ELECTROGRAVIMETRIC DETERMINATION OF CU2+, CO2+ and Ni 2+

Volume No. 3 Issue No. 2

ELECTROGRAVIMETRIC DETERMINATION OF CU2+, CO2+ and Ni 2+Dr. Kathlia De-Castro Cruza ; Elijah P. Ugaddanb

ABSTRACTElectrogravimetry or commonly referred to as electrodeposition has been a conventional method in quantitative analysis of of species such as metal ions. The quantity of the analyte is directly proportional to the applied charge to pursue an electrochemical reaction. Although this proves to be efficient, not all metal ions can be subjected in this analysis. Metals used are Cu, Ni and Co in their aqueous solutions of divalent salts. Prior to electrolysis, the samples were prepared such that the presence of chlorine in their divalent salt solution is eliminated through digestion of acid and in excess nitric acid as to the instant for Cu or ammoniacal solution which is in the case of Co and Ni. Electrodeposition is prone to certain phenomena known as IR drop, concentration overpotential and cathodic potential. This may affect the applied voltage towards the cell and incur significant deviation in the results. Elimination of it is done through constant stirring, the addition of depolarizers and complete removal of chlorine atoms. Range of percentage error obtained from subjecting individual metal solution in the Eberbach Electroanalyzer exhibited (1.27% - 4.11%). Separation of mixtures i.e. Cu-Co and Cu-Ni mixture is feasible although a very high percentage error is obtained.Keywords:electrochemistry, electrogravimetry, electrolysis, cathodic potential, concentration overpotential, Faradays Law, IR drop

ELECTROGRAVIMETRIC DETERMINATION OF CU2+, CO2+ and Ni 2+Page | 1

a. Professor of CHM115L, School of Chemistry and Chemical Engineering, Mapua Institute of Technology.b. Bachelor of Science in Chemistry and Chemical Engineering Student of CHM115L, Mapua Institute of Technology;

INTRODUCTION In quantitative analysis of ions specifically metals, most commonly used technique is electrolytic deposition or electrogravimetry. Ordinarily, metal is deposited on the electrode and an increase in the mass of electrode is determined. The principle underlying this analysis is electrochemistry predominantly the Ohms Law, Faradays Law and redox reaction. The Ohms Law states that the relation of current to voltage is directly proportional and inversely proportional to resistance. It is mathematically expressed as:

Where V is the potential measured in V; R is resistance in ohms(Equation 2.1) The Faradays Law states that the amount at which the sample deposits in the electrode is relatively directly proportional to the amount of the charge passed through it. This is expressed as

Where q is the charge measure in C; n is the mol of the analyte; F is the Faradays constant; N is the number of e-s involved in the redox reaction(Equation 2.2) There are two general types of electrogravimetric method: one of which is controlled current and controlled potential method. The first is maintaining the current and kept at a constant value while the potential of the indicator electrode is varied. On the other hand for a controlled potential, the current is varied usually starting at a highest value while the cell potential between electrodes is held constant. This is suitable for separating various metals in mixtures.The first part of the experiment will be applying the constant current electrogravimetric method. Its apparatus is given in Figure 2.1. In this electrolytic cell, the anode is where oxidation occurs and most of the deposited metals will be accumulated in the Pt cathode where reduction of the metal occurs.

Figure 2.1 Apparatus used in Constant Current MethodIn the anode the half- cell reaction is given by:4OH- O2+ 2H2O + 4eAnd on the cathode half-cell reaction are the following metals (Cu, Ni and Co) used:Cu2++ 2e- Cu(s)Ni2++ 2e- Ni(s)Co2++ 2e- Co(s)On the other hand in order to separate the mixtures given as Cu- Ni and Cu-Co, the second method, controlled potential is to be used. The purpose of the experiment is to be able to determine the amounts of metal in the aqueous solution from their divalent salts using electrogravimetric techniques and to be able to separate the mixture given by Cu-Ni and Cu-Co aqueous solution.

MATERIALS AND METHODOLOGYMaterialsThe reagents comprises of concentrated H2SO4, concentrated HNO3, concentration NH4OH, 85% hydrazine hydrate and aqueous solution of CuCl2, CoCl2 and NiCl2. The apparatus are 250 mL beaker, (1) hot plate, Bunsen burner and a pipet of 10 mL specification.

