Once in a Blue Moon

The saying, “Once in a Blue Moon” refers to two full moons occurring within the same month, a rarity. This recently occurred in January 2018. The actual moon appearing blue in color occurs from time to time as well. Usually it is the result of particulate matter in the atmosphere, such as the ash spewed from a volcanic eruption, scattering light such that only the blue light is visible to our eyes. Dr. R. Greenler discussed this phenomenon and showed how this could be demonstrated using hydrochloric acid and sodium thiosulfate. The demonstration is similar to “The Chemical Sunset” demonstration, except the growth rate of colloidal particles is much slower. This demonstration is a great application to a real word event and as a lead-in to inquiry investigations. It might even be a spooky demonstration for Halloween.

Materials

• 10 g of sodium thiosulfate pentahydrate
• 1 M hydrochloric acid
• Deionized (DI) water (used throughout)
• Overhead projector and screen, black paper and tape
• 125 to 147 mm crystallization dish (A large Petri dish can also be used and is less expensive.) 
• Roscolux filters (available from theatrical supply houses) #02 Bastard Amber, #54 Special Lavender, #63 Pale Blue, #98 Medium Gray

Crystallization dish set atop an overhead projector.

Safety

• Wear safety glasses or goggles, aprons and gloves.
• Hydrochloric acid is corrosive, avoid contact. Spills should be neutralized with baking soda and then the area rinsed with water.
• Small amounts of sulfur dioxide gas are produced. Perform this demonstration in a well-ventilated area. 

Advanced preparation

• Prepare the sodium thiosulfate solution by dissolving 10 g of sodium thiosulfate pentahydrate in 200 mL of water.
• Prepare 1 M hydrochloric acid solution by diluting 8.3 mL of concentrated acid in water to a final volume of 100 mL. Remember to always add acid to water slowly.
• Cut a hole in the black paper slightly smaller than the Petri dish slightly below the center of the black paper.
• Cut three 1-inch square holes near the top of the paper.

• Cut squares of each colored filter (amber, lavender and pale blue) slightly larger than the 1-inch square holes in black paper. Cover each 1-inch hole with the appropriate colored filter, securing with tape. Then cover each colored filter with the gray filter and secure with tape. This will allow the projected colors to match the colors projected on the screen by the actual reaction.

• Prepare 1 M hydrochloric acid solution by diluting 8.3 mL of concentrated acid in water to a final volume of 100 mL. Remember to always add acid to water slowly.
• Cut a hole in the black paper slightly smaller than the Petri dish slightly below the center of the black paper.
• Cut three 1-inch square holes near the top of the paper.
• Cut squares of each colored filter (amber, lavender and pale blue) slightly larger than the 1-inch square holes in black paper. Cover each 1-inch hole with the appropriate colored filter, securing with tape. Then cover each colored filter with the gray filter and secure with tape. This will allow the projected colors to match the colors projected on the screen by the actual reaction.

The order from left to right as viewed on the screen should be amber, lavender and pale blue. You can use clear tape to attach each square.

Performing the demonstration

• Place the cutout black sheet on the overhead projector so that the colors appear in the desired order.
• Place the Petri dish over the cut out round hole. 
• Add 13 mL of the prepared sodium thiosulfate solution to the dish.
• Dilute the 1 M hydrochloric acid solution by mixing 
• 5 mL with 28 mL of water. Add this solution to the sodium thiosulfate solution and stir briefly to mix.
• Turn off the room lights and turn on the overhead projector and allow the mixture to stand. In approximately 1 minute and 20 seconds the color on the screen will be amber. In about 2 minutes and 40 seconds the color will be lavender, and in about 4 minutes the color will be pale blue. 

​Cleanup

Check to see if the mixture is still acidic by adding baking soda or check with pH paper. When the solution is neutralized, it can safely be disposed of in the sink. Check with your local authorities before doing so.

Explanation

This demonstration is very similar to the “Chemical Sunset” published in Chem 13 News, February 2016. The reaction between sodium thiosulfate and hydrochloric acid produces colloidal sulfur particles that are approximately the same size. 

Na₂S₂O₃(aq) + 2HCl(aq) → S(s) + SO₂(g) + 2NaCl(aq) + H₂O(l)

The light is projected through the reaction mixture in the crystallization dish and is scattered by the colloidal sulfur particles. Initially, the particles are very small, and scatter the shorter wavelengths (blue) of light more than the longer wavelengths (red), thus resulting in the color amber being projected onto the screen. As the particles increase in size, the scattering maximum moves to longer wavelengths (red), the color projected onto the screen becomes lavender, then blue. The sulfur particles continue to grow until the light dims and, finally, no light is transmitted (black). This scattering of light is known as the Tyndall effect, and depends upon both the particle size and the thickness of solution through which the light travels. The sulfur nanoparticles absorb photons of light that have frequencies (and wavelengths) that correspond to their excited electronic states and, as they relax to their ground electronic states, photons of light are emitted in random directions, thereby resulting in scattering of the light.

As an inquiry activity, students can investigate how changing concentrations and/or depths can affect the rate of reaction and colors of light scattered and transmitted.

Reference

1. R. Greenler and R.K. Brandt, Optics & Photonics News, 1994, 5, 6-7, pages 66-67.

Kacey Hall is a chemistry graduate student and Dr. Kenneth Lyle is a Lecturing Fellow at Duke University, Durham, NC 27708.