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DANIEL CELL

Repetition of old experiments and new improvements are made by Lipsa Dorelia

Background and actual explanation

Daniel cell represents an improvement of Volta cell, avoiding both the use of sulfuric acid and the gas releasing during cell working. Because of no gases are involved at all, this cell does not suffer from the effect of polarization.

It consists of a central zinc anode dipping into a cylindrical porous clay pot containing 25% diluted ZnSO₄ solution. The porous pot is immersed in a saturated solution of copper sulphate contained in a copper can, which acts as the positive electrode of this electrochemical system. To keep the CuSO₄ saturation constant, copper sulphate crystals are added during cell working. The use of a porous barrier prevents the copper ions in the copper sulphate solution from reaching the zinc anode and undergoing reduction. This would render the cell ineffective by bringing the battery to equilibrium without driving a current.

Figure 1. Daniel’s cell porous vase version

Daniel cell has an approximate open circuit voltage of 1.07 Volt.

Many different Daniel cell variations are known and used.

In practice, a variant of cell patented by Callaud without any porous barrier, gained a large spread, at least in case of static generators of electricity. Instead of porous barrier, a layer of zinc sulfate sat on top of a layer of copper sulfate, the two kept separate by their differing densities as in fig. 2. The zinc anode was suspended in the top layer whilst the copper cathode sat in the bottom layer and this variant was known as the gravity cell.

Figure 2. Gravity cell

For laboratory studies, another variant of Daniel cell, with a salt bridge, is usually used for student demonstrations. In case of salt bridge variant, the zinc half-cell consisting of a zinc rod partially immersed in aqueous zinc sulphate, and the copper half-cell consisting of a copper electrode partially immersed in aqueous copper sulphate are connected by a salt bridge.

The salt bridge used may be a strip of a filter paper soaked in saturated aqueous sodium chloride, ammonium nitrate, potassium chloride, potassium nitrate, or other common salt. More complex salt bridges are formed by a salt sequestrated in an agar-agar gel.

The bridge completes the circuit by allowing ions that carry charge to move from one half-cell to another. When the salt bridge is removed, no current flows because no charge (whether it is carried by the ions or electrons) can flow around the circuit.

Figure 3. Salt bridge Daniell cell

Related to working principle, a Daniel Cell is a simple chemical cell that converts chemical to electrical energy, more precisely, produces electricity through redox reactions.

Many oxidation-reduction reactions occur spontaneously, giving off energy. An example involves the spontaneous reaction that occurs when zinc metal is placed in a solution of copper salt when the next reaction occurs:.

     Cu⁺² (aq)  +  Zn (s)  ------->  Cu(s)  +  Zn⁺² (aq)

The zinc metal slowly "dissolves" as its oxidation produces zinc ions which enter into solution. At the same time, the copper ions gain electrons and are converted into copper atoms which coats the zinc metal or sediments inside container. The energy produced in this reaction is quickly dissipated as reaction heat, but, in case of a electrochemical cell this energy is transformed in electrical energy.

In case of electrochemical cell, both half-cells are designed to contain the oxidation half-reaction and reduction half-reaction separately.

At anode, the oxidation of zinc occurs according to:

     Zn (s)                            Zn⁺² (aq)  +  2e^-

During the oxidation of zinc, the zinc electrode will slowly dissolve to produce zinc ions (Zn⁺²), which enter into the solution containing Zn⁺² (aq) and SO₄^-2 (aq) ions.

At cathode, copper reduction occurs according to:

     Cu⁺² (aq)  +  2e^-  Cu(s)

When the reduction of copper ions (Cu⁺²) occurs, copper atoms accumulate on the surface of the solid copper electrode or on the pot walls.

The reaction in each half-cell does not occur unless the two half cells are connected to each other. Practically, the difference between various types of Daniel cells regards the different connection between half cells.

In case of porous pot, it is accepted that pot itself permits the change of ions between copper and zinc sulfate solutions.

In case of gravity cell, the density difference between solutions permits a certain circulations of ions between lower and upper part of pot.

In case of salt bridge, this prevents the spontaneous mixing of the aqueous solutions in each compartment, but allows the migration of ions in both directions to maintain electrical neutrality. As the oxidation-reduction reaction occurs, cations ( Zn⁺²) from the anode migrate via the salt bridge to the cathode, while the anion, (SO₄)^-2, migrates in the opposite direction to maintain electrical neutrality.

The two half-cells are connected externally and electrons provided by the oxidation reaction are forced to travel via external circuit to the site of the reduction reaction. The reaction occurs spontaneously and therefore electromotive force (e.m.f) is measured in external circuit.

Myth and reality in Daniel cell working

In up presented description it is necessary to delimitate salt bridge cell for other variants of Daniel cell.

