A Hofmann voltameter is an apparatus designed for electrolysing water, and it was invented by August Wilhelm von Hofmann in 1866. This intriguing device consists of three interconnected upright cylinders, typically made of glass. Let me break down its components and how it works:

1. Cylinders: The Hofmann voltameter comprises three cylindrical chambers joined together. These cylinders are usually made of glass.

2. Inner Cylinder: The innermost cylinder is open at the top, allowing for the addition of water. This water contains a low concentration of a compound (such as sulfuric acid) to enhance conductivity and complete the electrical circuit.

3. Electrodes: Inside the bottom of each of the two side cylinders, there’s a platinum electrode. These electrodes are connected to the positive and negative terminals of an external source of electricity.

4. Electrolysis Process: When an electric current passes through the Hofmann voltameter, two significant reactions occur at the electrodes:

   • At the anode (positive terminal), gaseous oxygen forms.
   • At the cathode (negative terminal), gaseous hydrogen forms.

5. Gas Collection: The produced gases displace water and collect at the top of the two outer tubes. These tubes are often graduated, allowing the volume of gas to be measured.

6. Historical Use: Before the invention of the ammeter, voltameters were commonly used to measure direct current. According to Faraday’s law of electrolysis, the amount of metal electroplated onto the cathode (iron or copper electrodes) is directly proportional to the total coulombs of charge transferred. Thomas Edison even used voltameters as electricity meters.

7. Stoichiometric Demonstration: The Hofmann voltameter serves as a powerful teaching tool. The 2:1 ratio of the volumes of hydrogen and oxygen gas produced by the apparatus illustrates the chemical formula of water (H₂O). However, this ratio assumes that both hydrogen and oxygen gases are diatomic. If hydrogen were monatomic and oxygen diatomic, the gas volume ratio would be 4:1.