What is Shlenk flask?
, also known as a Schlenk tube
, is a reaction vessel invented by German chemist Wilhelm Schlenk that can be used for chemical reactions that require isolation from the air. It has a side arm fitted with a PTFE or hair glass screw plug which allows the vessel to be evacuated or filled with gas (usually an inert gas such as nitrogen or argon). These flasks are usually connected to the Schlenk production line
, which allows both operations to be easily accomplished.
Like most laboratory glassware, Schlenk flasks
and Schlenk tubes
are made of borosilicate glass (e.g. Pyrex). The Schlenk flask is a round-bottomed shape, while the Schlenk tube is an elongated shape. They can be purchased off-the-shelf from laboratory suppliers or made by skilled glass blowers from round-bottom flasks or glass tubes.
What is Schlenk line?
Schlenk line is a kind of vacuum gas manifold. It is also a commonly used chemical instrument developed by Wilhelm Schlenk. It consists of a double manifold with multiple ports. One manifold is connected to a purified inert gas source, while the other is connected to a vacuum pump. The inert gas line is vented through an oil bubbler, while solvent vapors and gaseous reaction products are prevented from contaminating the vacuum pump by liquid nitrogen or dry ice/acetone cold trap. Special plug valves allow the selection of vacuum or inert gas without placing the sample on a separate line.
Schlenk lines can be used to safely and successfully handle moisture and air-sensitive compounds. A vacuum is also often used to remove the last traces of solvent from the sample. Vacuum and gas manifolds often have many ports and lines, and carefully, multiple reactions or operations can be run simultaneously. When reagents are highly sensitive to oxidation, traces of oxygen can cause problems. Then, to remove oxygen below the ppm level, the inert gas needs to be purified by a deoxidation catalyst, typically a column of copper (I) or manganese (II) oxide that reacts with the trace oxygen present in the inert gas.
Function of Schlenk line
a. Counter-current addition, in which air-stabilized reagents are added to the reaction vessel in the presence of an inert gas flow.
b. Transfer of liquids and solutions using syringes and rubber spacers.
c. Cannula transfer, in which liquid or air-sensitive reagent solutions are transferred between different vessels plugged with septa using elongated tubes called cannulas. The liquid flow is supported by vacuum or inert gas pressure.
a. Glass vessels are usually connected by tight-fitting and grease-coated frosted glass joints. The circular bend of the glass tube with frosted glass joints can be used to reorient various vessels. The glassware must be purged of external air by alternately applying vacuum and inert gas. The solvents and reagents used also use various methods to remove air and water.
b. Filtration under inert conditions is a special challenge and usually requires the use of specialized glassware to solve. Schenk filters consist of sintered glass funnels equipped with fittings and plug valves. Filtration can be accomplished with minimal exposure to air by mounting the pre-dried funnel and receiving flask into the reaction flask against the nitrogen flow, carefully reversing the setup, and properly opening the vacuum.
c. The main hazard associated with the use of Schlenk pipelines is the risk of implosion or explosion. Implosions can occur due to defects in the use of vacuum and glass equipment.
d. Explosions may occur due to the use of liquid nitrogen, usually in cold traps, used to protect vacuum pumps from solvents. Liquid oxygen can be condensed into the cold trap as a light blue liquid if a reasonable amount of air is allowed into the Schlenk line. Explosions may occur due to the reaction of liquid oxygen with any organic compounds in the trap.
Emptying a Schlenk flask
Typically, the flask is dried and the flask atmosphere is exchanged with an inert gas before the solvent or reagent is introduced into the Schlenk flask
. A common method of exchanging the flask atmosphere is to flush the flask with an inert gas. The gas can be introduced through the side arm of the flask or through a large bore needle (connected to a gas line). The contents of the flask flow out of the flask through the neck of the flask. The advantage of the needle method is that the needle can be placed at the bottom of the flask to better flush the flask of air. Flushing the flask with inert gas is inefficient for large flasks and impractical for complex equipment.
Another way to exchange the atmosphere of a Schlenk flask
is to use one or more "vacuum refill" cycles, usually using a vacuum gas manifold, also known as a Schlenk line. This involves pulling air out of the flask and replacing the resulting vacuum with an inert gas. For example, evacuating the flask to 1 mmHg (130 Pa; 0.0013 atm) and then replenishing the atmosphere with 760 mmHg (1 atm) of inert gas would leave 0.13% ( 1 ⁄ 760 ) of the original atmosphere. Two such vac-refill cycles leave 0.000173% ( 1 ⁄ 760 2 ). Most Schlenk lines easily and quickly reach vacuum levels of 1 mmHg (~1.3 mBar).
