”). 
     The present applicant proposed, as disclosed in Japanese Patent Application Publication No. 2005-207887, a lead-free glass composition being useful as a material for an electrode supporting member of an electrochemical sensor, which has a good welding property to a sensitive glass membrane, ceramics or a platinum electrode. Also, this lead-free glass is relatively low-temperature softening, and is good in processability and workability. Further, this lead-free glass is good in water resistance, weather resistance and transparency. 
     However, according to the studies of the present inventors, it is revealed that in the case where a sensitive glass membrane is welded onto a tubular electrode supporting member, i.e. support tube (glass stem tube) formed of lead-free glass, or where ceramics for junction is welded onto the support tube, cracks may occur at the welding portion or sealing portion under some service conditions. 
     The CTE of the sensitive glass membrane somewhat varies with the kind of glass composition thereof. Particularly in a pH-sensitive glass membrane for high alkalinity, cracks tend to occur easily from the joint between the sensitive glass membrane and the support tube made of lead-free glass in the measurement for a long period of time longer than five hours at a high temperature higher than 100° C., although no particular problem in the measurement on a room-temperature level is posed. 
     With respect to a junction made of ceramics, cracks tend to occur easily at the sealing portion of the junction in the support tube made of lead-free glass, depending on ceramics composition, even in the measurement on a room-temperature level. Particularly, cracks tend to occur easily in the case where a junction formed of alumina-based ceramics is used. 
     The mechanism in which cracks tend to occur easily at the joint between the support tube made of lead-free glass and the sensitive glass membrane or the junction made of ceramics is not entirely clear. According to the studies carried out by the present inventors, however, it is considered to be caused by the following difference between lead glass and lead-free glass. 
     The lead-free glass composing the support tube in which cracks occur as mentioned above has a CTE of 94.5×10 −7 /° C. which is generally equal to the CTE of the typical conventional lead glass (94×10 −7 /° C.). Therefore, the lead-free glass is considered a suitable substitute for the lead glass. However, the lead-free glass has hardness in its molten state, and this is considered to prevent achievement of a sufficient processing accuracy. In other words, when forming the support tube from the conventional lead glass, the lead glass has softness in its molten state because of lead contained in its composition, and thus a extremely good processability is considered to be obtained. 
     The difference in softness in molten state between the lead-free glass and the lead glass seems to be particularly remarkable when an object to be welded has a CTE far different from that of the support tube (CTE; membrane for high alkalinity: 115 to 120×10 −7 /° C., alumina-based ceramics A-017: 84×10 −7 /° C.). 
     More specifically, lead glass is considered to permit welding of an object to be welded with a sufficient processing accuracy because of the easy processability resulting from softness in its molten state. Also, lead glass is considered to express flexibility by containing lead, and is considered to be able to absorb the difference in CTE from the object to be welded. As a result, a sufficient durability can be maintained even under severe service conditions such as a high temperature and a high alkalinity. In lead-free glass, in contrast, hardness in its molten state is considered to lead to an insufficient affinity with the object to be welded, and is considered to make it difficult to obtain a sufficient processing accuracy. Also, in lead-free glass, lack of flexibility is considered to prevent compensating for the difference in CTE from the object to be welded, and is considered to cause cracks during use. 
     Such an advantage of lead glass has not rather been noted since it has been the usual practice to manufacture a support tube for an electrochemical sensor from lead glass. 
     DISCLOSURE OF INVENTION 
     It is therefore an object of the present invention to provide an electrochemical sensor which can prevent occurrence of cracks at the welding portion between a support tube and a sensitive glass membrane or ceramics, and a method for manufacturing the same. 
     The foregoing object is achieved by the use of the image forming apparatus of the present invention. In summary, the first aspect of the present invention provides an electrochemical sensor wherein a sensitive glass membrane is welded to a support tube made of lead-free glass with a lead glass layer between the sensitive glass membrane and the support tube. According to an embodiment of the present invention, an annular lead glass layer is welded to an annular end face of the support tube, and the sensitive glass membrane is welded to an end face of the annular lead glass layer. Ceramics may further be welded to the support tube with a lead glass layer between the ceramics and the support tube. According to another embodiment of the present invention, the support tube has a double tube structure having an inner tube and an outer tube; a tube end of the outer tube is welded to a tube end of the inner tube and sealed thereto; and the annular end face is formed on the tube end of the support tube. Ceramics for a junction may be welded to the outer tube of the support tube with a lead glass layer between the ceramics and the outer tube of the support tube. The electrochemical sensor may be a pH electrode. 
     According to the second aspect of the present invention, there is provided an electrochemical sensor wherein ceramics for a junction is welded to a support tube made of lead-free glass with a lead glass layer between the ceramics and the support tube. The electrochemical sensor may be a reference electrode. 
     According to the third aspect of the present invention, there is provided an electrochemical sensor wherein a sensitive glass membrane or ceramics for a junction is welded to a support tube made of a material having a coefficient of thermal expansion (coefficient of linear thermal expansion) of 94±20×10 −7 /° C. with a lead glass layer between the sensitive glass membrane or the ceramics and the support tube. 
     According to the fourth aspect of the present invention, there is provided a method for manufacturing an electrochemical sensor comprising welding a sensitive glass membrane to a support tube made of lead-free glass with a lead glass layer between the sensitive glass membrane and the support tube. According to an embodiment of the present invention, the method comprises the steps of: (a) welding an annular lead glass layer to an annular end face of the support tube made of lead-free glass; and (b) welding the sensitive glass membrane to an end face of the annular lead glass layer. Step (a) may comprise a step of welding an annular member made of lead glass having substantially the same shape as that of the annular end face of the support tube to the annular end face of the support tube. Step (b) may comprise a step of welding a molten membrane seed of the sensitive glass membrane to the end face of the lead glass layer, and then swelling the membrane seed into a desired spherical shape by blowing air into the support tube. 
