Sodium Polyacrylate (0.5 G) Styrofoam Cup

Superabsorbent Polymers

Sodium polyacrylate and PolySnow are examples of superabsorbent polymers. Superabsorbents operate on the principle of osmosis: the passage of water through a membrane permeable only to water. In both sodium polyacrylate and instant snow powder, osmotic pressure results from the difference in sodium ion concentration between the inside of the polymer and the solution in which it is immersed. This osmotic pressure forces water into the solid polymer lattice in an attempt to equilibrate sodium ion concentration inside and outside the polymer membrane. The electrolyte concentration of the water will affect the osmotic pressure, subsequently affecting the amount of water absorbed by the polymer. For example, sodium polyacrylate will absorb approximately 800 times its own weight in distilled water, but will only absorb about 300 times its own weight in tap water, due to the high ion concentration of tap water.

Materials

Sodium polyacrylate (0.5 g) Styrofoam Cup

PolySnow (3.0 g) Distilled Water (250 mL)

2 Beakers (250 mL)

Procedure

1.  Add approximately 100 mL of distilled or deionized water to the second 250 mL beaker.

2.  Tell the students that the water will “disappear” when poured into the Styrofoam cup. (You can tip the cup containing sodium polyacrylate forward somewhat to show that it is “empty”; it will be difficult to see 0.5 g against the white Styrofoam cup.)

3.  Slowly pour the water into the cup containing sodium polyacrylate. Swirl the cup a bit (give the sodium polyacrylate time to absorb the water).

4.  Briefly tip the cup upside down to show the students that the water has “disappeared.”

5.  Set the beaker containing 3.0 grams of sow powder on a paper towel or tray.

6.  Add 150 mL of distilled or deionized water to the empty 250 mL beaker.

7.  Slowly add the water to the beaker containing 3.0 grams of PolySnow. The PolySnow will absorb the water, producing white powdery “snow” that overflows the beaker.

8.  Once all the water bas been absorbed, both cups can be emptied onto the paper towel or tray to show the class the gel and snow products.

9.  Add a small amount (1 gram) of sodium chloride to each of the 250 mL beakers. Add the gel to one and PolySnow to the other and stir each. The gel and the powder will release the water and each will transform into a slurry.

Source

Flinn Scientific Inc. The Wacky World of Superabsorbent Polymers. Catalog No. AP6303. Publication No. 6303.