JP2006162570A - Reference electrode for detecting degree of acidity and basicity of oil - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reference electrode for detecting the degree of acidity and basicity of oil, whose electrode potential will be substantially constant, even if the pH of the liquid to be tested varies. <P>SOLUTION: The reference electrode 70 for detecting the degree of acidity and basicity of the oil is used, in combination with a sensitive electrode which varies its potential difference in accordance with the degree of acidity and basicity of the oil, and a layer 90 which is made up of a prescribed metal and an oxide of the metal, such that their mass ratio is made substantially constant, is formed on the surface of an electrode substrate 80 of the reference electrode 70. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a reference electrode for detecting the acidity and basicity of oil.

  In the industry, various oils such as fuel, hydraulic oil, quenching oil and lubricating oil are used. It is known that during storage or use, the acidity gradually increases due to oxidation by the atmosphere or accumulation of fuel products, and eventually corrosion and other deterioration of initial performance are caused.

Therefore, prompt and accurate detection of oil alteration is extremely important for oil management. In order to detect such alteration of oil, conventionally, a sensitive electrode that changes its potential in response to the acidity and basicity of the oil, and a reference electrode that has a different degree of potential change slope are used. An electrode pair comprising the above is known (Patent Document 1).
JP-A-3-175350

  Since the reference electrode for detecting the acidity and basicity of the oil measures the pH of the oil by the potential difference from the sensitive electrode, it is desirable to show a constant potential regardless of the change in pH. In order to obtain a constant potential, a metal that is easily exposed by melting the metal base material of the reference electrode material is selected.

  In Patent Document 1, Zn (zinc) is used as a reference electrode for detecting the acidity and basicity of oil. However, when the electrode potential of the reference electrode is measured, a characteristic diagram as shown in FIG. 6 is obtained (the measurement method will be described in detail later). In this characteristic diagram, it was recognized that the electrode potential of the reference electrode changed by about 450 mV while the pH of the test solution changed from 6 to 3. As shown in FIG. 6, when the electrode potential changes with the change in pH, the acidity and basicity of the oil cannot be accurately detected.

  It is an object of the present invention to provide a reference electrode for detecting the acidity and basicity of oil so that the electrode potential becomes substantially constant even when the pH of the test solution changes.

  The present invention has been made based on the results of experimental studies on various reference electrodes. According to the first aspect of the present invention, there is provided a reference electrode for detecting the acidity and basicity of oil used in combination with a sensitive electrode whose potential difference changes in response to the acidity and basicity of the oil. A layer having a substantially constant mass ratio between a predetermined metal and an oxide of the metal is formed on the surface.

  In zinc (Zn), which is a kind of electrode metal used for the reference electrode, Zn oxide, that is, zinc oxide (ZnO) is formed in the vicinity of the surface. The metal oxide is formed near the surface of any metal. In general, the mass ratio of the metal oxide to the metal is the highest on the surface, and the mass ratio of the metal oxide is generally lowered toward the inside (see FIG. 9). In addition, when these metals are used as a reference electrode for detecting the acidity and basicity of oil, the metal surface dissolves due to the acidity and basicity of the solution. The mass ratio of changes. The inventor has focused on the fact that the change in the mass ratio affects the electrode potential of the reference electrode, and has reached the present invention.

  According to the first aspect of the present invention, even if the surface of the reference electrode is dissolved by the pH of the solution, the mass ratio between the electrode metal of the reference electrode and the oxide of the metal can be made substantially constant. The electrode potential can also be set to a substantially constant value.

  The invention according to claim 2 is characterized in that the metal according to claim 1 is a metal having a larger ionization tendency than hydrogen (H).

  Thereby, the reference electrode is easily dissolved in oil, and the potential difference becomes large when combined with the sensitive electrode. When the measured potential difference is large, the measurement resolution of the pH of the oil, that is, the degree of deterioration of the oil can be increased.

  Specifically, as described in claim 3, it is selected from zinc (Zn), lead (Pb), cobalt (Co), indium (In), tin (Sn), and nickel (Ni). It is characterized by being one kind of metal.

  Zinc (Zn), lead (Pb), cobalt (Co), indium (In), tin (Sn), and nickel (Ni) all have high solubility in oil and form an oxide film on the surface. Because it is a difficult metal, it is optimal for use as the metal of the layer of the reference electrode.

  The invention described in claim 4 is characterized in that the layer formed on the surface of the reference electrode is formed by sputtering or vapor deposition.