I don’t think one of all this writers of physics or chemistry books had the curiosity to see what energy produce a salt bridge cell by comparison with other variants of Daniel cells.. The present text analyzes at least in a qualitative manner the gravity cell and salt bridge cell.

It is very strange how a gravity cell works without any barrier between zinc and copper sulfate solutions. The ,,density” explanation, accepted by actual orthodox physics, is so absurd that does not worth to be discussed. A simple experiment without this copper and zinc sulfate solutions stratification can prove that a current is obtained in this case too. More elaborate experiment using a solution a NaCl instead of ZnSO4, where NaCl solution has the same density like CuSO4 solution will confirm the previous affirmation. The solution stratification is only a ,,theoretical explanation” offered for hiding the reality of an unexplained phenomena under a ,,hypothetical possibility”.

In practice, there are no conditions for this solution separation based on density. In fact a lot of these cells were built without any ZnSO₄ solution inside. Electrodes are fixed in certain position with Zn electrodes in upper part and Cu electrode in lower part and the pot is filled with distilled water. After that CuSO₄ crystals are dipped into pot and of course the crystals deposit at lower part of the pot. Slowly, they dissolve in water, and there is a gradient of concentration between lower and upper part of pot. As copper sulfate arrive in upper part of pot, a reaction between metallic Zn and copper sulfate occurs, with a delivery of an electric current between Cu and Zn electrodes.

A special attention must be granted to so called Daniel cell with a salt bridge. In gravity cell as time passes the zinc electrode diminishes in size, while the copper electrode increases in size, and the blue colour of the copper sulphate solution slowly fades. By comparison, in case of a cell build with a salt bridge no such visual effects are observed. Even the cell is short circuited and lived for weeks in this state, no consume of Zn is observed, no copper is deposited, no visual change of CuSO4 solution is observed.

If electric current delivered by a gravity cell and a bridge cell in quite identical conditions (the same size of electrodes, the same concentration of sulfate solutions) other surprising facts are observed. In case of a gravity cell, a current of about 25 mA is measured and the current is quite constant at least for half an hour. After that due to Zn electrode and CuSO4 solution consume, the current start to decrease going to zero when the Zn electrode is totally consumed.

By comparison, the bridge cell start with a current of 0,5 mA and after few seconds the current starts to decrease. After half an hour the current is about 0,1 mA and it becomes stable after another hour at about 0,06 mA. The current remain stable at this value for weeks, even the cell is short-circuited for all this time.

In proposed theory, the bridge cell is totally different from Daniel cell, because there are no conditions for a redox reaction able to power the cell. The concepts of electric current and the electrode phenomena need a drastically revision as was underlined with other occasions.

DANIELL CELL AND SALT BRIDGE

(The old material before 30 august 2009)

Daniell cell was considered a great improvement over the voltaic cell.

A variant of Daniell cell called gravity cell, was renamed as the Exchange Telegraph Cell because it was used by the Exchange Telegraph Company.

The purpose of this material is to demonstrate that a Daniel cell with a salt bridge does not work and consequently is only an imitation of original Daniel cell. In the modern representation of Daniell cell (found in every book of physics and chemistry), there is no reaction between components. Practically this Daniel cell is like a toy car for children. Has the same appearance, but the ,,engine” to work properly is missing.

Let’s analyze what does not fit in actual explanation of Daniel cell modeling.

In fig. 1 a simple Daniel cell similar with any other picture from a low level school is presented. The salt bridge is represented by 4 KCl species, for a simple description of phenomena.

In order to work properly, Daniell cell require a compensation of ions from the salt bridge as described in fig.2. Chloride ions compensate the Zn cations and potassium ions compensate sulfate anions in the other compartment. After a time it should exist a depletion of KCl in the bridge (in the picture are remaining 2 molecule instead of 4).

Figure 1. Daniel cell as represented in any scientific text

Figure 2. Daniel cell as represented in any scientific text - working

If this principle of cell working is correct, working with finite quantity of salt in the bridge it should be observed a depletion of this quantity, and formation of other compound in the compartments.

The study of trace components can be made with actual technological progress in order to identify if this reaction take place in reality.

But as will be described in the experimental part there are other and simpler possibilities to check the mechanism of cell working.

If the quantity of KCl in the bridge is varied, the current must depend on salt concentration. This means at low quantity of salt it will mean a low current, and a high quantity will mean a higher current. This is not the case when experiment is performed.

Let’s estimate the time of life for a Daniel cell in short circuit mode and the consequences of a pile working (the description of experiment is below at experimental part). As salt in the bridge was used a NaOH solution captured in a agar agar gel as described in below experiments.

The resistance of salt bridge is about 16 kΩ and the resistance of solution about 4 KΩ. Let’s exaggerate and consider that internal resistance of battery is 50 KΩ. In this condition considering the external circuit of resistance zero the short-circuit current is:

I =U/(R+r)= 1,07/50 000=21 micro A.