     According to the fifth aspect of the present invention, there is provided a method for manufacturing an electrochemical sensor comprising welding ceramics for a junction to a support tube made of lead-free glass with a lead glass layer between the ceramics and the support tube. According to an embodiment of the present invention, the method comprises the steps of: (i) putting the ceramics for the junction into a tubular member made of lead glass substantially engaging with the ceramics for the junction; (ii) inserting a composite member, formed in step (i), of the ceramics for the junction and the tubular member made of lead glass into a hole opened in the support tube made of lead-free glass; and (iii) welding the ceramics for the junction, together with the tubular member made of lead glass, to the support tube. The method may further comprise a step of partially welding the tubular member made of lead glass and the ceramics for the junction between step (i) and step (ii). According to another embodiment of the present invention, the method may comprise the steps of: (I) opening a first hole in the support tube made of lead-free glass; (II) closing the first hole by welding lead glass to the first hole opened in step (I); (III) opening a second hole substantially at the center of the lead glass having closed the first hole in step (II) so as to leave the lead glass around the second hole; and (IV) inserting the ceramics for the junction into the second hole opened in step (III), and then welding the ceramics to the second hole. 
     According to the sixth aspect of the present invention, there is provided a method for manufacturing an electrochemical sensor comprising welding a sensitive glass membrane or ceramics for a junction to a support tube made of a material having a coefficient of thermal expansion (coefficient of linear thermal expansion) of 94±20×10 −7 /° C. with a lead glass layer between the sensitive glass membrane or the ceramics and the support tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of the main part of an embodiment of the electrochemical sensor (combined pH electrode) according to the present invention; 
         FIG. 2  is a sectional view of the support tube  1  for describing the method for manufacturing the electrochemical sensor shown in  FIG. 1 ; 
         FIG. 3  is a schematic view illustrating the step of welding the sensitive glass membrane to the support tube in the method for manufacturing the electrochemical sensor shown in  FIG. 1 ; 
         FIG. 4  is a schematic view illustrating the step of attaching a binder tube to a junction in the method for manufacturing the electrochemical sensor shown in  FIG. 1 ; 
         FIG. 5  is a schematic view illustrating an example of the step of welding the junction to the support tube in the method for manufacturing the electrochemical sensor shown in  FIG. 1 ; 
         FIG. 6  is a schematic view illustrating another example of the step of welding the junction to the support tube in the method for manufacturing the electrochemical sensor shown in FIG.  1 .; 
         FIG. 7  is a schematic sectional view of the main part of another embodiment of the electrochemical sensor (pH glass electrode) according to the present invention; and 
         FIG. 8  is a schematic sectional view of the main part of yet another embodiment of the electrochemical sensor (reference electrode) according to the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The electrochemical sensor and the method for manufacturing the same according to the present invention will now be described further in detail with reference to the drawings. 
     Embodiment 1  
       FIG. 1  illustrates an embodiment of the electrochemical sensor of the present invention. In this embodiment, the electrochemical sensor takes the form of a pH electrode, and particularly, a combined pH electrode configured by integrally forming a measurement electrode and a reference electrode (hereinafter simply referred to as “pH electrode”). 
     The pH electrode  100  has a double tube structure with glass tubes (i.e., double glass tube), as a support tube (glass stem tube)  1  serving as a tubular electrode supporting member. The double tube structure is formed by integrally sealing tube ends  13  of an inner tube  11  and an outer tube  12  by welding. Furthermore, a sensitive glass membrane (pH-sensitive glass membrane)  2 A sensing a pH is integrally welded to the tube end  13  of the support tube  1 . 
     An internal electrode for measurement electrode  14  is arranged inside the inner tube  11 , and the interior of the inner tube  11  is filled with an internal liquid for measurement electrode. An internal electrode for reference electrode  15  is arranged inside an annular space formed by the inner tube  11  and the outer tube  12 , and the interior of the annular space is filled with an internal liquid for reference electrode. 
     A junction  2 B formed with ceramics is sealed in the side of the lower part of the outer tube  12 . The junction  2 B passes from the outside of the outer tube  12  to the inside of the outer tube  12  (i.e., to the tubular space between the inner tube  11  and the outer tube  12 ), thus permitting electric continuity between the outer tube  12  and the sample liquid outside the pH electrode  100 . 
     The pH measurement electrode is configured having the inner tube  11 , the sensitive glass membrane  2 A and the internal electrode  14 , and the reference electrode is configured having the outer tube  12 , the junction  2 B and the internal electrode for reference electrode  15 . 
     Although, the support tube  1  is the double glass tube having the inner tube  11  and the outer tube  12  in this embodiment, the support tube  1  may be a single tube as shown in  FIG. 7 , and the pH electrode  200  may of course be a single-type measurement electrode. In this case, the sensitive glass membrane  2 A is welded to the tube end  13  of the support tube  1  which is a single tube. As shown in  FIG. 8 , on the other hand, a single-type reference electrode  300  may be configured by providing a junction  2 B in a support tube  1  which is a single tube. In this case, the junction  2 B may be provided at the extremity of the support tube  1  as shown in  FIG. 8 , or on the side thereof. In  FIGS. 7 and 8 , the same reference numerals are assigned to elements having substantially the same or corresponding functions or configurations as those of the pH electrode  100  shown in  FIG. 1 . In the present specification, the term “electrochemical sensor” is used so as to encompass also the single-type reference electrode. 
     In this embodiment, the support tube  1  is manufactured from lead-free glass. A sensitive glass membrane  2 A is welded to the end face  13   a  of the tube end  13  of the support tube  1  made of lead-free glass, with a lead glass layer (hereinafter referred to as “binder layer”)  3 A between the sensitive glass membrane  2 A and the end face  13   a  of the tube end  13  of the support tube  1 . In this embodiment, the junction  2 B is also welded to the outer tube  12  of the support tube  1  with a binder layer  3 B between the junction  2 B and the outer tube  12  of the support tube  1 . 
