Abstract
The invention concerns a phosphate electrode with a base body (1) and a first coating (1a) provided at least in sections of the based body, wherein the base body comprises elemental cobalt and the first coating (1a) comprises a cobalt phosphate, wherein a second coating (1b) is applied at least in section onto the base body and/or the first coating, wherein the second coating binds protons and/or releases hydroxides. The invention further concerns a method for determination of the phosphate concentration with the phosphate electrode.
Claims
1-10. (canceled)
11. A phosphate electrode with a base body and a first coating (1a) provided at least on sections of the base body, wherein the base body comprises elementary cobalt and the first coating (1a) comprises a cobalt phosphate, characterized in that at least on sections of the base body and/or the first coating (1a) a second coating (1b) is provided, which binds protons and/or releases hydroxide ions.
12. Phosphate electrode according to claim 11, characterized in that the second coating (1b) sets a pH value between 7.5 and 9 in 50 ml of a 0.1 mM KCl solution at 25 C.
13. Phosphate electrode according to claim 11, characterized in that the second coating (1b) comprises a solid buffer system.
14. Phosphate electrode according to claim 11, characterized in that the second coating (1b) comprises a borosilicate glass, microcapsules and/or a functionalized carrier material.
15. Phosphate electrode according to claim 11, characterized in that the second coating (1b) comprises a borosilicate glass.
16. The phosphate electrode according to claim 11, characterized in that at least one gas feed line is provided with at least one opening, wherein the at least one opening is arranged such that, when a gas is introduced into the gas feed line the gas escapes from the at least one opening and flows around the base body.
17. Use of a phosphate electrode according to claim 11, for the determination of the phosphate concentration in activated sludge of a water treatment and/or sewage treatment plant.
18. Method for the determination of the phosphate concentration in an aqueous analyte with a phosphate electrode, characterized in that the phosphate electrode is immersed in an adjusting solution before the phosphate concentration is determined until the phosphate electrode outputs a measuring signal which does not vary with time, wherein Interfering ions and phosphate were added to the adjusting solution.
19. The method as claimed in claim 18, characterized in that the pH of the adjusting solution is between 5 and 9.
20. The method as claimed in claim 18, characterized in that the determination of the phosphate concentration is carried out at a constant gas partial pressure.
Description
[0041] Shown is:
[0042] FIG. 1 the voltage change of a half-cell of a phosphate electrode as described in DE 10 2009 051 169 with addition of nitrate (a, b), chloride (c, d) and sulfate (e, f),
[0043] FIG. 2a semi-logarithmic plot of the potential difference as a function of the change in the phosphate concentration at pH=8.8,
[0044] FIG. 3a the change in the potential difference as a function of the addition of interfering ions and the phosphate concentration for an initial concentration of 0.52 mM sulfate, 2.82 mM chloride and 0.01 mM phosphate,
[0045] FIG. 3b the change in the potential difference depending on the addition of interfering ions and the phosphate concentration for an initial concentration of 2.08 mM sulfate, 7.05 mM chloride and 0.01 mM phosphate,
[0046] FIG. 4 the change in the potential difference as a function of the addition of interfering ions and the phosphate concentration for an initial concentration of 2.08 mM sulfate, 7.05 mM chloride and 0.01 mM phosphate,
[0047] FIG. 5a the change in the potential difference of a phosphate electrode according to the invention as a function of the addition of phosphate for an initial concentration of 0.52 mM sulfate, 2.82 mM chloride and 0.01 mM phosphate,
[0048] FIG. 5b the change in the potential difference of a phosphate electrode according to the invention as a function of the addition of interfering ions (representation of the concentration gradient in mM) for an initial concentration of 0.52 mM sulfate, 2.82 mM chloride and 0.01 mM phosphate,
[0049] FIG. 6a the change in the potential difference of a phosphate electrode according to the invention as a function of the addition of phosphate for an initial concentration of 2.5 mM sulfate, 14.1 mM chloride and 0.01 mM phosphate,
[0050] FIG. 6b the change in the potential difference of a phosphate electrode according to the invention as a function of the addition of interfering ions (representation of the concentration gradient in mM) for an initial concentration of 2.5 mM sulfate, 14.1 mM chloride and 0.01 mM phosphate,
[0051] FIG. 7 schematically the structure of a base body with first and second coating,
[0052] FIG. 8 schematically shows a base body with coatings constructed as shown in FIG. 7,
[0053] FIG. 9a preferred embodiment of the invention wherein a constant gas partial pressure is generated,
[0054] FIG. 10 is a plan view of a phosphate electrode according to a preferred embodiment, and
[0055] FIG. 11a cross-section of a phosphate electrode as shown in FIG. 10.
