Sometimes the boiling flask ends up with burnt on junk, and normal cleaning methods just don’t cut it…
The Grasshopper’s Guide to Clean Glassware
A Chemistry Koan
A Chemistry Student approached the Master and asked, “Master, how do I achieve enlightenment in Chemistry?” The Master replied, “Wash your glassware.”
Clean glassware is essential in chemistry. The problem is that the tolerance for shmutz varies with the work you are doing, and sometimes a chemist does not know how important clean glassware is to an experiment until it has failed. This document is designed to give an undergraduate chemistry student a brief introduction to what chemists mean by “clean” and how it can be achieved.
There are two broad degrees of clean in chemistry; quantitative and normal. Quantitatively clean glassware is required for the most demanding applications where a quantity is being measured at high precision, such in analytical or physical chemistry. Glassware at this level of cleanliness has no residues (e. g., grease) or other impurities on it. Normal clean glassware is free of large amounts of impurities, but some grease may be tolerated. Glassware that has been cleaned normally is used where high degrees of precision are not required, such as in a synthesis.
The key to cleaning is doing it a timely manner; letting dirty glassware sit for long periods of time guarantees a harder cleaning job. Also take a minute to separate your glassware into a group which requires a higher degree of cleaning and one that does not.
Disassemble your apparatus as soon as possible after you are finished with it. Remove all stopcocks and stoppers from aliition funnels, separatory funnels and the like. Ground glass stopcocks and stoppers will freeze in place if certain reactants (e. g., bases) were used in them. Triple rinse all surfaces with an appropriate solvent to remove traces of solvents and reaction mixtures, place these in the appropriate waste container.
Graduated cylinders, beakers, Erlenmeyer flasks, burets and pipettes that were only used to dispense or store reagents generally only need to be triple-rinsed with a compatible solvent followed by tap water and a final DI water rinse, if desired. Air dry on a drying rack. In some cases you may need to be more thorough, as described below.
Büchner funnels, etc. should be rinsed with an appropriate solvent to remove substances that are clinging to them. Follow this by tap water and DI water rinses and air dry.
Health and Safety Considerations
Even a task as simple as washing glassware at the sink is potentially hazardous. You must wear eye protection, appropriate for the task, at all times. Gloves are recommended, even for general cleaning, if the glassware contained an irritant, lachrymator or toxic material. Before cleaning be sure that any excess reagent has been disposed of properly and the vessel in which it was contained has been triple-rinsed into the waste container.
General Cleaning Procedure
The following steps should be followed for glassware for which a simple solvent rinse is not sufficient. If you need quantitatively clean glassware, these should be the first steps toward this goal, and more aggressive cleaning methods may be required (vide infra).
Degrease your glassware’s ground glass joints by wiping them with a paper towel soaked in a small amount of ether, acetone or other solvent (CAUTION! wear appropriate gloves and minimize your exposure to the vapors).
Place the glassware in a warm concentrated aqueous solution of Alconox, or other detergent, and let sit for several minutes.
Scrub. Be sure that your brush is in good shape before scrubbing (not rusty, bristles are not matted down); replace it if necessary.
Rinse thoroughly with tap water and give a final rinse with DI water.
The water will sheet cleanly off the glass, if it is quantitatively clean. If water does not sheet off the glass, and you desire the glassware to be quantitatively clean, first repeat the above soaking and scrubbing steps. If, after a second cleaning, bits of solid still adhere to the glass, or if there is clearly a greasy residue on the glass, more aggressive action must be taken.
More Aggressive Cleaning Methods
The following cleaning methods are two of the more commonly used ways to remove contaminants from glassware. They are usually used after normal cleaning has failed, and they are often used together, because each is effective at removing different types of contaminants. Care must be taken using either one because of the corrosive nature of the solutions used.
If the contaminant is a metal-containing compound, soak the piece of glassware in a 6 M HCl solution. DANGER! this solution can cause severe burns! Wear appropriate gloves. Once the solid has dissolved, copiously rinse the item with tap water, and then repeat the general cleaning steps above. This method will also remove some organic residues (not grease).
If the contaminant is organic, submerge the item in a base bath (a saturated NaOH or KOH solution in ethanol or methanol). DANGER! the base bath will dissolve skin and alcohols are flammable! Wear butyl gloves and keep ignition sources away from the base bath. Be sure that the piece of glassware is completely filled with the solution and is sitting upright. After several minutes of soaking, carefully remove the item (it will be slippery), and rinse thoroughly. If the glassware is not quantitatively clean at this point, the general cleaning steps may need to be repeated, or a longer soaking time in the base bath, may be needed.