     Since lead glass and lead-free glass are welded satisfactorily to each other, cracks do not occur in the joint between lead glass and lead-free glass, not only under usual service conditions but also under severe service conditions such as a high temperature and a high alkalinity. As being described later in more detail with reference to the experimental examples, welding of the sensitive glass membrane  2 A and the junction  2 B to the binder layers  3 A and  3 B makes it possible to prevent cracks from occurring even under severe service conditions, not depending upon the glass composition of the sensitive glass membrane  2 A or the ceramics composition of the junction  2 B. 
     An embodiment of the method for manufacturing the pH electrode  100  will now be described with reference to  FIGS. 2 to 4 . The method for manufacturing the support tube  1  is not different from the conventional one except that the support tube  1  is manufactured from lead-free glass. In this embodiment, the support tube  1  is a circular tube (circular-section tube). While the support tube  1  is typically a circular tube, the cross-sectional shape of the support tube in the present invention is not limited to this. 
     First, the method of attaching the sensitive glass membrane  2 A to the support tube  1  made of lead-free glass will now be described. 
     As shown in  FIG. 2 , when welding the sensitive glass membrane  2 A to the support tube  1 , the binder layer  3 A is formed by welding lead glass in advance to the annular end face  13   a  of the tube end  13  of the support tube  1 , i.e. to the end face  13   a  to which the sensitive glass membrane  2 A is to be welded. Preferably, the binder layer  3 A welded to the end face  13   a  takes substantially the same shape as that of the end face  13   a,  i.e., an annular shape having substantially the same outside diameter and inside diameter as those of the end face  13   a.    
     A method of welding the binder layer  3 A to the end face  13   a  may be appropriately selected. Typically, the binder layer  3 A can be welded to the end face  13   a  by welding an annular member made of lead glass to the end face  13   a , wherein the annular member has substantially the same shape as that of the end face  13   a , i.e. substantially the same outside diameter and inside diameter as those of the end face  13   a.  More specifically, for example, a tubular member (binder tube) T 1  having an appropriate length (preferably, about 50 mm), manufactured from lead glass, having substantially the same shape as the end face  13   a  in cross-section, i.e. substantially the same outside diameter and inside diameter as those of the end face  13   a  in cross-section, is prepared in advance. The thus prepared binder tube T 1  is welded to the end face  13   a  in a manner of ordinary glasswork using a Bunsen burner. Then, an excessive lead glass portion is cut to adjust the thickness of the binder layer  3 A welded to the end face  13   a  to about 1 mm. 
     The binder layer  3 A should preferably be welded to the end face  13   a  so that the thickness t 1  is equal to or larger than 0.5 mm. When the thickness t 1  is smaller than 0.5 mm, uniform formation of the binder layer  3 A is difficult, thus reducing the crack-preventing effect at the joint between the sensitive glass membrane  2 A and the support tube  1 . 
     On the other hand, the thickness t 1  should preferably be equal to or smaller than 5 mm. A thickness t 1  larger than this is not desirable because it is difficult to adjust the shape. The thickness t 1  should more preferably be up to 2 mm, or further more preferably, up to 1 mm. 
     In this embodiment, the annular end face  13   a  of the tube end  13  of the support tube  1  has an outside diameter d 1  of 9 mm and an inside diameter d 2  of 8 mm. To this annular end face  13   a , the binder layer  3 A having substantially the same outside diameter and inside diameter as those of the end face  13   a  is welded so as to give a thickness t 1  of 0.5 mm. 
     After welding the binder layer  3 A to the end face  13   a  of the tube end  13  of the support tube  1  as described above, the sensitive glass membrane  2 A is welded via the binder layer  3 A to the support tube  1 . The method of welding (depositing) the sensitive glass membrane  2 A is not different from the conventional one. An outline of a typical such method will be described below. 
     First, as shown in  FIG. 3(   a ), a crucible F containing molten glass (glass membrane seed) G is installed in a furnace (electric furnace), and the support tube  1  is set at a prescribed position above the furnace. 
     Glass of an appropriate composition may be used for the glass membrane seed G, depending on the use of the pH electrode  100  to be manufactured. Generally, applicable glass compositions include: (a) for standard uses: SiO 2 , Li 2 O, Cs 2 CO 3 , BaCO 3 , TiO 2 , La 2 O 3 ; (b) for alkaline uses: SiO 2 , Li 2 O, Cs 2 CO 3 , La 2 O 3 , Pr 2 O 3 ; (c) for hydrofluoric acid resistant uses: SiO 2 , Li 2 O, BaCO 3 , Ta 2 O 5 , Cr 2 O 3 , La 2 O 3 ; and (d) for fermentation uses: SiO 2 , Li 2 O, BaCO 3 , TiO 2 , La 2 O 3 . Values of CTE for the glass membrane seed of these Li-based glass materials are usually within a range from 80 to 120×10 −7 /° C. 
     The temperature of the crucible F inside the furnace is adjusted to an appropriate level (for example, 1,000 to 1,550° C.), and the crucible F is heated for a prescribed period of time. Then, the support tube  1  is lowered at an appropriate speed (for example, 100 to 500 mm/sec) and the tube end of the support tube  1  is immersed in the glass membrane seed G in the crucible F. 
     As shown in  FIG. 3(   b ), the temperature of the crucible F in which the tube end of the support tube  1  is immersed in the glass membrane seed G is adjusted to an appropriate level (for example, 1,000 to 1,550° C.) by adjusting the temperature inside the furnace, and the glass membrane seed G is caused to adhere to the tube end of the support tube  1  while heating the crucible F for a prescribed period of time. 
     Then, as shown in  FIG. 3(   c ), the support tube  1  to which the glass membrane seed G adheres is lifted up at an appropriate speed (for example, 500 to 1,500 mm/sec), and then, as shown in  FIG. 3(   d ), air is blown into the support tube  1  to swell the glass membrane seed G adhering to the tube end of the support tube  1  into a desired spherical shape, thereby forming the sensitive glass membrane  2 A. 
     The method of attaching the junction  2 B to the support tube  1  will now be described. 
     As shown in  FIG. 4(   a ), the junction  2 B is put into a tubular member (binder tube) T 2  made of lead glass substantially engaging with the junction  2 B, in advance. Then, at least a part of the binder tube T 2  is preferably caused to melt by roasting by a Bunsen burner as shown in  FIG. 4(   b ), whereby the junction  2 B and the binder tube T 2  are partially welded to each other at the spot welding portion S ( FIG. 4(   c )), and the junction  2 B and the binder tube T 2  are fixed to each other. This prevents the junction  2 B from coming off the binder tube T 2  during the subsequent operation, thus improving the operability. However, this step of fixing the junction  2 B and the binder tube T 2  is not always necessary. 
