A. Title

: Electrogravimetry

B. Date

: Monday, 20th May 2013

C. Purpose of experiment:Separating and determining degree of Cu2+ and Zn2+ in quotation using electrogravimetry

D. Basic theory

:Electrogravimetry is a method used to separate and quantify ions of a substance, usually a metal. In this process, the analyte solution is electrolyzed. Electrochemical reduction causes the analyte to be deposited on the cathode. The cathode is weighed before and after the experiment, and weighing by difference is used to calculate the amount of analyte in the original solution. Controlling the potential of the electrode is important to ensure that only the metal being analyzed will be deposited on the electrode.

The process is similar to electroplating.It is known that in the phenomenon of polarization the products of electrolysis exerts a back emf, which reduces the actual emf of the cell. Thus electrolysis of an electrolyte is possible only when this back emf is overcome. Let us consider a case in which two more platinum electrodes are placed in a dilute solution of copper sulfate. If a source of potential is applied, no appreciable current will flow through the system, until some minimum potential is applied after which the current will increase as the applied potential increases. The applied voltage which is just sufficient to overcome the back emf due to polarization and also to bring about the electrolysis of an electrolyte without any hindrance is known as decomposition potential.

In gravimetric analysis determination of metal concentration is depends on the weighing process of the metal. But in electrogravimetery the metal concentration is determined by using electrochemistry. It is an easy and accurate method. The Electrogravimetric analysis is done by depositing the metal (whose concentration to determine) on a pre-weighed electrode, and the concentration can be calculated from the weight gained by that electrode.For example determination of copper is the most important application of electrogravimetery. The whole process is carried out in an electrolytic cell, which consists of two electrodes an anode and a cathode with an external electrical energy supply. On Cathode deposition of the metal takes place due to the reduction of metal, and it is connected to the ve terminal of the energy source. On Anode oxidation occurs, and it is connected to the +ve terminal of the energy source.

Now a days one of the electrode is used as a stirrer for stirring the analyte solution. Fisher electrodes are used in this process.

Types of Electrogravimetric Analysis, There are two types Electrogravimetric methods:Constant Current Electrolysis:

In this process the current is kept constant, and potential is increased. Here no control of the potential of the working electrode is exercised, and the applied cell potential is held at a more or less constant level but provides a large enough current to complete the electrolysis in a reasonable length of time. And a fixed amount of the current can be passing between the anode and cathode. The limitation of constant current electrolysis is it cannot be used for the separation of ion in a solution containing single species.

Constant Potential Electrolysis:

It is the simplest way of performing an analytical electrolysis is to maintain the applied cell potential at a constant value. It is more used in the separation of the components from a mixture in which the decomposition potentials are not widely separated.

For example the determination of copper from an acidic solution (either nitric acid or sulphuric acid solution or mixture of two acids) at constant current. Suppose an emf of 2-3 V is applied then the reaction taking place are given below.

At cathode:

Cu2+ + 2e- Cu

2H+ + 2e- H2At anode:

4OH- O2 + 2H2O + 4e-

Electro deposition is governed by Ohm's law and Faraday's two laws of electrolysis.

From ohms law the relation between current, voltage and resistance is given by,

Faraday's law states that,

a) In electrolysis the amount of substances liberated at the electrodes of a cell is directly proportional to the amount of electricity which passes through the solution.

Where:m is the mass of the substance liberated at the electrode,

Z is the electrochemical equivalent,

I is the current in ampere,

t is the time in seconds.

b)The mass of substances liberated is directly proportional to the relative equivalent weights of those substances at a particular quantity of electricity applied.

In electrogravimetery the determination of metal is carried out by above two procedures, at constant current or with controlled potential procedure. But constant current is limited.

E. Procedure

:

F. Tools and materialNameAmount

Trafo 500mA1

Dioda, resistor and elco1 set

AC and DC cableAs needed

Beaker glass 100mL1

Crocodile nipper1

Oven1

Analytical weight1

Copper plat40 cm

CuSO4 0,01M100mL

Aceton

Aquadest

G. Result of experiment

ProcedureResultHypothesisConclusion

CathodeAnode Hold with spiral electrode

Hold with straight electrode

Turn on the electroanaliser

Put the cathode and anode into beaker glass that had been filled with NaOH

Set the potential 270 V

Do the electrolysis 3 minutes

Take the cathode

Put into acetone

Balance the weight

Result

Mass of cathode

Wo = 2.728 gram

Wa1 = 2.735 gram

Wa2 = 2.744 gram

Wa3 = 2.754gram

Wa4 = 2.754 gram

Wa5 = 2.764 gram

W = Wa W0W1 = 2.735 2.728 = 0.007 gram

W2 = 2.744 - 2.735

= 0.009 gram

W3 = 2.754 - 2.744

= 0.010 gram

W4 = 2.754 - 2.754

= 0 gram

W5 = 2.764 - 2.754

= 0.010 gram

W = = = 0,0304 gramThe amount of Cu that form is 0.010 gram.

So % of redemen is 32.8947%.

