I found the following procedure to prepare starch indicator solution on a website:-

To prepare starch indicator solution, add 1 gram of starch (either corn or potato) into 10 mL of distilled water, shake well, and pour into 100 mL of boiling, distilled water. Stir thoroughly and boil for a 1 minute. Leave to cool down. If the precipitate forms, decant the supernatant and use as the indicator solution. To make solution long lasting add a pinch of mercury iodide or salicylic acid, otherwise it can spoil after a few days.

If I add salicylic acid to my starch solution, what would you recommend to be the expiry date of my solution? I would put it at 3 months or if the solution no longer turns blue with iodine, whichever is earlier.

What do you think?

Frankly, I've not had much success with so-called long lasting starch as an indicator solution. I prefer to make mine daily and discard at the end of the day. It takes only a minute or two to make good starch solution (although a little longer if you boil the solution and let it cool) and the little time it takes to prepare is a small price to pay for KNOWING that it is good.

Adding salicylic acid to the starch solution can help prolong its shelf life. Salicylic acid is a preservative that inhibits the growth of microorganisms, preventing spoilage. However, the exact expiry date of the solution will depend on various factors such as storage conditions and contamination.

Considering the information provided, setting an expiry date of 3 months or until the solution no longer turns blue with iodine, whichever comes earlier, seems reasonable. It is important to monitor the solution for any signs of spoilage or contamination and discard it if any such indication occurs before the 3-month mark.

Adding salicylic acid to the starch indicator solution can help extend its shelf life. Salicylic acid is a preservative that can inhibit the growth of microorganisms and prevent spoilage. However, the exact longevity of the solution after adding salicylic acid can vary depending on various factors such as storage conditions and contamination.

Considering your requirement to set an expiry date, 3 months would be a reasonable estimate for the shelf life of the solution. During this period, if the solution no longer turns blue with iodine, it indicates that the starch has degraded or the solution has spoiled. Therefore, you can consider that as the end point of usability, whichever comes earlier - either the 3-month mark or the loss of the blue color reaction.

To ensure the longest possible shelf life for your starch indicator solution, it is recommended to store it in a cool, dark place in a tightly sealed container to minimize exposure to air and contaminants. Regularly check the solution for any signs of microbial growth or changes in color or odor. If any of these signs are observed before the estimated expiry date, it is advisable to discard the solution and prepare a fresh batch.