Testing--AKA Titration--

[EvanWilliams]

Stupid process question but here goes.
When testing and adding the reagent (titrating) where is your end point?
Using the speedstir is frequently see the first hint of color change then a small but easily noticeable change, then one drop or more and a full color change.
I'm tempted to take the middle ground. Drop----tiny and slowly expanding faint color change then one drop or more gets me an easily noticeable color change that is still light but larger.

I don't my old eyes could be that precise without the speedstir.

any input appreciated.

 

[jblizzle]

You always add drops until the last one causes no further color change and then do not count that last drop.

 

[TheBraLady]

I've wondered about this too, thank you for asking the question!

 

[EvanWilliams]

You always add drops until the last one causes no further color change and then do not count that last drop.

I ges that makes sense because we are looking at quantity using known volume.
Were we simply doing qualitative--is it there or not--positive or negative----any color change would do.

Simple I guess but my old atrophied brain needed to have it articulated again.

Thank you.

 

[chem geek]

Technically in quantitative chemistry one has the endpoint at the halfway point between the color before vs. after the endpoint. However, there is enough error in the test kits and for simplicity the rule is to keep adding drops when there is a color change and not count the last drop that shows no change.

 

[Griswald]

Here is how I do it...

Add drop (as you stated with the Speedstir sometimes there will be a slight color change that will go away quickly, that is not anything to worry about) if color does not change, add another drop. Continue until there is a NOTICABLE change. If at 6 drops, the solution turns from blue to faint pink, that is not the color you are looking for. Add another until that solution changes to bright red, say at 7 drops, now add one more...if the bright red color does not get brighter, that was your endpoint, not the 8th drop, but the 7th, it was the last drop that did anything.

Also, do not wait for any long length of time between drops, no more than a few seconds. Solutions can change again given enough time and buffers.

Since I started going that extra drop or two, my results have been very consistent.

 

[EvanWilliams]

Thank you all. I learn every time I come here.