hoffman voltameter

I think graphite electrode can be used for electrolyzing water. Which electrode to be use is subject to what you want to be electrolyzed and the condition.

The ions that are successfully released at the electrodes depend on three factors

1.The position of the ion in the electrochemical series
2.The concentration of the ion in the solution
3.The nature of the electrode

1. The position of the ion in the electrochemical series
This is probably better expressed as the position of the redox equilibrium in the electrochemical series. All the redox equilibria are expressed as reductions going from left to right, for example:

Cu2+ + 2e- --> Cu

However, going from right to left this would be an oxidation. Hence it may be called a redox equilibrium.

The Copper ion | Copper equilibrium has an electrode potential of +0.34 V. This is more positive than hydrogens electrode potential of 0.00 V. This means that copper ions are more easily oxidised than hydrogen ions (remember that as we go down the series the redox equilibria have better oxidising properties i.e they can remove electrons from other things more easily - including electrodes) . Consequently in a competition between the two ions the copper ions will preferentially pick up the electrons.

As a rule of thumb, if the metal appears below hydrogen in the electrochemical series then it will be preferentially deposited.

2. The concentration of the ions
When two ions with similar reactivity are in competition then the relative concentration of the two ions becomes an important factor.

A good example of this is the electrolysis of sodium chlorides. When the chloride concentration is large the chloride ions lose electrons and chlorine gas is released at the electrode, but when it is in low concentration the hydroxide ions from the water are preferentially released

3. The nature of the electrode
Usually, inert electrodes such as graphite or platinum are used for electrolysis. These electrodes do not interfere with the reactions occuring at the surface of the electrode, they simply act as a point of connection between the electrical circuit and the solution.

However, if metal electrodes are used in metal ion solutions they can get involved in the reactions by dissolving as ions, leaving their electrons behind (this can only happen when the metal takes the place of the anode, the positive electrode) - this is called electrode participation

Summary
Reactive metals (more reactive than hydrogen) are never deposited during electrolysis of aqueous solutions. If the metal ion comes from a metal more reactive than hydrogen then hydrogen gas is liberated at the cathode.

Halide ions (chloride, bromide, iodide) are released preferentially and if these are not present, the hydroxide ions from the water are released at the anode.

Unreactive metals, such as copper or nickel, may participate in reactions at the anode.


The Hoffman Voltameter, was invented (how surprisingly) by Mister Hoffmann in the 19th century. It’s an apparatus for electrolyzing water. It consist of three, connected, vertical tubes. You can close the two tubes on the sides to collect the gasses which are formed, through the tube in the middle you can fill the Voltameter with an addition of water and an ionic compound in order to improve the conductivity.

When current is run through the Hoffman Voltameter, oxygen(gas) is formed at the anode and Hydrogen(H2 gas) is formed at the cathode. The electrode used to be of platinum but now a days they are often made of graphite. Each gas will displace water and collects at the top of the tubes. This is how we measured how much H2 was formed.

I hope this can help your understanding. :)

Because Hoffman is so rich that his voltameter is made with platinum and not with carbon, maybe diamond, but in this part conductivity is concerned and only platinum is used.