368 Journal of Chemical Education

_Vol. 87 No. 4 April 2010

_pubs.acs.org/jchemeduc

_r2010 American Chemical Society and Division of Chemical Education, Inc.

10.1021/ed800147c Published on Web 03/09/2010

Chemical Education Today

Making and Using an in Situ MicrofiltrationDevice

In a chemical laboratory, monitoring the instantaneouscomposition of the liquid (or solid) part of a heterogeneousmixture is a common occurrence, frequently carried out byinstrumental analysis of a sample consisting of a small fractionof the suitable part of the system. The publication of a letterdescribing a clever, fairly helpful microfiltration apparatus (1)prompts me to make known a somewhat reciprocal device,similarly mechanically aided, that is quite useful in obtainingfractional samples from heterogeneous mixtures by taking themdirectly from the reactor or container.

This in situ microfiltration device consists of a disposable,inexpensive polypropylene syringe of volume larger than that ofthe desired sample with the piston stem surrounded by anextended helical steel spring (see the supporting material, note1), resting on the base of the external end of the piston and thesyringe fins, of a stainless steel needle (we use a Luer needle that is120 mm long and 1.2 mm in diameter), and a miniature Buchnerfunnel turned out of a polytetrafluoroethylene (PTFE) rod (seethe supporting material, note 2) with exit hole diameter of 1.1mm to get a gastight connection to the pressure inserted 90� cut(and deburred) tip of the needle. Figure 1 shows these compo-nents. The funnel has an outer diameter of 9.8 mm to passthrough the SVL 15 (Sovirel France) borosilicate glass screwconnection (“butt joint”; see the supporting material, note 3),which is the largest screw connection commonly used in semi-micro apparatus, and a knife-edge border to facilitate detachmentof solution drops. Its inner diameter is 7 mm to allow use ofcircular filters cut from a sheet of suitablematerial (cellulose, glassfiber, nitrocellulose, PTFE, etc.) by means of a (three-) holepuncher (or cork borer) cutting disks of such diameter.

To take a sample, the spring is compressed of a lengthcorresponding to the desired volume of the sample; the pistonhead remains in the upper part of the syringe so that the filteredsolution will never contact the solvent-sensitive rubber head ofthe piston, making it generally unnecessary to use special(gastight, PTFE tip) syringes. Next, the needle previously con-nected to the funnel with the filter moistened with the appro-

priate solvent is connected to the syringe and immersed verticallyinto the suspension. Filtration begins as soon as the piston isreleased.

If the sample volume is small, say 0.1-0.2mL, as is typical ofour IR analyses of liquids (2), the short time required by thefiltration makes it convenient to draw the piston by hand,eliminating the spring.

When the phase of interest is the solid, it may be convenientto dip the funnel near the bottom of the container or to keepstirring the mixture rather than allowing it to sediment theparticulate. The filter disk with the solid is easily removed bysubstituting the syringe containing the solution with one filledwith nitrogen or argon and giving a short stroke to the piston.

Literature Cited

1. Rodriguez-Fern�andez, 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 availablevia the Internet at http://pubs.acs.org.

Bruno Lunelli

Istituto per lo Studio dei Materiali Nanostrutturati CNR, Sezione diBologna, I-40129 Bologna, Italyb.lunelli@bo.ismn.cnr.it

Figure 1. In situ microfiltration device with needle, funnel, filter disks,and syringe with piston in equilibrium position.