1. Introduction

Conducting polymers have received much attention due to their electroactive properties at electrode surface when they are doped by electrochemical oxidation-reduction reactions by chemical methods using electron donors or acceptors^(1,2). These properties allow the polymers to be used as modifiers of electrode surface to facilitate organic electrochemical reaction at the electrodes and to improve battery electrodes⁽³⁾.

Adding cations as doped elements improve the conducting properties of polymers and also play an important role in forming thin films⁽³⁾.

Conductivity is a good method for explaining the ion-interactions and the association of ions in different solutions⁽⁴⁾. In this work we apply new equation for the association constants for 1:3 asymmetric electrolytes by the use of conductivity data. This new equation is derived from Fuoss-Shedlovsky equation and Ostwald dilution law^(5-8).

Studying the effect of external magnet on the association constants of FeCl₃+10% PVA+50% ethanol-water solutions is very important because it is not studied before.

2. Experimental

Ferric chloride was provided from Merck and ethanol BDH was used without purification. Polyvinylalcohol, M.W, 17,000 water soluble polymer, from Arondale laborations, was used. 5 ml of mixed 50% ethanol-H₂O were put in test tubes then different salt concentrations of FeCl₃+10% PVA by weight were added and dissolved. The prepared solutions were left for two days in water thermostate of the type (Polyscience 8105, USA) at 298.15 K to reach the necessary equilibrium. Necessary volumes were withdrawn and measured using density techniques, conductivity and capacitance.

The density measurements were done by taking 1 ml of the prepared solutions and put in specific gravity bottle (1 ml capacity) and weighing them using Mettler-Toledo USA, four digital weighing balance. The density have been used to calculate the solvated radii^(9,10).

Conductances and capacitances were measured experimentally by the use of multimetter of the type [Macom (MX620)] with sensitivity of 1%. Dipping type cell with two carbon electrodes apart with 1 cm distance and with cell constant equal 0.96, was used. At least three readings were done. The conductance values of all solutions were corrected by substracting their values from that of pure solvents. The required temperature was adjusted at 298.15 K with a precision of ±1 K.

Two poles of permanent magnet was used with power 1.26 K Gauss (kG) measured by Gauss metter Model GM-54. The measured solutions were put between the two poles of the magnet and their conductivity were measured.

3. Results and Discussion

From the densities of FeCl₃+10% PVA in mixed 50% ethanol-water solutions at 298.15 K, the molar volumes were calculated by dividing the molecular weight of FeCl₃ by the densities of 50% mixed ethanol-H₂O solutions and the evaluated volume values are represented in Table 1, in presence of 10% PVA. From the molar volumes, the solvated radii r_s in Å units for FeCl₃+50% (EtOH-H₂O) in presence and absence of PVA were calculated by using equation 1.

(1)

where N_A is Avogadro’s number.

The relative capacitance of FeCl₃ solutions in 50% (EtOH-H₂O) solvents in absence and presence of PVA were evaluating by dividing the measured capacitances in microfarad (μF) by that of water and their values are given also in Table 1.

Table 1. Molar volumes (V), relative capacitance (∈), solvated radii (rs ) in, Born equation constants A, B and mean activity coefficients (γ±) of different FeCl3 solutions in 50% mixed EtOH-H2O+10% PVA solutions at 298.15 K Cmmole/L Vcm3/mol ∈ rsx10-8cm A B log γ± 0.084 209.30 11 4.36 9.71 0.88 -4.00 0.1 207.43 12 4.35 8.52 0.84 -3.75 0.15 200.25 13.5 4.30 7.14 0.79 -3.58 0.21 202.76 15.5 4.32 5.81 0.74 -3.24 0.25 204.03 17 4.33 5.06 0.71 -2.99 0.34 197.81 19.5 4.28 4.11 0.66 -2.72 0.42 202.76 22 4.32 3.43 0.62 -2.44 0.52 202.76 25.5 4.32 2.75 0.58 -2.13

Table 2. Molar conductance ( in ohm-1 cm2), dissociation degrees (α) and association constants (KA) for FeCl3 in 50% EtOH-H2O+10% PVA solutions in absence and presence of magnetic field of power 1.26 K Gauss at 298.15 K Cmmole/L A P Λx103 α KA Λx103 α KA 0.084 2.8 0.8 1.06x1014 3.01 0.717 2.81x1014 0.1 2.7 0.771 1.4x1013 2.9 0.690 3.6x1013 0.15 2.5 0.714 5x1012 2.65 0.631 6.1x1012 0.21 2.35 0.671 1,2x1011 2.4 0.571 3.65x1011 0.25 2.2 0.629 1.9x1010 2.2 0.524 6.2x1010 0.34 2.01 0.574 1.9x109 1.9 0.452 7.5x109 0.42 1.9 0.543 1.97x108 1.7 0.405 9.98x108 0.52 1.7 0.486 2.1x107 1.4 0.333 1.44x108 A: Λo = 3.5x103 P: Λo = 4.2x103

The mean activity coefficients (γ±) were calculated by the use of the modified Born equation⁽¹²⁾ as explained by equation (2)

(2)

(3)

Where

Z⁺, Z^- are the charges of the positive and negative ions of the electrolyte used, r_s the solvated radii and A, B are constants. ∈ is the experimental relative capacitance for FeCl₃ solutions.

The calculated mean activity coefficients are presented in Table 2 for FeCl₃+50% (EtOH-H₂O) solutions in presence of 10% by weight of PVA at 298.15 K are presented also in Table 1, with decreasing their values by increasing electrolyte concentrations.

For calculating the association constants for 1:3 asymmetric electrolytes new equation can be derived as follows:

(3)

(4)

(5)

By substitute α value from equation (5) in equation (4) we get:

(6)

This last equation is simple and derived from Fuoss-Shedlovsky theory and the Ostwald dilution law⁽⁸⁾. Knowing that S(Z) factor is approximately equal one for FeCl₃+50% (EtOH-H₂O) solutions in presence of PVA.

The molar conductance of FeCl₃ were estimated from the measured specific conductance by the apply the equation⁽¹³⁾ 8.

(7)

K_s is the measured specific conductance, K_cell is the cell constant which equal 0.96 and C_m is FeCl₃ concentrations in mole/L. On drawing the relation between molar conductances and square root of molar FeCl₃ concentrations, straight lines are obtained in 50% (EtOH-H₂O) solvents in presence of 10% by weight PVA. Extrapolating there lines to = 0, the limiting molar conductivity Λ_o was obtained.

The same relation was obtained from the measured specific conductivities in presence of two poles of permenant magnet of power 1.26 K. Gauss.

From Λ, Λ_o values, the association constants (K_A) for FeCl₃+50% (EtOH-H₂O)+10% PVA solvents in presence and absence of magnetic field were calculated by applying equation (7) and their data are presented in Table 2.

The dissociation degrees (α) were also evaluated for FeCl₃+50% (EtOH-H₂O)+10% PVA in absence and presence fo magnetic field (see Table 2) and for FeCl₃+50% EtOH-H₂O+10% PVA in absence and presence of magnet (see Table 2).

It was observed that K_A) values decrease with increase FeCl₃ concentrations due to the decrease in the dissociation degree. PVA favour very high FeCl₃ associations, i.e., increase the K_A values. This prove that the association of electrolyte ions took place easier through polymer media. Also the magnet increase also the association constants of FeCl₃+50% EtOH-H₂O+10% PVA which indicate that the magnet attract the cations and therefore facilitate the association through its surface.