Cyclic Voltammetric studies of Calcium acetate salt with Methylene blue Using Gold Electrode

A R T I C L E I N F O Article history: Received: 2018-Oct-25 Accepted: 2018-Nov-18 Available online: 2018-Nov-25 Manuscript ID: PCBR-1810-1001


Introduction
Calcium acetate is used to decrease and prevent blood phosphate levels in patients suffering from dialysis due to kidney disease [1].Hyperphosphatemia has a universal effect in patients with chronic disease [2].Dialysis can remove some phosphate from the blood, but it is hard to remove enough to control the phosphate level [1,2].Decreasing on the blood phosphate levels help to keep bones strong, prevent unsafe build up of minerals in the body and decrease the heart diseases.Calcium acetate is a natural mineral that works by extracting phosphate from the diet so that it passes out of the body [1].Theoretical calculations, using equilibrium constants should be an effective phosphate binder [3].In vitro experiments confirmed the theoretical binder [3].In vivo studies in normal subjects found calcium acetate to be more phosphorous binding than calcium carbonate [4].
Analytical determination of calcium acetate is therefore important to follow them in foods, environment and human bodies, which is our main target.Using cyclic voltammetry as electroanalytical tool for evaluation of calcium acetate is very important.Gold working electrode (GE) was prepared in our laboratory, facilitating the easy determination of calcium acetate in aqueous solutions.Calcium ions are well detected here using GE in opposite to that by using glassy carbon electrode which will appear at a high voltage area.

Reagents and solutions
The chemicals used are calcium acetate salt [Ca (CH3COO)2], HCl and methylene blue from Sigma Aldrich Company (purity 98%).Pure water was used by distillation.

Instrumentation
The cyclic voltammetric studies were done by using a potentiostat of the type DY2000, delivered from USA.It was connected to a cell of three electrodes, Ag/AgCl, KCl sat was used as reference electrode, gold electrode (GE) was used as working electrode and platinum wire, auxiliary electrode.GE was prepared in our laboratory from pure gold wire 18K bought from famous jewelling shop jointed with steel rod , isolated, covered with heat shrink polymer and polished good by woollen piece before using.Area of electrode is 7.853x10 -3 cm 2 .The system was applied from 1.6 to -1.6 V potential windows and 0.1, 0.02, 0.01 v/s as scan rates at constant temperature 294.85 K. Flow of purified N2 was done to ensure removal of O2 and diffusion experiment.+2 ions in absence of ligand, MB at 294.85 K.

Solvation and kinetic parameters in absence of MB.
First, the cyclic voltammogram of 30 ml HCl (0.1 M) as a supporting electrolyte was measured from 1.6 to -1.6 V of potential window, the current is measured in Ampere unit, whereas the scan is 0.1 V/S (volt per second) at 294.85 K, (Fig. 1).Then, the redox behavior of Ca +2 ions was examined in 0.1 M of HCl at 294.85 K.The Ca +2 ions solution is added step wisely from 0.2 ml (6.62 x 10 -4 M) to reach until 1 ml ( .  3 3 23 10 M)as shown in (Fig. 2).
The electrochemical redox behavior of Ca +2 ions in absence of MB at the GE was studied at the steady state current, cyclic waves were obtained and explained using the following equation (1) [5][6][7][8]: ip is the current in Ampere, A is the surface area of working electrode in cm 2 , D is the diffusion coefficient in cm 2 /Sec, ν is the scan rate in volts/Sec and C is the concentration of the Ca +2 ions.The peak potential difference, ΔEP is calculated from equation ( 2): If ΔEP will close to 59/n mv (at 25 ˚C) the reaction is reversible where, n is the number of electrons in redox reactions.The standard heterogeneous electron transfer rate constant ks in cm/sec was calculated by applying the following equation (3) [9,10]: Where, α is charge transfer coefficient and na is the numbers of electron transfer in the rate determining step.Assuming that α coefficient is equal to 0.5.Hence, αna will be as the shown in equation ( 4): Where Epc/2 is the half wave potential for cathodic peak.

Effect of different scan rates
Effect of different scan rates for the redox behavior of Ca +2 ions in 0.1 M HCl was studied in the range 0.1, 0.02 and 0.01 (V.s -1 ) at 294.85 K, (Fig. 4).The solvation and kinetic parameters (Ep, Ip, ΔEP, E½, D, kS, Γ and Q) of different scan rates of Ca +2 ions were presented in Table .2 (a,b).
Randless Sevicek equation (10)(11)(12)(13)(14)(15)(16)(17) was used to apply the relation between cathodic and anodic peak current Ip against the square root of scan rate in 0.1M HCl which gives straight lines indicating diffusion process as shown in Fig. 5.The diffusion mechanism is also supported by scan rate effect which shows an increase in height of the waves by increasing scan rate.

Electrochemical behavior of Ca +2 ions in presence of
ligand, MB at 294.85 K.

