# 1.5F: Pipette Filtration (Microscale)


For the separation of small volumes (< $$10 \: \text{mL}$$) of solid-liquid mixtures, pipette filters are ideal as filter papers absorb a significant amount of material. Pipette filtration may also be used if small amounts of solid are noticed in NMR or GC samples, as both instruments require analysis of liquids without suspended solids.

To create a pipette filter, use a long rod to wedge a small piece of cotton into the bottom of a Pasteur pipette (Figure 1.89a-c). For very small volumes (< $$2 \: \text{mL}$$), a GC vial makes a nice receiving flask. These vials are very easy to tip over, but can be held in place by an upside-down large septum (Figure 1.89e).

Pipette the solution to be filtered through the top of the filter pipette (Figure 1.89d). It's best to allow the liquid to trickle through the filter on its own, and to at first not use pressure from a dropper bulb, or else solid may be forced through. After the majority of the liquid has filtered, the residual liquid that stays in the pipette (Figure 1.89e), can then be gently pushed through with pressure from a dropper bulb.

Pipette filters can be filled with a centimeter or two of solid material on top of the cotton plug to achieve more than just a solid-liquid separation. They can be filled with drying agents (e.g. $$\ce{Na_2SO_4}$$) to remove small amounts of water from a solution, or basic solids (e.g. $$\ce{Na_2CO_3}$$) to neutralize a slightly acidic solution. With the addition of solid to the pipette filter, the liquid must then be gently pushed through with pressure from a dropper bulb as it will not drip through on its own.

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