Chemical Education Today
Making and Using an in Situ Microfiltration Device In a chemical laboratory, monitoring the instantaneous composition of the liquid (or solid) part of a heterogeneous mixture is a common occurrence, frequently carried out by instrumental analysis of a sample consisting of a small fraction of the suitable part of the system. The publication of a letter describing a clever, fairly helpful microfiltration apparatus (1) prompts me to make known a somewhat reciprocal device, similarly mechanically aided, that is quite useful in obtaining fractional samples from heterogeneous mixtures by taking them directly from the reactor or container. This in situ microfiltration device consists of a disposable, inexpensive polypropylene syringe of volume larger than that of the desired sample with the piston stem surrounded by an extended helical steel spring (see the supporting material, note 1), resting on the base of the external end of the piston and the syringe fins, of a stainless steel needle (we use a Luer needle that is 120 mm long and 1.2 mm in diameter), and a miniature Buchner funnel turned out of a polytetrafluoroethylene (PTFE) rod (see the supporting material, note 2) with exit hole diameter of 1.1 mm to get a gastight connection to the pressure inserted 90° cut (and deburred) tip of the needle. Figure 1 shows these components. The funnel has an outer diameter of 9.8 mm to pass through the SVL 15 (Sovirel France) borosilicate glass screw connection (“butt joint”; see the supporting material, note 3), which is the largest screw connection commonly used in semimicro apparatus, and a knife-edge border to facilitate detachment of solution drops. Its inner diameter is 7 mm to allow use of circular filters cut from a sheet of suitable material (cellulose, glass fiber, nitrocellulose, PTFE, etc.) by means of a (three-) hole puncher (or cork borer) cutting disks of such diameter. To take a sample, the spring is compressed of a length corresponding to the desired volume of the sample; the piston head remains in the upper part of the syringe so that the filtered solution will never contact the solvent-sensitive rubber head of the piston, making it generally unnecessary to use special (gastight, PTFE tip) syringes. Next, the needle previously connected to the funnel with the filter moistened with the appro-
368
Journal of Chemical Education
_
_
Figure 1. In situ microfiltration device with needle, funnel, filter disks, and syringe with piston in equilibrium position.
priate solvent is connected to the syringe and immersed vertically into the suspension. Filtration begins as soon as the piston is released. If the sample volume is small, say 0.1-0.2 mL, as is typical of our IR analyses of liquids (2), the short time required by the filtration makes it convenient to draw the piston by hand, eliminating the spring. When the phase of interest is the solid, it may be convenient to dip the funnel near the bottom of the container or to keep stirring the mixture rather than allowing it to sediment the particulate. The filter disk with the solid is easily removed by substituting the syringe containing the solution with one filled with nitrogen or argon and giving a short stroke to the piston. Literature Cited 1. Rodriguez-Fernandez, E.; Vicente, M. A.; Manzano, J. L. J. Chem. Educ. 2008, 85, 1051. 2. Lunelli, B.; Comelli, F. Rev. Sci. Instrum. 1987, 58, 305–307.
Supporting Information Available Schematic drawings with measurements. This material is available via the Internet at http://pubs.acs.org. Bruno Lunelli Istituto per lo Studio dei Materiali Nanostrutturati CNR, Sezione di Bologna, I-40129 Bologna, Italy
[email protected] _
Vol. 87 No. 4 April 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed800147c Published on Web 03/09/2010
