Sinter filtration

Sinter filtration is an effective filtration method where the residue, filtrate or both contained the desired compound(s). The glassware for these setups is available in a range of sizes for filtration volumes between ~10 mL and 25 L.

Sinter funnels consist of a glass funnel containing a sintered disc or frit, which is made of a fine mesh of glass fibres. The frit comes in several different porosities, which is a measure of the size of the holes in the frit, and thus, how fine a material can be filtered. These are generally referred to using a scale of 0-5, where 0 is a very coarse filter and 5 a very fine filter. The most commonly used frit is a 3, which is suitable for the isolation of most common laboratory precipitates and crystals from recrystallisations.

Sinter funnels are often used in active filtrations particularly with Celite and Silica.

Setup

Sinter filtrations are generally carried out using a sinter funnel fitted into a Buchner flask setup, although filter adapters fitted into a round bottomed flask can also be utilised. The step by step instructions below apply to the former setup.

1. Fit vacuum tubing to the vacuum outlet of the Buchner flask and seurely clamp the flask in place around the neck. Take care when clamping to ensure that the clamp is not above the top of the flask neck, especially when using a Buchner ring, as this may reduce the effectiveness of the filtration

2. Attach the vacuum tube to the vacuum connector, but keep the vacuum turned off at this stage.

3. Place a Buchner ring or cone into the top of the Buchner flask.

4. Insert the sinter funnel through the cone or ring into the flask.

5. Slowly open the vacuum and check that the setup is working. The sinter will be 'sucked' towards the flask and become secured, due to the reduced pressure inside the flask. Turn the vacuum off before starting filtration.

You may wish to check the sinter is clean and not blocked by running solvent thorugh the sinter. Generally, you should use the same solvent system that your material is suspended in. You don't want to introduce other solvents which may cause issues with your reaction, eg the introduction of water into an otherwise dry organic layer. If you do 'test' or clean the sinter, you should empty the flask before starting filtration. Carefully remove the sinter funnel and place somewhere safe where it will not be knocked / fall over, before removing the Buchner flask. The notes under 'drying material under vacuum' contains an appropriate step by step method for carrying this out.

Running

These instructions assume a conical flask containing a suspension of material to be filtered. The instructions may need to be adapted for other setups.

1. Carefully pour in some of your suspension into the sinter funnel and carefully open the vacuum. Check that the filtration is proceeeding.

2. Continue to top up the funnel with remaining material. It is a good idea to swirl the flask before pouring to try and get any solid material into the suspension so that it transfers into the funnel.

3. Once all the liquid has been transferred into the funnel, it is likely that some residual solid remains in the flask. Ideally the flask is 'rinsed out' using small volumes of additional solvent. The choice of solvent (and volumes used), often needs careful consideration (see notes below).

4. The isolated product is also often 'washed' in the solvent at this stage. Details on this procedure are listed separately.

5. Allow the vacuum to pull through all the solvent in the sinter to be left with a reside in sinter.

At this stage, the filtration is usually followed by a brief 'drying over vaccuum' in order to remove residual solvent in the material.

Where the filtrate is desired, the time over active vacuum should be minimised. This is to minimise the evaporation of solvent from the filtrate.

Washing a precipitate over vacuum

Often when collecting a product via vacuum filtration, it is desirable to wash the collected material using a solvent. Washing the residue allows the removal of unwanted material, such as small amounts of the filtrate to be removed from the residue. Careful choice of the solvent is important to ensure that product is not lost through dissolving portions of the residue (see notes below).

In order to effectively wash a residue:

1. Turn off the vacuum and release the vacuum pressure. The easiest way to release the vacuum is to carefully disconnect the rubber tubing from the vacuum pump connection, which will allow air to enter the previously evacuated system.

2. If using a different solvent, it may be sensible to empty the collection flask of the filtrate. This is especially important where the material would go into different waste streams.

3. Add some of the washing solvent into the sinter to form a suspension with the residue. Use a spatula to stir up the residue to ensure that the residue is thoroughly washed in the solvent. Make sure the sinter is supported during this process, as the release of the vacuum means that the sinter is no longer being secured into the flask.

4. Slowly turn the vacuum back on, allowing the washing solvent to be pulled through into the flask below.

5. Repeat the above steps as nescessary.

Drying material over vacuum

In order to effectively dry material over vacuum, it is nescessary to empty the collection flask of any solvent. As many of the solvents used in the lab are volatile, prolonged vacuum filtration would cause the flask contents to evaporate. This reduces the effectiveness of any attempts to dry the residue over vacuum.

1. Turn off and carefully release the vacuum to the setup. The easiest way to release the vacuum is to carefully disconnect the rubber tubing from the vacuum pump connection, which will allow air to enter the previously evacuated system.

2. Remove the sinter and place this somewhere secure, where it isn't going to be knocked over or broken.

3. Empty the collection flask of the filtrate.

4. Reclamp the flask back in place and reconnect the vacuum tubing to the pump.

5. Return the sinter to the setup.

6. Open up the vacuum and allow the residue to begin to be sucked dry for a few moments.

7. When the material looks visibly dry, break up the residue using a spatula whilst still over vacuum.

8. Periodically stir the residue with a spatula to allow the solid to fully dry over vacuum.

9. Once the residue is dry, turn off the vacuum and carefully release the vacuum.

10. Isolate the material.

Isolating the residue

When isolating the residue, consideration needs to be given as to where the residue wants to be for the next steps. For example, this might be to place the material into a watch glass or sample vial for characterisation, or it might be placed into a conical flask for further steps, such as recrystallisation.

