Development and calibration of an electrolytic cell for ion determination in a soil solution

An electrolytic cell was developed to monitor soil modifications after crop fertigation with wastewaters from agroindustrial plants. The device was first calibrated with different levels of potassium chloride dissolved in aqueous solutions at various temperatures. Nernst ́s model was used to fit the voltage indicated from the electrolytic cell versus the ionic activity of the potassium from the aqueous solutions of electrical conductivity and known ionic concentrations and the diluted wastewater samples. The equipment accurately indicated the tensions after appropriated correction of the electrical current and the temperature. The device estimated with accuracy the ionic coefficient of activity, the concentration of the potassium chloride and the concentration of the ion K dissolved in the agro-industrial wastewater.


Introduction
The storage, maintenance and treatment of waste water from agro-industrial plants in large ponds have become expensive.Environmental contamination frequently occurs and it has been necessary to find an appropriate disposal for these specific wastes.The alternative recommendation as a source for fertigation and plant nutrient (JIMENEZ-CISNEROS, 1995;TOZE, 2006;HEIDARPOUR et al., 2007;LEAL et al., 2009;QADIR et al., 2010;PEREIRA et al., 2011) during the dry season has been facing serious difficulties due to the incipient concern on the effects of the various ions leaching into the tropical soil profile (HARRIS; URIE, 1983;JALALI et al., 2008;KOLAHCHI;JALALI, 2007;ARIENZO et al., 2009).Ionic tolerance may be an indicator of environmental quality leading to rational applications for the maintenance of soil conditions suitable for a long-term cultivation and for the avoidance of any groundwater contamination (HARRIS; URIE, 1983;GRATTAN;GRIEVE, 1992).However, a limitation to the above technological application is the lack of a specific device to provide instantaneous readings of the chemical quality of wastewater and, consequently, a better management in this particular source of soil alteration.
Electrolytic cells in the soil profile but close to the root system of crops may be an efficient tool in monitoring the natural recycling of ions.As an alternative to direct methods of analysis or to the use of tensiometer (GHIBERTO et al., 2009), the surface gamma-neutron gauges (REICHARDT; TIMM, 2004) and the time domain reflectometry (LOPES et al., 2010), the electrolytic cells may estimate indirectly the leaching of hazardous substances, soil salinity, the electrical conductivity of soil solution, contents of potassium, nitrates, nitrites, and several heavy metals (YARON et al., 1996;SILVA et al., 2000).
Current experiment aims at the calibration of a manufactured electrolytic cell device for instantaneous readings at the correct point in the soil profile, sufficiently reliable for measuring the resistivity in the soil solution and not expensive for soil or environmental management.

