[naman1008]

I've just finished conducting an experiment to determine the caffeine content of 4 types of drinks, Coca-Cola, Tim Horton's brewed coffee pods, Matcha green tea and standard green tea which spectacularly failed when my indicator precipitated in my solution.

I begin by adding the caffeinated substance (250ml)

I proceeded to add the sulfuric acid as to create an acidic environment (10ml 1.0M)

Then added iodine (25ml 0.1M)

finally the potassium iodide (10ml 1.0M)

C8H10N4O2 + 2 I2 + KI + H2SO4 → C8H10N4O2 .HI .I4 + KHSO4 (the resultant solution has a dark tint in all experiments)

the indicator is then added to the resultant solution

this is where I have issues

the liquid starch indicator is then added, instead of changing the color of the solution to a dark blue it instead precipitates into black particulates.

addition of sodium thiosulfate appears to have no effect on the solution

I2 + 2 Na2S2O3 → 2NaI + Na2S4O6



I would like to know if anyone can think of what reaction could cause this sort of reaction by my indicator and how this could be fixed?

My research is based on the previous similar experiments,

http://www.hkasme.org/News/818/Report_6B_Christiam%2520Alliance%2520SC%2520Chan%2520Memorial%2520Secondary%2520School_Chemical_test_for_caffeine.pdf

https://pdfs.semanticscholar.org/eb16/5fcb4f857eb971c9c96f45191a14ccae83af.pdf

https://hackaday.com/2017/10/30/coffee-conspiracy-and-citizen-science-an-introduction-to-iodometry/

http://www.ijarse.com/images/fullpdf/1505545205_514.pdf

https://www.arcjournals.org/pdfs/ijarcs/v6-i9/2.pdf

[Borek]

It is a very well known problem with all iodometric titrations. Don't add the starch till titrating the solution to a very pale, yellow shade.

[naman1008]

great thank you so much, my chem teacher couldn't figure it out though I'm not sure how to get the substances to such a pale yellow color when they all have their own tint's in particular the coffee which is almost black after adding the 25ml of iodine. Any idea how I could facilitate such a color change?

[Borek]

I would try to to titrate with starch added earlier to do some rough estimate of the required volume, and in next titrations add about 90-95% of the required titrant before adding starch.