Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: sorincosofret on 10/08/2008 14:10:41
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Concentration cell and the salt bridge
To date, the subject of concentration cell using salt bridge is remind in the scientific book, without a detailed description. The purpose of the present text is to enter a little bit in the kitchen of these phenomena, more precisely to put in evidence the transfer of the ions from salt bridge in solutions.
Materials and experiments
1 M copper sulfate solution,
0.01 M copper sulfate solution
two small containers (fig.1 in section), with copper electrodes,
salt bridge made by saturating a piece of cotton in a CaCl2 solution and another one made with agar agar and CaCl2. The use of CaCl2 in the salt bridge instead of KCl is motivated only by the simpler analytical possibilities of Ca determination.
Two concentrations cells are made using the up presented salt bridges and their potentials are measured. After this operation, the Cu electrodes are short circuit with a simple metallic conductor in order to hurry up the electrochemical processes.
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After 3 days of working, the metallic conductor is removed and the difference of potential between electrodes is measured.
Contrary to expectations the cell with agar agar bridge presents the same difference of potential like initial.
After measurement again the electrodes are short circuit and leaved for another 5 days. After this period again the same difference of potential is counted.
The measurements of Cl and Ca concentrations in both compartments indicate a level of their concentration under the chemical limit of detection (precipitation with AgNO3 and Ba(OH)2) after both time intervals.
In the cotton piece bridge at the same time interval there is a diminishing of cell potential with approx. 0.02 V after 3 days and with approx. 0.07 V after another 5 days.
Both Ca and Cl species was detected in the anode and cathode compartments. A normal analytical procedure was followed for measuring their concentrations and it was found that:
Anode [Ca]=[Cl]/2
Cathode [Ca] =[Cl}/2
The concentrations of Ca and Cl species at anode are a little bit higher then at cathode region (probably due to the small different level of immersion of bridge into solution).
There is also a theoretical possibility for ions species migrations from a compartment to another, through salt bridge. To date, measurements of CuSO4 concentration in the anode and cathode space, using a calibrated curve and a VIS photometer deny this phenomenon. At the end of experiment, it is foreseen a dissection of bridges and a detailed analysis of Cu and SO4 ions. Preliminary analysis on other used bridges shows an absence of CuSO4 migration inside the bridge.
The experiment continues and I’m really interested when such cells are discharged.
Let’s analyze the theoretical consideration in the frame of actual physics for the up presented experiment.
Let’s consider the following concentration cell:
Cu(s) | CuSO4(1 M) || CuSO4(.01 M) | Cu(s)
with a salt bridge made by KCl.
This kind of cell, provide experimentally a potential difference closed by the Nernst equation prediction.
Nernst equation:
E = E° - (0.059/n) log Q = 0 - 0.029 log 0.01 = +0.058 V
The experimental measured difference was E’ = 0.048 in case of agar bridge and 0.046 in case of CaCl2, a little bit smaller then predicted. It seems that theory fit very well with practice. The difference can be very easy attributed to the contact potential existent in the circuit..
But is this appearance based on a consistent reality? How is this kind of cell working properly?
It is known and accepted by actual physics, the fact that an electrochemical cell needs an exothermic chemical reaction or a transport of an ionic species from a high concentration domain to a low concentration one as fundament of a potential difference generation.
Anyone can test the impossibility of a free (spontaneous) reaction between CuSO4, Cu and KCl , in a bi or tri component system reaction.
The actual explanation seems very strange, because it is necessary for cell working a transport of chloride in compartment and simultaneously a transport of potassium in another compartment according to the following picture.
(for insertion)
The reactions in the compartments are:
cathode:
Cu2+(1 M) + 2e– → Cu(s)
SO42- + 2K+ → K2SO4
anode: Cu(s) → Cu2+(.01 M) + 2e–
Cu2+ + 2Cl- → CuCl2
I live to ,,high authorities” the mission to explain how is impossible to have a spontaneous chemical reaction between CuSO4, Cu and KCl, but in case of electrochemical cell this is adopted as ,,de facto” phenomena.
