Communication Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Setup for Semimicro Pressure Filtration Bruno Lunelli*,† Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Consiglio Nazionale delle Ricerche (CNR), via Piero Gobetti 101, I-40129 Bologna, Italy
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Massimo Baroncini Center for Light Activated Nanostructures (CLAN), Università di Bologna and Consiglio Nazionale delle Ricerche, via Gobetti 101, I-40129 Bologna, Italy Dipartimento di Scienze e Tecnologie Agroalimentari, Università di Bologna, Viale Fanin 50, I-40127 Bologna, Italy S Supporting Information *
ABSTRACT: Semimicro pressure filtration is carried out by pressurizing a suspension through pushing the piston of a gas-filled plastic syringe whose needle perforates the closure of a cylindrical filter funnel. Basic features as well as pros and cons of vacuum and pressure filtration are pointed out and compared to support rational choice between the two procedures. KEYWORDS: Upper-Division Undergraduates, Laboratory Instructions, Safety/Hazards, Hands-On Learning/Manipulatives, Separation Science, Thermodynamics
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an ensemble of capillary channels (pores) of clearance sufficiently small to block the solid particles present in the slurry. Along such channels, the pressure decreases from Ps to Pf during the time of transit of the solution from entrance to exit. The feed is a slurry consisting of an equilibrium mixture of a solid and its saturated solution at temperature Ts and pressure Ps, so that the filtrate immediately below the filter channels quickly becomes supersaturated because Pf is lower than Ps. From there to the end of the funnel stem, the supersaturation increases as a result of (i) the effective partial removal of the solvent due to the lower pressure in that section of the apparatus and (ii) the consequent cooling of the filtrate due to the enthalpy of solvent evaporation. Therefore, vacuum filtration is only effective when (a) the concentration of the solution is so low that diffusion cannot provide sufficient material to grow crystals large enough to block the channels, (b) the speed of flow through the filter is sufficiently high to delay crystallization beyond the filter channels, or (c) the solute has a low tendency to crystallize. When these conditions are not fulfilled, pressure filtration4,5,11,14,17 becomes necessary. It has the additional advantages of preventing evaporation of the filtrate, offering easy operation in a controlled atmosphere, and avoiding unwanted emissions, as detailed below. Our implementation of pressure filtration is designed to handle small or semimicro size (20−200 mg of solid or 0.5−5 mL of liquid) samples, because in the current didactic and research activities, a few milligrams of material is generally sufficient for each measurement or experiment. Moreover, these operational scales are intrinsically safer because of the small amounts of material involved. The small size of the
n the undergraduate teaching or research chemical laboratory, solid−liquid separation is a frequent operation. Different laboratory techniques, such as decantation, centrifugation (or centrifugally enhanced decantation), clarification, and filtration, offer a variety of different approaches for this process, each one representing an optimal solution depending on the specific needs. “Filtration” in the following text refers to solid−liquid separations aimed at maximizing the recovery of both the residue and the filtrate of semimicro or small-size samples. It involves quite modest apparatus and training and, when assisted by the use of “vacuum” (pressure lower than atmospheric pressure) or pressure (higher than atmospheric pressure), can be much faster than other solid−liquidseparation techniques. However, filtration can become a significantly annoying task with low-boiling-point solvents; with hard-to-get, expensive, or air-sensitive substances; or with slurries that emit toxic fumes. As a consequence, extensive literature exists on semimicro filtration procedures1−17 that address one or more of the different problematic details of the process. The procedure described in most of the listed references is “vacuum filtration”,1,2,7−10,12,13,15,16 which accelerates solid− liquid separation by reducing the pressure on the filtrate side. Although there is mention in the literature of unwanted evaporation of the filtrate, crystallization in the stem of the funnel, and clogging of the filter support or of the filter itself, thus resulting in the need to abort the procedure when handling appreciably soluble materials or volatile solvents,11,13 we could not find an analysis of the causes of these troubles, despite the fact that such causes can be satisfactorily analyzed by following the values of the pressure from its highest (atmospheric) value (i.e., that at the slurry location, Ps) to its lowest value (i.e., that at the filtrate site, Pf). These two regions are separated by the filter, a layer which may be schematized as © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: March 18, 2019 Revised: April 18, 2019
