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Voltammetric Determination of Catalytic Effect of Nitrite in Nitrate Media by Graphite and Mercury Electrode on Acid Red-27 Cobalt Complex

In this study, the catalytic effect of nitrite on the cobalt complex of acid red 27, which is one of the most important azo dyes today, was investigated. Voltammetric studies were carried out with a Hanging Mercury Drop electrode and pencil graphite electrode using the Cathodic AdsSWV technique in nitrate media. First, in situ formation of the complex was determined by electrochemical method. It was then observed that the voltammetric peak current of the complex increased in the presence of nitrite anions. The deposition time, potential and nitrite concentration were investigated to optimize the conditions for both electrodes. According to the results, the reaction mechanism of the catalytic effect of nitrite has been proposed.


Introduction
Functional groups in aliphatic and aromatic structures that contain an azo -N=N-group and give color to the molecule are called Azo compounds. Azo dyes usually consist of conjugated systems that combine two or more aromatic rings in their structure. The compounds containing azo groups are called "azo dyes" to distinguish them from other dyes. They are also called monoazo, diazo, and triazo compounds depending on the number of Azo groups they contain (Murty et al., 2012).
Azo dyestuffs are used in many areas such as dye chemistry, medicinal chemistry, textile chemistry, trace element analysis, industrial wastes, food chemistry, biological studies, carcinogenetic properties and metabolic studies, because they have been deemed worthy of research (Patai, 1975;Griffith, 1976;Strandis and Glezer, 1979;Hart and Smyth, 1980). Because of these properties, it is important to determine and quantify azo dyes. Acid red 27 (AR-27) is dark red and purple azo dye in Figure1. It was early used in food in the United States, as a dyestuff and dye in cosmetics, but since 1976 it has been banned in the United States by the Food and Drug Administration (FDA) on charges of carcinogenesis. Voltammetry is of great importance in studies on azo dyes and their metal complexes (Karaman et al., 2018;Karaman et al., 2015;Karaman and Menek 2012;Menek et al., 2013;Menek et al., 2006a;Menek and Karaman 2005;Menek and Topçu, 2003;Menek et al., 2001). Voltammetric studies with carbon electrodes have recently been intensified since voltammetry is a fast and accurate technique (Karaman, 2014). In recent years, modern electrochemical techniques, especially square wave voltammetry (SWV) which improve sensitivity and selectivity have promoted the development of many electrochemical methods for ultra-trace measurements of a variety of organic and inorganic compounds. The popularities of adsorptive stripping voltammetry (AdsV) and catalytic adsorptive stripping voltammetry (CatAdsV) have been steadily increasing in the determination of ultra trace concentrations of metals. This is due to their excellent sensitivity, extremely low detection limits, good accuracy and precision, as well as low cost of instrumentation. The AdsV technique is based on the accumulation of the analyte on a suitable working electrode by potential controlled adsorption and subsequent electrochemical oxidation or reduction of the preconcentrated species (Krolicka et al., 2003;Menek and Karaman, 2006b;Topçu 2016). With the voltammetric catalytic effect, the analysis of low concentrations of substances can be easily done by increasing the reduction peaks with suitable catalysts (Bobrowski and Zarebski, 2000;Jain et al., 1981;Barek et al., 1985;Uzun et al., 2014;Menek et al., 2014, Menek et al., 2001. In this study, the cobalt AR-27 complex was determined by voltammetry and it was aimed to increase the amount of current by catalytic effect of nitrite on this complex. Mercury electrode and graphite electrode, which are of great importance in voltammetry, were used for this purpose. With the help of the data obtained, the electrode mechanism was proposed.

Materials and Methods
Acid Red 27 was obtained from Aldrich, Co(NO 3 ) 2 .6H 2 O is obtained from Merck company. 10 ml 0.5 M Potassium nitrate media was preferred as the supporting electrolyte. The deionized water used in the preparation of the solutions was obtained from the Sartorius Arium model ultrapure water system. Voltammetric measurements were performed at room temperature by using Metrohm 757 VA Computrace Electrochemical Analyser. The device consists of Ag / AgCl reference, Pt auxiliary and working electrodes. Hanging mercury Drop Electrode (HMDE) and Pencil Graphite Electrode (PGE) were preferred as working electrodes. 0.7 mm tombow brand pencil was used. Measurements were performed by dipping the pencil 1 cm into the voltammetric cell, and a new pencil was used for each measurement. Before each measurement, nitrogen gas was passed through the solutions for 5 minutes to remove oxygen. The study was conducted using the adsorptive square wave voltammetry (AdsSWV) technique.

The behavior of Acid Red 27 and cobalt on HMDE and PGE
Voltammetric behavior of AR-27 and Co 2+ ions on HMDE and PGE were investigated in 10 ml potassium nitrate media used as supporting electrolyte. Voltammograms formed by gradual addition of AR-27 to potassium nitrate medium are given in Figure  2. In the wide potential range (0.0 V) -(-1.2 V), only one reduction peak was found in the solution medium. The use of HMDE and PGE electrode has not changed the result. Considering the molecular structure of AR-27 azo dyestuff (Figure 1.), this reduction peak arises due to the reduction of the electroactive azo group. In addition, the voltammetric peak current increases with the increase of the AR-27 concentration.
Voltammetric measurements performed at concentration levels of 10 -5 M, 10 -4 M respectively on HMDE and PGE are shown in Figure 2. These measurements were taken in the cathodic region by the adsorptive SWV method. After examining the reduction signal of AR-27, the behavior of Co 2+ ion on PGE and HMDE was studied with voltammetry. The AdsSWV voltammograms of Co 2+ ion at different accumulation potentials and different accumulation times were examined and the signals are given in Figure 3. As can be seen in Figure 3, although no reduction signal was found in PGE, the reduction peak of cobalt appeared on HMDE. The interaction between AR-27 and Co 2+ ion and the complex structure that can form between them were studied by the AdsSWV technique (figure 4.). For this purpose, a series of 10 -4 M Co 2+ ions (2x10 -4 , 4x10 -4 , 6x10 -4 M) were added in 6x10 -4 M AR-27 in potassium nitrate media on PGE and a series of 10 -5 M Co 2+ (2x10 -5 , 4x10 -5 , 6x10 -5 M) ions were added in 6x10 -4 M AR-27 in potassium nitrate media on HMDE and the resulting voltammograms are given in Figure  4. When Figure 4 is examined, it is seen that if AR-27 is present in potassium nitrate medium, the reduction peak current increases upon addition of cobalt ion.