The isolation of material will be carried out using a spatula to transfer the contents of the sinter into another vessel. Note that it is usually a good idea to record the mass of the empty vessel prior to transfer, so that you can determine how much material has been isolated following transfer.

However, for narrow necked vessels, such as conical flasks, direct transfer runs the risk of spilling the compound. In these instances, transferring the material first onto a filter paper, before using the filter paper to pour the solid into the desired vessel is often advantageous.

When removing residue from the sinter, care should be taken to not 'scratch' at the sinter surface too much, as it is possible to break up the surface of the sintered frit, which results in fragments of glass fibre being isolated.

Isolating the filtrate

Where the filtrate is desired, the vacuum should be turned off as soon as possible after filtration in order to avoid evaporation of the solvent.

1. Turn off and carefully release the vacuum to the setup. The easiest way to release the vacuum is to carefully disconnect the rubber tubing from the vacuum pump connection, which will allow air to enter the previously evacuated system.

2. Remove the sinter and place this somewhere secure, where it isn't going to be knocked over or broken.

3. Disconnect the vacuum tubing from the Buchner flask.

4. Pour out the solvent from the neck of the flask into the desired vessel for the next step. Be aware of the flask side arm, and ensure that the flask is poured with the side arm remaining uppermost, so that solution is not lost via this arm. It is good practice to use a funnel when pouring to ensure the material isn't spilled.

5. Rinse the flask out with addtional small volumes of solvent to ensure that all of the filtrate is transferred.

Cleaning a sinter

Sinter funnels have particular challenges with cleaning, as material may be present inside the sintered frit, as well as in the funnel and stem. For general cleaning, the procedure listed below should be appropriate, however it is sometimes nescessary to follow cleaning procedures specific to the materials or procedure which have been used. the procedure assumes that the residue has been collected following filtration.

1. Rinse out any residual precipitate with small volumes of solvent into an appropriate waste stream. Ideally use a solvent the residue is known to be soluble in. In most cases, this is likely to be acetone.

2. Return the sinter to an empty Buchner flask setup. Add acetone into the funnel, then turn on the vacuum to draw the acetone through the sintered frit which allows this to be cleaned.

3. Empty the solvent flask into the appropriate waste stream.

4. Return the funnel into the flask and next draw water through the sinter.

5. Re-empty the flask, and repeat the steps to draw through a further portion of acetone.

6. The sinter should now be clean. The remaining glassware (Buchner flask) can be cleaned in a conventional manner.

Solvent choices for washing precipitate

Where material is known not to be soluble in the solvent forming the supsension, then volumes of solvent can be used without worrying about solubility issues. In other situations, such as following a recrystallisation, the precipitate could potentially be redissolved if lots of solvent was used to wash out the precipitate, or the solvent makup could be affected in a multi-solvent system recruystallisaiton which could cause problems. In general, to avoid redissolving material which is trying to be isolated, small volumes of ice-cold solvent of the recrsyallisation system (in the same proportions used) would be the most appropriate choice here.

Sometimes it is also nescessary to empty the flask between washing stages of the precipitate where different solvents are used. This is often driven by the need to consider the waste disposal routes for solvents. For example, a precipitate might be intitially isolated from water, before being washed with a water miscible solvent, and finally an organic solvent. It would be usual to empty the Buchner flask between each washing to ensure appropriate disposal of the filtrate in these cases.

Common issues

Flask becoming full

When filtering, it is possible that the receiving flask could become filled, and would need emptying before continuing. With a Buchner flask, you should not let the solvent level go above the stem of the sinter funnel, or let the solvent level approach the side arm of the Buchner flask. If you are approaching either of these situations, stop the filtration, and transfer the filtrate contents before resuming the filtration

Slow filtration

Sometimes sinter funnels become blocked with material, leading to slow or a stoppage in filtration. A blocked filter can be diagnosed by the sinter still being held in place by vacuum (ie the issue is not caused through poor vacuum), but no material is being drawn through.

Blocked filtrations can arise for a number of reasons:

The solid being isolated includes very small particles which are resulting in the sinter pores becoming blocked.

The filtration conditions are leading to precipitation in the sinter funnel. E.g. the use of an antisolvent to collect crystals from a recrystallisation is resulting in material being precipitated during the filtration in the sintered frit.

The solution to slow filtration depends on the precise cause. For very fine particulates, consider filtration using a filter agent, or an alternative filtration method (e.g. gravity). Note filter agents generally limit the filtration to processes where the filtrate is the desired product.

When collecting products, the solvent choices in washing are very important. When collecting products from recrystallisation, it is important to not affect the solvent composition to avoid causing precipitation. See the notes on solvent choices.

Presence of solid in the filtrate

This is generally not an issue from material passing through the sinter, but from the evaporative losses due to the vacuum nature of the filtration. This is especially common in recrystallisation where the filtrate (known as the 'mother liquour') will contain material in solution. Under vacuum, the solvent is liable to evaporate, which results in precipitation occurring. This material is likely to be of significantly lower purity than the crystals already isolated.

Isolation of sinter glass with the desired material

Scratching of the sintered glass surface can result in fragments of glass fibre being broken off the sinter. Take care when isolating product to avoid scratching at the surface.