Prepare a Sample Solution


These instructions are for the preparation of arachidic acid.  Adjustments may be made for the preparation of other chemicals.

Wear gloves, turn the hood on.
Use a clean vial and a micro-spatula cleaned with methanol .

The molecular weight (MW) for arachidic acid is 312.5 g/mol and the molecular formula is C20H40O2.
We need to know how much sample we need for spreading our monolayers.  The molecular area of arachidic acid on the surface of water is about 20 angstroms (A).  We want the monolayer to crystalize at a trough area of 130 cm2:

    130 (cm2) / 20x10-16 (cm2) = 6.5x1016 (molecules)

    6.5x1016 (molecules) / 6.022x1023 (molecules/mole) = 1.079x10-7 (moles)

We spread our monolayers with syringes that measure in the micoliters.  A convenient amount of solution to spread would be 30 to 50 microliters:
    molarity = moles / volume

    1.079x10-7 (moles) / 40x10-6 (L) = 2.6x10-3 (M)

Our volumetric flasks measure to 10 mL.  We need 10 mL of a 3 mM solution:
    molarity = moles / volume

    moles = mass / MW

    Therefore ==>  mass = molarity * volume * MW

    mass = 3x10-3 (M) * 10x10-3 (L) * 312.5 (g/mole) = 9.4x10-3 (g)

So our desired weight is about 9.4 mg of arachidic acid.  Use a balance accurate to at least .1 mg.  It is imparative that we work as cleans as possible due to the fact that we are measuring such small amounts of mass.  Weigh the vial and acid.  Record the sample's weight, cap and label the vial.  The label should contain the name of the sample, the date prepared, and it's weight, e.g. Arachidic Acid, 11-3-97, 5.7 mg in 10 mL of chloroform.

At this point, we dissolve the acid in 10 mL of HPLC grade chloroform. Be sure to do this step under the hood.  Also, be sure the instruments are clean for use with cholorform and waste is put in appropriate bottle.  When chloroform has been added, cover vial and put sample in refridgerator.  Put all chemicals away and clean up.