Solid state amperometric chloramine sensor

Abstract

A monochloramine microsensor includes an elongated housing defining a central axis and an open interior and having a capillary opening at one end. A semi-permeable membrane covers the capillary opening, the semi-permeable membrane allowing diffusion of chloramines there-through while preventing water from entering into the interior of the housing. A chloramine sensitive element in the form of a wire, fiber or nanotube is mounted within the housing, the chloramine sensitive element, when used in conjunction with an anode, outputs current in an amount proportional to the concentration of chloramine present in a liquid sample in which the chloramine sensitive element is immersed. The chloramine sensitive element extends along a length of the central axis to a first end adjacent to and spaced from the semi-permeable membrane. The chloramine sensitive element is a gold wire, a platinum wire, a carbon fiber or a carbon nanotube.

Claims

1. A microsensor for quantitative analysis of chloramines in aqueous systems, comprising: an elongated housing defining a central axis and an open interior and having a capillary opening at one end; a semi-permeable membrane covering the capillary opening, the semi-permeable membrane allowing diffusion of chloramines there-through while preventing water from entering into the open interior of the elongated housing; a chloramine sensitive element in a form of a wire, fiber or nanotube mounted within the elongated housing, the chloramine sensitive element, when used in conjunction with an anode, outputting current in an amount proportional to a concentration of chloramine present in a liquid sample in which the chloramine sensitive element is immersed, the chloramine sensitive element extending along a length of the central axis to a first end adjacent to and spaced at a constant diffusive length from the semi-permeable membrane.

2. The microsensor of claim 1 wherein the chloramine sensitive element is a gold wire, a platinum wire, a carbon fiber or a carbon nanotube.

3. The microsensor of claim 1 wherein the capillary opening at the one end has an inner diameter of 5-10 microns.

4. The microsensor of claim 1 wherein the first end of the chloramine sensitive element is spaced from the semi-permeable membrane by 5-6 microns.

5. The microsensor of claim 1 wherein the elongated housing includes a section which is tapered to the capillary opening.

6. The microsensor of claim 1 additionally comprising an anode, serving as a reference electrode, mounted within the elongated housing, wherein the open interior of the elongated housing is at least partially filled with an electrolyte.

7. The microsensor of claim 6 additionally comprising a conductive wire extending within the elongated housing over a length of the elongated housing and joined at a junction to a second end of the chloramine sensitive element.

8. The microsensor of claim 7 wherein the junction is a material which is electrically conductive and provides strong bonding of the conductive wire to the chloramine sensitive element.

9. The microsensor of claim 6 additionally comprising a seal closing a second end, opposite the one end, of the elongated housing.

10. The microsensor of claim 1 additionally comprising a conductive wire extending within the elongated housing over a length of the elongated housing and joined at a junction to a second end of the chloramine sensitive element.

11. The microsensor of claim 10 wherein the junction is a material which is electrically conductive and provides strong bonding of the conductive wire to the chloramine sensitive element.

12. The microsensor of claim 1 additionally comprising a seal closing a second end, opposite the one end, of the elongated housing.

13. A microsensor for quantitative analysis of chloramines in aqueous systems, comprising: an elongated housing defining a central axis and an open interior and having a capillary opening at one end; a semi-permeable membrane covering the capillary opening, the semi-permeable membrane allowing diffusion of chloramines there-through while preventing water from entering into the open interior of the elongated housing; a compact chloramine sensitive element in a form of a wire, fiber or nanotube mounted within the elongated housing, the compact chloramine sensitive element, when used in conjunction with an anode, outputting current in an amount proportional to a concentration of chloramine present in a liquid sample in which the compact chloramine sensitive element is immersed, the compact chloramine sensitive element extending along a length of the central axis to a first end adjacent to and spaced at a constant diffusive length from the semi-permeable membrane.

14. The microsensor of claim 1, wherein the microsensor is configured in a way such that the microsensor lacks dissolved oxygen sensitivity.

15. The microsensor of claim 1, wherein the microsensor is configured in a way such that the microsensor is operable with a positive applied potential.

16. The microsensor of claim 15, wherein the positive applied potential is between 0.15 and 0.45 V.

17. The microsensor of claim 15, wherein the positive applied potential is between 0 and 0.25 V.

18. A process for quantitative analysis of chloramines in aqueous systems, comprising the use of the microsensor of claim 1 with a reference electrode in a 5 mM pH 8.0 boric acid/sodium hydroxide buffer solution at a temperature of 23 C. with different DO and monochloramine (4:1 Cl.sub.2:N) concentrations, wherein said chloramine sensitive element comprises a gold wire.

19. A process for quantitative analysis of chloramines in aqueous systems, comprising the use of the microsensor of claim 1 with a reference electrode in a 5 mM pH 8.0 boric acid/sodium hydroxide buffer solution at a temperature of 23 C. with different DO and monochloramine (4:1 Cl2:N) concentrations, wherein said chloramine sensitive element comprises a platinum wire.

20. The process of claim 19 wherein a potential between +0.15 and +0.45 V is applied.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a first embodiment of the solid state amperometric chloramine microsensor of the present invention.

