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?
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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.
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