Introduction

The Hofmann electrolysis apparatus is used to decompose aqueous solutions of electrolytes using an electric current. Different reactions are observed at the cathode (negative electrode) and the anode (positive electrode). The amount of gases produced at each electrode may be compared and collected for further testing.

Materials

Safety Precautions

Do not operate the power supply with wet hands or in a wet area. Be sure that the working area is dry, and check the glass for chips and cracks before running the current. Wear chemical splash goggles and chemical-resistant gloves when working with the Hofmann electrolysis apparatus. Please review current Safety Data Sheets for additional safety, handling  and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of electrolyte solutions.

Procedure

Parts

1. Mouth
2. Reservoir
3. Stopcock (the picture shows it in the vertical, “open” position)
4. Clamp
5. Platinum electrode
6. Stopper
7. Contact point for connector cords

Setting up the Hofmann Apparatus

1. Set up a support stand with two support clamps.
2. Insert the rubber stoppers containing the electrodes tightly into the bottom of the graduated side arms (as shown in Figure 1).
3. Position the clamps about 6 inches apart on the support stand and secure the clamps to the center of the Hofmann apparatus.
4. With the stopcocks open, add the electrolyte solution slowly through the mouth of the apparatus until the graduated side arms are filled up to the base of the stopcocks. Approximately 60 mL of solution is needed. Note: Do not allow liquid to flow into the glass tips above the stopcocks. The center tube should not be filled completely—it will fill with more liquid as gases are produced at the electrodes and liquid is displaced from the side arms.
5. Close the stopcocks. 
6. Clip one end of the connector cord to the positive terminal of the battery or power supply. Connect the alligator clip on the opposite end of the same cord to one of the electrode connection points. This is the anode.
7. Clip one end of the other connector cord to the negative terminal of the battery. Connect the alligator clip on the opposite end of the same cord to the other electrode connection point. This will be the cathode.
8. If a DC power supply is used, turn it on.
9. Depending on the nature of the electrolyte, gas bubbles may begin to form at the electrode(s).
10. Disconnect the alligator clips from the Hofmann apparatus or turn off the power supply to stop the electrolysis reaction.
11. Compare the volume of gas collected at each electrode.
12. (Optional) Collect the gases that have been produced by placing gas collecting tubes over the stopcocks and open the stopcocks to release the gases (see Tips).
13. (Optional) The collected gases may be identified using glowing splints, burning splints or a match test depending on the gases present.

Teacher Tips

• In an electrolytic cell, the positive electrode is the anode (the site of oxidation), and the negative electrode is the cathode (site of reduction). The battery or power supply acts as an electron pump “pushing” electrons into one electrode and “pulling” them from the other electrode. Electric current flows through the electrolysis solution via the migration of ions. Anions move towards the anode, cations move toward the cathode.

• The most common electrolyte for electrolysis solutions is sodium sulfate. The rate of electrolysis increases as the concentration of sodium sulfate increases. Use a 1 M or a saturated solution for best results. A 1 M sulfuric acid solution gives good results in a quantitative electrolysis experiment (determining the value of the Farway constant). Electrolysis of aqueous sodium chloride solutions produces chlorine at the anode (oxidation of chloride ions competes with oxidation of the oxygen atom in water, which is a kinetically slow reaction).
• Ideally, solutions added to the Hofmann apparatus should be 20 °C at the time of electrolysis.
• Compare results between different concentrations of the same solutions.
• Luer lock syringes will not fit directly onto the stopcock tips for gas collection. Connect the stopcock tips to a syringe using a short piece of latex tubing. A test tube with a 1-hole rubber stopper will also work. When removing the test tubes from the stopcock, keep a finger over the hole in the stopper to prevent gas loss.
• Indicators may be added to the solutions in order to observe pH changes at the anode and cathode during electrolysis.
• This apparatus is fragile, store in original packaging or wrapped in protective material such as bubble wrap.

References

Please refer to Electrochemistry, Flinn ChemTopic™ Labs, Volume 17, for additional electrolysis experiments.