Electrochemical deposition of zinc using alanine

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Electrochemical deposition of zinc using alanine

Diana Aljendi, Master student; Mohammad Ali Alshikh Pro. Faculty of Sciences Department of Chemistry Al-Baath University

Annotation

In this research study, we examined the reaction of the amino acid alanine with zinc sulfate in hydroculture using physiochemical methods (measure of pH). As a result of this reaction, Zinc-Alanine complex forms with the ration (1: 2). We then studied electrochemical reduction of Zinc in presence of Alanine by polarography. The study showed the formation of Zinc-Alanine particle with the ration (1: 2). The structures of compounds formed have been identified and we calculated the constant of their formation in order to benefit from these Electrolytes in the galvanic plating with Zinc and depositing them on the solid electrodes at a later stage..

Keywords: Solution pH - Polarographic apparatus - polarography - dropping mercury electrode - electrochemical deposition

Аннотация

Электрохимический отложение цинка с использованием аланина

Диана Альдженди Магистр; Мохаммед Али Альших Про. факультет наук. Кафедра «Химия» Университет Аль-Баас

В этом исследовании мы исследовали реакцию аминокислоты аланин с сульфатом цинка в гидрокультуре, используя физикохимические методы (измерение рН). В результате этой реакции образуется комплекс цинк-аланин с соотношением (1: 2). Затем мы изучили электрохимическое восстановление цинка в присутствии аланина с помощью полярографии. Исследование показало образование цинк-аланиновой частицы с соотношением (1:2). Структуры образовавшихся соединений были идентифицированы, и мы рассчитали константу их образования, чтобы извлечь выгоду из этих электролитов в гальваническом покрытии с цинком и нанесении их на твердые электроды на более поздней стадии.

Ключевые слова: PH раствора - полярографический аппарат - полярография - капающий ртутный электрод - электрохимическое осаждение.

Introduction

Corrosion is the natural destruction of materials and alloys by chemical and/or electrochemical reaction with their environment [1 - 2]. It can also be defined as the dissolution of the metal by its reaction with the surrounding environment.

Corrosion causes a lot of damages and its economic effect is extremely bad as because of it, detrimental losses are estimated in billions annually due to the destruction it causes to industrial plants and equipment.

Electrochemical deposition refers to the process of accumulating/ depositing a thin layer of metal that has a certain electric charge onto the surface of an electrically-conducting body. Deposition of the ions takes place when an electric current - the electric potential of which is equal to potential of the reduction of the ion whose deposition is required - passes through its electrolyte.

Galvanic plating of Zinc is considered to be the optimum solution to the problem of iron oxidation and metal corrosion. The electrolytes of plating with Zinc are divided into:

1. Simple electrolytes: The most important of these electrolytes include (Sulfuric Acid - Halogenic Acids .. .etc.).

2. Compound electrolytes: The most important of these include:

- Cyanides: [Cd(CN)4]2- which have a number of advantages but yet have disadvantages including the toxic materials.

- Zinc Onitules: These are considered to be of the best electrolytes of plating with Zinc. They are not toxic and the plating glitters and is non-porous whose thickness is up to (20) microns. However, Onitules are unavailable and extremely expensive.

Aim of the study

This research aimed to study electrochemical reduction of Zinc from its complexes with alanine and to attempt to produce a new electrolyte for plating with Zinc that might contribute to solving the problem of corrosion.

1. Experimental:

2. Research Methods and Experiment Techniques

- Laboratory Glassware: beakers, Erlenmeyer flasks of different capacities, standard pipette, a scale

- The physiochemical study was conducted using the measure of pH and conductivity in order to identify the concentration of Hydrogen ions and to examine the electric conductivity. Then, the experiments were conducted at the lab temperature using recently-prepared solutions. The following materials were used: Alanine ,Zinc Sulfate (ZnSO4.5H2O), Sulfuric acid (H2SO4), Sodium Hydroxide (NaOH), Potassium Chloride (KCl), buffer solution (Ammonium Chloride NH4Q, Ammonium Hydroxide NH4OH), and distilled water.

- The electric reduction of Zinc (II) was studied using Volt-amperemeter station,

- To examine the electric deposition of Zinc, we used copper and steel electrodes.

3. Results and Discussion:

The Physio-Chemical Study in the system [alanine - Zinc (II) - water] To identify the bonding ratio between Zinc ion (II) and alanine acid, the changes in the solutions pH were examined as a method of physiochemical analysis. We prepared 10 ml of Zinc sulfate with concentration of (2 x 10^-2) M and the initial pH value 5,81. We also prepared 40 ml of alanine with the same concentration 2 x 10' ~²) m and the initial pH value of acid 6,3.