  Thereby, a desired layer can be easily formed on the surface of the reference electrode.

  Embodiments according to the present invention will be described below. FIG. 1 is a principle diagram of a normal oil acidity and basicity detector (hereinafter referred to as a pH sensor). FIG. 2 shows the measurement principle. FIG. 3 is a cross-sectional view of the reference electrode of the present embodiment.

  The pH sensor 10 includes an electrode pair 20 including a reference electrode 30 made of, for example, Zn and a sensitive electrode 40 made of, for example, stainless steel, and a potentiometer that detects a potential difference ΔE between the electrode pair 20. (Voltmeter) 60.

  Although not shown, the electrode pair 20 is normally installed in a test oil tank 50 that is attached to the bottom of the oil pan and into which oil can be introduced. A control device such as a microcomputer (not shown) calculates the potential difference ΔE detected by the potentiometer 60 based on measurement data obtained by measuring in advance the relationship between the potential difference ΔE between the electrode pair 20 and the pH in the standard pH solution at each level. The pH in the oil is calculated in terms of pH (see FIG. 2). Thereafter, if the calculated pH becomes a value at which the lubrication performance is considered to be lowered, the control device outputs a signal to a notification device such as an oil warning light.

  As described above, since the pH sensor 10 obtains the pH of the oil based on the potential difference ΔE of the electrode pair 20, the reference electrode 30 used for the electrode pair 20 has a constant potential regardless of the change in pH. It is desirable to show.

  In this embodiment, as shown in FIG. 3, as a reference electrode 70, for example, on the surface of silicon (Si) as the electrode base material 80, a mass ratio of Zn and ZnO is set to a predetermined value (for example, about 80% is ZnO). The adjusted layer 90 is formed. The thickness of this layer 90 is about 42 nm. This layer 90 is formed on the surface of Si80 by sputtering or vapor deposition. By this method, the layer 90 having a predetermined mass ratio of Zn and ZnO can be easily formed on the electrode substrate 80.

  In this embodiment, Si is used as the electrode base material 80, but of course, the same metal used for the layer 90 may be used. That is, Zn may be used as the electrode substrate 80, and the layer 90 made of Zn and ZnO may be formed on the surface of the electrode substrate 80.

  By using the reference electrode 70 having the structure as described above for the pH sensor 10, it becomes possible to make it almost constant regardless of the change in pH.

  Next, the performance as the reference electrode 70 for detecting the acidity and basicity of the oil in this embodiment will be described based on FIGS. 4 to 6 in comparison with a conventional reference electrode made of Zn. To do. FIG. 4 is a principle diagram showing a method for testing the electrode characteristics of the reference electrode. FIG. 5 shows the test results of the electrode characteristics of the reference electrode of this embodiment. FIG. 6 is a test result of electrode characteristics of a conventional reference electrode (comparative example).

  This embodiment in which a layer 90 having a predetermined mass ratio of Zn and ZnO is formed on the surface of Si80 using a test apparatus as shown in FIG. 4, and each of the conventional embodiments formed of Zn The electrode characteristics of the reference electrode were measured.

  The reference electrode 70 of this embodiment formed in a strip shape (for example, 5 cm in length, 3 cm in width, 2 mm in thickness) was immersed in a solution (test solution) whose pH was adjusted by adding hydrochloric acid into 2-propanol. The pH of the test solution was measured using a known pH meter 120 or the like.

  Thereafter, the reference electrode 70 of the present embodiment and the silver / silver chloride electrode 100 which is a known reference electrode are combined in the test solution (for example, the facing distance between the reference electrode 70 and the silver / silver chloride electrode 100 is about 3 cm). The potential difference ΔE generated between the reference electrode 70 of this embodiment and the silver / silver chloride electrode 100 was measured using a potentiometer (voltmeter) 110 at room temperature. For the reference electrode of the comparative example, the potential difference ΔE was measured under the same conditions as the reference electrode 70 of the present embodiment. FIG. 5 and FIG. 6 show the relationship between the potential difference ΔE measured using the silver / silver chloride electrode 100 as a reference and the pH of the test solution measured by the pH meter 120.