Supplementary the same ,,high authorities”, which are incapable to connect two wires experimentally, but they are self considering genies of the physics modeling, should explain why in the anode and cathode compartments the level of Cl and K (Ca) ions is under the limit of detections, when a bridge made by agar-agar is used.
When a bridge made by piece of cotton saturated with solution, the same ,,high authorities” must explain why a simple diffusion of KCl (CaCl2) appear in both compartments, and of course the concentration of chloride is stoichiometric with concentration of potassium (Calcium) in every compartment?
What is the time of discharging for such cell concentration?
Maybe actual science discovered already the perpetual mobile, but is not aware of this.
As usual the proposed explanation can be found in the chemistry book or in the new foreseen books …
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"It is known and accepted by actual physics, the fact that an electrochemical cell needs an exothermic chemical reaction or a transport of an ionic species from a high concentration domain to a low concentration one as fundament of a potential difference generation. "
In that concentration cell the Cu electrode in the more dilute solution dissolves into teh soulution. The other electode is plated with copper. The two solutions both move towards the same concentration. The electrodes are copper to start with, and copper whne they are finished. The overall reation is the transfer of copper from the more concentrated solution to the more dilute one.
Once again, the normal laws of electrochemistry are satisfied so the "new book" is redundant.
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It seems that a picture worth like miles of words so similar to Daniel cell a visual description of concentration cell phenomena is provided.
There is not a strictly stoichiometric representation, in order to simplify the representation.
In the first picture a concentration cell with a salt bridge formed by 4 molecules of KCl is considered.
In actual scientific texts it is considered that bridge cells delivers both anion and cations for the cell working according to the folowing picture :
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In the 1M compartment, copper deposits on the electrode. Th sulfate anion catch potassium from cell and K2SO4 is formed into solution.
In diluted compartment )0,01M), copper is released from electrode. The bridge deliver the chloride ion and CuCl2 is formed in solution.
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As is seen, the working of concentration cell in actual representation suppose a reaction between CuSO4 and KCl which is impossible (endhotermic) in principle.
What should be the difference of potential for a endhotermic reaction ?
Even the cell is accepted to work (we pass over this objection) the concentration of salt in bridge is a important factor for cell working. When the concentration of salt in bridge is exhausted, the cell stop to work.
Even we pass over this second argument, the most important observation must regard the impossibility of sulfate ion equilibration. It is accepted that a equilibration of activity (concentration) of sulfate ion represent the engine of a concentration cell. But the actual explanation does not give us any possibility for a sulfate ion to pass from a compartment to another.
Consequently, at least all scientific information already published must be corrected regarding this important experimental aspect.
Let's see how can a concentration cell insure a sulphate migration between compartments as is presented in the folowing picture.
The starting point is the same cell with 4 molecules of KCl in the bridge.
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The copper in concentrated compartment is deposited on electrode. The sulfate ion push the chloride ion from molecule to molecule until a chloride ion falls into the diluted solution solution compartment. Here, a copper from electrod is released and CuCl2 is formed.
After a time of working the cell is filled with K2SO4 as represented in the folowing picture.
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As is seen again the current in the cell and final compounds formed in this mechanism depends on the concentration salt in the bridge.
Does this mechanism work in reality?
From a chemical point of view this mechanism is not feasible.
Even we suppose that sulphate ion has enough energy to remove a chloride ion from a KCl molecule, it will be the first case in chemistry when a sulphate push a entire chain of chloride atoms.
In case of a concentrated KCl solution the cell is working for long time with a circuit of chloride from bridge to solution. In this case it is possible to have after a week the cell working and without any transfer of sulphate from a compartment to another. Does the intention of sulphate to move from a compartment to another really produce a countable energy?