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Electrolysis : Electrolysis Of Sodium Chloride Solution

Although the liberation of chlorine at the anode in the electrolysis of common salt could be explained by Berzelius's dualistic theory, since the oxygen usually liberated with salt solutions (Expt. 1) could be assumed to form oxymuriatic acid (chlorine) with the muriatic acid set free, the facts are better explained by assuming that sodium and chlorine are the actual products of electrolysis: NaCl = Na + Cl.

Expt. 2. - Repeat Expt. 1, with a solution of common salt in the U-tube. The litmus around the negative pole is turned blue, from liberation of caustic soda, but that around the positive pole is bleached, indicating that chlorine is evolved. Hydrogen is evolved from the negative pole.
It appears as if the salt is decomposed with liberation of chlorine, and the sodium set free at the negative pole then reacts with the water to give caustic soda and hydrogen, which are actually liberated at that pole. The primary production of sodium at the negative pole can, in fact, be demonstrated.

Expt. 3. - Pour mercury into a glass tube having a platinum wire sealed through the bottom.
Electrolysis with mercury cathode

Fig: Electrolysis with mercury cathode

Fill up the tube with sodium chloride solution, and connect the wire with the negative pole of a battery of two accumulators. The positive pole is connected with a piece of platinum foil dipping into the solution. The liquid soon smells strongly of chlorine, but very little gas is evolved from the mercury. The sodium liberated dissolves in the mercury and forms an amalgam. After a few minutes stop the experiment, and pour the mercury into water. Bubbles of hydrogen are evolved, and the water turns red litmus blue, showing that sodium was present, which reacts with the water.
It may be assumed that the current in Expt. 2 causes the deposition of sodium and chlorine at the negative and positive poles respectively. The atoms of chlorine combine to form molecules of chlorine gas, which is evolved. The atoms of sodium at once react with the water present, forming caustic soda and liberating hydrogen, which is evolved:

Neg. pole H2 + 2NaOH <- 2H2O + 2Na <- 2NaCl -> 2Cl -> Cl2 gas Pos. pole.

The primary products of the electrolysis are thus supposed to be sodium and chlorine; the sodium reacts with the water to give hydrogen and caustic soda, which are secondary products.

J. F. Daniell, of King's College, London, suggested in 1840 that the decomposition of all salts proceeds in this way, and that the acid and base, regarded as primary products by Berzelius, were really secondary products. Sodium sulphate he regarded, following Davy, as a compound of sodium and the radical SO4, instead of a compound of soda, Na2O, and sulphuric anhydride, SO3, so that its formula is Na2SO4. This is decomposed by the current, primarily, into its two radicals, which then react with water to form soda, sulphuric acid, and the two gases hydrogen and oxygen:

Neg. pole H2 + 2NaOH <- 2H2O + 2Na <- Na2SO4 -> SO4 + H2O -> H2SO4 + O gas Pos. pole.

All salts were therefore, constituted on the same plan as common salt whereas the latter was regarded by Berzelius as an exceptional type. This theory was extended by Darnell to the acids were salts of hydrogen. Daniell's theory was generally adopted, and the theory of Berzelius was given up. The theory of primary and secondary products of electrolysis just given has been modified as a result of experiments of Le Blanc. He considers that the liberation of hydrogen and oxygen from sodium sulphate solution occurs owing to a primary decomposition of water, the other constituent of which forms the alkali and acid, respectively, at the two electrodes. The hydrogen and oxygen (the latter really coming from a secondary reaction between discharged OH) are thus primary products. This theory will be explained in greater detail later.

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