Solvation and kinetic parameters in presence of MB.
The electrochemical behavior of the complexation between MB and Ca +2 ions in 0.1M HCl at 294.85 K with 1.6 V to -1.6 V potential windows and scan rate 0.1 V/S were shown in Fig. 6.
The electrochemical redox behavior of Ca +2 ions in the presence of MB at the GE was studied at the steady state current, cyclic waves were obtained and explained using equations (1)(2)(3)(4)(5)(6).The kinetic parameters [Da, DC ,Ks, Qa, Qc, Γa, Γc] for the interaction of MB with calcium acetate are smaller than that of calcium acetate solutions alone indicating complexation reaction between Ca ions and MB ligand.

Electrochemical behavior of the complexation between MB and Ca ions.
From Fig. 6, it is observed that the complex is formed as a result of decreasing in the anodic and cathodic peak beside the potential shifts to new values.Due to precipitating the complex during the process, no peak appeared.A stability constant is a measure of the strength of the interaction between the reagents that come together to form the complex.The stability constants (βMX) for bulk and nano cadmium chloride complexes for each addition are calculated by applying Eq. ( 10) [5,16]: ΔE˚ = E˚C -E˚M = 2.303 (RT/nF) * ( log βMX + j log Cx ) (10) Where E˚M is the formal peak potential of metal at finally adding in the absence of ligand, E˚C is the formal peak potential of metal complex after each addition of MB, R is a gas constant (8.314J.mol -1 .degree - ), T is the absolute temperature, j is the coordination number of the Stoichiometric complex and Cx is the concentration of MB in the solution.
The thermodynamic parameters βj and ∆G for the interaction of MB with calcium acetate are increased by increasing the concentration of MB in solutions indicating more complexation.

Table(4):
The values of (Formal potential E˚, stability constant βMX and Gibbs free energy ΔG) for Ca +2 ion complexes at 294.85 K and scan rate 0.1

Effect of different scan rates
Effect of different scan rates on the interaction between Ca +2 ions and MB was studied in 0.1, 0.02 and 0.01 V.s -1 , (Fig. 8).The solvation and kinetic parameters (Ep, Ip, ΔEP, E½, D, kS, Γ and Q) of different scan rates of Ca +2 ions in the presence of MB were presented in Table 5.(a,b).Randless Sevicek equation was used to apply the relation between cathodic and anodic peak current Ip against the square root of scan rate in 0.1M HCl which gives straight lines indicating diffusion process as shown in Fig. 9.All scan rate parameters for interaction of MB with calcium acetate are smaller than that of the absence of ligand indicating also the complexation character (Fig. 9).It indicates less slope obtained for the complex than the metal supported also the complexation behavior.

Conclusion
The cyclic voltammetry of the supporting electrolyte (0.1M HCl) was obtained by using Gold electrode (GE).One reduction wave was obtained at ~ -1 V corresponding to the reduction of gold chloride to gold metal.By adding different concentrations from calcium acetate appearance of two new waves happened, one reduction peak at ~ 0.0V and one oxidation peak at ~0.25 V corresponding to the reduction and oxidation of calcium ions.All the kinetic parameters [anodic diffusion coefficient Da, cathodic diffusion coefficient DC ,electron rate constant Ks, anodic quantity of electricity Qa, cathodic quantity of electricity Qc, anodic and cathodic surface coverage, Γa, Γc] for calcium acetate concentrations are increased by increasing in the concentrations of calcium acetate favoring diffusion controlled reactions.The calcium peaks were increased in intensity by adding methylene blue (MB) to calcium ions, the potential of reduction peak was shifted to more negative and the potential of oxidation peak was shifted to more positive values.All the results, indicate that the solvation and kinetic parameters [Ks, Qa, Qc, Γa, Γc] of the complexation reaction between MB ligand and Ca +2 ions are smaller than that for Ca +2 ion in HCl solution as supporting electrolyte The thermodynamic parameters [stability constant, βj and Gibbs free energies of complexation, ∆G] for the interaction of MB with Ca +2 ions are increased by increasing the concentration of MB in solutions indicating more complexation.

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H I G H L I G H T S  Detecting of calcium ions in form of acetate by cyclic voltammetry. Good detection of calcium ions at suitable potentials and not large potentials as usual. Complexation study with methylene blue which help in analytic determination of calcium ions.Determining the thermodynamic parameters for interaction of calcium ions with methylene blue.G R A P H I C A L A B S T R A C T A R T I C L E I N F O 12 |2018, 1, 11-18

Fig. 3 Fig. 3 .
Fig.3.The relation between peak current Ip (ip,a -ip,c) against different concentrations of Ca +2 ions at 294.85 K and scan rate 0.1 V/S.

Fig. 5 .
Fig.5.The relation between peak current Ip (ip,a -ip,c) against the square root of different scan rates for Ca +2 ions at 294.85 K.