We also titrated Zinc sulfate solution by adding alanine acid in batches, the results were represented graphically as follows:

Figure (1): Change of the solution pH of the system [alanine - Zinc - water] in terms of the volume of alanine added as Ala]=[Cd]=(2 x 10^-2)M

When zinc ions react with alanine, they displace hydrogen ions from the carboxyl group and take their place. This leads to an increase in the concentration of hydrogen ions and a decrease in the value of the solution pH. according to the concentration of alanine in the solution, the compounds (Zn:Ala) might form with the following Mole fractions [1:1] [1:2] [3:1] and this might reflect differently on the solution pH. pH value continues to decrease until the ratio of solution ingredients become (66.6%) of alanine and (33.3%) of zinc sulfate. The solution pH then becomes (5.38).

This means that the increase in percentage of the amino acid alanine in the solution from (0%) to (66.6%) leads to decreasing the solution pH. The persistence of this increase in the percentage of alanine leads to an increase in the solution pH. This can be explained by the fact that when the solution ingredients become (66.6%) of alanine and (33.3%) of zinc, the complex zinc-alanine [Zn-Ala] forms and the ration of the ingredients is [2:1]. Hydrogen ions are released from the group COOH which is in the amino acid as per the following equation:

As for the range in which the percentage of alanine is from (66.6%) - (100%),. the range rich in alanine, the addition of the amino acid leads to an increase in the solution pH due to accumulation of the unbonded molecules of alanine acid. pH continues to increase until it reaches the value specific for the amino acid alanine. Therefore, the formation of other compounds in the solution such as [3:1] is considered unlikely, which is identical with the reference data [6,5,4].

We then calculated the concentration of Hydrogen ions resulting from the reaction in equation (1) and the equilibrium concentrations of the reacting materials and those resulting from the reaction using the data from the graph. The solution pH changed in terms of the alanine volume added (Figure 1). Then the constant of the complex [2:1] formation was calculated depending on Beurma Method [6,2,1]

It was found that the formation constant of the formed compound [Zn:Ala] [2:1] equals K=2 x 10⁴ i.e. the decomposition constant of the compound Kq=5 x 10^-5. This value matches well the reference data [7-5].

Thus, the physiochemical study (measuring pH) allowed gaining a primary understanding of the reaction happening in the system [alanine - Zinc (II) - water]. It appeared that when mixing the studied ingredients, one compound formed in the solution, which is alanine-zinc [2:1]

2-2- The Electro-chemical Study

4-2-1- Studying the Electrochemical Reduction of Zinc (II) in the Presence of Alanine by Polarography:

We studied the electrochemical of zinc (II) in the presence of alanine on the dropping mercury electrode using the following electrolyte solutions: (H2SO4 0,5mol/l) (KCL 0.5mol/L) and (NH4Q 0.5mol/l + NH4OH 0.5mol/l) as earths or pregnant solutions to increase solutions conductivity. These solutions were selected so that the study would be inclusive to pH range (from 1 to 12). a- Electrochemical Behavior of the amino acid alanine:

It was necessary to know the behavior of alanine acid during its reduction from previous electrolyte solutions in order to guarantee accuracy when interpreting the experimental data. The experimental studies show that there is one polarographic value of the acid in each of the media.

Figure (2): Reduction of the amino acid Alanine on the following backgrounds: (a) (0.5 mol/l H₂SO4), (b) (0.5 mol/L KCl), (c) (0.5 mol/l NH4Cl + 0.5 mol/l NH4OH)

b- The Electrochemical Behavior of Zinc (II) and the Effect of the Electrolyte pH on the Reduction Process:

Through drawing the polarographic graphs of zinc sulfate solutions, we obtained waves in the presence of alanine in the solution being studied and in its absence. Therefore, we drew the Differential Pulse Polarogram (DPP) on the three on Hie background (H2S04) where:

[A\a\= [Zn+2]=2 X 10 ^mol/l [H2SO4]=0.5 mol/l.

We notice from the previous figures that as the concentration of alanine increases, the limiting current increases when moving from the acidic medium to alkaline medium passing through the neutral medium. We also noticed the shift in the half-wave potential to the more negative potentials as seen in Figure (6).

