WO2009144331A1

WO2009144331A1 - Reconditioning of glass electrodes for sodium trace detection

Info

Publication number: WO2009144331A1
Application number: PCT/EP2009/061814
Authority: WO
Inventors: Peter Wuhrmann
Original assignee: Swan Analytical Instruments
Priority date: 2009-09-11
Filing date: 2009-09-11
Publication date: 2009-12-03

Abstract

In the proposed method for operating an apparatus (1) for determining trace amounts of sodium ions in a liquid sample (2) by potentiometry, a reference electrode (4) and a sensing electrode (5) comprising a glass body (5a) are used. For reconditioning said sensing electrode (5), the glass body (5a) is exposed to a fluorine ions containing reconditioning liquid (8) which is nominally free from sodium ions. In particular, the reconditioning liquid (8) comprises less than 100 ppb of sodium ions. Preferably, said reconditioning liquid (8) is an aqueous solution of 1 % by weight of acetic acid and 1 % by weight of ammonium fluoride.

Description

RECONDITIONING OF GLASS ELECTRODES FOR SODIUM TRACE DETECTION

Technical Field

The invention relates to the field of electroanalytical chemistry and potentiometry and more particularly to the determination of traces of sodium in liquids and electrodes used therefor. Specifically, the invention relates to the reconditioning of glass electrodes used for determining traces of sodium in liquids, in particular in aqueous solutions, by means of potentiometry.

The invention relates to methods, uses and apparatuses according to the opening clauses of the claims.

Background of the Invention

It is well-known in the art how to determine traces of sodium, more particularly of sodium ions, by means of potentiometry; speaking of traces or trace amounts in the present patent application, we speak of less than 10 ppm, usually less or by far less than 1 ppm. A reference electrode and a sensing electrode are immersed in a liquid sample, and the electrical potential between these electrodes is monitored. A negligible or no current is flowing during determination of said potential. From said potential, the amount or concentration of sodium in the sample can be readily deduced.

E.g., the apparatuses "SODITRACE Analyzer", "Analyzer AMI Sodium A" and "Analyzer AMI Sodium P" of Swan Analytische Instrumente AG (see, e.g., http://www.swan.ch/Products/HighPurityWater/Sodium) are such apparatuses.

As such as sensing electrode, usually, a glass electrode is used which has a doped glass membrane and which is considered to be sensitive to a specific type of ion, namely in the present case to sodium ions. Such a glass electrode tends to become increasingly inert after having been exposed to sodium-containing liquid as it is the case during measurements. This manifests in an increase of its response time, i.e. in the time it takes from a change in concentration in the sample until this change reflects in the above-mentioned potential. Usually, one mentions the T90-value which indicates the time it takes until the potential value reflects 90 % of the new concentration. E.g., a fresh glass electrode can have a T90 of several seconds, but after a couple of days of measuring, T90 may amount to several minutes. Since a slow response (high T90- value, e.g., above 5 or 10 minutes) is usually very undesired, the glass electrode is reconditioned from time to time, so as to achieve a shorter response time again. Already for decades, one immerses the glass electrode in an acid aqueous solution containing sodium ions for reconditioning it. Most prominently, an NaF-containing solution is used. The believe is that because of the measuring, the glass electrode becomes largely depleted of Na⁺ ions, and one therefore has to provide new sodium ions which can overcome the depletion, thus making it easier again for the glass electrode to release sodium ions which again results in a faster reaction of the glass electrode. In the before-mentioned case, the Na⁺ in the NaF solution is the sodium that is offered to the glass electrode and believed to be incorporated therein.

Another suggestion has been made and introduced into the market which follows the same idea. In this case, an aqueous solution of NaNO₃ is used as the sodium provider. Obviously, a glass electrode can be successfully reconditioned when being immersed in a suitable NaNO₃ solution on a daily basis.

Summary of the Invention

It is one object of the invention to create a different way of reconditioning glass electrodes for sodium trace sensors. In particular, a corresponding method, use and apparatus shall be provided.

