Introduction to Gravity Filtration

Gravity filtration is the simplest filtration method, useful for separating small amounts of unwanted solids from a solution. A typical set-up is shown:

Alternative gravity filtrations

When filtering some materials, all that is required is a quick, coarse filter. A small ball of cotton wool can be pressed into the narrowing of the funnel using a glass rod and this can provide a rapid and effective filtration. A cotton wool filter funnel is particularly useful for removing (eg magnesium sulfate) and enables filtering straight into a round-bottomed flask prior to .

Set-up for a gravity filtration

The receiving conical flask should be clamped, as the complete filtration apparatus is top-heavy and would be easy to knock over. An appropriately-sized conical funnel is placed in the neck of the flask; the funnel must be fitted with a filter paper.

Filter paper

A circular piece of filter paper should be folded to fit inside the funnel. The right size paper should be chosen: after folding, the edge of the paper should not extend beyond the edge of the funnel. A simple method is to fold the circular filter paper twice, into a quarter circle, then partially opened to form a cone. A better, more time-consuming method is to “flute” the filter paper, by folding the quarter circle several more times, alternating the direction of folding each time; consult a demonstrator if unsure.

If the funnel fits snugly into the neck of the receiving flask, this can cause a problem during the filtration, as the filtrate enters the flask, but the displaced air in the flask cannot easily escape. This can be avoided by clamping the funnel in place above the flask, or by breaking the contact between funnel and flask with a small object (a piece of filter paper, or a bent paperclip, may be suitable).

At this stage, the suspension can be filtered by pouring it carefully into the filter funnel. It is good practice to pour the suspension in several portions, swirling the suspension before each pour, so as to maximise the transfer of material into the filter funnel each time. The suspension flask should be rinsed out with the same solvent, and these washings also poured into the filter funnel. The filtrate is then collected.

The most common problem that arises with a gravity filtration is that the filtration becomes very slow, due to the filter paper being blocked with material; hence, this technique is most suitable for removing small amounts of solid. If this happens, the remaining suspension can be poured into a fresh filter paper/funnel; this is much easier if the suspension is in the flask, than if it has all been poured into the original filter funnel in one portion.

Cotton Wool

Hot Filtration

Hot filtration is a gravity filtration in which the solution is kept hot throughout. This is most often done as the first step in a recrystallisation (see below), in order to remove any insoluble impurities by filtration; the solution must be kept hot, otherwise the desired compound may begin to crystallise during the process, leading to some of the desired compound being removed by filtration, along with the insoluble impurities. A typical set-up (using a hotplate – see point 2, below) is shown:

Set-up for a hot filtration

The set-up for hot filtration is very similar to gravity filtration, with one key difference: a stemless funnel is used, as otherwise, it is likely that the desired compound would crystallise in the stem of the funnel and block it.

In general, the procedure is also very similar to the gravity filtration, with the flasks of suspension and rinsing solvent kept hot until the moment they are used. There are two additional techniques, described below, which can be used to maintain hot solutions throughout, if necessary.

1. The apparatus may be heated in an oven, then removed, assembled and clamped just before the filtration takes place.

2. The apparatus is assembled with the receiving flask on a heat source such as a hotplate, and a few cm3 of solvent placed inside. The solvent is heated, and the hot vapour warms the apparatus from the inside.

When the entire suspension has been filtered, it is good practice to use hot rinsing solvent to wash the insoluble solid material trapped in the filter paper – this washes any residual solution into the filtrate, and should also redissolve any desired compound that may have crystallised out.

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