(2) FIG. 2 is a schematic diagram of a second embodiment of the solid state amperometric chloramine microsensor of the present invention with an internal reference electrode.

(3) FIG. 3 shows a cyclic voltammogram (CV) for an embodiment of the solid state amperometric chloramine microsensor of the present invention wherein the sensing material is a gold wire.

(4) FIG. 4 is a representative monochloramine calibration curve for the first embodiment of the solid state amperometric chloramine microsensor of the present invention using gold wire as the sensing material.

(5) FIG. 5 shows a 3-D calibration curve for a modification of the chloramine sensor disclosed by W. H. Lee et al (2011) (prior art).

(6) FIG. 6 shows multiple calibration curves of the chloramine sensor of W. H. Lee (2010) (prior art).

(7) FIG. 7 is a representative monochloramine calibration curve of an embodiment of the solid state microsensor of the present invention wherein the sensing material is a platinum wire.

(8) FIG. 8 is a cyclic voltammogram (CV) of an embodiment of the solid state microsensor of the present invention wherein the sensing material (element) is platinum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) FIG. 1 shows a solid state amperometric sensor 10 which is an embodiment of the present invention which may be used in combination with a conventional Ag/AgCl reference electrode to measure the amount of chloramine in engineered or natural waters and aqueous solutions. The sensor 10 includes an elongated housing 12, in the form of a glass capillary, extending along a central axis A. The elongated housing 12 tapers to a capillary at the tip end 23 (lower end in FIG. 1). A conductive wire 14, for example a copper wire, extends through an upper seal 16 into the sealed interior space within the housing 12, along the central axis A, to a junction 18 within the housing 12. The upper seal 16 may be any conventional seal forming material such as glue or an elastomer, e.g., an epoxy rubber, as in the second embodiment shown in FIG. 2 and can have an internal diameter between 100 and 1500 microns, and preferably about 1100 microns.

(10) A junction 18 within the housing 12 serves to electrically connect and bond the conductive wire 14 with a sensing element 20 which extends from the junction 18 to an end 21 slightly short of the capillary tip end 23 (lower end) of the housing 12. The inner diameter X of the open capillary tip end of the housing 12 is suitably 5-7 microns. It is appreciated that the inner diameter X of the open capillary tip end of the housing 12 can be between 1 to 100 microns. A semi-permeable membrane 22 covers and seals off the tip end 23 of the housing 12. The lower end 21 of the sensing element 20 is separated (recessed) from the semi-permeable membrane by a distance Y which is between 1 and 100 microns, but preferably 5-6 microns. The semi-permeable membrane 22 selectively allows the monochloramine to diffuse there-through from an aqueous solution sample and into contact with the lower end of the sensing element 20. Thus, Y defines a constant diffusive length for the monochloramine. The semi-permeable membrane also serves to protect the sensing element 20.

(11) Suitable materials for the sensing element 20 include gold and platinum wires, carbon fibers and carbon nanotubes.

(12) The junction 18 may be any material which has high electrical conductivity and strong conductive bonding. Examples of such a material include bismuth alloys and silver-filled epoxies.

(13) The primary use of the chloramine sensor is in testing and/or monitoring engineered and natural aqueous solutions (e.g., chloraminated drinking water distribution systems or any chloramine system). Therefore, the general operating conditions for chloramine measurement are pHs between 6 and 9 and chlorine to nitrogen mass ratios (Cl.sub.2:N) less than 5:1, resulting in the predominant chlorine species present being monochloramine. The applied potential for the monochloramine microsensor is a positive potential which does not result in dissolved oxygen interference.

(14) FIG. 2 shows a second embodiment of the chloramine sensor of the present invention wherein a chloramine sensor 30 is provided with an internal reference electrode (e.g., Ag/AgCl reference electrode) 26. In this second embodiment the housing (outer casing) 12, in the form of a glass capillary, is at least partially filled with a suitable internal electrolyte 24. The reference electrode 26 used in testing by the present inventors was a 0.25 mm silver wire coated with AgCl and the internal electrolyte 24 was 0.1 M KCl+0.3 M K.sub.2CO.sub.3+0.2 M KHCO.sub.3, pH 10.3. In the second embodiment the inner diameter of the capillary tip end is 7-10 microns, slightly larger than that of the first embodiment.

(15) FIG. 3 shows a cyclic voltammogram (CV) obtained using the first embodiment, i.e., FIG. 1, of the monochloramine sensor (10 m tip diameter) with a solid gold wire for various monochloramine and dissolved oxygen (DO) concentrations. With no monochloramine and between 0% and 21% DO, positive (+) potentials after approximately 0 V when scanned in the positive direction (FIG. 3) result in no changes in electrode response with changes in DO concentration, indicating a lack of DO sensitivity. After 25 mg Cl.sub.2 L.sup.1 of monochloramine addition, the electrochemical oxidation and reduction of monochloramine were observed, indicating two ranges for potential monochloramine detection.

