Illustrated Guide to Home Chemistry Experiments

84 DIY Science: Illustrated Guide to Home Chemistry Experiments

the funnel. Make sure that all of the solid and liquid is

transferred into the funnel.

When all of the liquid has drained through the filter paper,

rinse the solid filtrand several times with a few mL of

water to make sure that all of the soluble material has

been rinsed into the receiving vessel.

If the filtrand is waste material, remove the funnel and

dispose of the filter paper and filtrand properly.

If the filtrand is your product, carefully remove the filter

paper and filtrand from the funnel and transfer them to

a Petri dish or similar container for drying. Once the filter

paper and filtrand are thoroughly dry, you can determine

their combined mass and subtract the mass of the filter

paper to determine the mass of the filtrand.

Wash and dry the funnel and any other vessels you used

and put them away.

FIGURE 5-13:

Making a filter paper cone

FIGURE 5-14:

Proper filtration procedure

SATIEpAR oNS

In a laboratory sense, separation means physically dividing two or more immiscible liquid layers,

usually an aqueous layer and an organic layer. A separation most commonly follows an extraction,

in which a substance dissolved or suspended in one solvent is extracted from that solvent by

agitating the solution with a quantity of a second, immiscible solvent. If the solute is more soluble

in the second solvent than in the first, it is preferentially extracted into the second solvent.

Because the two liquids are immiscible, they separate upon standing into two physical layers,

which can subsequently be divided and isolated from each other.

For example, in one of the laboratory sessions later in this
book, we produce an aqueous solution that contains elemental
iodine. Iodine is relatively insoluble in water, but is very soluble
in many organic solvents. Adding a quantity of an organic
solvent to the reaction vessel and agitating the mixture allows
the iodine to migrate from the aqueous solution to the organic
solvent. Upon standing, the solvent layers separate, with the
less dense organic layer—which now contains nearly all of the
iodine—floating on top of the aqueous layer, which still contains
nearly all of the other reaction products. By drawing off and later
evaporating the organic layer, we can isolate nearly all of the
iodine in relatively pure form.

In formal laboratories, most separations are done with a
separatory flask, usually called a sep flask. If you don’t own a
sep flask, you can still do a good separation using inexpensive
standard glassware and a disposable plastic pipette. In the
following example, assume that you begin with two layers,
each of about 100 mL, in a 250 mL flask or beaker. If the
actual quantities differ significantly, you can modify this
procedure accordingly.

1. First, make absolutely certain that you know which layer
   contains your product and which layer is waste. Many
   novice chemists have embarrassed themselves by
   pouring the product layer down the drain and saving the
   waste layer. In this example, we’ll assume that the top
   layer is the product layer.


2. After allowing the two layers to separate completely,
   carefully pour the top layer into your 100 mL graduated
   cylinder. Make sure to get all of the top layer, and as little
   as possible of the bottom layer—at most, a few mL.


3. Allow the layers to separate completely again, and then
   pour as much as possible of the top layer into a receiving
   beaker or flask, making sure that none of the bottom
   layer is transferred. At this point, a few mL at most should
   remain in the 100 mL graduated cylinder.


4. Pour the entire remaining contents of the 100 mL
   graduated cylinder into your 10 mL graduated cylinder
   and allow the layers to separate.


5. Use the disposable plastic pipette carefully to draw up as
   much as possible of the top layer, as shown in Figure 5-15,
   and transfer that liquid to the receiving beaker or flask.

13.

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