Purify Sulfuric Acid by Distillation

In this video we purify sulfuric acid by distillation using a heating mantle we built in a previous video.

The reason why we need a special heating mantle is because most don't get hot enough to boil sulfuric acid.

Click here for our original video on how to make a high temperature heating mantle:

   • Make a High Temperature Heating Mantle  

The heating mantle in the video is connected to a variac autotransformer off-camera. But a light dimmer switch can provide the same power control for a fraction of the cost. Alternatively, a PID controller may be used.

In this heating mantle, a 100 mL flask is placed that has been half way with dirty sulfuric acid from drain cleaner. This is connected to a Liebig condenser via a 90 degree elbow joint that's been lubricated with high-temperature grease. A receiving beaker is placed at the end of the condenser to collect the distillate. Water is circulated through the condenser and the power is turned on.

The power is slowly ramped up until refluxing is observed in the round bottom flask.

At first, water is condensed as well as any volatile impurities. To tell when concentrated acid begins condensing, occasionally apply a few drops of the distillate to a tissue. if the distillate is mostly water then the tissue will just get wet. When the distillate is concentrated acid, the tissue will decompose. The receiver beaker is changed for an empty one and the acid is collected.

At this point the acid is concentrated, but not yet the typical 98% concentration of azeotropic sulfuric acid. To know when this point is reached, observe the receiver and note when the collected distillate is refracting light. If it is then the acid is still increasing in concentration and density and the receiver should be changed out. When the receiver is collecting acid that is homogeneous without refracting light then the acid is of uniform density corresponding to the azeotrope of 98%.

Keep collecting until about 20% of the dirty acid remains in the round bottom flask. It's not recommended to continue as the acid tends to bump at this point.

Turn off the power and allow the apparatus to cool.