because,starch undergoes a photochemical reaction. So, with the passage of time it can under go to the photochemical reaction and starch wouldnot be starch but it might have changed in anyother product. so...we should use freshly prepared starch.
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Freshly prepared starch solution is used in iodometric titration as an indicator to detect the endpoint of the titration. Starch forms a complex with iodine that leads to the formation of a blue-black color, indicating that all the iodine has been consumed in the reaction. Using a freshly prepared starch solution ensures the accuracy of the endpoint detection, as older solutions may have degraded and produce unreliable results.
FeSO4 solution is typically prepared freshly because iron (II) ions are prone to oxidation in aqueous solutions, leading to the formation of insoluble Fe(OH)2 or Fe(OH)3 precipitates. This oxidation can result in inaccurate concentration measurements and affect the experimental results. Therefore, freshly prepared FeSO4 solutions ensure the stability and reliability of the solution for experimental purposes.
To prepare 1% starch indicator weigh 1g of starch powder.Measure 100ml of water and keep it for heating.by using some of 100ml water taken,make a paste of starch in watchglass and mix this paste with little and continuous strring of water which is kept for heating.when all the paste is added,allow for heating 5more minutes then starch indicator is ready for titration.Note:no lumps of starch should present in water
Freshly prepared solution of ferrous sulfate should be used because it is prone to oxidation when exposed to air, leading to the formation of ferric ions. This can result in inaccurate test results due to the presence of ferric ions instead of ferrous ions. Additionally, solutions that have been stored for extended periods of time may also be contaminated with impurities, affecting their effectiveness.
The pH of a freshly prepared solution of urea is around 7, which is considered neutral. However, urea can hydrolyze over time to form ammonia and carbon dioxide, which can increase the pH of the solution.
Sodium nitroprusside is light-sensitive and can decompose over time, leading to a loss of potency. Freshly preparing the solution ensures that it is at its maximum effectiveness and concentration for use in medical treatments, particularly in managing acute hypertension or heart failure.