Another object of the invention is to provide a way of reconditioning the before-mentioned glass electrodes which allows to conduct measurements already a relatively short time after a reconditioning of the glass electrode.

Another object of the invention is to provide a way of reconditioning the before-mentioned glass electrodes which enables a relatively large up-time of the corresponding sodium trace measuring apparatus. The up-time is the time during which measurements can be made - usually expressed relative to the absolute time.

Another object of the invention is to provide a way of reconditioning the before-mentioned glass electrodes which can be carried out relatively safely, in particular without exposing personnel to particularly dangerous chemicals.

Further objects emerge from the description and embodiments below. At least one of these objects is at least partially achieved by methods, uses and apparatuses according to the patent claims.

The method for operating an apparatus for determining trace amounts of sodium ions in a liquid sample by potentiometry using a reference electrode and a sensing electrode comprising a glass body comprises the step of reconditioning said sensing electrode by exposing said glass body to a fluorine ions containing reconditioning liquid which is nominally free from sodium ions. The inventor has found out that the common believe of what happens during a reconditioning process of such a glass body is not correct. The picture that the glass body of the sensing electrode has to be exposed to sodium ions (Na⁺) for reconditioning cannot be correct, since the inventor found out that an at least essentially sodium-free solution can be used for reconditioning. Whereas in the state of the art, there are always considerable amounts of sodium in the employed reconditioning liquids such as 1 weight %, the inventor found that essentially no sodium has to be present in the reconditioning liquid. For example, reconditioning liquids containing less than 1 ppm of sodium ions and reconditioning liquids containing less than 100 ppb of sodium ions work very well. In fact, the less sodium ions in the reconditioning liquid, the less time has to pass until measurements can be made again after reconditioning.

The reason for that is, that, in the state of the art reconditioning solutions, a considerable amount of sodium is still present at the glass body after reconditioning (since the reconditioning solution contains so much sodium) . And all that excess sodium has to disappear (usually in sample liquid passing by) before the envisaged low sodium concentrations in the sample can be detected properly, wherein those low sodium concentrations are nearly always below 1 ppm, in many cases below 1 ppb and beloa 100 ppt . In the state of the art, typically about an hour or even more time has to pass after reconditioning before reliable trace measurements can be made again. Since the inventor has not observed any particular beneficial effect of sodium in the envisaged nominally sodium-free reconditioning liquid, it can be advantageous to have as little as below 10 ppb of sodium, more preferably below 1 ppb of sodium in the reconditioning liquid, because the less sodium is in the reconditioning liquid, the sooner one can return to measuring after reconditioning. The inventor found that with the nominally sodium free reconditioning liquid, in a typical setup, reasonable sensitivities are reached already after about 10 min after reconditioning, and after about half an hour, sodium trace contents below 100 ppt can be reliably sensed. This is a remarkable improvement and increases the up-time of the corresponding sodium trace detecting apparatus. It is assumed by the inventor that the fluorine ions etch off an outer layer of the glass body, which presumably is the only or at least the main effect of the reconditioning. It is assumed by the inventor that during measurements, a presumably sodium-depleted outer layer forms at the glass body which can be etched off, thus exposing a fresh surface which enables measurements with a low T90.

Viewed from a (slightly) different angle, the before- addressed method can be referred to as a method for reconditioning a glass body comprised in or suitable for use in a sensing electrode suitable for use in an apparatus for determining trace amounts of sodium ions in a liquid sample by potentiometry .

Typically, said reconditioning liquid is an aqueous solution. This has at least several practical advantages. Nevertheless, other solvents could - at least theoretically - be used, e.g., an alcohol.

In case the term "nominally free" would be considered an unsuitable term, the upper limits for the sodium content as mentioned in the present patent application could be employed instead.