Material and methods
The components consist of an electrolytic cell, a thermocouple, a vacuum system (Figure 1) and an electrical circuit performing a continuous current (Figure 2).The electrolytic cells have a ceramic cylinder, 0.01 high and diameter 0.0175 m, featuring permeability to soil solution up to 0.080 MPa.Two opposite gold electrodes, diameter 0.005 m, were attached to the underside of the cylinder, 0.005 m from the center and close to the copper constantan thermocouple wire, optimizing the cell performance.
Terminals, thermocouple and display were connected by an electric circuit trespassing the upper side of the cylinder, whereas the vacuum system was connected by a polyethylene tube.A digital multimeter Minipa, model ET-2042, performed readings in voltage and temperature.The conditioning circuit was similar to Wheatstone bridge (EISBERG; LERNER, 1981;MIODUSKI, 1982;FIALHO, 2002) with the electrical resistance having an identical potential in the two circuit branches.The electrolytic cell was disposed as a by-pass in one of the resistors so that any redox reaction (ATKINS; PAULA, 2002) between electrodes and electrolyte inside the cell caused a difference in the electric potential which was indicated on the voltmeter display (EISBERG; LERNER, 1981;MIODUSKI, 1982;FIALHO, 2002).A 5 V tension in the bridge was the voltage which trespassed the resistors; the resistance in R1 in the conditioning circuit was 4.7 kΩ, while in R1 the 6.8, 15, 150 and 470 kΩ were tested to find the correct resistor rate for the present application.The calibration temperatures were 274, 281, 286, 291 and 298 K.
Three replicates of the device (C 1 , C 2 , C 3 ) were manufactured and compared to each other with F-ratio tests in which F = (difference MS r )/s 2 for F 0.01 (2, 19) after linear analysis of regression (MEAD et al., 1993).
After the readings from standard solutions of potassium chloride at concentrations ranging between 10 -4 and 10 -1 mol L -1 at 25 o C, the voltage data sets versus the concentrations were individually fitted by linear regression procedure; similarly for the cassava wastewater (Table 1) for dilutions between 8.2x10 -3 and 8.2x10 -5 mol L -1 .Potassium chloride was chosen for the calibration due to its widespread occurrence in the cassava industrial wastewaters.Under the general law of Debye-Hückel, the relation between ion activity and electric potential, following Nernst´s model, was employed to determine the coefficient of activity of K + after appropriate corrections of the thermal effect and electrical current, according to the Equation ( 1) where: in which E cel is the potential of electrolytic cell V; E 0 is the standard potential of 4.85 V; R is the gas constant 8.31447 J K -1 mol -1 ; F is Faraday´s constant, 96485.3C mol -1 ; T is the temperature, K; a is the ionic activity, a-dimensional; C f is the correction of the model.
where: ρ 0 , ρ 1 , ρ 2 are the parameters, I is the electric current in the electrolytic cell, A.

Results and discussion
There were no significant differences when the data sets from the three electrolytic cells were compared by the F-ratio test (p > 0.01), which suggested reproducibility in the manufacture of the present device (Figure 3).Under calibration conditions of the aqueous potassium chloride solution, identical and more sensitive response was found with 150 or 470 kΩ than with 6.8 or 15 kΩ in R2 when R1 had 4.7 kΩ (Figure 4).
After the better resistor choice of 4.7 kΩ for R1 and 150 kΩ for R2, the steady tension 4.85 V found in the empty cell became the standard potential.The overestimated potentials were found by Nernst´s equation, corroborated by Skoog et al. (1997).Under thermal conditions between 273 and 373 K, the viscosity of aqueous solution was changed (ATKINS; PAULA, 2002;REICHARDT;TIMM, 2004) and Nernst´s equation overestimated the potential, probably due to the characteristics of the continuous electric current (SKOOG et al., 1997;ATKINS;PAULA, 2002).Correction according to the electrical current and the temperature is necessary, and a linear trend for the electrical current (I) as well for the temperature (T) was found in which where: E cel is the potential from the electrolytic cell, V; a is the ionic activity by the general law of Debye-Hückel, making possible accurate measurements for wastewaters from cassava industries (R 2 = 99.99%), as indicated in Figure 5.

Conclusion
Similar performance of electrolytic cells indicates reliability in manufacturing the device.Potassium from cassava wastewaters may be estimated with accuracy after appropriated calibration for electrical current and temperature.

Figure 1 .
Figure 1.Diagram of the electrolytic cell indicating in the sectional plane Z -Z the cell base A, the electrode and thermocouple supporter B, the ceramic cylinder C, the upper face of the cell D, the electrodes E, the thermocouple F, the tubing system for the electrical circuits G, and the vacuum system H.

Figure 2 .
Figure 2. Diagram indicating the bridge and the electrolytic cell, in which R1 and R2 are circuit resistances.

Figure 3 .Figure 4 .
Figure3.Readings of potential (Rr) versus estimates (Ee) from equation V L = βC 1/2 , in which V L is the electrolytic potential from each cell; β is a coefficient; C is the concentration of the potassium chloride in the solution.

Figure 5 .
Figure 5. Reading potential (Rr) from electrolytic cells and estimates (Ee) from Nernst´s model after the correction factor for potassium from diluted wastewater solutions.

Table 1 .
Chemical composition in wastewater from cassava agroindustrial plants.