NEVER soak the following items in a base bath for prolonged periods:
Glassware contaminated with metal-containing compounds
Glass fritted funnels
Volumetric glassware (pipettes, volumetric flasks)
Any glassware contaminated by an oxidizing agent
Anything that has not been washed according to the above steps first
Glass fritted funnels and volumetric glassware can be soaked briefly with the base bath solution to remove small amounts of grease, but prolonged exposure to the caustic solution can damage these items.
Even More Aggressive Cleaning Methods
Sometimes 6 M HCl and a base bath are not sufficient, and even more aggressive methods must be employed. CAUTION! all of these methods will do severe damage to the eyes, skin, mucous membranes and lungs. Extreme caution should be exercised when using these methods. Wear butyl gloves (NOT latex or nitrile exam gloves), eye protection and a lab coat. Work in the hood.
Undergraduate students must check with their faculty supervisor before using these methods, and they must be under the direct supervision of a faculty member at all times when using these methods (no exceptions).
This is an extremely powerful oxidizing solution prepared from 1 part concentrated HNO3 and 3 parts concentrated HCl (it is recommended that 1 part H2O be added if the aqua regia will be stored to minimize the generation of Cl2). It is the only acidic solution that will dissolve gold and will oxidize just about everything else. Extreme caution must be used when working with aqua regia because it generates Cl2 and NOx gases in addition to causing severe tissue damage. Clean the glassware before soaking in aqua regia and then rinse thoroughly with water.
Acidic Peroxide Solution
This is most conveniently prepared by dissolving the commercially-available “NoChromix” mix in concentrated H2SO4 per the package directions. An alternative preparation is to prepare a solution by mixing equal proportions of concentrated H2SO4 and aqueous H2O2 solutions (remember to add the H2O2 to the acid). A 3% H2O2 solution is usually sufficient, and under no circumstances should H2O2 solutions greater than 10% be used. The H2O2/H2SO4 solution is both a strong oxidant and a strong reductant, so care must be taken when using it. Another acidic peroxide solution for cleaning can be prepared by dissolving 36 g (NH4)2S2O8 (ammonium peroxydisulfate) in 2.2 L of 98% H2SO4 (can be made right in the bottle of H2SO4, if the bottle is loosely stoppered). The procedure for these solutions is the same as for aqua regia as are the precautions for their use.
This is a solution of CrO3 in concentrated H2SO4. A premeasured mix is available under the name “Chromerge”, which should be treated in the same way as aqua regia or acidic peroxide solutions. Because high-valent chromium is carcinogenic, teratogenic and causes severe environmental damage, the use of chromic acid is not recommended.
Concentrated solutions of HF will remove just about everything from glass and will even etch the surface of the glass itself. It should not be used on calibrated volumetrics. HF causes severe, painful burns that do not heal well, and prolonged or intense exposure can lead to a very slow, painful death. It is not to be used by any students at Truman under any circumstances.
Generally, you only need to rinse a cuvette in the appropriate solvent and wipe the outside with a Kimwipe immediately after use. If something has adhered itself to a cuvette, it is best to soak the cuvette in solvent first and gently coax the solid off the side with a cotton swab. Never> use a brush on a cuvette! If this fails, one of the acidic cleaning solutions mentioned above can be used (but never HF!). It is not recommended that base bath be used on cuvettes, because it tends to etch glass surfaces.
These can generally be cleaned by inverting and allowing to solvent to flow by gravity through the frit in reverse. Solvent can also be pulled through the frit under vacuum. Recalcitrant gunk can usually be removed by soaking in acid, followed by copious rinsing with water under vacuum. Because HF and the base bath solution etch glass, they should not be used on fritted funnels (a brief exposure to a base bath is not usually fatal to a frit, but prolonged soaking should be avoided).
Usually proteins can be removed scrubbing with detergent, but occasionally protein defies removal. In that event, you can proceed to the more aggressive acidic solutions, or you can prepare a peptidase solution (an enzyme that degrades proteins). The enzymatic approach is a bit slower than the forcing methods, but it is gentler and so can be used in situations that the contaminated item is incompatible with acid.
© 2018 Alison Frontier, University of Rochester. Supported by a grant from the Nati