[0056] FIG. 1 shows the potential difference change V as measured by a phosphate electrode according to DE 10 2009 051 169 as a function of different interfering ion concentrations at a pH value of 7.4 and 8.8. The potential difference is determined in this case against a reference electrode whose half-cell potential is not influenced by the phosphate concentration. The analyzed analyte solutions contained dipotassium hydrogenphosphate (K.sub.2HPO.sub.4) with a concentration of 0.01 mM. The interfering ions nitrate (a, b), chloride (c, d) and sulfate (e, f) were added in the indicated concentrations. The potential difference V was recorded outgoing from an initial value (V=0). A significant change in the voltage difference was observed for all interfering ions at a pH of 7.4. This shows that the prior art phosphate electrode has a strong cross-sensitivity to other anions.
[0057] The potential difference change at pH=7.4 follows essentially a saturation kinetic and can be described very well with the Langmuir equation used in the absorption processes:
[00001]
[0058] K.sub.L is the bond constant for the interfered interstitial ion, V.sub.max is the maximum deflection of the potential difference and c.sub.A the concentration of the interfering ion. The matching of a corresponding Langmuir equation and the obtained binding constant for the investigated interfering ion are also shown in FIG. 1 for pH=7.4 (b, d, f). Correspondingly, for neutral environments at pH=7.4, the interfering ions on the phosphate electrode appear to be absorbed, which leads to an undesirable change in the potential difference and makes the determination of the phosphate concentration considerably more difficult.
[0059] At an elevated pH of 8.8, the electrode's response to increasing chloride, nitrate and sulfate concentrations is strongly damped compared to more neutral conditions (pH=7.4). Especially in the lower concentration range (<1 mM) hardly any change in the potential difference is observed.
[0060] At the same time, the sensitivity for phosphate is maintained, as shown in FIG. 2. For a pH value of 8.8, the voltage difference is determined as a function of the phosphate concentration. In the semi-logarithmic plot shown, a linear progression is observed, where the slope corresponds to a value which would typically be expected under these conditions for a divalent anion (here: the hydrogen phosphate HPO.sub.4.sup.2).
[0061] In a further series of experiments, the phosphate electrode was examined for the effect of a change in concentration of anions on the electrode potential. The results are shown in FIG. 3.
[0062] Two ion environments (a, b) were tested, which can simulate, for example, the situation in the sewage water of a water treatment plant. In the results shown in FIG. 3a, 0.52 mM of sulfate and 2.92 mM of chloride were added to the analyte solution. In the results shown in FIG. 3b, 2.08 mM of sulfate and 7.05 mM of chloride were added to the analyte solution. Both represent extreme cases of typical interfering ion concentrations, a typical minimum concentration being shown in FIG. 3a and a typical maximum concentration in FIG. 3b. In particular, such interfering ion concentrations are present in the phosphate concentration determination in water treatment plants. In both situations (a and b), the addition of nitrate (as potassium nitrate) and chloride (as potassium chloride) did not have any measurable effect on the electrode potential. The change in the phosphate concentration (upper axis), however, caused the expected potential change, demonstrating that the electrode can be used to determine the phosphate concentration.
[0063] A sulfate addition of 0.5 mM caused a slight potential change (FIGS. 3a and b). At lower changes in sulfate concentration (FIG. 4), however, no potential changes were recorded. In general, it has been found that the higher the starting concentration of the corresponding interfering ion or all interfering ions, the lower the potential change due to a certain interfering ion concentration change is. This observation is explained by saturation effects.
