The Grasshopper's Guide to Cleaning Glassware

Author: J. M. McCormick

Last Update: June 30, 2006

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.”

Introduction

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.  At this level of cleanliness there are no residues (e. g., grease) or other impurities on the glassware.  Normal clean glassware is free of most contamination but some contaminates (e. g., grease) are tolerated.  Glassware that has been cleaned normally is used where a high degree of precision is not required, such as in a synthesis.

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 Tips

The key to cleaning is doing it a timely manner; letting dirty glassware sit for long periods of time guarantees a harder cleaning job. 

•Disassemble your apparatus as soon as possible after you are finished with it. Remove all stopcocks and stoppers from addition 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 the rinses in the appropriate waste container.

 

•Separate glassware that must be quantitatively clean from that which does not.  In this way you do not waste time trying to quantitatively clean items that do not need to be. 

 

•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.  Running solvent through them backwards using gravity (never use vacuum to speed up this process!) can help remove contamination from the inside of the funnel and from the surface of fritted funnels.  Follow this by tap water and DI water rinses and air dry.

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 it 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, methanol or isopropanol). 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

 

»Cuvettes

 

»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 rinsed 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).

Aqua Regia 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 NO_x 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 acid to the H₂O₂).  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 cleaning solution 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.

 

Chromic Acid 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.

 

Hydrofluoric Acid 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.

Special Cases

Cuvettes 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.

 

Fritted Funnels These can generally be cleaned by inverting and allowing to solvent to flow by gravity through the frit in reverse (do not use vacuum to speed this process).  Solvent can also be pulled through the frit (in the normal direction) 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).

 

Protein Contamination 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.

Drying Glassware

Wet glassware can be dried by 1) placing it on the drying rack (or invert on a paper towel), 2) placing it in the drying oven (for items that are water-wet only, no flammable solvents) or 3) rinsing with a solvent such as acetone, methanol or ethanol and then gently blowing compressed air into the vessel until it is dry.  The first method is preferred for drying quantitatively clean glassware (provided that the prongs of the drying rack are not inside the item, thus contaminating it). Volumetric glassware and cuvettes are never to be placed in drying ovens, even if they are not quantitatively clean.  The third method  is acceptable only when the compressed air supply is known to be free oil and other contaminants.  An alternative to blowing air into the item is to use an aspirator, or house vacuum, to pull air into the item.

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