H. Analysis The aim of this experiment is to separate and determine degree of Cu2+ and Zn2+ in quotation using electrogravimetry. The first thing that we did is arrange the tools, that consist of trafometer 500mA, AC and DC cable, beaker glass, resistor, and crocodile nipper.In this experiment we use Cu as an electrode, there are two electrode Cu, each of them has 20 cm long. One of the electrode make as coil and the other one is straight. The straight shape copper placed on the positive pole as oxidation side (anode), while coil-shaped copper placed on the negative pole as reduction side (cathode). But, before we use it, the copper as anode must be rubbed by sandpaper until the copper color more shine, than the copper is washed by using aquadest and aceton, so the impurities contained in the copper lost because aceton is volatile so the impurities can easily volatile. After that, weight the copper and we got 2.728 g for the initial weight. This coil is connect in the cable of electroanalyzer in reduction side. And the other electrode in oxidation side. The solution that we used is CuSO4 0.001 M that flows electricity have current 0.5 A by 4.5 volt. The time is 180 second.The reaction that happened is electrolysis process is on:

Anode : 2 H2O(l) ( 4H+(aq) + 4e + O2(g)

Cathode : Cu2+(aq) + 2e (Cu(s)After 180 second, put the coil-sharped electrode and dipped in acetone solution. Then weight as W1. Repeat this step as many as 4 times and in different time is: t2 = 600 se, t3 = 720 sekon, t4 = 840 sekon, t5 = 960 sekon.

Based on data result experiment, we get Wcu : t = 3 menit Wcu = 0.007 t = 10 menit Wcu = 0.009 t = 12 menit Wcu = 0.010 t = 14 menit Wcu = 0 t = 16 menit Wcu = 0.010Based on data result of experiment, we can calculate the WO2 dan WCu by formula :

I. DiskusiBased on data result of experiment that have been done to compare amount of electron O2 and Cu through experiment by amount of electron that occur in reaction.

In this experiment, there are some differences of weight Cu based on experiment and theory. In our experiment, we get the changing of weight in experiment is 0.007 gram, 0.009 gram, 0.010 gram, 0 gram, and 0.010 gram, but in our calculation we should get the changing of weight is 0.03 gram, 0.102 gram, 0.122 gram, 0.1423 gram, and 0.1626 gram. It is happened because there are many mistakes. In this experiment, we dip cathode in acetone. The characteristic of acetone can solute many of compound even the compound is synthetic polymer. So, it is possible if the metal is losing when it is dipped in acetone. Besides that in experiment that we have done, in the certain time mass of Cu not increase. This thing can influence to seek weight of electron Cu that involve in the experiment.

Arrange the electrolysis tools that are less precise, or when do electrolysis process is less true or less sanding the copper electrode by sandpaper are made result less precise.J. ConclusionBased on data result experiment that we have done, the weight that we have got is not suitable with the theory. The weight of copper that we have gotten is 0.007 gram, 0.009 gram, 0.010 gram, 0 gram, and 0.010 gram. The average of weight copper we have gotten is 0.0072 gram. While weight copper based on theory is 0.03 gram, 0. 102 gram, 0.122 gram, 0.1423 gram, 0.1626 gram. The average of weight copper based on theory is 0.112 gram. So % of redemen is 32.8947%.K. ReferenceAzizah, Utiya. dkk. 2007.Panduan Praktikum Mata Kuliah Kimia Analitik II: Dasar-Dasar Pemisahan Kimia. Surabaya: Universitas Negeri Surabaya.

Svehla, G. 1979. Vogel: Buku Teks Analisis Anorganik Kualitatif Makro dan Semimikro. Edisi Kelima. Terjemahan oleh Ir. L. Setiono dan Dr. A. Hadyana Pudjaatmaka. 1985. Jakarta: PT. Kalman Media Pustaka.

Underwood, A. L. dkk. 1986. Analisis Kimia Kuantitatif. Edisi Keenam. Jakarta: Erlangga.L. Attachment

Weight of copper

Wo = 2.728 gram

Wa1 = 2.735 gram

Wa2 = 2.744 gram

Wa3 = 2.754gram

Wa4 = 2.754 gram

Wa5 = 2.764 gram

W = Wa W0W1 = 2.735 2.728 = 0.007 gram

W2 = 2.744 - 2.735 = 0.009 gramW3 = 2.754 - 2.744 = 0.010 gramW4 = 2.754 - 2.754 = 0 gramW5 = 2.764 - 2.754 = 0.010 gram

t 1= 180 sekon

g PO2 = XO2 x P0O2 = 0,33 x 0,67 = 0,223 atm

t2 = 600 sekon

g PO2 = XO2 x P0O2 = x 0,67 = 0,2227 atm

t3 = 720 sekon

g Tekanan oksigen PO2 = XO2 x P0O2 = x 0,67 = 0,5583 atm t4 = 840 sekon g PO2 = XO2 x P0O2 =x 0,67 = 0,2206 atm

t5 = 960 sekon

g PO2 = XO2 x P0O2 =x 0,67 = 0,223 atmM. Pictures

Wire of Cu

Coil of Cu

Straight Wire

Cut the wire 2 piece @ 20 cm

Polish with sand paper

1st wire is form as coil

Coil of Cu

W0

Weigh it

Coil of Cu

Connect with cable of electroanalyzer as reduction part

Plug in the cable into source of current

The process do 3 minutes

Plug out the cable

Observe what happen in that coil

Syringe with H2O

Dip into acetone

Oven 2 minutes with 150o C

Weigh it

Repeat the process with different time

W1

Gambar 1.

Set of tools to do electrogravimetry

Gambar 2.

Electode

Gambar 2.

Balancing

Gambar 2.

Dipped electode into CuSO4 solution

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