Expt:Preparation of "chamber acid"

Purpose

As an instructor demonstration, to demonstrate the potential of concentrated sulfuric acid to cause serious tissue burns.

Principle behind the experiment

The dilution of concentrated sulfuric acid in water is highly exothermic. The greatest heating effect that can be safely achieved is the preparation of 10 M sulfuric acid, formerly known as “chamber acid”. Borosilicate glass is an extremely poor conductor of heat, allowing the demonstrator to instructor to safely hold the neck of a conical (Erlenmeyer) flask containing liquid at about 80 ºC (170–180 ºF).

Experimental

Materials and apparatus used

• Concentrated sulfuric acid (ca. 98%)
• Water (tap water is adequate for demonstration purposes)
• two 100 mL measuring cylinders
• one 250 mL borosilicate glass conical (Erlenmeyer) flask

Necessary planning

The demonstration should be incorporated into the normal safety presentation. As there is a necessary pause of about thirty seconds (or more, if desired), the instructor should have suitable discussion to cover this “gap”.

Procedure

Approx. 80 mL of water is measured out in one of the measuring cylinders, and placed in the conical (Erlenmeyer) flask. Approx. 80 mL of concentrated sulfuric acid is measured out in the other measuring cylinder. The sulfuric acid is added to the water at moderate rate (neither too fast nor too slow) with slight agitation of the flask. After the addition is complete, the flask is gently agitated to ensure thorough mixing, then set aside for at least thirty seconds.

After thirty seconds (or longer if desired), the instructor takes the flask by the neck and demonstrates to the students that they should gently and briefly touch the outside of the flask (the part containing the liquid). The demonstrator then takes the flask to each student in turn, to allow them to touch the outside of the flask gently and briefly.

It is important that the students realize that they should not try to grasp the flask (nor even touch it for too long) as the surface temperature in the part containing liquid is about 70 ºC (160–170 ºF), well above the pain threshold for prolonged contact but safe for very brief contact.