     Next, as shown in  FIG. 5 , a composite member (complex) of the junction  2 B and the binder tube T 2  is inserted into a hole  12   a  opened in the outer tube  12  of the support tube  1 . The hole  12   a  is opened in advance by roasting with the Bunsen burner, for example, so as to permit substantial engagement of the composite member of the junction  2 B and the binder tube T 2  with the hole  12   a . Then, the junction  2 B is welded to the support tube  1 , together with the lead glass of the binder tube T 2 , by roasting the peripheral edge of this hole  12   a  and the proximity of the binder tube T 2  with the Bunsen burner. This forms the binder layer  3 B and seals the junction  2 B in the support tube  1 . 
     After sealing, portions of the junction  2 B and/or the binder tube T 2  projecting outside the outer tube  12  may be removed by filing, for example. Of course, the lengths of the junction  2 B and the binder tube T 2  may be set to be equal to or shorter than the thickness of the outer tube  12 . 
     For the same reason as that in the case of the binder layer  3 A provided at the welding portion of the sensitive glass membrane  2 A, a binder layer  3 B formed on the junction  2 B has a thickness t 2 , when it is welded to the support tube  1 , of 0.5 mm to 5 mm, or more preferably, from 0.5 mm to 2 mm, or further more preferably, from 0.5 mm to 1 mm. The binder tube T 2  should preferably have an inside diameter d 4  substantially equal to an outside diameter d 5  of the junction  2 B. An outside diameter d 3  of the binder tube T 2  is selected so as to obtain a desired thickness t 2  of the binder layer  3 B after the aforementioned welding. A column-shaped junction  2 B having an outside diameter d 5  of 1 mm to 1.5 mm is usually used. In this case, usually, a circular tube having an inside diameter d 4  substantially equal to the outside diameter d 5  of the junction  2 B, i.e. of about 1 mm to about 1.5 mm, and an outside diameter d 3  of about 1.5 mm to about 2.5 mm, may be suitably used as the binder tube T 2 . 
     In this embodiment, as one example, a substantially columnar ceramics having a diameter of about 1 mm cut into length of about 3 mm is employed for the junction  2 B. Thus the binder tube T 2  used has the outside diameter d 3  of 2 mm, and the inside diameter d 4  of about 1 mm. This results in the thickness t 2  of the binder layer  3 B of 0.5 mm. 
     The shape of the junction  2 B is not limited to a columnar shape. Although the binder tube T 2  should preferably have a hole having a cross-sectional shape of substantially the same shape as the cross-sectional shape of the junction  2 B, it is not limited to this. Any shape of the binder tube T 2  is applicable so far as the binder tube T 2  is substantially engageable with the junction  2 B, and can cover the junction  2 B prior to inserting into the hole  12   a  of the support tube  1 . 
     In the present invention, the method of providing the binder layer  3 B is not limited to the above-mentioned method of covering the junction  2 B with the binder tube T 2  composing the binder layer  3 B in advance, inserting the thus covered junction  2 B into the hole  12   a  of the support tube  1 , and then, welding the junction  2 B, together with the binder tube T 2 , to the support tube  1 . For example, after providing the binder layer  3 B on the outer periphery of the junction  2 B in the usual method of glasswork with a Bunsen burner employing a glass rod consisting of lead glass as a primary material, the composite member (composite body) of the binder layer  3 B and the junction  2 B may be inserted into the hole  12   a  of the support tube  1 , and then it may be welded to the support tube  1 . 
     Alternatively, the junction  2 B can be attached to the support tube  1  as mentioned below. 
     First, as shown in  FIG. 6(   a ), an appropriate hole (first hole)  12   a  is opened in the outer tube  12  of the support tube  1  made of lead-free glass, at a position where the junction  2 B is to be attached. This hole  12   a  is opened so that the junction  2 B and the binder layer  3 B fit thereinto. This hole  12   a  can be opened in the usual method of glasswork using a Bunsen burner. More specifically, the hole can be opened by blowing air, in a state in which glass is softened by roasting it by a Bunsen burner, through a fine tube such as a rubber tube into the softened portion. 
     Next, as shown in  FIG. 6(   b ), the lead glass  16  is welded in the usual method of glasswork using a Bunsen burner to the hole  12   a  opened as described above, thus once closing the hole  12   a.  The lead glass  16  having closed the hole  21   a  is usually once hardened by cooling. 
     Subsequently, as shown in  FIG. 6(   c ), another hole (second hole)  16   a  is opened substantially at the center of the lead glass  16  having closed the hole  12   a  as described above so as to leave the lead glass  16  therearound. This hole  16   a  is opened so that the junction  2 B adapts thereto. This hole  16   a  also can be opened in the usual method of glasswork using a Bunsen burner as in the aforementioned case. 
     Then, as shown in  FIG. 6(   d ), the junction  2 B made of ceramics is inserted into the hole  16   a  opened in the lead glass  16  as described above, and the junction  2 B is welded to the hole  16   b  of the lead glass  16  by roasting the outer peripheries of the junction  2 B and the lead glass  16 . This permits sealing of the junction  2 B, via the binder layer  3 B, into the outer tube  12  of the support tube  1 . 
     As in the aforementioned case, the portion of the junction  2 B projecting outside the outer tube  12  may be filed off after sealing. Of course, the length of the junction  2 B may be set to be equal to or shorter than the thickness of the outer tube. 
     In the present invention, the composition of lead glass applicable for the binder layer  3  ( 3 A and  3 B) is not critical, but any commercially available lead glass can be used without particular limitation so far as it is applicable for an electrochemical sensor. More specifically, lead glass containing PbO of 20 wt. % to 50 wt. % is suitably used. The PbO content smaller or larger than this makes it difficult to obtain functions as the binder layer  3  ( 3 A and  3 B). Preferably, lead glass should have a CTE of 94±20×10 −7 /° C. on the same level as the CTE of the object to be welded (the sensitive glass membrane  2 A and ceramics  2 B in this embodiment). So as to be good in workability, lead glass should preferably have a softening point (a temperature at which welding of glass becomes possible in this disclosure) lower than or equal to 900° C. 
     EXPERIMENTAL EXAMPLE 1  
     To confirm advantages of the present invention, the pH electrode having the binder layers  3 A and  3 B as manufactured by the foregoing manufacturing method (Concrete Example of the Present invention), and the pH electrode having the sensitive glass membrane  2 A and the junction  2 B welded by the conventional method to the support tube  1  made of lead-free glass without providing the binder layers  3 A and  3 B (Comparative Example), were compared as to the tendency of crack occurrence relative to the service conditions. 
     The composition of lead-free glass of the support tube  1  and the composition of the binder layers  3 A and  3 B are shown in Table 1 below. The glass composition of the support tube  1  was same in all the pH electrodes. The compositions of the binder layers  3 A and  3 B provided respectively at the welding portions of the sensitive glass membrane  2 A and the junction  2 B were identical to each other. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Lead glass 
                 Lead-free glass (1) 
               