[0064] These results show that at a suitable pH value, in particular of approximately 8.8, the cross-sensitivity of the phosphate electrode to the constitutively occurring interfering ions is reduced and the phosphate concentration determination is only insignificantly impaired. The stated pH value represents an optimum. If the pH value is increased to values >9, the potential change of the phosphate electrode decreases with respect to a change in the phosphate concentration so that the phosphate electrode loses its sensitivity.
[0065] FIG. 5a shows the potential change in the course of the measurement time of a phosphate electrode according to the invention at a disturbance ion concentration of 0.52 mM K.sub.2SO.sub.4 and 2.82 mM KCl with a changing the phosphate concentration (upper axis). It becomes apparent that the phosphate electrode according to the invention is suitable for determining the phosphate concentration. The calibration curve of the phosphate electrode according to the invention obtained from the measured data is shown as an insertion. From a measurement time of approx. 200 min, the phosphate electrode was transferred to a further solution with the initial concentration of 0.01 mM phosphate. An increase in the measured potential difference (in mV) was observed. After a measurement time of at most 1300 minutes, the measured potential difference of the phosphate electrode according to the invention is returned to the starting value (for 0.01 mM phosphate). This demonstrates the function and good reversibility of the phosphate electrode according to the invention.
[0066] FIG. 5b shows the potential change of a phosphate electrode according to the invention at an interfering ion concentration of 2.08 mM sulfate and 7.05 mM chloride as a function of the concentration of KNO.sub.3, KCl and K.sub.2SO.sub.4. It becomes clear that the addition of further interfering ions leads only to negligible potential changes. Thereby the highest remaining cross-sensitivity for the divalent sulfate is observed. An addition of 1.02 mM K.sub.2SO4 (to a total of 3.1) leads to a potential change below 10 mV, which results in a small measurement error with respect to the phosphate concentration.
[0067] FIGS. 6a and b show, analogously to FIG. 5, the potential change of a phosphate electrode according to the invention with a higher interfering ions concentration. As interfering ions, 2.5 mM K.sub.2SO.sub.4 and 14.1 mM KCl were introduced into the analyte solution. FIG. 6a again shows the change in the potential with changing phosphate concentration. In addition, the reversibility of the potential change was also tested by transferring the phosphate electrode according to the invention into a solution with a concentration of 0.01 mM phosphate at a measurement time of 275 min. Here again, after a measuring time of at the latest 1350 min, the output value at 0 min measuring time is reached.
[0068] FIG. 6b shows the change in the potential at the same interfering ions concentration as FIG. 6a and the indicated interfering ions concentrations. The low influence of the interfering ions on the potential of the phosphate electrode according to the invention is also evident here.
[0069] FIG. 7 shows schematically the structure of a base body 1 made of cobalt of a phosphate electrode according to the invention with a first coating 1a and a second coating 1b. The second coating 1b is preferably hydrophilic and waterpermeable, which facilitates the diffusion of phosphate onto the base body or the first coating 1a. In addition, the second coating 1b must establish a basic pH value in the electrode environment and should quickly compensate for changes in the pH value in the boundary layer of the electrodes surface. In a preferred embodiment, pulverized borosilicate glass is used for the second coating, e.g. as offered by Trovotech GmbH (Edisonstr.3, D-06766 Bitterfeld-Wolfen). Said company produces borosilicate glass powder in defined grain sizes, wherein the pH value in the boundary layer can be adjusted in a targeted manner by chemical modification of the particle surface.
[0070] FIGS. 7 and 8 schematically illustrate a preferred, already tested construction of the base body 1 with coatings 1a and 1 b of a phosphate electrode according to the invention. The other measurement setup corresponds to the specifications in DE 10 2009 051 169 and is typical for ion-selective electrodes. A mixture of cobalt powder (Fluka. 60784, Sigma-Aldrich) and cobalt hydrogen phosphate (mixing ratio 1:1) is applied as coating onto a cobalt plate (thickness 0.1 mm, from Alfa-Aesar, Karlsruhe) to obtain a first coating 1a on the base body 1. Then, a second coating 1b comprising the borosilicate glass powder (TROVOpowder B-K20_8.8) was applied. For this purpose, the borosilicate glass powder was suspended in water and the suspension was applied with a Pasteur pipette onto a filter paper of glass fiber (which was adapted to the dimensions of the electrode, MN85/70, from Macherey-Nagel, Duren). The glass particles are transported with the penetrating water into the filter pores and fixed therein. Powder remaining on the surface is carefully spread out with a spatula and powder residues are removed. The thus prepared filter paper is then applied on both sides to the base body 1 and the first coating 1a in a moist state, and is then immediately introduced into a filter pocket 2 made of cellulose. Two hard plastic meshes 3, which are rigidly connected to each other by clamps 3a and mechanically stabilize the coatings 1a and 1 b, are finally attached as an outer boundary.
