In materials science and chemistry, a colloid represents a stable solution of small, solid particles suspended in a liquid. Colloids typically exhibit particle sizes less than 100 nanometers (nm). The key to successful preparation of a colloid lies in the proper selection of a stabilizing agent that binds to the particles as soon as they form to prevent the particles from binding together into large agglomerates. Gold colloids represent one of the easier colloids to synthesize. As an added benefit, colloidal gold solutions assume different colors, ranging from red to pale blue, based on the average particle sizes of the gold. Consequently, the color of the gold solution provides a convenient indication of a successful colloid synthesis.
Instructions
Solution Preparation
1. Put on gloves and safety goggles. Weigh out 0.01 g of tetrachloroauric acid, HAuCl4, on a balance and transfer it to a clean, empty, 125-mL Erlenmeyer flask. Fill the flask to the 50-mL mark with distilled water and swirl it until the solid HAuCl4 has completely dissolved.
2. Measure out 0.05 g of trisodium citrate on a balance and transfer it to a clean 50-mL Erlenmeyer flask. Measure out 10 mL of distilled water in a 10-mL graduated cylinder, add it to the trisodium citrate and swirl the solution until the citrate has dissolved.
3. Transfer the HAuCl4 solution to an amber bottle, or a clear glass bottle wrapped with aluminum foil, cap it tightly and label it "HAuCl4 solution." The solution is sensitive to light, so place it in a dark location, such as inside a cabinet, until you need it. Label the flask containing the sodium citrate solution as "citrate."
Colloid Synthesis
4. Measure out 20 mL of the HAuCl4 solution using a 50-mL graduated cylinder and add it to a 50-mL Erlenmeyer flask.
5. Place the flask containing the HAuCl4 on a hot plate and bring it to a boil. Measure out 2 mL of the citrate solution in a 10-mL graduated cylinder and add it to the boiling HAuCl4 solution.
6. Boil the reaction mixture in the 50-mL Erlenmeyer flask until the solution takes on a deep red color. Remove the flask from the hot plate using a set of tongs and place it on a heat-resistant surface to cool. The flask contains colloidal gold.
7. Obtain a better visualization of the color of the solution by placing about 3 mL of distilled water and 3 mL of the colloid in a small vial or beaker.
Tags: Erlenmeyer flask, HAuCl4 solution, 50-mL Erlenmeyer, 50-mL Erlenmeyer flask, distilled water