Figure (3): The Differential Pulse Polarogram (DPP) of the Reaction of Zinc Reduction from its Solutions with Alanine Acid where: 1) - ZnSO4.5H2O, 2) - [Zn-Ala 1:1], 3) - [Zn-Ala 1:2], 4) - [Zn-Ala 1:3]

Figure (4): The Differential Pulse Polarogram (DPP) of the Reaction of Zinc Reduction from its Solutions with Alanine Acid on the Background (KCl) where: 1) - ZnSO4.5H2O, 2) - [Zn-Ala 1:1], 3) - [Zn-Ala 1:2], 4) - [Zn-Ala 1:3]

Figure (5): The Differential Pulse Polarogram (DPP) of the Reaction of Zinc Reduction from its Solutions with Alanine Acid on the Background (NH4OH+NH4O) where: [Ala]=[Zn⁺²]=2 x 10^-3 mol/l [(0.5 mol/l NH4OH)+(0.5 mol/l NH4O)], 1) - ZnSO4.5H2O, 2) - [Zn-Ala 1:1], 3) - [Zn-Ala 1:2], 4) - [Zn- Ala 1:3]

Figure (6): The Limiting Current of the Electrochemical Reduction Process of Zinc (II) in terms of the Concentration of Zinc Sulfate on the following Earths:1) - 0,5 mol/l H2SO4 2) - 0,5 mol/l KCl 3) - 0,5 mol/l NH4OH + 0,5 mol/l NH4O

This refers to the diffusion system of zinc ions reduction process (i.e. the defined stage for the velocity of the electrode process is diffusion) [9-8]. The figure also shows that the limiting current of zinc (II) reduction decreases with the increase in the electrolyte pH.

In order to know the rate of reversibility of the electrode process, we used Tamamushi Theorem. The functions Log i/id-I were drawn in terms of E, (Figure 4). All potentials were taken as per the standard potential of Hydrogen electrode. This figure shows that the process of zinc ions reduction in the absence of the amino acid alanine is reversible. which undoubtedly indicates the reversibility of Zinc ions reduction process from its simple salts solutions, which matches well with the reference data [10 - 9].

Figure (7): The Function of the Amount log i/id -i of the Dropping Mercury Electrode Potential for the Electric Reduction Reaction of Zinc (II) from its Solutions with the Amino Acid Alanine with Mole Ratio: 1) -1 (1:0) 2) - (1:1) 3) - (1:2) 4) - (1:3)

Figure (4) shows that when adding the amino acid alanine to the solution being studied polarographically so that its ratio to zinc ions is [1:1], a slight decrease can be noticed in the limiting current value of zinc ions reduction and the shift in the half-wave potential whose reduction is by (30 - 35 mv). When the ratio of zinc ions and amino acids glycine becomes [2:1], the value of the limiting current decreases more, whereas the potential of the reduction half wave remains practically stable. The accompanying polarization is linked to the process of emptying zinc ions from alanine-zinc electrolyte [1:1]. The presence of zinc ions in a fixed cyclic compound leads to a decrease in its efficiency and to shifting its deposition potential in the direction of negative potentials. We identified the number of electrons contributing to the zinc ions reduction process in two ways:

1-From the differential polarographic curve in the relation used for reversible and semi-reversible processes: n ⁹⁸ Where: 98 mv is a fixed amount S is the breadth of the half peak [10].

2-And from the inclination of the lines presented in Figure (4) and the data of equation of polarographic reversible and irreversible wave:

E=Ei --(00“ )-l°9 i/id -i

The conversion factor whose value ranges between zero and one (based of the degree of reversibility) [11]. We considered that (6 =1) for the reversible polarographic waves (6 =0.5) for semi-reversible waves. It appeared to us that (n=2), i.e. the reduction process happens in one stage: [Zn⁺² + 2e

zinc alanine electrode galvanic

Discussion

The potential of half wave of zinc reduction process from its solutions with amino acid alanine is shifted compared to the potential of the reduction of the hydrous charge in all the solutions being studied. The amount of this shift is one "equal", which is (30 - 35 mv), which is in zinc alanine [2:1].

pH is considered to be an important factor in identifying the dynamism of the reduction of zinc-alanine (As this leads to a decrease in the height of the reduction wave and the shift in the potential of half wave towards the negative values of the potential). However, it is possible to notice the effect of zinc hydroxyl charges of the surface of the electrode, which leads to the negativity of the electrode - partially - and thus decreases the yield as per the current (60% - 50%).

Finally, the process of zinc-alanine reduction happens by sharing two electrons in each stage. This is a semi-reversible process where the cathodic polarization is relatively high.

Conclusions

1- The physio-chemical study (measure of pH) indicated the occurrence of a reaction in the system (alanine - zinc - water). This reaction is accompanied by the formation of the complex alanine-zinc with the ratio [2:1].

2- The electro-chemical study confirmed the results of the physio-chemical analysis which were found: The polarographic method revealed that the electrochemically active particles which ensure the formation of excellent zinc plating are alanine-zinc with the ration [2:1].

References

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