[0071] In another variant, the base body 1 and the first coating 1a are separated from the second coating 1b, in particular a borosilicate layer, by a fine-pore, hydrophilic membrane (for example, of synthetic fiber) of a few m thickness (not shown).
[0072] In a further, preferred version, only non-biodegradable material is used, which has a favorable effect on the stability and the lifetime of the electrode. For example, filter bags 2 made of synthetic fibers are used instead of those made of cellulose. Further, filter papers of glass fiber, e.g. Munktell 3.1101.047 of thickness 250 m from the company Munktell Filter AB may be used. If a filter paper made of glass fiber is used, an additional wrapping by a filter bag can be omitted, which allows a more cost-effective production of the electrode. In addition, the liquid exchange between the electrode surface and the analyte solution can be improved.
[0073] In a further variant, instead of borosilicate glass powder, microparticles are used, whose surface has been doped with amino groups in order to buffer the local pH value in the basic range. These microcapsules may be coated and/or filled such that they continuously release hydroxide ions.
[0074] FIG. 9 schematically shows a base body 1 (with coatings) according to FIG. 8 and additional gas line 4 with corresponding opening 5 in two perspectives. In this case, an opening 5 can be provided for each gas line 4 as well as a plurality of openings 5 for a gas line 4. An oxygen-containing gas, in this case air, is passed through the gas line 4, for example a commercially available PVC hose, and is distributed via opening 5 in the vicinity of the phosphate electrode according to the invention. This is shown schematically in FIG. 9 by the circles. Thereby, a constant oxygen partial pressure (pO.sub.2) is set in the vicinity of the phosphate electrode, and the cross-sensitivity of the electrode potential against the oxygen in the analyte can be reduced. For introducing the air, for example, a commercially available aquarium pump can be used.
[0075] A supply of oxygen-containing gas around the phosphate electrode is particularly advantageous when the oxygen partial pressure on the electrode surface deviates strongly from that in the analyte (for example, under anaerobic conditions in the clarification basin of a sewage treatment plant).
[0076] FIGS. 10 and 11 show schematically a preferred embodiment of the phosphate electrode.
[0077] FIG. 10 shows a plan view of a phosphate electrode according to the invention with an additional gas feed line (PVC hose) with openings 5, reference electrode 6, additional temperature sensor 7 and phosphate electrode measuring head 8.
[0078] FIG. 11 shows a cross-section of the phosphate electrode shown in FIG. 10. As described above, the base body 1 has two coatings, is arranged horizontally in the image plane and forms the reactive surface of the phosphate electrode on the side facing the analyte. A basic pH value of above 7.4 (namely between 7.5 and 9) is generated around this surface by the second coating (not shown). In addition, air is released via the gas line 4 on the reactive surface of the base body 1, as a result of which a constant oxygen partial pressure is generated in the phosphate electrodes environment.
[0079] Both the reference electrode 6 and the phosphate electrode measuring head 8 are connected via BNC sockets 9 and cables 10 to a preamplifier 11. which amplifies the measurement signal and outputs it to an amplifier (not shown). For sealing the electronic components, a plurality of sealings 12 are provided, which prevent the analyte from penetrating into the electrode.
LIST OF REFERENCE NUMERALS
[0080] 1 base body [0081] 1a first coating [0082] 1b second coating [0083] 2 filter bag [0084] 3 hard plastic mesh [0085] 3a clamps [0086] 4 gas feed line [0087] 5 opening [0088] 6 reference electrode [0089] 7 temperature sensor [0090] 8 phosphate electrode measurement head [0091] 9 BNC connectors [0092] 10 cable [0093] 11 preamplifier [0094] 12 sealing
