Squeaky clean

There comes a point in every chemist’s life when clean NMR tubes are ever more difficult to come by and the gigantic mountain of unwashed glassware becomes hard to ignore. Washing glassware is hardly anyone’s favourite activity; trying to avoid this tedious task can turn even the most honest chemists into glassware thieves, who ‘borrow’ from tidier colleagues.

Working with organometallic complexes, failed reactions left my flasks more often than not with shiny metal mirrors or rock-solid black residues, tempting me to throw the whole setup in the bin rather than clean it. But giving up easily on my favourite pieces of glassware would’ve felt like betraying an old friend. After all, when setting up a reaction that stubbornly failed to yield anything for weeks, what other vessel would I choose than my one ‘lucky’ round-bottomed flask?

Fortunately, there are a few options to remove stubborn stains when detergent, a good scrub and some acetone just won’t do it. One of them is to render stubborn contaminants soluble by changing their oxidation state. Piranha solution keeps true to its name; the mixture of sulfuric acid and hydrogen peroxide is a powerful oxidant that eats almost anything, even elemental carbon. However, it is difficult to handle – often heating vigorously upon preparation – and not very good with metals.

Aqua regia, the famed ‘king of acids’, a mixture of nitric acid and hydrochloric acid, is slightly tamer than piranha, though no less corrosive. Freshly prepared, it dissolves even noble metals such as gold and platinum. Over time it loses its potency, decomposing into gaseous nitrosyl chloride and chlorine – the reason to never keep aqua regia in a tightly closed container. Finding a Winchester bottle containing the communal aqua regia waste broken, the acid not only having flooded the fume hood but also having started to dissolve the metal cabinet below, is hardly the best way to start a Monday morning.

When facing even more indelible stains, it is time to bring out the heavy cleaning artillery – things that will literally remove the glass layer the stains are stuck to. Borosilicate glass is fairly resistant, but certain chemicals like hydrofluoric acid or base bath mixture (potassium hydroxide in isopropanol) manage to scratch its surface. Having fostered a perhaps irrational fear of HF since I had to use it as an undergraduate chemist under the watchful eye of my supervisor, wearing three layers of gloves and a full-face splash mask, base baths were my method of choice. Most stains come off easily after soaking glassware overnight in a bucket of the caustic mixture – making this option almost feel like a lazy way out. Clean glassware, however, meant a dirty base bath, and when it was time to renew the mixture, finding your washing up in the brown opaque slurry became something of a treasure hunt.

Sadly, something as essential as glassware cleaning isn’t part of the chemistry education curriculum and there is surprisingly little reliable literature on the topic. Most of my collective knowledge has been acquired from the occasional tip by a senior group member who has watched me struggling for some time with a brush and some apparently unwashable glassware. This makes the best cleaning procedures somewhat legendary, not dissimilar to voodoo and magic – never written down but passed down through generations of frustrated chemists faced with the never-ending quest for clean flasks.