The use is a use of a fluorine ions containing liquid which is nominally free from sodium ions, in particular which comprises less than 100 ppb of sodium ions, for reconditioning a glass body comprised in or suitable for use in a sensing electrode suitable for use in an apparatus for determining trace amounts of sodium ions in a liquid sample by potentiometry . The apparatus is an apparatus for determining trace amounts of sodium ions in a liquid sample by potentiometry, wherein the apparatus comprises

— a reference electrode;

— a sensing electrode comprising a glass body; — a container for holding said sample;

— a container holding a reconditioning liquid for reconditioning said sensing electrode;

Therein, said reconditioning liquid contains fluorine ions and is nominally free from sodium ions, in particular it comprises less than 100 ppb of sodium ions.

Speaking of an apparatus for "determining" trace amounts of sodium ions, this includes, among others, detecting, sensing, measuring, monitoring of trace amounts of sodium ions . The invention comprises uses and apparatuses with features of corresponding methods according to the invention, and vice versa. The advantages of the uses and apparatuses basically correspond to the advantages of corresponding methods and vice versa.

In one embodiment which may be combined with one or more of the before-addressed embodiments, said reconditioning liquid has a pH-value below 6. In an acid environment, HF (hydrofluoric acid) can form, which is assumed to be important for the reconditioning, as explained above.

In one embodiment which may be combined with one or more of the before-addressed embodiments, said reconditioning liquid has a pH-value between 1 and 5.5. Although also even lower ρH-values would probably work, the reconditioning process can be handled more easily in case of not too low pH-values. In particular, said reconditioning liquid may have a pH-value between 3 and 5. Near and below pH 5, the reconditioning process can be carried out efficiently; pH 3 and above is suitable in practice. More particularly, said reconditioning liquid has a pH-value between 4 and 5. With pH not below 4, safety and security measures that have to be taken are rather low; the concentration of free HF is relatively low. So, for practical purposes, pH between 4 and 5, e.g., pH 4.5 ± 0.3 can be advantageous in practice.

In one embodiment which may be combined with one or more of the before-addressed embodiments, said reconditioning liquid comprises NH_1J ⁺ ions and F^~ ions. Besides dissolved ammonium fluoride, also other substances can be used for providing the fluorine ions, e.g., fluoride salts of organic ammonium bases or HF . A possible advantage of ammonium fluoride (NH₄F) is, that in most or all _ g -

applications, NH₃ vapors or vapors of even stronger ammonium bases like Diisopropylamine are blown into the sample during measurements in order to strongly increase the pH value of the sample in order to avoid that the sensor rather works as an H+ sensor than as a Na⁺ sensor. Accordingly, the glass body is accustomed to being exposed to ammonium bases (during measurements), and NH₄F will not introduce new ions that might in some way or the other influence measurements (in particular in a first time span after reconditioning the sensing electrode) .

In one embodiment which may be combined with one or more of the before-addressed embodiments, said reconditioning liquid comprises dissolved acetic acid. Dissolved CH₃COOH allows to readily adjust the pH-value in a suitable range. Alternatively, any other sodium-free acid could be used, e.g., an anorganic acid such as HCl or H₂SO₄, or an organic acid such as formic acid or priopionic acid. In case of acids distinctly stronger than acetic acid, one has to be more careful preparing the reconditioning liquid; a pH buffer could in principle be used, but pH buffers mostly contain and set free sodium, rendering their use in the present context unsuitable.

In one embodiment which may be combined with one or more of the before-addressed embodiments, said reconditioning liquid is obtainable or obtained substantially by adding acetic acid and NH₄F to water (preferably deionized water) .

The concentration of acetic acid can be between

0.2 weight % and 8 weight %, more particularly between

0.4 weight % and 5 weight %, even more particularly between 0.5 weight % and 2 weight %.

The concentration of NH₄F can be between 0.2 weight % and 8 weight %, more particularly between 0.4 weight % and 5 weight %, even more particularly between 0.5 weight % and 2 weight %.

