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Standardize with 0.1 M sodium thiosulfate

Standardization of sodium thiosulfate 0.1 M by USP method:

Weigh the amount of 210 mg of potassium dichromate, which has been heated to 120 degrees Celsius for 4 hours and cooled in a desiccator, and write down the exact amount.

Dissolve it in 100 ml of water in a 500 ml flask with a lid.

Stir until completely dissolved.

Immediately add 3 grams of potassium iodide, 2 grams of sodium carbonate and 5 ml of concentrated hydrochloric acid to it and close the lid of the flask. Stir until dissolved.

After exactly 10 minutes, titrate the solution with the prepared thiosulfate. Near the end point where the solution turns yellow-green, add 3 ml of starch glue and continue until the titration solution becomes colorless.

Starch glue must be fresh.


Important points:

The effective factors in the stability of sodium thiosulfate solution are: pH, micro-organisms, impurities, environmental oxygen, exposure to sunlight.

If the pH of the solution is 5 or less, the following reaction will take place:

The experiment shows that the stability of thiosulfate solutions is maximum at pH between 9 and 10. However, in most cases pH=7 is also sufficient.

The most important factor in the instability of thiosulfate solutions can be considered “bacteria” which causes the metabolism of thiosulfate ion and turns it into sulfite, sulfate and sulfur.

The activity of bacteria is minimized in pH between 9 and 10.

Final points:

As mentioned, thiosulfate is sensitive to microorganisms. Therefore, it is better to prevent contamination in the solution as much as possible. This is one of the reasons for boiling water before making the solution.
If you want to increase this level of accuracy, you can sterilize your thiosulfate container inside it.

It is recommended to keep the thiosulfate solution in a glass container with a screw cap and keep it cool (if possible in the refrigerator). Because not closing the lid of the container well and keeping it in a place with high temperature will increase the possibility of evaporation during the next few days and the molarity of the solution will change.

*Tiosulfate standardization should be done at least every week.*