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DOI: 10.1021/acs.jchemed.9b00230 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
by means of an additional syringe through the septum without the need to disassemble the system. In our experience, a pressure 0.5 atm higher than atmospheric pressure was sufficient to keep the time of filtration of about 2 mL of suspension within 2 min. When necessary, the feed was kept warm by blowing air from a heat gun. Solid residues lacking air sensitivity were typically removed by disassembling the system, collecting the sample with a spatula and allowing it to dry in air. With air-sensitive samples, the funnel containing the residue was introduced into a previously argon-filled container just larger than the filter funnel. The container was first evacuated to remove the solvent adsorbed on the solid and then filled with argon at atmospheric pressure from a gas bag. Subsequently, the container was used to store the product. We avoided the use of ground-glass joints, because grease is necessary to make them truly gastight, and they are plagued by capillary condensation19 of solvent vapors, which dissolve traces of grease and thus can contaminate the products. The procedure by Rodriguez et al.,4 is useful for atmospheric filtering of aqueous suspensions. However, we found it particularly convenient to use a filter-disk support plate, built by cutting the plastic end (not the rubber end) of the plunger of a syringe to avoid crimping of the filter disk as a result of the usually conical shape of the syringe bottom.
apparatus also allows easier control of temperature, whose propagation is faster over smaller distances where heat transfer is dominated by conduction.18 To carry out semimicro (20−200 mg of solid) filtrations of slurries of high vapor pressure or air sensitivity, we initially used a standard cylindrical fritted glass filter funnel with an internal diameter of 15 mm and a 16−40 μm nominal pore size (FAVS Bologna Catalogue 480.001153) closed at the top with an injection stopper. The syringe was pushed against it to keep it in place when the funnel interior was pressurized by use of the gas-filled syringe (see Figure 1). Subsequently we modified
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HAZARDS The procedure is not suitable and must not be used to handle slurries of extremely aggressive liquids, such as SbF5, H2SO4− SO3 (oleum), HSO3F, HSO3Cl, HF, and HNO3. When not in use, insertion of the tip of the syringe needle into an old septum avoids accidental pinpricks and unwanted influx of air into the syringe due to temperature changes consequent to temporary hand contact.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.9b00230. Pictures of the apparatus (PDF, DOCX)
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Figure 1. Scheme and components of the apparatus. The light-blue color indicates the sections pressurized when the syringe plunger is pushed.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bruno Lunelli: 0000-0001-5944-0682 Massimo Baroncini: 0000-0002-8112-8916
the funnel with the help of a glassblower (cost $30) by using a GL 18 screw-thread tube as the body of the funnel. A screwcap with a septum at the top was then added to enable higher and pulsed pressures (see Figure 1). Pressures are always modest, as they are generated by controlled hand manipulation of a syringe of volume comparable to that of the funnel. In the case of air-sensitive samples, the receiving container was previously filled with argon and connected to a deflated, gastight bag, as schematized in Figure 1. The funnel closure was perforated by an insulin needle (29−31 gauge, 5−8 mm long) fitted to the Luer nozzle of a syringe filled with nitrogen or argon. The plunger was pressed by hand or by silicone rubber bands4 to adjust the pressure on the feed to a convenient value. The residue was rinsed with solvent injected
Present Address †
B.L.: Via Monte Vodice 34, I-33100 Udine, Italy
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are indebted to Paolo Mei of the ISMN Mechanical Workshop for the precise construction of the connection to the gas bag and to Lucy S. Scioscia for language and style correction. B
DOI: 10.1021/acs.jchemed.9b00230 J. Chem. Educ. XXXX, XXX, XXX−XXX
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REFERENCES
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DOI: 10.1021/acs.jchemed.9b00230 J. Chem. Educ. XXXX, XXX, XXX−XXX