In one embodiment which may be combined with one or more of the before-addressed embodiments, said step is carried out automatically, said step being carried out alternatingly with phases during which a content or concentration of sodium ions in a sample is determined.

In one embodiment which may be combined with one or more of the before-addressed embodiments, said glass body substantially consists of NaS 11-18 type glass. Such a glass, e.g., available from Corning, is used in known sodium trace sensors. Other types of glass suitable for sodium detection can be used, too.

In one embodiment which may be combined with one or more of the before-addressed embodiments, said glass body forms a hollow body. In one embodiment which may be combined with one or more of the before-addressed embodiments, said glass body forms a membrane .

In one embodiment which may be combined with one or more of the before-addressed embodiments, said exposing is carried out for at least 20 s, in particular for at least 30 s, more particularly for at least 50 s, and moreover, said exposing is carried out for at most 5 min, more particularly for at most 2 min, more particularly for at most 80 s. With fluorine in concentrations and pH values as specified above, a minimum reconditioning time of 20 to 30 seconds turned out to be required for good results, and above 1 to 2 minutes reconditioning time, results did not seem to noticeably improve any further. In one embodiment which may be combined with one or more of the before-addressed embodiments, the apparatus comprises a potential determining device operationally connected to said reference electrode and to said sensing electrode.

In one embodiment which may be combined with one or more of the before-addressed embodiments, the apparatus comprises a flow control unit structured and configured for automatically alternating phases in which said glass body is exposed to said reconditioning fluid and phases during which a content or concentration of sodium ions in a sample is determined. Since sodium trace measuring apparatuses are often used for monitoring purposes which extend over long periods of time such as weeks or months or years, a high degree of automatization is desirable.

In one embodiment which may be combined with one or more of the before-addressed embodiments, said flow control unit comprises at least one flow control device such as, e.g., a valve or a pump, and a controller such as, e.g. a controller chip, operationally connected to said at least one flow control device. In one embodiment which may be combined with one or more of the before-addressed embodiments, the apparatus is structured and configured for determining trace amounts of sodium ions of the order of 1 ppb or of the order of 10 ppt or both. In power plant applications (in which, e.g., water steam cycle samples are monitored) , traces at least in the range of 5 ppb to 500 ppt, more particularly at least in the range of 20 ppb to 200 ppt, and even more particularly at least in the range of 100 ppb to 100 ppt are determined. In semiconductor applications (in which, e.g., rinsing water for photolithographic processes is monitored) , traces at least in the range of 50 ppt to 5 ppt, more particularly at least in the range of 200 ppt to 2 ppt, even more particularly at least in the range of 500 ppt to 1 ppt are determined.

Further embodiments and advantages emerge from the dependent claims and the figures.

Brief Description of the Drawings

Below, the invention is described in more detail by means of examples and the included drawings. The figures show:

Fig. 1 a schematic diagram of an apparatus; Fig. 2 a schematic flow chart of a method.

The reference symbols used in the figures and their meaning are summarized in the list of reference symbols. The described embodiments are meant as examples and shall not confine the invention. Detailed Description of the Invention

Fig. 1 shows a schematic diagram of an apparatus 1 for monitoring traces of sodium ions (Na⁺) in a liquid sample 2. Such an apparatus 1 is suitable for use in monitoring sodium contaminations in water, e.g., in the semiconductor industry and in power plants. Typical Na⁺ concentrations to be detected in these fields are in the range of 10 ppt and 1 ppb, respectively. The apparatus comprises a container 3 for holding sample 2 having an inlet 3a and an outlet 3b, a reference electrode 4, a sensing electrode 5 comprising a glass body 5a of NaS 11-18-type glass or of a similar glass type, a potential determining device 6 operationally connected to said reference electrode 4 and said sensing electrode 5, such as a suitable high-impedance voltmeter, a container 7 for holding a reconditioning liquid 8, valves 2a, 3a and 7a and a controller 9. Controller 9 and valves 2a, 3a and 7a constitute a flow control unit. The apparatus 1 is designed for automated long-term measurements in flowing-through mode. The need for periodically reconditioning sensing electrode 5 and glass body 5a, respectively, has been explained further above in the present patent application. During measurements, valve 2a is open and valve 7a is closed, as controlled by controller 9 (cf. the operational connections depicted in Fig. 1 by broken lines) . Accordingly, sample 2, i.e. the water to be analyzed, is fed into and out of container 3 via inlet 3i and outlet 3o, whereas no reconditioning liquid 7 is admitted into container 3.

