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Electrolysis of Molten Sodium Chloride

Electrolysis of Molten Sodium Chloride

Background and actual explanation

In the example we will use the most common of the salts, sodium chloride. Solid sodium chloride, in normal condition, does not conduct electricity, because there are no electrons which are free to move.
However, molten sodium chloride does. According to actual interpretation, when sodium chloride and heated and melted, the sodium ions and the chloride ions can separate from one another somewhat, and they are free to move throughout the liquid.
Let’s analyze in detail the phenomena of molten NaCl electrolysis in an electrolytic cell. The cell is driven by a battery or some other source of direct current. The battery acts as an electron pump, pushing electrons into one electrode and pulling them from the other. The electrode from which the electrons are withdrawn is labeled as positive. The one receiving the electrons is labeled as negative (fig.1).

moltensalt001Figure 1 Electrolytic cell

The following equation represents the breaking apart of NaCl(l):
2NaCl(l) → 2Na(l) + Cl2 (g)
The half-reactions involved in this process are:
Reduction: 2Na+(l) + 2e- → Na(s) E° = -2.71 V
Oxidation 2Cl-(l) → Cl2 (g) + 2 e- E° =-1.36V

net voltage required = - 4.07V
The negative sign of voltage tells us that the overall reaction will NOT be spontaneous, and a minimum of 4.07 volts will be required for this reaction to occur.

Experimental part

The experiment has as purpose to measure the conductibility of molten NaCl at a potential lower then 4V, when no chemical reaction takes place at electrodes. 

In this purpose a ceramic crucible is filled with NaCl and in the solid NaCl two inox electrodes are inserted. The electrodes are home made inserting two inox materials into two glass tube, which serve as mechanical protection and insulator {fig.2). In the experiment NaCl of chemical purity was used, but the results are the same with kitchen NaCl.
Electrodes are connected to a series circuit formed by a alkaline battery of 1,5 V, and ammeter. Having solid NaCl in the crucible, the ammeter registers a null current (0 mA) fig.3.
The crucible is heated with a flame coming from metan gas (the flame must be blue in order to have high temperature). After 40 seconds circa, the current starts to flow into circuit, gradually, from 0,2 mA, and arriving to 40 mA after circa 1 minute. After that the increasing of current is less evident, arriving to about 45 mA in another minute. The picture below  with a current of 23 mA, is taken after I removed the flame because I was working alone and I could not heat and make pictures in the same time. 


Figure 2. Detail of the experiment

Figure 3 Experiment details

Experiment interpretation

          It is inconceivable to explain this simple experiment in the frame of actual physics and chemistry. Because if melted NaCl is formed by ions, there is no possibility to discharge these ions at electrodes at a voltage lower then 4 V. In this case the electrolytic cell should comport like a capacitor.

From electrochemistry we know that for NaCl electrolysis approx. 4 Volts are necessary. In our experiment the voltage is lower than value necessary for electrode reactions and for electron transfer, fact confirmed also visually, because no reactions are observed at electrodes. In this case according to actual physics the ions must migrate to electrodes and at beginning the intensity must be great due to the movement of charges in solution; in time around the electrodes are formed charged regions (fig 4.) and intensity of electric current must decrease like in fig 5, admitting a constant velocity of ions in solution. After a time interval the intensity of electric current must became zero and the solution transforms in a capacitor in this conditions.


Figure 3. Charge displacement in solution 



Figure 4. Expected variation for electric current 

The reality is opposite; with a stabilized source, the intensity of current remains indefinitely constant in time. The accumulation of charge around the electrodes and capacitor comportment of solution is not observed in these conditions

The actual theoreticians should choose one option from two possible:
• A electric current can flow through a melted NaCl salt at a potential lower then potential necessary for electrode reaction
• The series of electrode potential is wrong.

In proposed theory, an electric current does not mean a charge displacement. Therefore an electric current can pass through a molted salt without having electrode processes. Of course, even there are not electrode processes, the electrolytic cell has a resistive comportment and some power is consumed into circuit. The Faradays lows of electrolysis need some structural corrections.

Experiment repetition using graphite electrodes in order to avoid any secondary reaction at electrode (for example between chloride and iron, etc)
Using graphite electrode, dry and grounded salt (reagent degree), as is indicated in fig. 4 in absence of heating, the current through system is 0 micro A.


Figure 4

Then the crucible is heated as it can be seen in the fig. 5. It is difficult to believe that NaCl 99% become conductor due to humidity at this temperature. In fact the water is eliminated at maximum 400 C, when the process of melting does not start.


The maximum observed current was about 45 mili A.
In the following picture a part of crucible and ammeter are visible at few seconds after flame removing. The current is about 23 mili A.


About two years after this experiement was done, I have found in Electricite, G. Bruhat, septieme edition, Paris, 1959, that a half a century ago, the phenomena of solution conductibility under the value necessary for electrode reaction, was known. In this book, it is called invisible electrolysis. Of course there is no explanation for this phenomenon.
During time, in order to hide the deficiencies of actual theories, these kind of things were left aside and did not appear at all in eyes of the scientists....