Cliff Kubiak

Clifford P. Kubiak

Filtration Techniques

Suction Filtration

To perform suction filtration in the drybox, a filter flask fitted with an O-ring adapter and a glass frit is securely clamped to a ring stand. Vacuum is applied to the side arm of the filter flask . A T-tube assembly (Figure 1) is connected to the filter flask via one of the two available hose outlets. (It is assumed that a liquid N2 trap has already been set-up and the pump has been turned on. If a sample is already being dried under vacuum prior to filtration, it is wise to place that sample under N2 via the T-tube assembly before you start filtration to prevent possible contamination of that sample.)

To begin filtration, check if the T-tube is in the "closed" position (Figure 4-1). Turn on the "main" vacuum source at the back of the dry box. Gradually bring the T-tube stopcock into the "open" position after placing enough solution in the frit. Upon completion of the filtration, make sure that any previous samples being dried are returned to vacuum. If no samples were previously connected to the T-tube, return to "closed" position and turn off the main valve on the drybox wall.

Pressure Filtration

Glass Frit

This method is a quick alternative to suction filtration for filtering small quantities of solution (≤ 20 mL). Fill a glass frit, securely mounted in a clamp, approximately half full with your solution. A rubber pipet bulb is inserted into the glass frit until a seal is made (Figure 4-2). Gradually apply hand pressure to the bulb and collect the filtrate in an appropriate vessel.

Pipet Mini Filter

This method is a quick alternative to suction filtration. This method is good for small quantities of solution when the solid is not desired. A disposable glass pipet is layered with glass wool and Celite (Figure 4-3). The filtration is facilitated by forcing the solution through the Celite and glass wool with a disposable pipet bulb. This filtration method is good for the preparation of NMR and IR solution samples.

Schlenk Filtration

The basic problem in Schlenk filtration is to minimize the exposure of the reaction mixture to moisture or oxygen. Normally there are three pieces of glassware involved in the operation. There are two variations, similar in concept but different in equipment, pictured in Figures 4-4 and 4-5. The trick is to connect the flask containing the reaction mixture to the Schlenk frit which is in turn is fastened to a large receiver Schlenk flask. In most cases where the desired product is the precipitate, it is wise to use a receiver flask whose volume is at least twice that of the reaction flask to allow for washing of the product. The Schlenk frit and receiver assembly should be evacuated and filled with inert gas at least three times before anything else is done.

Using equipment in Figure 4-4, there are essentially two ways of transferring the mixture into the frit. If the product is fine grain in nature, the mixture may simply be transferred via a cannula of suitable size by applying a partial vacuum to the collecting flask. (The cannula should be thoroughly purged with inert gas before connecting it to the reaction flask.) The more delicate way is to open one end of the reaction flask (the bent side) and with dexterity and quickness connect this to the mouth of the Schlenk frit under a positive flow of nitrogen. This is best done by greasing the bent side arm joint, removing septa from both the reaction flask and the Schlenk frit and quickly combining the two with a large nitrogen flow present. This latter method of joining the frit with the reaction flask involves a certain degree of mobility so care must be taken not to clamp things too tightly. When doing this the first time, it is wise to practice. The equipment in Figure 4-5 can be assembled anaerobically in the dry box, avoiding the "gymnastics" of Figure 4-4 assembly of reaction flask and frit. Washing of the product is simply achieved by using a cannula to transfer solvent from a reservoir flask or bottle onto the filtered product. Note: For other variants of glassware equipment and set-up consult the references in part A.

Kramer and Cannula Filtration

Air-sensitive samples and solutions may be conveniently filtered on the bench top using Kramer and Modified Kramer Filters with cannula techniques. Finely divided suspensions of impurities in air-sensitive solutions (such as those generated in Grignard reactions, sodium amalgam reductions, etc.) can be removed by using a disposable version of the Kramer filter. This filter, Figure 4-6, is comprised of a disposable syringe and needle, glass wool, and

Celite. The disposable syringe and needle is packed with a layer of glass wool, a layer of Celite and then another layer of glass wool. The syringe barrel is topped with a rubber septum. The filter is degassed by vacuum/inert gas charge cycles or by an inert gas purge. The solution to be filtered is transferred via cannula through the rubber septum, passed through the filter and collected in a receiving flask. The filter and its contents can then be discarded in an appropriate manner. (A commercially available Kramer filter is shown in Figure 4-7.)

If the solid is to be isolated the Kramer filter can be substituted by a glass frit. The top of the glass frit is capped with a rubber septum. The stem of the funnel is inserted into a predrilled hole of another septum which is in the receiving flask. The filter can be used as previously described for the Kramer filter.

Filtered Stick (Gas-Dispersion Tube)

This method consists of a dry, coarse gas-dispersion tube immersed in a liquid and connected to the receiving flask by flexible tubing (Figure 4-8).

The liquid is forced into the receiving flask by nitrogen pressure. If the liquid is the desired component the flexible tubing should be made of an inert material such as Teflon. An alternative, is to place a rubber septum on the end of the gas-dispersion tube and use a cannula to transfer the liquid.

This method has the advantage of allowing the solution being filtered to be heated or cooled during filtration. Disadvantages are that this method may be slower than others and that the small surface area of the gas-dispersion tube may clog easily.

Millipore Filtration

Millipore filters are thin, porous structures made of pure and biologically inert cellulose esters or related polymeric substances. They are available in more than 20 pore sizes ranging from 14 μm to 25 nm (0.025 μm); in disks from 13 to 293 mm in diameter. Their characteristics include very uniform pore size, high porosity and flow rate, high chemical stability (even toward concentrated acids and bases), negligible residue after incineration and transparency when pores are filled with an immersion oil of matching refractive index. Write for Catalog MC/1, Millipore Corporation, Ashby Road, Bedford, Mass. 01730. (See Scale 4-1 at end of this section.) Other membrane filters can be purchased through chemistry supply houses such as Matheson Scientific, Inc.

Due to their durability, Belman, Alpha Metrical Filters have been found to be extremely reliable and stand up to most organic solvents. These membranes are small diameter, small pore size syringe filters which limits their use to very small scale reactions or NMR/EPR sample preparation in which small particulate matter is removed from a 1-5 mL liquid sample. With the use of syringe/Schlenck techniques, this can be an anaerobic filtration (Figure 4-9).