Applications of Metal Complexes Dyes in Analytical Chemistry

*Mariame Coulibaly*

### **Abstract**

Trace elements, especially heavy metals, are considered to be one of the main sources of pollution in the environment since they have a significant effect on ecological quality. Commonly, the analytical methods for the determination of trace metals are the spectrometry techniques. While, the electroanalytical methods are recognized as a powerful technique for trace metals owing to its remarkable sensitivity, relatively inexpensive instrumentation, ability for multi-element determination at trace and ultra trace level. New alternative electrode materials are highly desired to develop sensitive stripping sensors for meeting the growing demands for on-site environmental monitoring. Dyes aromatic heterocyclic compound, used in food, textile and cosmetic industries has been used for spectrophotometric determination of metals. In electrochemitry, methods for metals determination based on their complexation with dyes were proposed. In this chapter, a brief summary of spectrometry methods and electrochemical sensors for heavy metals detection based on the formation of metals dyes complexes is presented.

**Keywords:** heavy metals, dyes, dye/complexation, electrochemical analysis, metal complex dye, spectrometry

#### **1. Introduction**

Dyes are known to be used in the textile industry, printing, food industry, as well as cosmetic industry. Since the invention of synthetic dyes in 1856, chemistry has been enriched by these large group of chemical compounds. More than 800,000 tons are manufactured by year [1–4]. Dyes are organic substances with chromophore and auxochromic groups (**Figure 1**) classified into several groups (indigoid dyes, xanthene dyes, etc.) of various structures for different applications.

These applications depend on dyes chemical structure, their hue and their entire light absorbing system. The chromophores are the groups of atoms responsible for the dye colour and auxochromes are an electron withdrawing or donating substituents that cause or intensify the colour of the chromophores [6] in shifting the adsorption towards longer wavelength along with an increase in the intensity of absorption. Some commonly known chromophores groups are: azo (–N=N–), carbonyl (–C=O), methine (–CH=, nitro (–NO2) and quinoid groups. The auxochromes are acids or bases; the most important are amine (–NH3), carboxyl (–COOH), sulfonate (–SO3H) and hydroxyl (–OH).

The use of dyes in analytical chemistry is well known. Dyes applications in analytical chemistry are feasible because of the presence of chromophores and auxochromes [7]. Most of dyes form complexes with pollutans in aqueous media [8].

#### **Figure 1.** *Structure of the azo reactive dye [5].*

They are used as titrations indicator in analytical chemistry, and their complexes with the metal ions in aqueous media are used in spectrophotometric analysis. The complexation between dyes and some essential metals including Cu(II), Hg(II) allows the detection of them by the spectrophotometric method or cromatography [9–11].

As in spectrophotometry, titration, colorimetry or chromatogarphy, dyes are also used in electrochemistry specially in metal ions detection. Electrochemical analysis is recognized to be a method for industrial process control, environmental monitoring, and different applications in medicine [12–16]. The electrochemical technique, especially stripping voltammetry for the trace analysis of metal ions, obtained considerable interest because of its low cost, easy operation, good sensitivity, high selectivity and accuracy [17]. The usual working electrode for stripping voltammetry was a mercury electrode [18] and bare electrodes. However mercury is toxic and causes harm to the environment and human bodies. Concerning bare electrodes, they have numerous limitations such as poisoning, low sensibility, poor stability. Therefore, many groups tried to develop mercury free-electrodes and modified electrode to determine metal ions by voltammetric analysis [19–22]. These chemically modified electrodes (CME) have received an increasing attention in recent years in the fields of electroanalysis due to well recognized advantages in comparison with conventional electrodes [23]. Several reagents and techniques are used to modify the electrodes surfaces [24–28]. The complexation reactions with organic or inorganic reagents on electrodes surfaces, incorporate in electrode paste or in solution for electrochemical anaysis have been reported [29–32]. Among them, the dyes which are important complexing agents for metals. Electrochemical methods for metal ions determination based on their reaction with dyes, their complexation with dye film on electrode surface or inside of electrode paste have been studied [33–35].

### **2. Metal-complex dyes in spectrophotometric analysis**

#### **2.1 Overview**

Many studies have been based on the spectrophotometric determination of metal ions after their reactions with complexing reagents including dyes [36–38]. The dyes are organic substances with chromophore and auxochrome groups which can be classified in different type. Among these different type of dyes, azo dyes represent the largest production volume. They make up about 70% of all synthesized dyes annually [39]. The importance of these may increase in the future and also their use in a variety of applications such as complexing agent in spectrophotometric analysis. However, their stability causes environmental pollution once the dyes are discharged with liquid effluents without adequate treatment before release into the natural environment.