For both, sample 2 and reconditioning liquid 8, a constant volume flow can be ensured, e.g., by using gravity, as shown in Fig. 1 for reconditioning liquid 8.

The electrodes 4, 5 are immersed in the liquid contained in container 3, which during measurements is the sample 2. In the depicted case, the formation of H⁺ ions is largely suppressed in container 3 by letting flow vapors 10 of NH₃ into the liquid in the container. Alternatively, vapors of even stronger ammonium bases like Diisopropylamine could be used. This way, a potential between the electrodes 4 and 5 determined by potential determining device 6 is strongly related to the sodium ion concentration in sample 2 and largely independent of the H⁺ concentration in sample 2.

After about a week of measuring, sensing electrode 5 has to be reconditioned. For this purpose, sensing electrode 5 could be removed from container 3 and immersed in reconditioning liquid 8 elsewhere. But that way, it would be hard to reach the desired detection ranges, and considerable operation and/or equipment efforts would be required. It is simpler to let sensing electrode 5 in place and fill reconditioning liquid 8 into container 3, as shown in Fig . 1. Accordingly, for reconditioning, valves 2a and 3a are closed, and valve 7a is open, as controlled by controller 9. Alternatively, since the envisaged samples are very clean, also valve 2a could be open during reconditioning, and the liquid in container 8 could be of a higher concentration than needed in container 3, such that an appropriate reconditioning liquid 7 would be present in container 3, formed by mixing sample 2 with the liquid in container 7; therein, it is possible to carry out reconditioning in a flow-through mode with valve 2a open virtually all the time during reconditioning, or to produce a suitable reconditioning liquid using sample 2 and then close valve 2a while continuing reconditioning. In order to enable the presence of a suitable pH value during reconditioning though, it is important to have valve 3a closed during reconditioning.

For about one minute, at least glass body 5a will be immersed in reconditioning liquid 7, which - during measurements taking place thereafter - will result in a faster response of the measured potential to changes in the Na⁺ concentration of the measured sample 2 (lower T90 because of no Na⁺ contamination) .

In order to return to measuring mode, valve 7a will be closed, and valves 2a and 3a will be opened. Or, in case valve 2a was open during reconditioning, valve 2a would remain open and valve 3a would be opened.

With the proposed reconditioning liquid 8, reliable measurements will be possible already after typically 10 to 30 minutes, which is faster by a factor of at least 2 to 3 when compared to the use of reconditioning liquids known in the art .

The proposed reconditioning liquid 8 is an aqueous solution of 1 % by weight of acetic acid (CH₃COOH) and 1 % by weight of ammonium fluoride (NH₄F) . That reconditioning liquid 8 has a pH-value of about 4.5 and contains hydrofluoric acid (HF) in an appropriate amount to etch off enough of the surface of glass body 5a so as to appropriately regenerate sensing electrode 5. No sodium has been provided in reconditioning liquid 8, unlike in case of the reconditioning liquids known in the state of the art, where, e.g., NaCl, NaF or NaNU₃ has been added to water in order to create the reconditioning liquid. With the proposed nominally sodium-free reconditioning liquid 8, which - mainly for contamination purposes - still might contain sodium traces of below 100 ppb or below 10 ppb or so, the time passing after reconditioning before reliable measurements can be accomplished again, is strongly reduced with respect to the sodium-containing reconditioning liquids of the state of the art.