               
                   
                 (Binder) 
                 (Support tube) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Glass 
                 SiO 2   
                 58.5 
                 70 
               
               
                 composition 
                 Al 2 O 3   
                 1 
                 2 
               
               
                 (wt. %) 
                 B 2 O 3   
                 — 
                 2 
               
               
                   
                 BaO + PbO 
                 28 
                 — 
               
               
                   
                 Na 2 O + K 2 O 
                 12.5 
                 — 
               
               
                   
                 RO 
                 — 
                 11 
               
               
                   
                 (R: bivalent 
               
               
                   
                 metal element) 
               
               
                   
                 R 2 O 
                 — 
                 15 
               
               
                   
                 (R: monovalent 
               
               
                   
                 metal element) 
               
            
           
           
               
               
               
            
               
                 Coefficient of linear 
                 94 
                 94.5 
               
               
                 thermal expansion 
               
               
                 (×10 −7 /° C.) 
               
               
                   
               
            
           
         
       
     
     For the purpose of comparing tendencies of crack occurrence depending on the difference in the glass composition of the sensitive glass membrane  2 A, pH electrodes were prepared respectively using a standard membrane (having the binder layer  3 A: Concrete Example 1; not having the binder layer  3 A: Comparative Example 1), and a membrane for high alkalinity (having the binder layer  3 A: Concrete Example 2; not having the binder layer  3 A: Comparative Example 2) with the glass compositions shown in Table 2 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Standard 
                 Membrane 
               
               
                   
                 membrane 
                 for higt alkalinity 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Glass 
                 SiO 2   
                 50~55 
                 55~60 
               
               
                   
                 composition 
                 Li 2 O 
                 28~30 
                 28~30 
               
               
                   
                 (wt. %) 
                 BaO 
                 2~5 
                 2~5 
               
               
                   
                   
                 La 2 O 3   
                 3~6 
                 3~6 
               
               
                   
                   
                 Cs 2 O 
                 0~1 
                 2~3 
               
               
                   
                   
                 TiO 2   
                 3~8 
                 3~8 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Coefficient of linear 
                   
                 100~110 
                 115~120 
               
               
                   
                 thermal expansion 
               
               
                   
                 (×10 −7 /° C.) 
               
               
                   
                   
               
            
           
         
       
     
     For the purpose of comparing tendencies of crack occurrence depending on the difference in the ceramics composition of the junction  2 B, pH electrodes were prepared respectively using alumina-based ceramics (having the binder layer  3 B: Concrete Example 3; not having the binder layer  3 B: Comparative Example 3), cerium-based ceramics (having the binder layer  3 B: Concrete Example 4; not having the binder layer  3 B: Comparative Example 4), magnesia-based ceramics (having the binder layer  3 B: Concrete Example 5; not having the binder layer  3 B: Comparative example 5), and zirconia-based ceramics (having the binder layer  3 B: Concrete Example 6; not having the binder layer  3 B: Comparative Example 6) with the chemical compositions shown in Table 3 below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Almina- 
                 Cerium- 
                 Magnesia- 
                 Zirconia- 
               
               
                   
                 based 
                 based 
                 based 
                 based 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Glass 
                 Al 2 O 3   
                 90~98 
                 15~20 
                 — 
                 0~3 
               
               
                 composition 
                 CaO 
                 0~2 
                 — 
                 0~5 
                  3~10 
               
               
                 (wt. %) 
                 MgO 
                 1~5 
                  5~10 
                 60~80 
                 0~3 
               
               
                   