A little bit deeper into actual physic:

I = Q/t = ne/t

Where n = number of charge in our case electrons

e the charge of electron

t – time of working in short-circuit

The actual Daniel battery is working in short circuit already from 8 days which means 691200 s.

The number of electrons which has already passed by external conductor is:

n = I*t/e=6,912/1,6 * 10 exp (-19) =0,9072 *10 exp(20) electrons

These numbers of electrons are coming from the Zn dissolution reaction

Zn (s) → Zn^{2+ +} 2e^-

Which means a number of 0,4536*10 exp(20) atoms of Zn are already in solution.

For this number of Zn cations a double number of hydroxyls ions must come from salt bridge.

Mass of Zn passed in solution is :

0,4536*10 exp(20)*65,37/(6,023*10exp(23)) =4.92 mg

Let’s calculate the size of hydroxyl quantity present in the cell

For the salt bridge 5 ml of 0,1 M NaOH were diluted to 75 ml agar agar solution.

After that the tube was filled with 20 ml of this solution.

The quantity of NaOH in 5 ml solution is: 20 *10 exp (-3) g

In the tube there is a quantity is 5 *10 exp (-3) g

From this mass the mass of hydroxyl ions are 5*17/40 = 2.125 mg

The stoechiometry of reaction:

Zn²⁺ + 2HO^-= Zn(OH)₂

65,37 mg 2* 17 mg

4.92……… y

y = 2,55 mg

So in order to neutralise the 4,92 mg of Zn²⁺ there are necessary 2,55 mg OH^-.

But the total quantity of hydroxyl available in the entire bridge is only 2,125 mg.

Considering that all this quantity is flowing into Zn compartment, there will remain still an excess of Zn cations already moving into solution and searching for anions able to maintain the neutrality of solution. Of course in the Cu compartment there is excess of anions moving into solution and searching for cations able to maintain the neutrality.

The cell is still working at full potential after 8 days, and this means there is a transfer of cations and anions from one compartment to another through salt bridge.

Can actual science confirm this?

In the same time if all hydroxyl from the bridge is passed into solution the Zn compartment should have an alkaline pH.

This is not confirmed by direct measurements - both compartment are at pH about 5, in acid region and not alkaline.

How is possible for formed Zn(OH)₂ to have a week acid pH?

Further analysis must regard the formation of Zn(OH)₂ precipitate.

The product of solubility of Zn(OH)₂ is K_SP = 3 *10 exp (-17) so it is very easy to be observed visually a increasing of hydroxyl concentration.

It reality the Zn(OH)₂ precipitate does not appear even in other repetitions of experiment the concentration of NaOH was increased to 4M.

In some contradictory discussions with other physicists it was argued that only sulfate migrate through the cell in order to have a chemical reaction at Zn electrode.

Starting from the same number of KCl species as is represented in first picture, let's analyze this possibility, more precisely only the sulfate is moving.

As is presented in the fig. 3, Zn react with chloride coming from the salt bridge.

Another sulfate take place of chloride and a sulfate is going up in the bridge.

At Cu electrode, simultaneously a copper atom is deposited (not figured in the picture). In solution a SO₄ anion remain free. Potassium goes down from the salt bridge and compensates the charge of the sulfate.

Who is going up to compensate the charge of chloride?

Is copper dissolved again from the electrode?

In the same time this kind of movement does not work properly because a layer of K₂SO₄ is formed and it blocks the flow of chloride. If a chloride anion from inside the bridge must remove another hundreds of sulfate ions in order to arrive at the baker solution... there is no energy to remain for having an external electrical current.

Figure 3. Daniel cell – sulfate pumping

The problem with the Daniel cell having a salt bridge in its constitution regards the absence of a chemical reaction. In this case only a static (contact) difference of potential is measured which lasts forever. The subject is detailed in the book, due to its relative importance from theoretical point of view.

If someone mix Zn, ZnSO4 and KCl (CaCl2, NaOH, etc) no reaction take place.

The same if Cu and CuSO4 and KCl are mixed.

The original Daniell cell, invented in 1836 by John Frederic Daniell, does not have a salt bridge, even the ingredients are quite the same.

From simple elementary school manuals to the high scientific treatise, the Daniel cell is presented with a salt bridge ….but this cell has never worked and will never work.

3. 1 Daniell cell - Experimental part

Experiment 1

Materials for the experiment:

ü Cu and Zn metals strips;

ü CuSO4 and ZnSO4 as 1M solutions;

ü Salt bridges made after the following recipe:

ü 3 g of agar-agar are boiled in 250 ml deionised water.