The formation of dye complex depends of the number of ligands in the dye structure, and the coordination number of the metal. The electron donating ligand or ion

**119**

**Figure 2.**

*The light absorption system of dyes [43].*

*Applications of Metal Complexes Dyes in Analytical Chemistry*

combines with the metal ion to form the complex. For instance, for copper ion which is a bivalent ion with coordination number four, it can complexed with two bidentate ligands in an acid dye or a trivalent or a tetravalent one [40]. Metal-complex dyes formed may be broadly divided into two classes: 1:1 metal complexes and 1:2 metal complexes [41]. These complexes have versatile application in various fields include the dyeing of nylon and protein fibers, paint, toners for photocopiers, laser and ink-jet printers, photoconductors for laser printers, nonlinear optics, singlet oxygen generators, dark oxidation catalysts, and high-density memory storage devices [42].

By UV–Vis Spectrophotometry, the absorption spectra of solutions allows the determination of metals concentration. The absorption spectra of dye soltution and metal ions solutions are measured first Then, after the mix of the dye and the metal ion, the formation of coloured complex between the both compound give a new

Their colors span the entire spectrum allowing their use in spectrometry.

**2.2 Spectrophotometric determination of trace metal by formation of** 

It well know that dyes can form a stable complexes with metal ions. Dyes applications in spectrophotometric analysis are possible because of the presence of chromophores and auxochromes (**Figure 2**) [7]. Dyes especially the azo dyes are used as spectrophotometric chemosensor. These compounds interact easily with metal ions through the heteroatoms S, N, and O and can chelate with a large number of metal ions to form a metal-dye complex (**Figure 3**). Numerous works have been dedicated to the synthesis and spectral characterization of new azo dyes and their metal complexes [44, 45]. These studies allow to establish the optimal conditions of formation of the complexes (ratio metal: dye, pH, temperature, the maximum light absorption, the influence of foreign ions …) and the determina-

The complex formation equilibrium and formation constant of the complex can

[ ] [ ][ ] *n n*

*ML*

*K*

*M nL ML* + → *<sup>n</sup>* (1)

*M L* <sup>=</sup> (2)

*DOI: http://dx.doi.org/10.5772/intechopen.95304*

color peaking and a new absorption spectra.

**complexes with dye**

tion of the constants of complexes.

be represented by Eqs. (1) and (2) [46].

#### *Applications of Metal Complexes Dyes in Analytical Chemistry DOI: http://dx.doi.org/10.5772/intechopen.95304*

*Dyes and Pigments - Novel Applications and Waste Treatment*

cromatography [9–11].

*Structure of the azo reactive dye [5].*

**Figure 1.**

They are used as titrations indicator in analytical chemistry, and their complexes with the metal ions in aqueous media are used in spectrophotometric analysis. The complexation between dyes and some essential metals including Cu(II), Hg(II) allows the detection of them by the spectrophotometric method or

**2. Metal-complex dyes in spectrophotometric analysis**

Many studies have been based on the spectrophotometric determination of metal ions after their reactions with complexing reagents including dyes [36–38]. The dyes are organic substances with chromophore and auxochrome groups which can be classified in different type. Among these different type of dyes, azo dyes represent the largest production volume. They make up about 70% of all synthesized dyes annually [39]. The importance of these may increase in the future and also their use in a variety of applications such as complexing agent in spectrophotometric analysis. However, their stability causes environmental pollution once the dyes are discharged with liquid effluents without adequate treatment before release into the natural environment. The formation of dye complex depends of the number of ligands in the dye structure, and the coordination number of the metal. The electron donating ligand or ion

