Why must thiosulfate solution be standarized every couple of days? Please explain with an equation. Thanks.

I quote from Fundamentals of Analytical Chemistry by Skoog/West/Holler, sixth edition (1992) page 374-5--

"Although sodium thiosulfate solutions are resistant to air-oxidation, they do tend to decompose to give sulfur and hydrogen sulfite ion:
S2O3^2- + H^+ ==> HSO3^- + S(s)

Variables that influence the rate of this reaction include pH, presence of microorganisms, solution concentration, presence of copper(II) ions, and exposure to sunlight. These variables may cause the concentration of a thiosulfate solution to change by several percent over a period of a few weeks. On the other hand, proper attention to detail will yield solutions that need only occasional restandardization."

My biggest problem through the years has been the microorganism problem and I always advised my students to restadardize if the solution was more than a week old.

Thank you DrBob222

Thiosulfate solution is commonly used as a titrant in analytical chemistry, particularly in the determination of the concentration of various substances, such as iodine, chlorine, and dissolved oxygen. The standardization process involves determining the exact concentration of the thiosulfate solution, which can vary over time due to several factors.

To understand why thiosulfate solution needs to be standardized, let's consider the reaction it undergoes in a typical titration:

Na2S2O3 + I2 -> Na2S4O6 + 2NaI

In this equation, thiosulfate (Na2S2O3) reacts with iodine (I2) to form tetrathionate (Na2S4O6) and sodium iodide (2NaI). The reaction is conducted in the presence of a starch indicator, which turns blue-black in the presence of excess iodine.

During the titration, the iodine is gradually consumed by the thiosulfate until the reaction is complete. The endpoint of the titration is reached when the blue-black color of the starch indicator disappears. At this point, the amount of thiosulfate used can be determined by balancing the equation and calculating the moles of thiosulfate consumed.

However, three main reasons require regular standardization of the thiosulfate solution:

1. Oxidation: Thiosulfate solutions are sensitive to oxidation by air and light. The thiosulfate ion can be oxidized to form sulfate (SO4^2-) and other species. This leads to a decrease in the concentration of the thiosulfate solution over time. By standardizing the solution regularly, you can determine the current concentration accurately.

2. Decomposition: Thiosulfate solutions can decompose over time, forming sulfur and other by-products. This decomposition leads to a decrease in the concentration and affects the accuracy of subsequent titrations. Standardizing the solution allows you to account for any decomposition that may have occurred.

3. Contamination: Thiosulfate solutions can become contaminated with other substances, such as metal ions, which can interfere with the titration reaction. Regular standardization ensures that any contamination is detected and the concentration adjusted accordingly.

By standardizing the thiosulfate solution every couple of days, you can ensure that the concentration is accurate, and any changes due to oxidation, decomposition, or contamination are accounted for. This allows for precise and reliable titrations in analytical chemistry.