This year’s award winners will be receiving 500 ml Erlenmeyer flasks made of borosilicate glass, etched with Armour Etch. There is some interesting chemistry involved!
Borosilicate glass is made of silica (SiO2), sodium oxide (Na2O), aluminum oxide (Al2O3), and boron oxide (B2O3) giving it a very low coefficient of thermal expansion and making it more durable and resistant to chemical corrosion than soda lime glass (about 90% of common glass is soda lime glass).
Pure silica has a high melting temperature and is difficult to work with. Soda lime glass is composed of ~70% silica, ~15% soda (sodium oxide), and ~9% lime (calcium oxide), with smaller amounts of other compounds. The soda decreases the melting point of silica but results in increased water solubility and therefore lime is added to increase chemical durability.
Armour Etch contains barium sulfate, sulfuric acid, ammonium hydrogen difluoride and sodium hydrogen fluoride. The centrosymmetric triatomic hydrogendifluoride anion features the strongest known hydrogen bond, with a F−H length of 114 pm and a bond energy greater than 155 kJ/mol.(Wikipedia)
In solid NH4HF2, each ammonium cation is surrounded by four fluoride centers in a tetrahedron, with hydrogen-fluorine hydrogen bonds present between the hydrogen atoms of the ammonium ion and the fluorine atoms.
The ammonium hydrogendifluoride attacks the silica component of glass:
SiO2 + 4NH4HF2 → SiF4 + 4NH4F + 2H2O
The removal of minute amounts of glass causes the characteristic rough surface and translucent quality of frosted glass. The mechanism involves hydrofluoric acid, which is known as a weak acid, but is extremely hazardous. HF differs from other acids because the fluoride ion readily penetrates the skin, causing the destruction of deep tissue layers. This process may continue for days if left untreated. Fluoride ions form insoluble salts with calcium and magnesium in bodily tissue. Soluble salts can form with other cations, which dissociate rapidly causing further disruption and damage to tissue. The severe, throbbing pain associated with HF burns is thought to result from nerve irritation due to potassium cations entering the extracellular space to compensate for reduced calcium ion concentrations.
Fluoride poisoning is associated with hypocalcemia (low calcium levels), hyperkalemia (high potassium levels), hypomagnesemia (low magnesium levels), and sudden death. Systemic hypocalcemia should be considered a risk whenever the body surface area of skin burns from concentrated HF exceed 160 cm2, or about the size of the palm of your hand. Concentrated HF burns can be fatal if only 2% of the body surface area is exposed.
(Harvard University, Guidelines for the safe use of Hydrofluoric Acid)
Molecular models provided by Ryler Enterprises: https://www.rylerenterprises.com