Development of real-time detecting device for nitrogen concentration of liquid fertilizer.

For the realization of real-time detection of nitrogen concentration in the process of automatic mixed liquid fertilizer, a detection device was designed, which was mainly composed of a nitrate ion selective electrode, an electrode signal conditioning circuit, a temperature sensor, and a data acquisition and display circuit. There was a one-to-one correspondence between the response potential of a nitrate ion selective electrode and nitrate ion concentration. For this reason, the detection device could obtain the level of nitrate ion concentration by detecting the electrode's response potential, and then the nitrogen concentration of liquid fertilizer had been acquired indirectly. Measuring accuracy for the electrode's potential was a key influence factor of the device's performance. A 1 mV error of the electrode's potential led to a 4% concentration measuring error of monovalent ions. To evaluate the device's measuring accuracy for an electrode's potential, the electrode had been placed in a series of prepared potassium nitrate solutions whose NO3- concentration were 10-6, 5×10-6, 10-5, 5×10-5, 10-4, 5×10-4, 10-3, 5×10-3,10-2, 5×10-2, and 10-1 mol/L respectively, and the response potential of the electrode was measured by the device and a standard millivoltmeter PHS-3CT which was used as the comparative reference respectively, and it was known from the test results that the device's measuring result for the electrode's potential in each NO3- concentration was closed to that of PHS-3CT, and its maximum and minimum relative error were 5.2% and 1.2% respectively. Meanwhile, the response potential of the electrode would produce a temperature drift when the temperature varied, which would result in a measuring error of ion concentration. For reducing the influence of temperature variation, the electrode's temperature variability had been analyzed with 25°C as a reference, by measuring its response potential in a series of prepared solutions with different NO3- concentrations in the range of 10-5-10-1 mol/L and different temperatures within the scope of 5-45°C. The results showed that the response potential of the electrode had a linear change with the temperature difference of the solution to be examined and the maximum variability rate of the potential was 8.8%. Based on the response potential of the electrode measured in a series of prepared solutions with different NO3- concentrations and different temperatures, a temperature parameter model had been established by using the method of the least squares fitting step by step. Verification experiments of the temperature parameter model had been carried out, by using three kinds of prepared potassium nitrate solutions whose NONO3- concentration were 4×10-1, 4×10-2 and 4×10-3 mol·L-1 severally, and it turned out that the average relative measurement error and the maximum relative measurement error of the temperature parameter model were 4.49% and 9.2% separately. Furthermore, there may be some interfering ions (such as Cl-, SO4 2-, H2PO4 - and HPO4 2-) in a mixed liquid fertilizer in addition to NO3-, and the nitrate ion selective electrode had cross sensitivity. It would allow the electrode's response for interfering ions to come into being, which could bring about some impact on the device's measuring results. In order to determine the degree of influence of the interfering ions for the device's measuring results, the fixed interference method had been used to measure the electrode's selectivity coefficient for Cl-, SO4 2-, H2PO4-, and HPO4 2- respectively. The results indicated that the selectivity coefficient for 4 interfering ions were 5.1×10-2, 1.0×10-4, 2.0×10-4, and 5.0×10-5 singly, and that the bigger the selectivity coefficient was, the greater the impact of interfering ions on the electrode; moreover, it was found from experiments that the effects of interfering ions on the electrode weakened gradually with the decrease of the interfering ions concentration, and the measuring error of the device caused by Cl- was 4% when Cl-: NO3- = 1:100, that was to say, a small amount of interfering ions in a liquid fertilizer could not constitute a significant effect for the measured results. From the above, the device developed could meet the application requirements of a nitrogen concentration test in the process of automatically mixed fertilizer in engineering. [ABSTRACT FROM AUTHOR]