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1.1 Reagents and Instruments
The chemical reagents used were all analytically pure and the solutions were prepared from distilled water. The electrode potential was measured on a Beckhman Model 512 pH meter and the resulting electrode potentials were all relative to saturated calomel electrodes. The buffer was composed of 0101 mol LH3PO42H3BO32CH3COOH. The pH change of the buffer was achieved by adding 1 mol of LKOH or HCl dropwise. The pH of the buffer was simultaneously monitored by a 231 pH electrode (Shanghai Dianguang Instruments).
1.2 Synthesis of Nano Metal Oxides
Nano-sized MnO2, Co3O4 and PbO were synthesized by solid-state reaction at room temperature. MnO2 is obtained by the redox reaction of solid state KMnO4 and a divalent manganese salt containing crystal water (such as MnCl2?4H2O) at room temperature. Co3O4 or PbO is prepared by mixing a certain amount of cobalt acetate or lead acetate with an amount of citric acid or oxalic acid, and fully reacting at room temperature and decomposing at 300-600 DEG C. to obtain a product. The synthesis of nano-TiO2 is based on sol-gel process using n-butyl titanate as raw material.
1.3 Preparation of Oxide Electrodes
In this experiment, a semi-automatic screen printing machine (packaging and printing industry) was used, and the base material was a PVC sheet. The electrode strip is composed of three parts, namely the electrical contact terminal, the branch circuit and the working electrode terminal. Each part is made by printing carbon slurry (imported from Japan). Coating thickness is controlled at about 20mm. In order to reduce the resistance of the branch, it is also possible to brush a silver paste on the branch. After heating and curing, apply a layer of carbon paste and oxide to the working electrode end (control the oxide content to 2-10). Heat at 70°C for half an hour to cure the printing paste.
Before the electrode is used, it is necessary to cover the end of the branch with an insulating layer (which can be replaced with nail polish). At this time, the electrode strip exposes only the working electrode end and the electric contact end. The working electrode area of ​​each electrode strip is 0.045 cm2.
2 Results and Discussion
2.1 Characterization of Synthetic Oxide Powders
Solid-phase reaction synthesis at room temperature is a new method for preparing ultrafine powders. The results of X-ray diffraction show that the crystal form of MnO2 produced by the solid-state reaction of KMnO4 with MnCl2 is Α? Mixed type. The results of TEM indicate that the prepared powder has a nanometer size and a good dispersion degree. XPS and chemical analysis results show that Mn in MnO2 is mainly tetravalent. Co3O4 and PbO were prepared by reheating and decomposing the complexes by solid-state reaction at room temperature. The corresponding Co3O4 and PbO particles were spherical and the size was about 20 nm. The TiO2 prepared by the sol-gel method is mainly composed of anatase, with a particle size of about 5 nm and a spherical shape.
2.2 Test of pH Response of Oxide Electrodes
The electrode exhibited a linear response in the pH range of 2 to 12, with a response sensitivity of approximately -7813mVpH and a correlation coefficient of Χ = 0. The pH response of the MnO2 electrode was similar to that of the coated electrode. When the pH of the solution was changed from two directions, the electrode exhibited different response sensitivities (-78.3 and -5517 mV pH, respectively). This indicates that the pH response of the MnO2 electrode has a hysteresis effect.
The pH response of the Co3O4 electrode has a certain relationship with the synthesis route of the oxide. The pH range of the Co3O4 electrode prepared from acetate was linear from pH 2 to 12, and the response sensitivity was -38 mV pH and did not have a Nernst response. The response sensitivity of the Co3O4 electrode prepared from cobalt citrate at pH 2-12 is -5614 mVpH, which can be approximated as the Nernst response. The linear response sensitivity of the Co2O3 electrode prepared from cobalt oxalate is -5318 mVpH.
It can be seen that the pH response performance of the oxides prepared by the solid phase reaction route is significantly higher than that of the oxides obtained by the direct heating of cobalt salts. This shows that the response of the oxide has a certain relationship with the structural parameters of the oxide. The response time of the electrode is less than 1min. When the pH value of the solution is changed from two directions, the oxide electrode has almost no hysteresis effect. The sensitivity of the PbO electrode made from citrate in the range of pH 3 to 12 is -3713mV pH, and the lead oxalate is used. The sensitivity of the prepared PbO electrode is -3111mVpH, and the sensitivity of the PbO electrode made of lead acetate is -6143mVpH, which is similar to the Nernst response. This may be due to the crystal structure and complex decomposition of PbO prepared from lead acetate. The crystal structure of PbO is different. For the TiO2 electrode, its pH response performance is worse than that of the above three oxide electrodes, the linear response interval is only 4 to 9, and the response sensitivity is far from Nernst.
2.3 Discussion and Application of Response Mechanism
At present, the understanding of the pH2 potential response characteristic of the oxide electrode is still shallow, even though the response mechanism of the most studied IrO2 electrode has not been clearly described. It is generally believed that when the oxide electrode is in contact with a buffer, the surface hydrolysis or proton exchange process generates an interfacial potential difference between the oxide and the solution phases, and this potential may be related to the pH of the solution. The pH response mechanism of the oxide electrode may follow: (1) the ion exchange mechanism; (2) the presence of an H-reactive redox reaction between oxides of high or low valence state or between metals and their oxides; (3) the occurrence of oxides The proton or electron intercalation reaction produces a solid solution; (4) Oxygen occurs in the electrode reaction of oxygen incorporation.
Since the pH-responsive properties of manganese oxide, cobalt oxide, lead oxide, and titanium oxide electrodes are not disturbed by K, Na, Ca2, Cl-, Br-, and F-plasma, oxidizing or reducing agents such as K4Fe (CN) exist in the solution. 6) K3Fe(CN)6 and I- will interfere with the determination of pH [9,10], indicating that when these oxides are H-sensitive, the response mechanism should not be simple ions. Switching theory should be more reasonable in the form of mechanism (2) or (3). Since the Co3O4 and PbO electrodes exhibited a variety of response characteristics, it was shown that the electrode potential of this type of electrode has a certain relationship with the structural parameters and properties of oxides, which needs further study.
The prepared oxide solid pH electrode can serve as a primary electrode and can also be used repeatedly. However, since the MnO2 and PbO electrodes have different degrees of hysteresis, it is more desirable to use the primary electrode. However, the cobalt oxide electrode has a Nernst response and does not exhibit hysteresis due to pH measurement, so it can be used repeatedly.
In addition, we have found that manganese oxide and cobalt oxide electrodes can still exhibit a stable pH2 potential response in HF systems. This facilitates the pH measurement of some special solutions such as the strong agitation system and the wastewater treatment of the microelectronics processing industry.
1 experimental section