Cleaning glassware
Many different reagents, reagent mixtures and tips have to be used to thoroughly wash glassware. It is important to know them to keep your glassware clean of impurities for your experiments. A good thread about it can be found here.
Contents
Preliminary glass cleaning
First, remove any solid impurities with a paper towel, if there are any. If the dirt is soluble in water or can be removed with a water jet, use water. Use detergent to remove any other impurities that haven't been removed with plain water. Use a brush to remove persistent dirt.[1]
Common impurities
Graphite
Graphite is a problem as it is not soluble in any solvent and has low reactivity. It will clog fritted glassware. Aqua regia and piranha solution are good at removing graphite, though the reaction takes a while.
Limescale
Tap water generally presents a high mineral content, and it's also known as "hard water". The build-up of lime on glassware can be removed by dissolving them with an acid, preferably a strong one. It's best however to avoid sulfuric and oxalic acids, as the resulting salts are poorly soluble and require more effort to remove them. Hydrochloric acid or concentrated acetic acid are ideal and cheap. To prevent a build-up of lime or the glassware, the glassware can be further cleaned with either distilled water or steam. Although nitric acid is also very good at removing limescale, it's too expensive (compared to the other mentioned acids) to be wasted this way, so it's best to only use it if all the others before it have failed.
Metal and metal oxide traces
The traces left from Wood's metal or another eutectic alloy on glassware can be removed either chemically or by rubbing the glassware with a wet insoluble/poorly soluble salt, such as sodium sulfate or bicarbonate. Using anything harder will erode the glass.
Persistent stains and burns on the outside
A cooktop cleaner for ceramic oven works like a charm to clean the outside of glassware stained with hard to remove stains. It can only be used on the outside though, because it needs to be rubbed correctly to remove the stain and the inside of glassware is impossible to reach to rub correctly.
Rust and iron deposits
For deposits of rust, iron, carbonates, and oxides which are hard to remove, prepare a solution of ~19 parts vinegar to 1 part store bought isopropyl alcohol. Add sulfuric acid or sodium bisulfate to catalyze the production of isopropyl acetate, a solvent. Put this solution in your stained glassware to clean it of debris and residue.
Scraping the glassware with a toothpick or skewer can help remove deposits.
Another cheap way to remove the rust deposits is to add the glassware in a diluted solution of HCl. After several hours the rust deposits will vanish. Oxalic acid can also be used.
Cleaning baths
Acid bath
A diluted solution of a strong acid, such as hydrochloric acid, can be used to clean glassware, usually after they were cleaned in a base bath. It will remove most of metal impurities. Excellent for cleaning glass and plastic. Metal items however, must never be put in an acid bath as they will rust.
Base bath
A concentrated solution of alkali hydroxide in 95% ethanol or isopropanol is a very powerful cleaning solution. This solution will dissolve glass after a period of time, so care must be taken to ensure the glass is not left in it too long. This solution should not be used on ground glass joints as this may alter their size.
Fritted glass is particularly sensitive, as the strong alkali solution will slowly dissolve the sintered bonds and increase the pore size or even cause them to crumble.
Solvent bath
This type of bath is rarely used, as it has the unwanted effect of releasing large amounts of flammable or toxic vapors in the room. It is very useful for cleaning objects that would be destroyed by an alkaline or an acidic bath. Items made of aluminium, zinc, cellulose, cork, natural rubber or ceramic can be cleaned from various organic or even inorganic stains, without damaging them. The best solvent to use is acetone, and chloroform should be used only as a last resort. The solvent bath is safe to use only on small scale (~250 ml) and when not in use, it's best to not keep it in the cleaning container, but rather in a labeled bottle, to limit the solvent evaporation. Since the bath gives off flammable and potentially explosive fumes of solvents, it's best to keep it inside a fumehood or outside.
Other solutions
Aqua regia
Aqua regia solutions are good for removing organic material from fritted glass and graphite to a degree. Does not attack glass, but will damage metal and plastic items.
Bleach
Bleach is useful in the removal organic traces from glassware, mostly because it's cheap and readily available. However, removing the chlorine traces is difficult. The best way is to wash the glass with a diluted solution of hydrogen peroxide or a salt such as sodium thiosulfate or metabisulfite. If you're using a bleach bath (not recommended due to its strong chlorine smell), add the bleach container inside a larger one to reduce the chlorine smell, and keep it away from people. Always check the glassware for any traces of acids, as contact with bleach will release dangerous amounts of chlorine and chlorine dioxide. Properly label the bleach container.