As in spectrophotometry, titration, colorimetry or chromatogarphy, dyes are also used in electrochemistry specially in metal ions detection. Electrochemical analysis is recognized to be a method for industrial process control, environmental monitoring, and different applications in medicine [12–16]. The electrochemical technique, especially stripping voltammetry for the trace analysis of metal ions, obtained considerable interest because of its low cost, easy operation, good sensitivity, high selectivity and accuracy [17]. The usual working electrode for stripping voltammetry was a mercury electrode [18] and bare electrodes. However mercury is toxic and causes harm to the environment and human bodies. Concerning bare electrodes, they have numerous limitations such as poisoning, low sensibility, poor stability. Therefore, many groups tried to develop mercury free-electrodes and modified electrode to determine metal ions by voltammetric analysis [19–22]. These chemically modified electrodes (CME) have received an increasing attention in recent years in the fields of electroanalysis due to well recognized advantages in comparison with conventional electrodes [23]. Several reagents and techniques are used to modify the electrodes surfaces [24–28]. The complexation reactions with organic or inorganic reagents on electrodes surfaces, incorporate in electrode paste or in solution for electrochemical anaysis have been reported [29–32]. Among them, the dyes which are important complexing agents for metals. Electrochemical methods for metal ions determination based on their reaction with dyes, their complexation with dye film on electrode surface or inside of electrode paste have been studied [33–35].

**118**

**2.1 Overview**

combines with the metal ion to form the complex. For instance, for copper ion which is a bivalent ion with coordination number four, it can complexed with two bidentate ligands in an acid dye or a trivalent or a tetravalent one [40]. Metal-complex dyes formed may be broadly divided into two classes: 1:1 metal complexes and 1:2 metal complexes [41]. These complexes have versatile application in various fields include the dyeing of nylon and protein fibers, paint, toners for photocopiers, laser and ink-jet printers, photoconductors for laser printers, nonlinear optics, singlet oxygen generators, dark oxidation catalysts, and high-density memory storage devices [42]. Their colors span the entire spectrum allowing their use in spectrometry.

By UV–Vis Spectrophotometry, the absorption spectra of solutions allows the determination of metals concentration. The absorption spectra of dye soltution and metal ions solutions are measured first Then, after the mix of the dye and the metal ion, the formation of coloured complex between the both compound give a new color peaking and a new absorption spectra.

#### **2.2 Spectrophotometric determination of trace metal by formation of complexes with dye**

It well know that dyes can form a stable complexes with metal ions. Dyes applications in spectrophotometric analysis are possible because of the presence of chromophores and auxochromes (**Figure 2**) [7]. Dyes especially the azo dyes are used as spectrophotometric chemosensor. These compounds interact easily with metal ions through the heteroatoms S, N, and O and can chelate with a large number of metal ions to form a metal-dye complex (**Figure 3**). Numerous works have been dedicated to the synthesis and spectral characterization of new azo dyes and their metal complexes [44, 45]. These studies allow to establish the optimal conditions of formation of the complexes (ratio metal: dye, pH, temperature, the maximum light absorption, the influence of foreign ions …) and the determination of the constants of complexes.

The complex formation equilibrium and formation constant of the complex can be represented by Eqs. (1) and (2) [46].

$$M + nL \to ML\_n \tag{1}$$

$$K = \frac{\left[ML\_n\right]}{\left[M\right]\left[L\right]^n} \tag{2}$$

**Figure 2.** *The light absorption system of dyes [43].*

**Figure 3.** *Chemical structure of the azo-metal chelates [44].*

[M], [L] and [MLn] represent the molar equilibrium concentrations of the metal ion, ligand dye and the complex, respectively.

Thus, several new spectrophotometric methods for determination of metal ions based on their complexation with dyes have been developed and tested in real samples [47–49]. Bonishko et al. [48] have developed, a simple spectrophotometric method for the determination of osmium (IV) ions, based on the formation of a complex of this metal with Congo Red. While, the orange G has been used as a complexing reagent in spectrophotometric determination of osmium(IV) by Rydchuk et al. [49]. They showed that the optimum conditions for the formation of coloured complex compound between Os(IV) and acidic monoazo dye Orange G (OG) were: the stoichiometric ration in the complex was 2:1 at pH = 5.80. Moreover, their study showed that at the room temperature Os(IV) practically did not interact with OG. Os(IV)-OG compound was almost fully obtained after 30 min of heating on a boiling water bath (~98°C).

In general, the formation of complexes lead a significant decreases in the absorption band of the dyes and the emergence concomitantly of a new absorption band with different absorbance. Thus, the formation of complex species between mercury and indigo carmine((Hg)IC and (Hg)2IC) allowed a optical determination of mercury [36]. The interaction between Cu(II) ions and indigo carmine forms Cu2(IC) complex characterized by the stoichiometric ratio between indigo carmine and copper 2:1, the molar absorptivity 1.17 x 104 mol L−1 cm−1 at 715 nm and the stability constant of the complex log K = 5.75, at pH 10, obtained by spectrophotometric data. This complex has been successfully tested for determination of copper in pharmaceutical compounds [37].

