Overview
- Sodium Polyacrylate is used to demonstrate an osmosis-like effect, specifically in the context of diapers.
Purpose
To illustrate the flow of water attempts to even out concentration gradients.
Procedure
- Introduction:
- Hold up a diaper and discuss its function.
- Reveal that it contains sodium polyacrylate.
- Experiment Setup:
- Use approximately 2 g of sodium polyacrylate with 50 mL of deionized (DI) water, then add another 50 mL of DI water.
- Repeat with a fresh 2 g of sodium polyacrylate and a saturated salt solution.
- Draw the structure of sodium polyacrylate for students to see, highlighting that sodium is trapped in the “cells.”
- Demonstration:
- Take a few scoops of sodium polyacrylate into the bottom of a beaker and add water. Control the ratio to create a gel within seconds, allowing the beaker to be turned upside down.
- Ask students why sodium polyacrylate absorbed the water.
- Show an illustration of the swollen sodium polyacrylate.
- Discuss how to extract the water by adding salt, which alters the concentration gradient.
Key Concepts
- Hydrogels: Cross-linked polymers with hydrophilic groups, such as carboxylic acid groups, which are water-attracting.
- Chemical Structure:
- Sodium polyacrylate, also known as poly(sodium propenoate), is a long, randomly coiled polymer.
- Removal of Na+ causes negative charges on oxide ions along the polymer chain to repel, leading to uncoiling. Water molecules are attracted to these charges via hydrogen bonding.
- Sodium polyacrylate can absorb over five hundred times its weight in pure water, but less in salty water.
Diapers and Leakage
- Diapers may leak due to pressure forcing water out of the polymer beads. Manufacturers increase cross-link density to mitigate this.
- The presence of salts in urine complicates absorption. Sodium polyacrylate can’t absorb all the water when salt concentration is balanced both inside and outside the polymer.
Osmotic Pressure Experiment
- Use a thistle tube with a membrane in a 2 L graduated cylinder next to the cabinet.
Additional Experiment
- Dialysis Tubing:
- Fill one bag with 25% glucose and 2% starch; place it in a beaker with deionized water and KI/I2 solution.
- Fill another bag with deionized water and iodine; place it in a beaker with a saturated salt solution and about 5 mL of starch solution.
- Observe the flow of water and the iodine/starch complex formation.
Chemical Data Table
| Compound | Mass (g) | Chemical Formula | Freezes at (°C) | Kf (°C/m) |
|---|---|---|---|---|
| Acetic acid | 25.0 | C2H4O2 | 16.6 | 3.90 |
| Benzoic acid | 2.44 | C7H6O2 | (to be determined) | (to be determined) |
| Benzene | – | – | 5.5 | 5.12 |
| Camphor | – | – | 179.8 | 39.7 |
| Carbon disulfide | – | – | −112 | 3.8 |
| Carbon tetrachloride | – | – | −23 | 30 |
| Chloroform | – | – | −63.5 | 4.68 |
| Cyclohexane | – | – | 6.4 | 20.2 |
| Ethanol | – | – | −114.6 | 1.99 |
| Ethyl ether | – | – | −116.2 | 1.79 |
| Naphthalene | – | – | 80.2 | 6.9 |
| Phenol | – | – | 41 | 7.27 |
| Water | – | – | 0 | 1.86 |
References
- Website for Images: TutorNext (may be subject to copyright).