Determination of the optimum deposition potential for Acid Red 27-cobalt complex
In Section 3.1, it was seen that AR-27 and cobalt ion formed a complex by the AdsSWV technique. There are many variable parameters in adsorptive techniques. One of these parameters is the accumulation potential. In order to determine the optimum accumulation potential, accumulation was performed in the range of (-0.4 V) -(0.0 V) potentials with 5 seconds in PGE and 15 seconds in HMDE in potassium nitrate media with 6x10 -4 M AR-27 and cobalt ion. The highest reduction peak current was observed to be at -0.4 V (Figures 5 and 6).

Determination of the optimum deposition time for Acid Red 27-cobalt complex
In Chapter 3.2, the optimum accumulation potential of -0.4 V was selected for the determination of AR-27 and cobalt complex by AdsSWV technique. In order to determine the optimum deposition time, the deposition was performed by AdsSWV technique in a potassium nitrate media containing AR-27 and cobalt ion between 0-20 seconds on PGE and 0-120 seconds on HMDE. Voltammograms and current values of these changes are given in Figures 7 and 8. According to these changes, the current increases with the increment of the deposition time up to 6 seconds in the deposition process in the 0-20 s interval on PGE and, the current value remains constant despite the deposition time increases after 6 seconds. The current increases until 15 s accumulation and after 15 seconds the current value is fixed for HMDE. As a result, the optimum accumulation time was determined as 5 s for PGE and 15 s for HMDE.

Optimum nitrite concentration for catalytic effect on Acid Red 27-cobalt complex
In order to observe the optimum catalytic effect on the cobalt-AR-27 complex, a series of 5 M nitrite were added to voltammetric cell Acid Red 27 in the potassium nitrate support electrolyte medium and 5 M nitrite on the cobalt ion were added to a series of voltammetric cells which consist of Cobalt-Ar-27 complex in nitrate media. The deposition processes were performed at -0.4 V for 5 seconds for PGE and -0.4 V for 15 seconds for HMDE which were selected as the optimum condition in the previous sections. Voltamograms and % current values of the resulting changes are given in Figures 9 and 10 respectively.

Results
In this study, the catalytic effect of nitrite on cobalt-AR-27 complex formation in potassium nitrate medium was investigated by voltammety on PGE and HMDE. For this purpose, the current and potential of the complex were investigated in first step.When metal ions in the solution phase are complexed with ligands other than water molecules, their reduction waves change in two different ways. First, the reduction potential almost always shifts to more negative potentials. Second, its current changes and usually decreases (Crow, 1969). The complexation of ligands with metal ions in the solution media has two effects: a negative shift in the potential of the reduction wave and a significant decrease in the diffusion coefficient (Nepomnyashchii et al, 2008). Reduction signals of complexes can occur at a potential independent of the reduction signal of metal and dyes, and sometimes very close to the reduction potential of the dye or at the same place. Voltammetric studies also include determination of shifts in currents or halfwave potentials of metal ions in the presence of increasing amounts of complexing ligand (Crow, 1969). In this study, the reduction signal of the complex Acid Red 27 was very close to the reduction potential and shifts in the peak potential were observed. Although the cobalt ion did not give a reduction signal that could be evaluated analytically in the graphite pencil electrode, when Co 2+ was added to the potassium nitrate media with AR-27, the signal increased in a region close to the reduction potential of AR-27. As can be seen in Figure 2, when cobalt ion is added to the nitrate media in the presence AR-27, the peak current increases and the peak current becomes more pronounced (Khatri and Singh, 1981;Nezhadali and Sharifi, 2010). In this way, cobalt ions that do not signal alone on PGE emerge by signaling in the presence of AR-27. This phenomena provides an opportunity for indirect determination of cobalt ions in PGE. After determining the cobalt-AR-27 complex, optimal voltammetric parameters were defined for PGE and HMDE. In order to determine the optimum deposition potential, accumulations were performed in the range (0.0 V)-(-0.4 V), and the optimum deposition potential -0.4 V was selected for both PGE and HMDE. Another parameter that needs to be examined in adsorptive techniques is the deposition time. In voltammetry, a difference in the reduction current is observed depending on the deposition time.
Depending on the deposition time, the amount of material that can be reduced on the electrode surface is different, so there are variations in the amount of current accordingly. In addition, desorption can be observed with increasing deposition times (Bobrowski and Zarebski, 2000). However, after accumulating on the PGE in the range of 0-20 s at the potential of -0.4 V deposition was performed and it was decided that the optimum deposition time was 5 s. After the optimum deposition times are selected, the electrode surface is saturated and the current value is fixed (Figures 7 and 8.). The optimum deposition time for HMDE is determined as 15 s. As a result, nitrite catalytically increases the voltammetric peak current of the cobalt-AR-27. This phenomenon provides convenience for analysis at lower concentrations. A reaction mechanism is proposed that depends on the data obtained in Figure 11 and is compatible with the literature (Zhao et al., 1991).