Source: Laboratory of Dr. Neal Abrams — SUNY College of Environmental Science and Forestry
Glassware is a regular appearance in the professional chemistry laboratory, because it has a relatively low cost, extreme durability, and specific levels of precision. While some labware is being supplemented with plastic or even everyday kitchen materials, glass is still the standard material by which laboratory work is done. While there are few rules about glassware, there are some best practices for use that set the groundwork for good techniques in the lab.
Glass is ubiquitous in the chemistry laboratory, but not all glass is the same. Standard consumer-grade glass is known as "soda-lime" or "float" glass. It is good for many applications, but cracks under rapid heating and cooling applications due to expansion/contraction. Borosilicate glass is used to solve this problem in the lab. Made with an introduction of small amounts of boron, borosilicate glass has a very low coefficient of expansion, which prevents internal stresses. The most common trade name for borosilicate glass is Pyrex, the same type of glass used in some kitchen bakeware.
While borosilicate glass is thermally robust, the impurities found in borosilicate and standard glass lead to a limited temperature range and optical quality. Fused silica, or quartz, is used in situations where glass needs to be heated above 450 °C or to be transparent to UV light. Fused silica is chemically-pure silicon dioxide with no impurities and a very high melting point above 1,600 °C. The easiest way to tell the difference between borosilicate glass and fused silica in the lab is to look down the long axis of a piece of glassware. A greenish color is indicative of borosilicate impurities, whereas fused silica is optically clear and colorless.
Standard laboratory glassware, like beakers and flasks, has a limited accuracy of measuring volume, typically ±5%. Volumetric glassware, however, is considered very accurate. This accuracy is known to the user through a few different pieces of information on the glassware. For one, an etched line or volume marking is typically located on volumetric glassware to indicate a volume. The next piece of information is the temperature at which the glassware is accurate, typically 20 °C. This is important because the density (and volume) of a liquid are dependent on temperature. Thirdly, the notations "TD" or "TC" are used to indicate "to deliver" or "to contain", respectively. When a piece of glass is marked as "TD", it is calibrated to accurately deliver the stated volume, whereas glassware with the "TC" marking only contains a specified volume, but it may not transfer to another vessel accurately.
Glassware can be sealed using a variety of stoppers, typically rubber, cork, or glass. Rubber and cork stoppers fit into standard glass necks, though cork is being phased out, and newer stoppers made of neoprene are taking over. Stoppers are conical in shape and fit like a wedge into the glassware. Stoppers can have anywhere from 0 – 3 holes, allowing for connections to tubing or inserting thermometers and stirrers. A variation of the stopper is the septum, which can be used to seal glassware and allows for easy access with a syringe needle. The downside of most flexible stoppers is that they break down over time, though newer Teflon stoppers are more robust but lack the physical flexibility. Ground glass stoppers are used to seal flasks that have ground glass fittings. While the seal is very good, glass-to-glass connections are known to seize, so joint grease (vacuum, Krytox, etc.) is often used to prevent this. Rubber stoppers are sized by number, ranging from 000 – 10, whereas glass stoppers are sized by the diameter and length of the sealing section. For example, a stopper marked as 24/40 is 24 mm in diameter at its widest part and 40 mm long on the tapered edge, which would fit into a flask with a 24/40 opening.
Connections between pieces of glassware are made using a variety of ground glass joints including a standard taper, ball-and-socket, and O-ring. The standard taper is the most common fitting. Glass joints are sized to fit into one another and a variety of size adapters are available. Like all other glass joints, grease is required to prevent seizing. While the joint may be sealed, it is not a mechanically strong connection and can fall apart. To prevent glass pieces from separating, connector clips are used, which are sometimes referred to as Keck clips. These clips are color-coded for the size of the joint. Alternatives to connector clips include springs and wire.
Clamping and supporting glassware is a vital part of a successful experiment. While some pieces of glassware, like beakers and Erlenmeyer flasks, have flat bottoms that can sit flat on a hotplate, other pieces of glassware, like round-bottom flasks, need to be supported using clamps. Even with flat-bottom glassware, it can be far too easy for something like a vacuum filtration flask to fall over. Metal clamps are connected to the neck of a piece of glassware using either a three-finger or a standard clamp. The other end of the clamp is then attached to a ring stand (or retort stand). Other clamps exist for special purposes, like chain-style for large pieces or water-bath clamps for thermometers. The lab jack uses a scissoring action to raise or lower a piece of glassware. This is very convenient for large or heavy items and, when used in conjunction with a cork ring, can also be used to move round-bottom flasks.
Just like in the kitchen, soap and water are typically used to clean glassware in the lab. When that fails, organic solvents, like acetone, are sometimes employed to remove sticky and insoluble organic deposits. Even then, some compounds adhere to glassware so well that they are impossible to remove without some form of chemical etching. In the case of organic carbon-containing deposits, glassware can be soaked in a base bath composed of an alcohol (ethanol) and a strong base (sodium hydroxide). This bath etches thin molecular layers of glass from the vessel, taking the stubborn deposits with it. It is very important to never place volumetric glassware in a base bath, which could lead to etching and a change in volume. When a metal has plated or infused into a piece of glassware, an acid bath made with a dilute strong acid, like hydrochloric, is used. The amphoteric nature of glass and the general oxidation of metal in acid lead to its cleaning power. Regardless of the bath type, 24–48 h is required for effective deposit removal.
1. Glassware for Qualitative Uses
2. Glassware for Measuring
3. Procedural Glassware
While there are few rules to how glassware must be used, each piece of glassware was designed for a general set of procedures. Unique situations create some flexibility on the application, and nearly all glassware can be further adapted and customized with the assistance of a professional glassblower.
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