The entire volume is divided in four and to every quarter is added:

1) NaOH 0,1M or higher concentrations - aprox 5 ml

2) CaCl2 about 0.1 g

3) CaCl2 about 2 g.

4) CaCl2 about 2 g.

In this way 4 different salt bridges are made.

4 tubes of PVC (or another material) are curved in a U shape and filled with up presented solution. At the ends of the tube some glass wool is fitted in order to have a good contact between bridge and solution.

The tubes are leaved to cool down and after the gel apparition, the bridge is ready.

The 4^th bridge is a repetition of 3rd bridge, but instead of glass wool at the ends of the tube, 2 pieces of graphite recovered from some old carbon zinc battery are used as presented in down figures.

The experiment is designed to check the movement of the ions from bridge to solution.

First, the diluted CaCl₂ bridge is used. The potential registered is about 1,1 V. In solution there is no confirmation of a precipitate apparition after 2 hour of working with a multimeter connected to it.

Another cell is formed with concentrated CaCl₂ solution. The potential registered is about 1,1 V. In solution there is no confirmation of a precipitate apparition after 2 hour of working with a multimeter connected to it.

The third cell is formed with a NaOH bridge. Before bridge insertion and after 2 hours of cell working the pH is measured with pH paper. Before bridge insertion the pH of ZnSO₄ was approx 5 and the pH of CuSO₄ was about 9. After two hours the same values were registered.

The 4^th cell is formed with a bridge having a ,,mixed” conduction. The graphite is an electronic conductor and the gel is an ionic conductor (according to actual classification). The contact between bridge and CuSO₄ and ZnSO₄ solutions is made only by graphite portion of the bridge. The potential registered is about 0,8 V.

How can be interpreted these results?

If an ion circulation is necessary for cell working, the calcium ion should pass in sulfate solution. Similarly chloride ion should pass in the other baker. A reaction between sulfate and Ca should occur and a precipitate should appear. In the first case the concentration of CaCl₂ is to low to arrive to a product solubility of CaSO₄, but in time the salt bridge should be exhausted. In reality even after days of working the cell shows the same potential

How is cell working in this case?

In case of concentrated CaCl₂ bridge, in the CuSO₄ baker should appear a precipitate of CaSO₄. The quantity of Cl and Ca gained by every solution can be determined analytically. After two hours of cell working a small quantity of Cl can be put in evidence using AgNO₃ as reagent. The CaSO₄ precipitate does not appear. The diffusion of CaCl₂ from the bridge in solution is due to the Fick diffusion and not due to an electrochemical transport. After long time of cell working the quantity of Cl does not increase in the solution.

In the third case the pH of the solution should appear basic in a Zn compartment, and also Zn(OH)₂ should precipitate, which is not the case.

In the same time the logical aspect of problem is not solved.

For every atom of Zn which passes in solution, two hydroxyls ions should come from bridge. In the bridge there are not enough ions (5 ml of NaOH 0.1M diluted to 75 ml agar agar solution) to satisfy this necessity.

Do ions circulate from a baker to another?

What is the mechanism of this circulation?

In the fourth cell is completely inexplicable how an ion can pass through a graphite material.

Experiment 2

Materials for the experiment:

Cu and Zn metals strips;

CuSO₄ as 1M solutions;

H₂SO₄ as 1 up to 3 M solution.

The purpose of experiment is to dissect how a ,,formal” Daniel cell works and the importance of every electrode in the process.

For beginning the simplest electric cell is build using CuSO₄ as electrolyte and a strip of Cu and Zn inserted into solution.

At immersion the quite normal potential (1.054) is displayed. After few seconds from immersion, something strange begins to manifest. The Zn strip starts to become dark (even black). It was considered this only a visual effect due to the dissolution of Zn atoms from strip. But, at a detailed analysis, it can be observed that Cu ions from solution form a deposit on the Zn strip. At the Copper electrode there is no visual sign of any chemical reaction.

This is a picture of cell after one hour of working with voltmeter connected to it.

After another 2 hours the electrodes in glass appear as follows:

After about 4 hours the Zn electrode is completely disintegrated, but in its region a brown-dark fluffy compound remain.

When this effect was observed quite long time ago ( about 3 years), it was considered a experimental error.

But, numerous repetition of experiment convinces about the reality of this effect.

Of course the effect is the same if the compounds are changed. For example putting in a H₂SO₄ solution a Cu and Zn strip connected to a voltmeter, the hydrogen is developed at Zn electrode too.

Faced with this problem, in evident contradiction with known percepts of electrochemistry - oxidation and reduction took place at different electrodes-, a reasonable explanation was searched.

The deposit remaining in the Zn region was separated, washed with diluted nitric acid in order to eliminate the traces of Zn and after that reacted with concentrated nitric acid. As anyone can suppose, the deposit dissolves in concentrate nitric acid, so the deposit is formed by copper.