| The total phosphorus content is one of the important indicators for judging the water pollution index.It needs to be oxidized and digested to orthophosphate before measurement.Compared with the traditional spectrophotometry and ion chromatography,the ion selective electrode?ISE?method has the advantages of fast response,simple operation,low cost and easy portability.Therefore,the research and development of a phosphate ion selective electrode that can achieve in-situ measurement and high measurement accuracy is of great significance for the online detection of total phosphorus.In this paper,the detection method of phosphate ions is studied based on molybdenum electrode.The main research work is as follows:?1?In order to solve the problem that the response potential of the molybdenum electrode changes with the pH of the solution during the measurement process,an electrochemical impedance spectroscopy?EIS?method was proposed to analyze the influence mechanism of the detection performance of the phosphate ion based on the molybdenum ion selective electrode when the pH value of the tested solution changed.First changed the test bias potential and the pH of the solution to test the molybdenum electrode/solution surface and analyzed the electrochemical reaction process;on this basis,an R?QR?or R?Q?RZw??circuit was established to perform equivalent impedance fitting on the test data,and the trend of each fitting parameter with the change of pH of the solution was analyzed;finally,the measurement performance of phosphate ions by the molybdenum electrode at different pH was tested and compared.The experimental results showed that when the pH of the Na2HPO4solution was 11,the charge transfer resistance Rct of the electrode reaction system was the lowest,and the molybdenum electrode had a higher detection slope,reaching-28.29 mV/dec.?2?Aiming at the problem that the pH value and temperature of the solution interfere with the detection of phosphate ions by the molybdenum electrode,a measurement model of molybdenum electrode was established by using the phosphate ion distribution coefficient and multiple linear regression method.First analyzed the influence of phosphate ion concentration,pH value and temperature in the measured solution on the output potential of the molybdenum electrode,and then based Nernst equation,the linear compensation term of pH and the temperature correction item were introduced to establish a compensation model using the distribution coefficient of phosphate ion under different pH conditions.The experimental test results showed that the measurement error of the phosphate ion content in the 10-6?10-1 mol/L Na2HPO4 solution using this measurement model was within 6.33%;compared with the ion chromatography detection,the maximum relative error was 5.50%.?3?In view of the problem that the molybdenum electrode cannot detect the phosphate ions of the acidic solution obtained by the total phosphorus digestion device because the molybdenum electrode can only woks under alkaline conditions,the electrodialysis method was proposed to increase the pH value of the digested solution to above 11.Using the dilute triethyl phosphate solution as the detected solutions,the phosphate ion detection device based on the molybdenum electrode was used in conjunction with the online digestion equipment,and an orthogonal test was designed to determine the best electrodialysis parameters.At the same time,the measurement performance of the molybdenum electrode was tested by the established compensation model.The experimental results showed that when the concentration range of phosphate ions in the digested solution was 2.220×10-6?9.481×10-5mol/L,the detection slope of the molybdenum electrode was-25.38±0.5 mV/dec,and the lower detection limit was 1.56×10-6 mol/L.It had a good detection effect,and provided a reference direction for the molybdenum electrode for the online detection of phosphate. |
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