                 CeO 2   
                 — 
                 70~80 
                 — 
                 — 
               
               
                   
                 SiO 2   
                 — 
                 — 
                 20~40 
                 0~4 
               
               
                   
                 ZrO 2   
                 — 
                 — 
                 — 
                 80~95 
               
               
                   
                 HfO 2   
                 — 
                 — 
                 — 
                 0~3 
               
            
           
           
               
               
               
               
               
            
               
                 Weldeng property 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 to lead-free glass 
               
               
                   
               
            
           
         
       
     
     RESULT  
     1) Processability and Workability: 
     The lead-free glass used had a softening point lower than 900° C., and processability and workability themselves were good as the conventional lead glass. 
     The cases where the binder layer  3 A or  3 B was provided (Concrete Examples 1 to 6) showed good welding property between lead glass and lead-free glass. Welding property between lead glass and the sensitive glass membrane  2 A or the junction  2 B was also good. 
     2) Sensitive Glass Membrane: 
     When the standard membrane was used as the sensitive glass membrane  2 A, crack occurrence was not observed at the joint between the support tube  1  and the sensitive glass membrane  2 A, even after a measurement for a long period of time (for longer than 5 hours; same applies to others hereafter) with any of room-temperature (25° C.; same applies to others hereafter) water, high-temperature (100° C.; same applies to others hereafter) water, and high-temperature and high-alkalinity solution (aqueous solution of sodium hydroxide with a pH of 13; same applies to others hereafter), in both cases where the binder layer  3 A was provided (Concrete Example 1) and where the binder layer  3 A was not provided (Comparative Example 1). 
     When the membrane for high alkalinity was used as the sensitive glass membrane  2 A, in the case where the binder layer  3 A was not provided (Comparative Example 2), cracks occurred at the joint between the support tube  1  and the sensitive glass membrane  2 A after a measurement for a long period of time with high-temperature water, or high-temperature and high-alkalinity aqueous solution, while no problem was encountered with room-temperature water. In the case where the binder layer  3 A was provided (Concrete Example 2), in contrast, crack occurrence was not observed at the joint between the support tube  1  and the sensitive glass membrane  2 A, even after a measurement for a long period of time with any of room-temperature water, high-temperature water, and high-temperature and high-alkalinity aqueous solution. 
     As described above, it is found that, when the binder layer  3 A is not provided, cracks occur more easily in the case where the membrane for high alkalinity is used as the sensitive glass membrane  2 A than in the case where the standard membrane is used. When using the standard membrane, it seems that it is difficult for cracks to occur because the CTE of the standard membrane is close to that of the lead-free glass. On the other hand, the membrane for high alkalinity has a larger CTE, and thus the lead-free glass cannot absorb the difference in the CTE, and this is considered to leads to easier occurrence of cracks under high-temperature condition. High-alkalinity condition seems to foment this easier occurrence of cracks. 
     Among the compositions of the sensitive glass membrane shown in Table 2, components contributing to the CTE are SiO 2 , Li 2 O, and Cs 2 O. There is almost no difference in the SiO 2  and Li 2 O compositions between the standard membrane and the membrane for high alkalinity, whereas there is a large difference in Cs 2 O. This difference largely contributes to the value of CTE, and is considered to be related with the tendency of crack occurrence. However, since a less Cs 2 O content leads to increase of alkaline error at a high alkalinity, the quantity of Cs 2 O cannot be reduced in the membrane for high alkalinity. 
     When welding the sensitive glass membrane  2 A to the support tube  1  via the binder layer  3 A in accordance with this embodiment, it is possible to prevent occurrence of cracks, irrespective of the service conditions both for the standard membrane and the membrane for high alkalinity. Although the present invention is not bound by a theory, the reason is considered as follows. By providing the binder layer  3 A which can be welded satisfactorily both to the support tube  1  and to the sensitive glass membrane  2 A, between the support tube  1  and the sensitive glass membrane  2 A which is the object to be welded, the support tube  1  and the sensitive glass membrane  2 A do not come into direct contact with each other. As a result, a sufficient processing accuracy is available from flexibility of lead glass in its molten state as described above, and flexibility of lead glass is considered to enable the binder layer  3 A to absorb the difference in CTE between the lead-free glass and the sensitive glass membrane  2 A. 
     As is understood from the above, when the membrane for high alkalinity is used as the sensitive glass membrane  2 A, or more specifically, when the glass composition of the sensitive glass membrane  2 A contains Cs 2 O of at least 2 wt. %, cracks tend to occur more easily. The advantages of the present invention therefore become very remarkable especially when using such a membrane for high alkalinity as the sensitive glass membrane  2 A. However, as is clear from the aforementioned result of experiment, durability under severe service conditions can be improved by providing the binder layer  3 A. Therefore, even when using the standard membrane which makes it relatively difficult to generate cracks as the sensitive glass membrane  2 A, it is very favorable to provide the binder layer  3 A in accordance with this embodiment, considering, for example, the case where a measurement is performed for a long period of time under a condition of higher temperature, and this permits improvement of reliability of the electrode. 