(16) Starting at approximately +0.8 V, the electrochemical monochloramine oxidation (Eq. 1 and Eq. 2) proceeds by reaction with gold surface oxides as follows:
2Au+2OH.sup..fwdarw.2AuOH+2e.sup.(Eq. 1)
2AuOH+2NH.sub.2Cl.fwdarw.N.sub.2H.sub.4+2HCl+O.sub.2+2Au(Eq. 2)

(17) However, as reported by A. N. Tsaousis, Amperometric Determination of Hypochlorous Acid and Monochloramine at Gold Electrodes. M.S. thesis, University of Wisconsin, Milwaukee, Wis., USA, (1985), gold surface oxide accumulation has been shown to lead to the loss of electrode activity, limiting its application for continuous monochloramine monitoring. Therefore, the monochloramine reduction reaction (Eq. 3), occurring between 0 and +0.25V, was chosen to measure monochloramine [3] without DO interference.
NH.sub.2Cl+2H.sub.2O+2e.sup..fwdarw.NH.sub.4.sup.++Cl.sup.+2OH.sup.(Eq. 3)

(18) FIG. 3 shows a cyclic voltammogram (CV) obtained with a microsensor 10 according to the first embodiment of the present invention having a gold wire as the sensing element 20, with the microsensor immersed together with a reference electrode in 5 mM pH 8.0 boric acid/sodium hydroxide (buffer) solution and 23 C. with different DO and monochloramine (4:1 Cl.sub.2:N) concentrations. The scan was initiated at +1.5 V vs. Ag/AgCl in negative direction at 200 mV s.sup.1. The inset in FIG. 3 is a zoomed window showing between +0.3 V to 0.4 V of scanned potentials.

(19) FIG. 4 is a representative monochloramine calibration curve obtained with the first embodiment, i.e., FIG. 1, of the present invention having a gold wire as the sensing element 20 and using a 5 mM borate buffer solution, pH 8.0, 4:1 Cl.sub.2:N ratio, and at 23 C. FIG. 4 shows that the gold microsensor of the present invention has an output that is linear with monochloramine concentration and is not subject to DO interference. The baseline signal (i.e., signal without monochloramine) is close to zero. The sensor 10 has a fast (<5 seconds), highly stable, and excellent amperometric electrode response to monochloramine over a wide concentration range (0-10 mg Cl.sub.2 L.sup.1), including the primary range of interest (0-4.2 mg Cl.sub.2 L.sup.1) with a sensitivity of at least 520.7 pA mg Cl.sub.2.sup.1 L and a lower detection limit of at most 0.120.013 mg Cl.sub.2 L.sup.1.

(20) FIGS. 5-7 present a comparison of monochloramine calibration curves obtained using modifications of previous chloramine sensors, W. H. Lee et al (2011) and W. H. Lee et al (2010), FIGS. 5 and 6, respectively, and the present invention with a platinum wire, FIG. 7. The multiple calibration curves of FIGS. 5 and 6 result from DO interference, while the single calibration curve of FIG. 7 indicates lack of any DO interference for monochloramine measurement (using a 5 mM borate buffer solution, pH 8.0, 4:1 Cl.sub.2;N ratio, and at 23 C.). The current invention using platinum wire has been optimized and, as shown in FIG. 7, does not exhibit oxygen interference.

(21) FIG. 8 shows a cyclic voltammogram (CV) of platinum (Pt) microsensor immersed in a 5 mM pH 8.0 boric acid/sodium hydroxide (buffer) solution and 23 C. with different DO and monochloramine (4:1 Cl.sub.2:N) concentrations. It represents a scan initiated at +1.5 V vs. Ag/AgCl in negative direction at 10 mV s.sup.1. Inset is a zoomed window between +0.5 V to 0.0 V of scanned potentials. FIG. 8 shows a representative platinum sensor CV of the invented monochloramine sensor (10 m tip diameter) with solid platinum wire. Similar to that seen with gold, the electrochemical oxidation and reduction of monochloramine were observed, indicating two ranges of potential monochloramine detection (FIG. 8). Starting at approximately +0.5 V, the electrochemical monochloramine oxidation (Eq. 4 and Eq. 5) has been shown to proceed where platinum oxides react with monochloramine. See D. A. Davies, M.S. thesis, University of Wisconsin Anodic Voltammetric Determination of Monochloramine in Water, M.S. thesis, University of Wisconsin, Milwaukee, Wis., USA, 1985.
2Pt+2OH.sup..fwdarw.2PtOH+2e.sup.(Eq. 4)
2PtOH+2NH.sub.2Cl.fwdarw.N.sub.2H.sub.4+2HCl+O.sub.2+2Pt (Eq. 5)

(22) Monochloramine reduction at platinum was observed at decreasing potentials from approximately +0.45 V with the same reaction (Eq. 3) as gold [3]; however, DO interference was observed with potentials lower than approximately +0.15 V. Consequently, any potential between +0.15 and +0.45 V may be used as the applied potential, resulting in no DO interference while producing a highly stable electrode response to monochloramine. The monochloramine concentration is determined from the measured current using a calibration curve (FIG. 7).

(23) The present invention may be embodied in other forms without departing from the spirit and scope of the appended claims. The foregoing embodiments are intended to be illustrative only and do not limit the scope of the appended claims.