In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Using a Disposable Pipet for Preparing Air-Sensitive Compounds for Melting Point Determinations or Storage Martial Sanz Departamento de Química Inorgánica, Universidad de Alcalá, Edificio de Farmacia, Campus Universitario, 28 871 Alcalá de Henares, Madrid, Spain;
[email protected] When chemists need to measure melting points of compounds, they normally do it at atmospheric pressure using devices available in their laboratories. But, when melting point measurements involve hygroscopic, air-sensitive substances, chemists are faced with the problem of determining melting points in anhydrous and oxygen-free conditions to prevent the measurement errors due to premature decomposition. Devices and methods have been published in this Journal for handling and preparing air-sensitive compounds either for melting point determinations (1) or for storage, employing ampules (2) without using a glove box. Although these devices and methods are interesting and efficient, they are relatively complex and somewhat expensive, requiring materials for their construction that are not necessarily available in a laboratory. To circumvent this problem, a thin-wall disposable Pasteur pipet is transformed into a capillary tube by sealing the end of the pipet. A funnel-shape is made at the other end of the pipet to facilitate the filling of samples. Using a Quickfit adapter with a side arm (derived from standard Schlenk technique) and rubber septa, the capillary tube formed from the pipet is ready to receive air-sensitive compounds and be sealed under vacuum.
the male NS14兾23 joint of the adapter. Another NS14兾23 rubber septum is attached to the top of the adaptor (Figure 2C) and the system evacuated and filled with purified argon gas. Air-sensitive samples stored in a Schlenk-type vessel can be introduced under purified argon gas into the capillary tube by means of an adapter; a Schlenk-type vessel (with a female NS14兾23 joint) in which the sample is placed can be connected to the system (Figure 2C, without the rubber septum on female 14兾23 joint) by means a short Quickfit adapter male兾male 14兾23 joint if the sample is ultra air sensitive or can be only in contact to the system by female兾female NS14兾23 joint under a flush of argon gas. By gently tapping or gently vibrating the tube, the sample moves to the bottom of the capillary tube and the system is placed under vacuum before sealing. When maximum vacuum is reached, the capillary tube is sealed about 2 cm below the septum and is ready for melting point determinations.
Method
Sample Storage To prepare a sample for storage, the technique can be modified in the following manner. The smaller-diameter segment of the pipet is separated from the larger-diameter segment by melting the glass. The smaller-diameter glass tube is introduced by piercing through the hole in the septum previously made (Figure 3A). The shape of the funnel is formed
Melting Point Determinations A thin-wall disposable Pasteur pipet of 22.9-cm length, 1-mm small diameter, and 6-mm larger diameter is used. The first step consists in forming the shape of the funnel as shown in Figure 1. At about 2 cm from the widening of the larger diameter, the glass is melted and the two glass pieces are separated. The end is melted again, forming a small glass bubble, and rapidly blown to give roughly the shape of a funnel. The glass is melted for the third time and the funnel is expanded to a diameter of ca. 10–12 mm by means of a spatula. The opposite end of the pipet is then sealed. The modified pipet is dried in an oven (120 ⬚C) overnight prior to use. The dried capillary tube is coupled with a Quickfit adapter with a side arm by means of a rubber septum, NS14兾23, in which a thin hole is made with a needle (Figure 2A). The rubber septum is placed on the end of the adaptor. The capillary tube is then introduced through the opening of the female NS14/23 joint of the Quickfit adapter and pushed through the hole in the septum. Pressure of a finger on top of the capillary tube is necessary to force it into the rubber septum (Figure 2B). To ensure vacuum tightness, stainless steel wire is used to securely attach the septum and
Figure 1. Pipet before (left) and after (right) modification.
106
Journal of Chemical Education
•
Vol. 81 No. 1 January 2004
•
www.JCE.DivCHED.org
In the Laboratory
A
B
C
A
B
C
Figure 2. Apparatus for melting point determination: (A) separated components, (B) pipet positioned in the septum, and (C) second septum attached to the top of the adapter to create a closed system.
Figure 3. Apparatus for sample storage: (A) attaching the pipet part to the septum, (B) septum attachment to the adapter, and (C) closed system.
as described above, taking care that the diameter of the funnel will be smaller than that of the male NS14兾23 joint of the adapter. The opposite end of the pipet is sealed. The septum with the pipet is attached to the male NS14兾23 joint of the adapter (Figure 3B) and secured as previously described. After attaching another NS14兾23 rubber septum to the top of the adapter, the system (Figure 3C) is dried under vacuum by means of a heat gun and gradually warmed to room temperature under vacuum. The system is filled with purified argon gas. Several cycles of vacuum–filling with purified argon gas are repeated to ensure an oxygen-free system. The previously mentioned procedure is used to introduce samples under purified argon gas and to seal the tube below the septum under purified argon gas or vacuum. This technique can be also used for rubber septa NS29兾32 and corresponding NS29兾32 Quickfit adapter joints using larger diameter tubes.
Conclusion
www.JCE.DivCHED.org
•
This method presents several advantages. It is simple, inexpensive, efficient (vacuum up to 5.8 Pa), grease-free, flexible, and can be extended to the storage of air-sensitive compounds using a uniform diameter glass tube, as an alternative to the method previously published (2). This method has been used successfully for several years to routinely prepare air-sensitive samples for melting point determinations without using a glove box and noticing any decomposition of the products while handling. Literature Cited 1. Kramer, G. W.; Stocky, T. P. J. Chem. Educ. 1977, 54, 264. 2. Minas da Piedade, M. E.; Dias, A. R.; Martinho Simões, J. A. J. Chem. Educ. 1991, 68, 261.
Vol. 81 No. 1 January 2004
•
Journal of Chemical Education
107
