Home Chemistry: Sodium Thiosulfate Synthesis via NaOH + S Method
3,285 views · Published 27 August 2018 · 9:06 · Indexed 6 October 2026
Channel: Matthew Wilson · 2018 · Howto & Style
This is a solution of sodium thiosulfate and sodium polysulfides which I created by boiling a concentrated solution of sodium hydroxide with elemental sulfur. The initial production of thiosulfate is disproportionately abundant but is highly contaminated by sodium sulfide and a mixture of polysulfides. Polysulfides readily oxidize with O2 in the air but the process is slow. A fish tank air bubbler will suffice and is likely the most efficient route as well as the cheapest. Pure oxygen can be bubbled through solution by a simple oxygen generator utilizing hydrogen peroxide and a manganese dioxide catalyst. As the polysulfide(s) solution is oxidized to thiosulfate, the solution pH will drop and if it becomes too close to the neutral range, hydrogen sulfide production will result along with the precipitation of elemental sulfur. Using sodium hydroxide to keep the solution pH between 11 and 13 has so far given me the best results in the final product. The solution will gradually lighten as it oxidizes from the starting red or orange color depending on how saturated it is with sulfur, and will eventually turn colorless. I find it best to mix the initially created solution with a volume of distilled water in a 5:1 ratio. This gives a much larger surface area for the oxygen bubbles to travel through. This has resulted in successful runs previously for me, but in this video I only diluted the solution by 50%. In order to achieve successful results I may end up adding the approximate 20mL of solution I have in this test tube to a larger flask with 100mL of distilled water.--I'm not thrilled with the current pace of oxidizing. In regards to more rapid oxidation, the use of hydrogen peroxide will result in the formation of sodium sulfate, precipitated sulfur and a larger production of hydrogen sulfide which is very toxic. From a previous experiment the entire flask of polysulfides solution was completely oxidized to sodium sulfate. The test I used to determine this hypothesis was dropping some solution onto a piece of copper. The darkened oxide layer already present was quickly removed revealing clean bright copper. Had the solution contained any sulfide/polysulfides, black copper sulfide would immediately appear. This is also the test I perform fo confirm sulfide formation.