As has been described before in the preset patent application, variations of the composition of the reconditioning liquid 8 are well possible, in particular with respect to the concentrations of the solved constituents and the pH-value and also with respect to the cation coming with the fluorine (i.e. a replacement for the NH₄ ⁺) and with respect to the acid (i.e. a replacement for the acetic acid) .

Fig. 2 shows a schematic flow chart of a method. Step 100 is a measuring step as described above. Step 110 is a reconditioning step as described above. In step 120, after reconditioning (step 110), the electrodes are exposed to the sample again, and actual measuring (step 130) is then carried out some time after reconditioning (step 110) , as described above.

Aspects of the embodiments have been described in terms of functional units. As is readily understood, these functional units may be realized in virtually any number of hardware and/or software components adapted to performing the specified functions.

In the Figures, some parts and details have been omitted, in particular some of those which are not closely related to the reconditioning and more particularly to the reconditioning liquid. Such parts and details can be embodied in the way known in the art.

List of Reference Symbols

1 apparatus 2 sample

2a valve

3 container 3a valve

3i inlet 3o outlet

4 reference electrode

5 sensing electrode 5a glass body

6 potential determining device 7 container

7a valve

8 reconditioning liquid

9 controller

10 vapor

100,110,120,130 steps

Claims

Patent Claims :

1. A method for operating an apparatus for determining trace amounts of sodium ions in a liquid sample by potentiometry using a reference electrode and a sensing electrode comprising a glass body, said method comprising the step of reconditioning said sensing electrode by exposing said glass body to a fluorine ions containing reconditioning liquid which is nominally free from sodium ions, in particular which comprises less than 100 ppb of sodium ions.

2. The method according to claim 1, wherein said reconditioning liquid has a pH-value below 6.

3. The method according to claim 1 or claim 2, wherein said reconditioning liquid has a pH-value between 1 and 5.5, in particular between 2 and 5, more particularly between 4 and 5.

4. The method according to one of the preceding claims, wherein said reconditioning liquid comprises NH₄ ⁺ ions and F^" ions .

5. The method according to one of the preceding claims, wherein said reconditioning liquid comprises dissolved acetic acid.

6. The method according to one of the preceding claims, wherein said step is carried out automatically, said step being carried out alternatingly with phases during which a content or concentration of sodium ions in a sample is determined.

7. The method according to one of the preceding claims, wherein said glass body substantially consists of NaS 11-18 type glass.

8. The method according to one of the preceding claims, comprising carrying out said exposing for at least 20 s, in particular for at least 30 s, more particularly for at least 50 s, and comprising carrying out said exposing for at most 5 min, more particularly for at most 2 min, more particularly for at most 80 s.

9. Use of a fluorine ions containing liquid which is nominally free from sodium ions, in particular which comprises less than 100 ppb of sodium ions, for reconditioning a glass body comprised in or suitable for use in a sensing electrode suitable for use in an apparatus for determining trace amounts of sodium ions in a liquid sample by potentiometry.

10. The use according to claim 9, wherein said liquid contains dissolved NH₄F and dissolved CH₃COOH and has a pH- value between 1 and 5.5, in particular between 2 and 5, more particularly between 4 and 5.

11. An apparatus for determining trace amounts of sodium ions in a liquid sample by potentiometry, comprising

— a reference electrode;

— a container holding a reconditioning liquid for reconditioning said sensing electrode; said reconditioning liquid containing fluorine ions and being nominally free from sodium ions, in particular comprising less than 100 ppb of sodium ions.

12. The apparatus according to claim 11, comprising a flow control unit structured and configured for automatically alternating phases in which said glass body is exposed to said reconditioning fluid and phases during which a content or concentration of sodium ions in a sample is determined.

13. The apparatus according to claim 11 or claim 12, wherein said apparatus is structured and configured for determining trace amounts of sodium ions of the order of 1 ppb or of the order of 10 ppt or both.