Methodology I. Procedure for Cu in CuCl2

II. Procedure for Co in CoCl Procedure for Ni in NiCl2III. Separation of Metals

RESULTS AND DISCUSSIONDuring the preparation of the solution prior to electrolysis, CuCl2, CoCl2 and NiCl2 was digested with concentrated sulfuric acid. In this manner the chloride ions in these solutions formed the precipitate of CuSO4 , NiSO4 and CoSO4. The following reactions are given below: CuCl2(aq) + H2SO4(aq) ->CuSO4 (s) + HCl(g)(white ppt.)NiCl2(aq) + H2SO4(aq) ->NiSO4 (s) + HCl(g)(yellow ppt.)CoCl2(aq) + H2SO4(aq) ->CoSO4 (s) + HCl(g)(pink ppt.)The purpose of this is to remove the chloride ions from the original divalent salt solution since this Cl- will give off erratic results upon electrolysis. The amount of metal deposited will not be optimized since Cl- will also attach to the cathode part of the Pt electrode and accounts for the mass after electrolysis.

Table 2.1 shows the different changes occurring in the solution after the following processes were doneThe solution was then heated to boiling to dissolve the precipitate in the aqueous solution. It is necessary to heat the sides of the beaker since this is the condensed state of the HCl that affixed on the beaker. HCl are then released as fumes in the fumehood. In order to ensure the complete removal of HCl, nitric acid was then added for Cu since HCl forms an azeotrope, boils off any nitriles and improves plating. Upon which is subjected to electrolysis. As for the case of Co and Ni, addition of hydrazine and concentration NH4OH was done after the color changes throughout heating. In electrogravimetry, as time increases, more metal ions are deposited on the cathode part of the electrode therefore more are reduced. After some time, when lesser metal ions are present in the solution, lesser current will then be applied to carry out electrolysis as shown in Figure 2.2.

Figure 2.2 shows the current-time relationship in electrolysisSince the current decreases over time, the IR drop (potential developed when resistance is applied) in the cell decreases as well. In order to sustain the shift in the IR drop and maintain the applied potential, the cathode potential (Ecath) is shifting to a more negative value. Because of this, the cathode is less likely to be reduced and the codeposition of the hdyrogen ions in the anode will then occur. The hydrogen ions in the anode is said to depolarize the copper in the cathode.

Figure 2.3 shows (a) the IR drop decreases as the (b) Ecathode also shifts to a decreasing value

The reduction of the anode will result to an erroneous measurement of the amount of substance that will deposit on the electrode for this will account the codeposition of the hydrogen ions via gas bubble formation. In order to remove this depolarization effect, a cationic depolarizer is then added to the solution. This cationic depolarizer is easily reduced or oxidized and helps to maintain the applied potential in the cell. Instead of reducing the hydrogen ions in the anode, the cationic depolarizer will be more favored to be reduced. Hydrazine and sulfuric-nitric acid solution which forms nitrates are examples of cationic depolarizer that are used in the experiment.

Figure 2.4 Structure of HydrazineThe addition of ammonium hydroxide in both Ni and Co allows the formation of complexes which is neutral in solution. Ammonia being the ligand in the Ni and Co complex allows the formation of their ppt.

Table 2.2 shows the results in the mass of metal sample after electrolysisIn Table 2.2, the % error of the experiment was obtained through the theoretical value of the following metals from their aqueous solution which is at a theoretical value of 2M. From this concentration, the theoretical weight of the sample deposit was calculated. It has been shown that Co has shown the least % error among the other metal samples, therefore Co was efficiently deposited in the electrode throughout the process. During the electrolysis, a stirrer was used in order to overcome the concentration overpotential. The concentration overpotential implies the diffusion of the metal ions in the Pt electrode and their relative concentration. Concentration is not constant all throughout electrolysis. The proximity of the ions in the solution to the Pt electrode dictates its concentration and they are said to be directly proportional. The smaller the distance is between the ions to diffuse to the electrode, the lesser is the concentration of it as compared to those that have greater distance from the electrode. This difference between the concentration near the electrode and the ones away from the electrode is known as the concentration overpotential. Taking the case for Cu ions as seen in Figure 2.5 the [Cu2+] electrode is less than that of the [Cu2+]bulk.