     3) Junction: 
     When ceramics other than alumina-based ceramics, i.e. cerium-based ceramics, magnesia-based ceramics or zirconia-based ceramics was used as the junction  2 A, crack occurrence was not observed at the joint between the support tube  1  and the junction  2 B, even after a measurement for a long period of time with any of room-temperature water, high-temperature water, and high-temperature and high-alkalinity solution, in both cases where the binder layer  3 B was provided (Concrete Examples 4 to 6) and where the binder layer  3 B was not provided (Comparative Examples 4 to 6). 
     When alumina-based ceramics was used as the junction  2 B, cracks occurred at the joint between the support tube  1  and the junction  2 B after a measurement for more than 24 hours even with room-temperature water in the case where the binder layer  3 A was not provided (Comparative Example 3). Cracks tended to occur more easily with high-temperature water, and high-temperature and high-alkalinity aqueous solution. In these cases, crack occurrence was observed in a measurement for more than an hour. In the case where the binder layer  3 B was provided (Concrete Example 3), in contrast, crack occurrence was not observed at the joint between the support tube  1  and the sensitive glass membrane  2 A, even after a measurement for a long period of time with any of room-temperature water, high-temperature water, and high-temperature and high-alkalinity aqueous solution. 
     As described above, it is found that, when the binder layer  3 B is not provided, cracks occur more easily in the case where alumina-based ceramics is used than in the case where the other kind of ceramics is used. High-alkalinity condition seems to accelerate this easier occurrence of cracks. 
     As shown in Table 3, applicable ceramics as the junction include alumina-based ceramics, cerium-based ceramics, magnesia-based ceramics and zirconia-based ceramics, in terms of the main component, i.e. in terms of the component contained in the largest amount. Alumina-based ceramics is experimentally-found to provide the best performance as the junction. However, cracks most easily occur when using this alumina-based ceramics, in the case where the binder layer  3 B is not used. Occurrence of cracks is more difficult when using ceramics other than alumina-based ceramics, i.e. cerium-based ceramics, magnesia-based ceramics and zirconia-based ceramics. However, these ceramics other than alumina-based ceramics are experimentally-found to be inferior to alumina-based ceramics in terms of the performance as the junction  2 B. 
     When the junction  2 B is sealed in the support tube  1  via the binder layer  3 B in accordance with this embodiment, it is possible to prevent occurrence of cracks irrespective of the service conditions, not depending upon which kind of ceramics is used as the junction  2 B. Although the present invention is not bound by a theory, the reason is considered as follows. As in the above-mentioned case of the welding portion of the sensitive glass membrane  2 A, by providing the binder layer  3 B which can be satisfactorily welded to any of the support tube  1  and the junction  2 B between the support tube  1  and the junction  2 B which is the object to be welded, the support tube  1  and the junction  2 B do not come into direct contact with each other. As a result, a sufficient processing accuracy is available from flexibility of lead glass in its molten state as described above, and flexibility of lead glass is considered to enable the binder layer  3 B to absorb the difference in CTE between the lead-free glass and the junction  2 B. 
     According to this embodiment, as is understood from the above, the crack-preventing effect becomes very remarkable particularly when using alumina-based ceramics as the junction  2 B. However, as is clear from the aforementioned result of experiment, durability under severe service conditions can be improved by providing the binder layer  3 B. Therefore, even when using ceramics other than alumina-based ceramics, i.e. cerium-based ceramics, magnesia-based ceramics or zirconia-based ceramics which makes it relatively difficult to generate cracks, it is very favorable to provide the binder layer  3 B in accordance with this embodiment, considering, for example, the case where a measurement is performed for a long period of time under a condition of higher temperature, and this permits improvement of reliability of the electrode. 
     EXPERIMENTAL EXAMPLE 2  
     Experiments similar to those of the aforementioned Experimental Example 1 were carried out by using each lead-free glass shown in the following Table 4 in place of the lead-free glass shown in Table 1 as the lead-free glass for the support tube  1 . 
     As the lead-free glass shown in Table 1, each lead-free glass shown in Table 4 had a softening point lower than 900° C., and processability and workability themselves were good as the conventional lead glass. As the lead-free glass shown in Table 1, welding property between lead glass and lead-free glass was good when providing the binder layer  3 A or  3 B. Welding property between lead glass and the sensitive glass membrane  2 A or the junction  2 B was also good. 
     The tendency of crack occurrence in the case where the binder layer  3 A or  3 B was not used, and the crack-preventing performance in the case where the binder layer  3 A or  3 B was provided were substantially the same as the results of the above-mentioned Experimental Example 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Lead-free glass (2) 
                 Lead-free glass (3) 
               