  FIG. 5 shows the test results of the electrode characteristics of the reference electrode of this example, and FIG. 6 shows the test results of the electrode characteristics of the reference electrode of the comparative example. The evaluation of each electrode characteristic is determined by linear approximation of the data of the potential difference ΔE when the plotted pH is changed. It can be seen from FIG. 6 that the potential difference ΔE of the conventional reference electrode changes by about 450 mV while changing from pH 6 to pH 3, for example. On the other hand, it can be seen that the potential difference ΔE of the reference electrode 70 of the present embodiment changes by about 60 mV while changing from pH 6 to pH 3, for example. From this test result, it was confirmed that the reference electrode 70 of the present embodiment has higher performance as the reference electrode than the conventional reference electrode.

  Next, FIG. 7 to FIG. 9 show the results of measurement of the state of the surface layer portion of each reference electrode according to the present embodiment and the prior art by the XPS method (X-ray photoelectron spectroscopy). FIG. 7 is a result of measuring the state of the surface layer portion of the reference electrode of the present embodiment in which the pH of the test solution is relatively high, and FIG. 8 is the result of the embodiment in which the pH of the test solution is relatively low. It is the result of having measured the state of the surface layer part of a reference electrode. FIG. 9 shows the result of measuring the state of the surface layer portion of the reference electrode of the prior art. In the measurement results of FIGS. 7 to 9, the vertical axis represents the element ratio (%) of Zn and O, and the horizontal axis represents the depth (nm) from the surface.

  From FIG. 9, the state near the surface of the conventional reference electrode shows that the element ratio of Zn and O changes as the depth from the surface increases, that is, the mass ratio of Zn and ZnO changes. I understand. In this case, when the pH of the test solution is lowered and the surface of the reference electrode is dissolved, the mass ratio of Zn and ZnO on the surface of the reference electrode also changes, and the fluctuation of the potential difference ΔE increases as shown in FIG. .

  On the other hand, in the reference electrode 70 of this embodiment in which the layer 90 having a constant element ratio of Zn and O, that is, the mass ratio of Zn and ZnO, is formed on the surface of the Si 80, the pH of the test solution is changed. 7 and 8 that the mass ratio of Zn and ZnO on the surface of the reference electrode 70 can be kept substantially constant even if the surface 90 is dissolved. Thereby, as shown in FIG. 5, the fluctuation | variation of electric potential difference (DELTA) E can be suppressed rather than the reference electrode of a prior art.

  In the present embodiment, the metal used as the layer 90 of the reference electrode 70 is described as Zn. However, the metal used as the layer 90 may be Pb, Co, In, Sn, or Ni. Since these metals are easily dissolved in oil and are difficult to form an oxide film on the surface, they are optimal as the metal used as the reference electrode 70 for detecting the acidity and basicity of the oil.

It is a principle diagram of a normal oil acidity and basicity detector. It is a measurement principle figure of the acidity and basicity detector of normal oil. It is sectional drawing of the reference electrode of this embodiment. It is a principle figure which shows the test method of the electrode characteristic of a reference electrode. It is a test result of the electrode characteristic of the reference electrode of this embodiment. It is a test result of the electrode characteristic of the conventional reference electrode. It is the result of having measured the state of the surface layer part of the reference electrode of this embodiment in the state where pH of a test solution is comparatively high. It is the result of having measured the state of the surface layer part of the reference electrode of this embodiment in the state where pH of a test solution is comparatively low. It is the result of measuring the state of the surface layer part of the reference electrode of a prior art.

Explanation of symbols

10 pH sensor 20 Electrode pair 30 Reference electrode 40 Sensitive electrode 50 Test oil tank 60 Potentiometer 70 Reference electrode (this embodiment)
80 Electrode substrate (this embodiment)
90 layers (this embodiment)
100 Silver / silver chloride electrode 110 Potentiometer 120 pH meter

Claims (4)

A reference electrode for detecting the acidity and basicity of oil used in combination with a sensitive electrode whose potential difference changes in response to the acidity and basicity of the oil,
A reference electrode for detecting acidity and basicity of oil, wherein a layer having a substantially constant mass ratio between a predetermined metal and an oxide of the metal is formed on the surface of the electrode substrate. The metal is
The reference electrode for detecting acidity and basicity of oil according to claim 1, wherein the reference electrode is a metal having an ionization tendency larger than that of hydrogen (H). The metal is
2. The metal according to claim 1, wherein the metal is selected from zinc (Zn), lead (Pb), cobalt (Co), indium (In), tin (Sn), and nickel (Ni). A reference electrode for detecting the acidity and basicity of the oil according to claim 2. The layer formed on the surface of the reference electrode,
The reference electrode for detecting acidity and basicity of oil according to claim 1, wherein the reference electrode is formed by sputtering or vapor deposition.

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