Types of Schlenk flask
When using Schlenk systems (including Schlenk flasks), grease is usually required at the plug valve and gross glass joints to provide a gas-tight seal and to prevent glass fragments from melting. In contrast, Teflon plug valves may have a trace amount of oil as a lubricant, but usually no grease.
A. Standard Schlenk flask
Standard Schlenk flasks are round-bottomed, pear-shaped or tubular flasks with ground glass fittings and side arms. The side arm contains a valve, usually a greased plug valve, to control the contact of the flask with the manifold or atmosphere. This allows the material to be added to the flask through the ground-mouth glass fitting, which is then capped with a septum. This operation can be done in a glove box, for example. The flask can then be removed from the glove box and taken to the Schlenk production line. Once connected to the Schlenk line, inert gas and/or vacuum can be applied to the flask as needed. While the flask is connected to the line under positive pressure with inert gas, the septum can be replaced with other equipment, such as a reflux condenser. After the operation is complete, the contents can be vacuum dried and placed under a static vacuum by closing the side arm valves. These evacuated flasks can be placed back in the glove box for further manipulation or storage of the flask contents.
B. Schlenk bomb flask
The "bomb" flask is a subcategory of the Schlenk flask, which includes all flasks that have only one opening accessible by opening a Teflon plug valve. This design allows a Schlenk bomb to be more completely sealed than a standard Schlenk flask, even if its septum or glass cap is wired. Schlenk bombs include structurally sound shapes such as round bottoms and thick-walled tubes. Schlenk bombs are typically used to react as closed systems at high pressures and temperatures. In addition, all Schlenk bombs are designed to withstand the differential pressure generated in the front chamber when pumping solvents into the glove box.
In practice, Schlenk bomb flacks can perform many of the functions of standard Schlenk flasks. Even when the opening is used to attach the bomb to the manifold, the stopper can still be removed to add or remove material from the bomb. However, in some cases, Schlenk bombs are not as convenient as standard Schlenk flasks: they do not have an accessible fritted glass fitting to connect additional equipment; the opening provided by the plug valve is difficult to open with a spatula, and it is easier to use a diaphragm designed to fit a fritted glass fitting than a Teflon plug.
The name "bomb" is often used for containers used under pressure, such as bomb calorimeters. While glass does not equal the pressure rating and mechanical strength of most metal containers, it does have several advantages. The glass allows visual inspection of ongoing reactions is inert to a variety of reaction conditions and substrates, is often more compatible with ordinary laboratory glassware, and is easier to clean and check for cleanliness.
C. Straus flask
Straus flasks are a subclass of "bomb" flasks, typically used for storing dry and degassed solvents. Straus flasks are sometimes referred to as solvent bombs - a name that applies to any Schlenk bomb that is specifically designed to store solvents. The main difference between Straus flasks and other "bombs" is their neck structure. There are two necks in a round-bottom flask, one larger than the other. The larger neck ends in a hairy glass joint and is permanently separated by blown glass that does not allow direct access to the flask. The smaller neck section includes the threads needed to screw the Teflon stopper vertically into the flask. The two necks are connected by a glass tube. The hair glass fitting can be connected directly to the manifold or through an adapter and hose. Once connected, the plug valve can be partially opened to allow for the vacuum transfer of solvent from the Straus flask to other vessels. Alternatively, once connected to the line, the neck can be placed under a positive pressure of inert gas and the plug valve can then be completely removed. This allows direct access to the flask through a narrow glass tube now protected by an inert gas curtain. Then the solvent can be transferred through the casing into another flask. In contrast, other bomb flask stoppers are not necessarily ideally located to protect the flask atmosphere from external atmospheres.
D. Solvent pot
Strauss flasks are different from "solvent tanks", which are flasks containing solvents and drying agents. Solvent tanks are usually not bombs, not even Schlenk flasks in the classic sense. The most common configuration of a solvent pot is a simple round-bottom flask connected to a 180° adapter with some form of valve. The pot can be attached to a manifold and then distilled or vacuum transferred to other flasks without soluble drying agents, water, oxygen, or nitrogen. The term "solvent pot" can also refer to a flask containing a desiccant in a classical solvent distillation system. Due to the risk of fire, solvent distillers have been largely replaced by solvent columns where a degassed solvent is forced through an insoluble desiccant prior to collection. The solvent is usually collected from the solvent column, e.g. in the case of Strauss flasks, by piercing the septum of the flask with a needle attached to the column or by means of a hair glass fitting attached to the column.
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