Reprinted with permission of the Corning Corporation General Cleaning Sterilizing Glassware Glassware Cleaners
Safe Use of Chromic Acid Rinsing Handling & Storing
Good Laboratory Technique demands clean glassware, because the most carefully executed piece of work may give an erroneous result if dirty glassware is used. In all instances, glassware must be physically clean; it must be chemically clean; and in many cases, it must be bacteriologically clean or sterile.
Glassware Must be Absolutely Grease-Free. The safest criteria of cleanliness is uniform wetting of the surface by distilled water. This is especially important in glassware used for measuring the volume of liquids. Grease and other contaminating materials will prevent the glass from becoming uniformly wetted. This in turn will alter the volume of residue adhering to the walls of the glass container and thus affect the volume of liquid delivered. Furthermore, in pipettes and burettes, the meniscus will be distorted and the correct adjustments cannot be made. The presence of small amounts of impurities may also alter the meniscus.
Wash Labware as Quickly as Possible after Use. If a thorough cleaning is not possible immediately, put glassware to soak in water. If labware is not cleaned immediately, it may become impossible to remove the residue.
Most new glassware is slightly alkaline in reaction. For precision chemical tests, new glassware should be soaked several hours in acid water (a 1% solution of hydrochloric or nitric acid) before washing.
Brushes with wooden or plastic handles are recommended as they will not scratch or abrade the glass surface.
Glassware which is Contaminated with blood clots, such as serology tubes, culture media, petri dishes, etc., must be sterilized before cleaning. It can best be processed in the laboratory by placing it in a large bucket or boiler filled with water, to which 1 to 2% soft soap or detergent has been added, and boiled for 30 minutes. The glassware can then be rinsed in tap water, scrubbed with detergent, rinsed again.
You may autoclave glassware or sterilize it in a large steam ovens or similar apparatus. If viruses or spore-bearing bacteria are present, autoclaving is absolutely necessary.
When washing, soap, detergent, or cleaning powder (with or without an abrasive) may be used. Cleaners for glassware include ALCONOX®, DURAL®, MICRO-90®, LUX®, TIDE®, and FAB®. The water should be hot. For glassware that is exceptionally dirty, a cleaning powder with a mild abrasive action will give more satisfactory results. The abrasive should not scratch the glass.
During the washing, all parts of the glassware should be thoroughly scrubbed with a brush. This means that a full set of brushes must be at handbrushes to fit large and small test tubes, burets, funnels, graduates, and various sizes of flasks and bottles. Motor driven revolving brushes are valuable when a large number of tubes or bottles are processed. Do not use cleaning brushes that are so worn that the spine hits the glass. Serious scratches may result.
Scratched glass is more prone to break during experiments. Any mark in the uniform surface of glassware is a potential breaking point, especially when the piece is heated. Do not allow acid to come into contact with a piece of glassware before the detergent (or soap) is thoroughly removed. If this happens, a film of grease may be formed.
If glassware becomes unduly clouded or dirty or contains coagulated organic matter, it must be cleansed with chromic acid cleaning solution. The dichromate should be handled with extreme care because it is a powerful corrosive and carcinogen.
When chromic acid solution is used the item may be rinsed with the cleaning solution or it may be filled and allowed to stand. The length of time it is allowed to stand depends on the amount of contamination on the glassware. Relatively clean glassware may require only a few minutes of exposure; if debris is present, such as blood clots, it may be necessary to let the glassware stand all night.
Due to the intense corrosive action of the chromic acid solution, it is good practice to place the stock bottle, as well as the glassware being treated, in flat glass pans or pans made from lead or coated with lead, or plastic polymer pans determined compatible with the concentration of chromic acid you are using. Extra care must be taken to be sure chromic acid solution is disposed of properly.
Special types of precipitates may require removal with nitric acid, aqua regia, or fuming sulfuric acid. These are very corrosive substances and should be used only when required.
Chromic Acid Cleaning Solution
Use powdered commercial or technical grade sodium dichromate. If the compound is in the form of crystals, grind to a fine powder in a mortar. To 20 grams of the powder in a Liter beaker, add a little water, sufficient to make a thin paste. Slowly add approximately 300 mL of commercial concentrated sulfuric acid, stirring well. Transfer to a glass-stoppered bottle.
Larger amounts can be made in the same proportions. Use the clear supernantant solution. Chromic acid solution can be used repeatedly until it begins to turn a greenish color. Dispose of in accordance with appropriate regulations. Dilute with large volumes of water before discarding, or carefully neutralize the diluted solution with sodium hydroxide. Chromic acid solution is strongly acidic and will burn the skin severely. Use care in handling it.
It is Imperative that all soap, detergents and other cleaning fluids be removed from glassware before use. This is especially important with the detergents, slight traces of which will interfere with serologic and cultural reactions.
After Cleaning, rinse the glassware with running tap water. When test tubes, graduates, flasks, and similar containers are rinsed with tap water, allow the water to run into and over them for a short time, then partly fill each piece with water, thoroughly shake and empty at least six times. Pipettes and burettes are best rinsed by attaching a piece of rubber tubing to the faucet and then attaching the delivery end of the pipettes or burettes to a hose, allowing the water to run through them. If the tap water is very hard, it is best to run it through a deionizer before using.
Rinse the Glassware in a large bath of distilled water. Rinse with distilled water. To conserve distilled water, use a five gallon bottle as a reservoir. Store it on a shelf near your clean-up area. Attach a siphon to it and use it for replenishing the reservoir with used distilled water.
For Sensitive Microbiologic Assays, meticulous cleaning must be followed by rinsing 12 times in distilled water.
To Prevent Breakage when rinsing or washing pipettes, cylinder, or burettes, be careful not to let tips hit the sink or the water tap.
Dry test tubes, culture tubes, flasks, and other labware by hanging them on wooden pegs or placing them in baskets with their mouths downward and allowing them to dry in the air; or place them in baskets to dry in an oven.* Drying temperatures should not exceed 140°C. Line the drying basket with a clean cloth to keep the vessel mouths clean.
Dry burettes, pipettes, and cylinders by standing them on a folded towel. Protect clean glassware from dust. This is done best by plugging them with cotton, corking, taping a heavy piece of paper over the mouth or placing the glassware in a dust-free cabinet.
Store Glassware in specially designed racks. Avoid breakage by keeping pieces separated.
Do not store alkaline liquids in volumetric flasks or burettes. Stoppers or stopcocks may stick.