## **3. Electrochemical method for the determination of trace metal by formation of complexes with dye**

#### **3.1 Electrochemical behaviour of dye**

The electrochemical behaviour of dyes depends of their chemical characteristics, the working electrodes and the pH of supporting electrolyte. According

**121**

**Figure 4.**

*250, 500 and 750 mV/s. [51].*

*Applications of Metal Complexes Dyes in Analytical Chemistry*

nature of electrodes, the voltammograms of dyes exhibited irreversible oxidation peaks [50] or can be involved in a two or more steps redox reaction [51]. The voltammetric response of indigo carmine shows two well separated peak pairs on graphite electrode (**Figure 4**) at pH 7, while the first pair of peak disapear at ph

As indicated previously, azo dyes are an important class of organic dyes which consist of at least a conjugated chromophore azo (–N=N–) group. This is the largest and most versatile class of dyes. These dyes are characterised by the presence in their molecules of one or more azo groups —N=N— which form links with organic groups, of which at least one is usually an aromatic nucleus (**Figure 2**). Taking account their potential toxicity, electrochemical methods was developed for the analyzing of azo dye. The mechanism based on the reduction of the azo group with a classical dropping mercury electrode or static mercury drop electrode has been described in detail [52]. Recently, several modified electrodes have been used to study the electrochemical characteristics of azo dyes and their electrochemical determination [54–57]. On a glassy carbon modified, the voltammograms exhibited a irreversible oxidation peaks and a well-resolved oxidation wave was observed at approximately 0.74 V for the azo dye sudan I, sudan II, sudan III, and sudan IV [50] and similar irreversible oxidation peaks was obtained with the congo red on graphene oxide modified electrode [56]. However, the release potential of anodic peak

These studies show that some of azo dyes are electrochemically reactive. They can reduced or oxidized on different bare or modified electrode (**Table 1**). These electrochemical behaviour allows the detection of dye by voltammetric technique but also the detection of trace metals based on the decrease of dyes oxidation/

*Cyclic voltammograms recorded at graphite electrode for 10–3 M IC. Experimental conditions: supporting electrolyte 0.1 M phosphate buffer (pH 7); start potential,* −*1.0 V vs. Ag/AgCl,KClsat; scan rates: 25, 50, 100,* 

*DOI: http://dx.doi.org/10.5772/intechopen.95304*

depends of dye and electrode.

reduction peak after their complexation.

more basic.

*Dyes and Pigments - Novel Applications and Waste Treatment*

ion, ligand dye and the complex, respectively.

*Chemical structure of the azo-metal chelates [44].*

**Figure 3.**

and copper 2:1, the molar absorptivity 1.17 x 104

in pharmaceutical compounds [37].

**formation of complexes with dye**

**3.1 Electrochemical behaviour of dye**

[M], [L] and [MLn] represent the molar equilibrium concentrations of the metal

mol L−1 cm−1 at 715 nm and the

Thus, several new spectrophotometric methods for determination of metal ions based on their complexation with dyes have been developed and tested in real samples [47–49]. Bonishko et al. [48] have developed, a simple spectrophotometric method for the determination of osmium (IV) ions, based on the formation of a complex of this metal with Congo Red. While, the orange G has been used as a complexing reagent in spectrophotometric determination of osmium(IV) by Rydchuk et al. [49]. They showed that the optimum conditions for the formation of coloured complex compound between Os(IV) and acidic monoazo dye Orange G (OG) were: the stoichiometric ration in the complex was 2:1 at pH = 5.80. Moreover, their study showed that at the room temperature Os(IV) practically did not interact with OG. Os(IV)-OG compound was almost fully obtained after 30 min of heating on a boiling water bath (~98°C). In general, the formation of complexes lead a significant decreases in the absorption band of the dyes and the emergence concomitantly of a new absorption band with different absorbance. Thus, the formation of complex species between mercury and indigo carmine((Hg)IC and (Hg)2IC) allowed a optical determination of mercury [36]. The interaction between Cu(II) ions and indigo carmine forms Cu2(IC) complex characterized by the stoichiometric ratio between indigo carmine

stability constant of the complex log K = 5.75, at pH 10, obtained by spectrophotometric data. This complex has been successfully tested for determination of copper

**3. Electrochemical method for the determination of trace metal by** 

The electrochemical behaviour of dyes depends of their chemical characteristics, the working electrodes and the pH of supporting electrolyte. According

**120**

nature of electrodes, the voltammograms of dyes exhibited irreversible oxidation peaks [50] or can be involved in a two or more steps redox reaction [51]. The voltammetric response of indigo carmine shows two well separated peak pairs on graphite electrode (**Figure 4**) at pH 7, while the first pair of peak disapear at ph more basic.

