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In order to improve plant factories, an appropriate control system on fertilization is urgently required. An automatic management system to control nutrient concentration was constructed using a programmable logic controller (PLC) and ion selective electrodes (ISEs) of nitrate, phosphate, and potassium ion. The concentration of nutrient components in a culture solution was monitored using these ISEs. When the concentration of nutrient components diminished to the threshold set as an optimum condition (0.1-2.0 mM), an appropriate amount of a concentrated solution of each nutrient component was added to the culture solution using solenoid valves connected with the PLC. The present cultivation system was simply constructed without any computers and pumps. Three kinds of automatic control systems simultaneously worked and did not influence each other.

Determination of phosphate ions in aqueous solutions attracts a great deal of interest in the areas of environment, medicine, and agriculture. As phosphoric acid is a poly basic acid, the different forms of existence at different pH result in direct determination facing a big challenge. Herein, we reported a potentiometric phosphate ion sensor based on a surface-modified tungsten electrode. Pure tungsten was electrodeposited at a constant potential of 0.2 V versus Ag|AgCl in Na2HPO4. WO3 and H3O40PW12·xH2O were electrodeposited on the surface of the tungsten electrode. The modified tungsten electrode was used as a working electrode in a two-electrode system to detect the concentration of phosphate ions in aqueous solutions. The detection limit of the modified tungsten electrode for phosphate ions is 10–6 M from pH 7 to pH 8 and 10–5 M from pH 9 to pH 10. It has good selectivity to other common anions. The long-term monitoring experiment showed that the potential fluctuation was less than ±3 mV in 24 h. Compared to conventional determination methods, the current phosphate ion sensor showed a close value in a real sample. The mechanism of phosphate ion response was investigated in detail. This sensor possesses advantages of simple manufacture, low cost, a wide pH range for detecting, and good selectivity.

A phosphate ion-selective electrode using molybdenum metal was constructed. The modified molybdenum electrode responded to HPO4 2in the presence of molybdenum dioxide and molybdophosphate (PMo12O40 3-) on the surface. The electrode exhibited a linear response to HPO4 2in the concentration range between 1.0 × 10 -5 and 1.0 × 10 -1 M (mol dm -3 ) in the pH range from 8.0 to 9.5 with a detection limit of 1.0 × 10 -6 M. The sensor showed near Nernstian characteristics (27.8 ± 0.5 mV dec -1 ) at pH 9.0. Since the responding potential was attributed to the activity of HPO4 2-, the potential at a given concentration of phosphate depended on the pH. The electrode indicated a good selectivity with respect to other common anions such as NO3 -, SO4 2-, Cl -, HCO3and CH3COO -. The modified molybdenum electrode can be continuously used for over a 1 month with good reproducibility. The feasibility of the electrochemical sensor was proved by successful for the detection of phosphate in real samples.