Chromic acid
A solution of concentrated sulfuric acid and a hexavalent chromium salt, such as potassium dichromate, is one of the best cleaning solutions. It doesn't dissolve glass, but care must be taken as this solution is extremely corrosive, toxic, and carcinogenic. This solution will dissolve many impurities as their dichromate salt, and will oxidize many other hard to remove stains, including carbon.
Chromic acid may be made by dissolving chromium trioxide in water.
A more potent version of chromic acid is chromonitric acid, which can be prepared by dissolving hexavalent chromium compounds in concentrated nitric acid. This reagent is comparable in oxidizing and cleaning power to piranha; it was widely used in countries of the Soviet bloc, where piranha was almost unknown. Be sure to use chromonitric acid only against the most inert and resistant "goats", because it is known to react with active, oxidizable organics with dangerous vigor.
Fenton's reagent
Fenton's reagent is excellent for removing organic traces from glassware, especially solvent traces and organic impurities.
Piranha solution
The Caro's acid solution is a very effective way of removing extremely hard to clean impurities from glassware, but it is much more dangerous to handle than the other solutions mentioned. It will dissolve and destroy any organic compound and is good for removing graphite. The base form of piranha solution needs to be heated to 60 °C before use.
Rinsing
After the glassware has been cleaned of impurities, check for any impurities that haven't been washed away. Use a brush to further remove the impurities, or if they cannot be reached directly, put them again in the cleaning bath until they vanish. If the removal of hard to clean traces was done with a solvent insoluble in water, or soluble/miscible but with a high boiling point or toxic (toluene, DMSO, DMF e.g.), further rinse the glassware with a water miscible solvent, such as ethanol or acetone to remove the traces before rinsing it with water.
After the glassware has been thoroughly cleaned, the last step before drying it is to rinse it with water to remove any last traces. Good old tap water is effective and cheap, but it has the main disadvantage of containing dissolved minerals, aka hard water. If the tap water is very hard, it is best to run it through a deionizer before using. Distilled water, either bought from the store or home made is necessary to rinse the glassware to ensure no lime will be remain on the glassware. Steam can also be used.
Drying
The final step in cleaning involves drying the glassware. Plastic and rubber items should be placed in a container, labeled to avoid confusion and left in open air for several hours.
Glass and metal items can be dried in a drying oven or stove, at a temperature of around 120-130 °C, for at least 2 hours. Avoid adding any glassware that has plastic or rubber parts, except for syringe needles as their plastic connector can tolerate such temperatures.
Best way to hold the glassware in the oven is in a stainless steel wire basket, with the glassware being added bottom up, so that the resulting steam will wash away any leftover lime. Tubes should be placed in a flask, also bottom up. This is not required if the glassware was rinsed previously with distilled water.
After they're dry, take the glassware out of the oven and let them cool on a table or other clean surface and once they're cool, place them in their resting place.
Other tips
- Using baths of other solvents, such as acetone, ethyl acetate, THF, chloroform, carbon disulfide, is not recommended, as, apart from wasting large amounts of valuable solvent, the standard basic and acid baths will eventually remove the impurities. Most solvents tend to be volatile, either toxic and/or flammable, which could lead to a build-up of hazardous or even explosive vapors. Small acetone baths however can be used if the material they're made of reacts with most acids or alkali, but resists organic solvents (ex: aluminium, zinc). However, acetone will slowly condense in the presence of certain substances, so it's best to change it after a few washes.
- The base bath strength will decrease over time, as the NaOH/KOH absorbs carbon dioxide from air and breaks down organic compounds. The concentration of the alkali should be checked periodically.
- Wipe the joints from grease before adding them in the base bath; otherwise the grease will react with the alkali and a goo will result in the bath, that will contaminate other glassware that are being cleaned.
- Try to use warm water instead of hot water if you're washing traces of volatile compounds.
- Certain glassware items, such as volumetric and graduated pipettes, Pasteur pipettes, stirring rods, and burettes can be cleaned more efficiently by adding them in a large graduated cylinder filled with the washing bath. While they can be introduced in a standard bath, these type of glassware are very fragile and may break if another heavy glassware is put on top. The same technique can also be used for other similar glassware such as eudiometers, Thistle tubes and NMR tubes. This method also limits the amount of cleaning bath used, saving valuable reagent.
- Glassware used for trace analysis should not be cleaned with strong alkali, as it may cause the glass to leach its components in the solution and alter the solution concentration of salts.