As indicated previously, azo dyes are an important class of organic dyes which consist of at least a conjugated chromophore azo (–N=N–) group. This is the largest and most versatile class of dyes. These dyes are characterised by the presence in their molecules of one or more azo groups —N=N— which form links with organic groups, of which at least one is usually an aromatic nucleus (**Figure 2**). Taking account their potential toxicity, electrochemical methods was developed for the analyzing of azo dye. The mechanism based on the reduction of the azo group with a classical dropping mercury electrode or static mercury drop electrode has been described in detail [52]. Recently, several modified electrodes have been used to study the electrochemical characteristics of azo dyes and their electrochemical determination [54–57]. On a glassy carbon modified, the voltammograms exhibited a irreversible oxidation peaks and a well-resolved oxidation wave was observed at approximately 0.74 V for the azo dye sudan I, sudan II, sudan III, and sudan IV [50] and similar irreversible oxidation peaks was obtained with the congo red on graphene oxide modified electrode [56]. However, the release potential of anodic peak depends of dye and electrode.

These studies show that some of azo dyes are electrochemically reactive. They can reduced or oxidized on different bare or modified electrode (**Table 1**). These electrochemical behaviour allows the detection of dye by voltammetric technique but also the detection of trace metals based on the decrease of dyes oxidation/ reduction peak after their complexation.

#### **Figure 4.**

*Cyclic voltammograms recorded at graphite electrode for 10–3 M IC. Experimental conditions: supporting electrolyte 0.1 M phosphate buffer (pH 7); start potential,* −*1.0 V vs. Ag/AgCl,KClsat; scan rates: 25, 50, 100, 250, 500 and 750 mV/s. [51].*

#### *Dyes and Pigments - Novel Applications and Waste Treatment*

**123**

**Table 1.**

*Applications of Metal Complexes Dyes in Analytical Chemistry*

Reactive Brilliant Red x (RBR x-3b);

**Electrode Dye/structure Electrochemical** 

**behaviour**

Irreversible oxidation

Reversible oxidation reduction

**Reference**

[59]

[60]

**3.2 Detection of metals using their reaction with dyes**

*Electrochemical behaviour of some dyes investigated in electrochemistry.*

The coordination complexes of metals with azo-ligands are used in several applications due to the interesting material properties synthetized. The metal complexation by dyes modify the photophysical and coloristic properties of dyes. The formation of complexes are influenced by several by parameters such as dye concentration, dye

*DOI: http://dx.doi.org/10.5772/intechopen.95304*

Acid Red 6b (AR 6b); Reactive Yellow x-rg (RY x-rg); Reactive Orange x-gn (RO x-gn)

Novacron Deep Red C-D;

Novacron Orange C-RN

Glassy carbon electrode.

Graphite carbon electrodes.

#### *Applications of Metal Complexes Dyes in Analytical Chemistry DOI: http://dx.doi.org/10.5772/intechopen.95304*

*Dyes and Pigments - Novel Applications and Waste Treatment*

**Electrode Dye/structure Electrochemical** 

Sunset yellow Irreversible

Amaranth Irreversible

Allura Red AC Irreversible

Congo red Irreversible

Sudan I, II, III and IV Irreversible

Graphite Indigo carmine Two pairs

**behaviour**

reversible oxidation and reduction

oxidation

reduction

reduction

oxidation

oxidation

**Reference**

[51]

[58]

[52]

[52]

[56]

[55]

**122**

Nanoclay modified carbon electrode

Amalgam electrodes

Amalgam electrodes

Graphene oxide casted glassy carbon electrode

Carbon nanotube ionic liquid gel modified glassy carbon

**Table 1.**

*Electrochemical behaviour of some dyes investigated in electrochemistry.*