               
                   
                 (Support tube) 
                 (Support tube) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Glass 
                 SiO 2   
                 66 
                 63 
               
               
                 composition 
                 Na 2 O 
                 8 
                 11 
               
               
                 (wt. %) 
                 Li 2 O 
                 4.9 
                 4.9 
               
               
                   
                 K 2 O 
                 4.8 
                 4.8 
               
               
                   
                 SrO 
                 5.5 
                 5.5 
               
               
                   
                 CaO 
                 3.8 
                 3.8 
               
               
                   
                 BaO 
                 3 
                 3 
               
               
                   
                 MgO 
                 2 
                 2 
               
               
                   
                 ZnO 
                 0 
                 0 
               
               
                   
                 Al 2 O 3   
                 2 
                 2 
               
            
           
           
               
               
               
            
               
                 Coefficient of linear 
                 94 
                 105 
               
               
                 thermal expansion 
               
               
                 (×10 −7 /° C.) 
               
               
                   
               
            
           
         
       
     
     In the present invention, the composition of lead-free glass applicable as the material for the support tube  1  is not critical, but any commercially available lead-free glass can be used without particular limitation so far as it is glass not containing lead, and is suitable for forming the support tube  1  of the electrochemical sensor. Preferably, lead-free glass should have a CTE of 94±20×10 −7 /° C. on the same level as the CTE of the object to be welded (the sensitive glass membrane  2 A and ceramics  2 B in this embodiment), and a glass composition not containing lead. Apart from the above, it is important that the material for the support tube  1  for the electrochemical sensor is good in easiness of manufacture (processability and workability), water resistance, weather resistance, and transparency. So as to be good in workability, lead-free glass should preferably have a softening point lower than or equal to 900° C. 
     The present applicant proposed the lead-free glass composition suitably applicable for the electrochemical sensor, which is relatively low-temperature softening, good in processability, workability, water resistance, weather resistance and transparency, as is disclosed in Japanese Patent Application Publication No. 2005-207887. In the present invention, the lead-free glass disclosed in the foregoing prior art document is suitably employed. Each lead-free glass shown in Table 4 is the one which is prepared in accordance with the invention disclosed in the aforementioned prior art document. 
     More specifically, as lead-free glass composing the support tube  1 , the glass composition containing, in weight percentage, 60 to 75% SiO 2 , 2 to 14% Na 2 O, 0 to 9% Li 2 O, 1 to 9% K 2 O (where, 10 to 25% Na 2 O+Li 2 O+K 2 O), 0 to 9% SrO, 1 to 9% CaO, 1 to 6% BaO, 0 to 6% MgO, 0 to 6% ZnO (where, 10 to 25% SrO+CaO+BaO+MgO+ZnO), and 0 to 6% Al 2 O 3  is suitably applicable. 
     As is disclosed in the above-mentioned prior art document, the lead-free glass should preferably have the aforementioned composition for the following reasons. 
     SiO 2  is a skeleton component composing a glass network, and the SiO 2  content smaller than 60 wt. % makes it difficult to achieve vitrification. On the other hand, if the SiO 2  content is larger than 75 wt. %, the glass softening temperature becomes too high, leading to lower processability and workability. 
     Both Na 2 O and K 2 O have a function of increasing the low-temperature softening property. Further, these components increase CTE. When taking into account processability and workability of the electrode supporting member, glass should preferably have a softening point equal to or lower than 900° C. Therefore, at least 2 wt. % of Na 2 O and at least 1 wt. % of K 2 O should be contained. On the other hand, excessively high contents of these components lead to degradation of water resistance and weather resistance, and to an excessively high CTE. Therefore, the Na 2 O content should be up to 14 wt. %, and the K 2 O content should be up to 9 wt. %. 
     As in the case of Na 2 O and K 2 O, Li 2 O has a function of increasing the low-temperature softening property. If Na 2 O and K 2 O are contained within the aforementioned ranges, the Li 2 O content should be up to 9 wt. %. If this range is exceeded, water resistance and weather resistance are degraded, and CTE becomes too high. 
     In order to achieve the low-temperature softening property, avoid degradation of water resistance and weather resistance, and prevent CTE from becoming too high, the aforementioned Na 2 O, K 2 O and Li 2 O should be contained within a range from 10 to 25 wt. % in total. 
     SrO is a glass network modifying oxide and effective for improving water resistance and weather resistance. It is useful, when used in an appropriate amount, for preventing devitrification and improving processability. SrO furthermore increases CTE. When CaO and BaO are contained within the following range, the SrO content should be up to 9 wt. %. If this range is exceeded, devitrification may be caused, bubble removal may be degraded, and CTE becomes too high. 
     As SrO mentioned above, each of CaO and BaO is a glass network modifying oxide, and effective for improving water resistance and weather resistance. These are useful, if used in an appropriate amount, for preventing devitrification and improving processabiltiy. Further, these components raise CTE. In order to obtain a desired effect, the CaO content should be up to 1 wt. %, and the BaO content should be at least 1 wt. %. On the other hand, these components, if contained too much, may cause devitrification, lead to degradation of bubble removal, and excessively raise CTE. The CaO content should therefore be up to 9 wt. %, and the BaO content, up to 6 wt. %. 
     MgO is useful for improving processability, but, if contained too much, it may causes degradation of weather resistance and water resistance. Therefore, the MgO content should be up to 6 wt. %. MgO has no marked effect on CTE. 
     ZnO is effective for decreasing the softening point. Further, ZnO lowers CTE. An excessively high ZnO content reduces water resistance. Therefore, the ZnO content should be up to 6 wt. %. 
     However, in order to improve water resistance and weather resistance, prevent devitrification, improve processability, as well as to limit CTE within a desired range, it is preferable to limit the total content of SrO, CaO, BaO, MgO and ZnO within a range from 10 to 25 wt. %. If this content exceeds 25 wt. %, weather resistance and water resistance are reduced. Also, if this content is smaller than 1 wt. %, there cannot be attained a sufficient effect. 
     Al 2 O 3 , if contained in a small amount, is effective for inhibiting devitraification and improving water resistance and weather resistance, but a high content impairs the low-temperature softening property. The Al 2 O 3  content should therefore be up to 6 wt. %. Al 2 O 3  has no marked effect on CTE. 
     In this embodiment, the description has been made based particularly on the case where the glass electrode serving as the electrochemical sensor is the pH electrode having the pH glass electrode sensing pH as the sensitive glass membrane. However, the present invention is equally applicable to the case where the glass electrode is the ion electrode other than the pH electrode. In this case, the sensitive glass membrane may have a known glass composition sensitive to ion to be measured. 
     Manufacturing the support tube serving as the electrode supporting member from glass is very favorable because of many advantages including the good workability, and the possibility to select the material having a CTE close to that of the object to be welded. However, as is understood from the above, provision of the binder layer displays the function of absorbing the difference in CTE between the support tube and the object to be welded. Therefore, the support tube can be manufactured from an arbitrary material not containing lead, including the lead-free glass in this embodiment, so far as the material has a CTE close to that of the lead glass of the binder layer, or more specifically, a CTE of 94±20×10 −7 /° C. 
     According to the present embodiment, as described above, it is possible to prevent occurrence of cracks at the welding portion between the support tube  1  made of lead-free glass and the sensitive glass membrane  2 A or the junction  2 B made of ceramics. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, as described above, the electrochemical sensor has the configuration in which the sensitive glass membrane is welded to the support tube made of lead-free glass with the lead glass layer between the sensitive glass membrane and the support tube, or the configuration in which ceramics for the junction is welded to the support tube made of lead-free glass with the lead glass layer between the ceramics and the support tube. It is therefore possible to prevent occurrence of cracks at the welding portion between the support tube and the sensitive glass membrane or ceramics.