MICRODENSITY OF PLASTICS
Introduction:
Density is defined as the mass per unit volume of a substance. It is one of the most
important properties (physical constants) used in the identification of substances. However, if
the sample of material is very small and of irregular shape, determination of mass and volume
may be difficult to accomplish with precision.
Usually, density is found by massing the object, measuring its volume, and then dividing
mass by volume. However, this lab demonstrates a different technique of determining density —
an “indirect” method. It uses the relative densities of substances and the properties they
demonstrate. An object with a low relative density will float on top of a liquid with a high
relative density. But, if the liquid’s density is somehow lowered, a point could be reached where
the densities are equal. At this point, the object will be suspended in the liquid, neither sinking
nor floating. This same idea is used for a relatively low-density liquid and a high-density object,
only the liquid’s density must be increased to suspend the material. Once the object is
suspended, the object and the liquid possess exactly the same density. The density of the object
can be determined indirectly by measuring the density of the liquid in which the object is
suspended.
Purpose:
To find the density of a small piece of plastic indirectly, and use the density to
identify the type of plastic.
Recyclable Group
1 = PETE
2 = HDPE
3 = PVC
4 = LDPE
5 = PP
6 = PS
7 – Other
Type of Plastic
Polyethylene terephthalate
High density polyethylene
Polyvinyl chloride
Low density polyethylene
Polypropylene
Polystyrene
(often a mixture)
Acceptable Density Range
1.23 – 1.32 g/mL
0.95 – 0.97 g/mL
1.28 – 1.32 g/mL
0.92 – 0.94 g/mL
0.90 – 0.91 g/mL
1.02 – 1.05 g/mL
varies
Equipment/Materials:
small piece of plastic
small test tube
dropper bottle of saturated NaI solution
dropper bottle of methanol
1 mL micropipet and plastic tips
forceps
mini-weight boats
analytical balance
stirring rod
Safety Considerations:


Always wear safety goggles and aprons in the chemistry lab.
Never eat or drink in the chemistry lab.
Procedure:
1.
Fill a test tube about one-fourth full with water.
2.
Obtain a piece of plastic from the samples available. It should be small, about the size of
a grain of rice. Record the number of your sample in the data table.
3.
Place the plastic in the test tube of water. Shake the test tube and use a stirring rod to
remove all of the air bubbles that want to cling to the plastic.
4.
If the plastic floats:
Add methanol - a few drops at a time to the test tube and carefully shake the
test tube until the liquid is homogeneous.
Use the stirring rod if necessary. Repeat
this process until the plastic is suspended.
If you add too much and the plastic sinks,
add a few drops of NaI.
If the plastic sinks:
Add NaI - a few drops at a time to the test tube and carefully shake the
test tube until the liquid is homogeneous.
Use the stirring rod if necessary. Repeat
this process until the plastic is suspended.
If you add too much and the plastic floats,
add a few drops of methanol.
(Caution: Be careful with the sodium iodide solution. It may discolor your skin or
clothing.)
5.
Once the plastic is suspended in the liquid, the density of the plastic should equal that of
the surrounding solution (provided there are no air bubbles clinging to the plastic to keep
it afloat). Place a mini-weigh boat on the pan of the analytical balance and tare (with all
doors closed, of course). Measure 1.000 mL of the solution with the micropipet and
place it in the weight boat. Record the mass and volume of the solution in the data table.
6.
Repeat the procedure for an unknown plastic sample which will be supplied by your
teacher.
7.
All solutions may be disposed of in the drain, but be careful not to allow the piece of
plastic to go down the drain. Rinse out your test tube and leave inverted in your rack.
Micropipet tips, weight boats, and plastic samples may be thrown in the garbage.
MICRODENSITY OF PLASTICS
Data Sheet
Names:_____________________________________
______________________________________
Data: Known Plastic Sample # _______
Date: ___________________
Period:___________________
Accepted Density Range: ________________
Mass of solution
__________g
Volume of solution
__________mL
Density of solution
__________g/mL
Density of plastic
__________g/mL
How does the density of the plastic compare with the accepted density range for your
plastic sample?
(Note: The acceptable density ranges are given in the chart on the front of the
procedure. If your sample does not agree, repeat until you get an acceptable value.
This will ensure that you are performing the procedure correctly before you test an
unknown sample.)
Unknown Plastic Sample:
Mass of solution
___________g
Volume of solution
___________mL
Density of solution
___________g/mL
Density of plastic
___________g/mL
By comparing the density you calculated to the reference list, what group of recyclable
plastics does your unknown sample most probably belong to?