Figure 2.5 Concentration overpotential exhibited by Cu ions in bulk solutionIn order to eliminate the offset in the measurements caused by this phenomenon, constant stirring is initiated. Since not all atoms are of the same mobility due to their proximity in the electrode, it is highly necessary for the solution to be stirred to maintain a constant concentration between the bulk and the electrode. The electrode is made up of platinum (Pt) because this metal does not readily react with any other metals and said to be inert. Therefore when current is applied, no plating or deposition of Pt will be observed and results will only be contributed from the participating metal ions. When Cu was subjected to electrolysis, a controlled current of 1A was set and the electroanalyzer operated for almost 45 minutes. During the analysis of Co and Ni, current was increased and maintained constant at 2A and the machine operated for almost an hour. In the half cell reaction at the cathode, it is given by these following reactions and their following standard potentials: Cu2++ 2e- Cu(s) E0= + 0.339 VNi2++ 2e- Ni(s) ) E0= -0.236 VCo2++ 2e- Co(s) ) E0= -0.282 VThe reason why Cu was operated at a much lower current than Ni and Co is because amongst the three solutions, it is more likely to be reduced because of its (+) value on its reduction standard potential. Ni and Co would prefer more to be oxidized than reduced relative to Cu. Increasing the current for Ni and Co allows the applied potential to overcome the (-) values associated to its standard potential and permits electrodeposition. According to the relationship between Ohms law and Faradays Law, increasing the current also increases the amount of time needed for the Ni and Co solution to complete electrolysis therefore these two (Co and Ni) have more reaction time than Cu.Moreover, completion of electrolysis was determined after the respective colored of solutions of each metal became colorless. Drying of the electrode and first weighing was done. Afterwards, continuous weighing was observed to ensure that no more metal will deposit in the Pt electrode.At the latter part of the experiment is the separation of mixtures. Tabulated results are shown in Table 2.3.

Table 2.3 shows the electrolysis results with Cu-Co and Cu-Ni mixtureAlthough the results were not appreciable due to the reasons that the amount of time needed for completion in electrolysis where disregarded, the possibility of separating this mixture is feasible. Another reason may also arise from the significant error in the preparation of the sample. The method applied was a controlled potential electrogravimetry. Current was readjusted from 1A to allow deposition of Cu to 2A to ensure the deposition of the other metal, Co and Ni. The difference in their applied potential allows the separation of mixtures.

CONCLUSIONElectrogravimetry is a quantitative analysis of species particularly metals through electrodeposition. It is classified by two methods; controlled current and controlled potential electrogravimetry. Although electrogravimetry still proves to be efficient, selective metals can only be applied through this analysis. Cu, Co and Ni are the metals analyzed along with their divalent salts. Samples were prepared prior to electrolysis to ensure accurate measurements in determining the quanity of the analyte in the sample. This quantity is relative to the current or charge added into it. Electrodeposition may be subjected to different phenomena such as IR drop, concentration overpotential and cathodic potential that may contribute to erratic results. Prevention may include constant stirring, addition of cathodic depolarizers and elimation of impurities. The variation of current plays a great role in separation of mixtures. The amounts of this metals obtained were in a range of 1.27-4.11% error. In the separation mixture part of the experiment, a very high percentage error was found due to errors in the experimental methodology. Although poses with unappreciable result, separation of mixture (Cu-Ni mixture and Cu-Co mixture) is feasible.

REFERENCES[1] D.A. Skoog and J.J. Leary, Principles of Instrumental Analysis, Fourth Ed., Saunders College Publishing, Orlando, FA, 1992, Chap. 22, particularly pp. 499-511.[2] Willard, Merritt, Dean and Settle, Jr., Instrumental Methods of Analysis, Seventh Ed., Wadsworth, 1988, Chap. 22 and particularly pp. 682-691[3] Electrogravimetry; In Encyclopedia Britannica, 2011; Retrieved from http://www.britannica.com/EBchecked/topic/183095/electrogravimetry[4] Electrogravimetric Estimation of Metals; Value at Amrita, 2011; Retrieved from http://amrita.vlab.co.in/?sub=2&brch=200&sim=367&cnt=1[5] Harris, D. C., (1995) Quantitative Chemical Analysis 4th Ed., W. H. Freeman and Company, New York Chapter 17[6] D.A Skoog and West, Principles of Analytical Chemistry. Ninth Ed., Saunders College Publishing, Orlando, FA, 1992, Chap. 21