Science Buddies: "Ask an Expert"

Hi, I have just completed the practical side of my advanced higher chemistry investigation. I was investigating whether or not the quality/price of orange juice is linked to the concentration of vitamin C contained in the orange juice. I was advised by my teacher to add the startch indicator just before the end point and not at the beginning of the titration. Does anyone know why it is best to do this as I am really struggling to find this answer! Thanks for any replies!

Good question, Becky. Struggling to find the answer to this one myself.

I'm assuming this is roughly the method you are using:

http://www.outreach.canterbury.ac.nz/ch ... iodine.pdf

An immediate question that comes to mind is, "How do you know when you are near the endpoint of the titration if you don't have any indicator? There won't be any color change at that point, correct?"

Any chemists on the board with a better answer to this?

zzzzdoc wrote:Good question, Becky. Struggling to find the answer to this one myself.

I'm assuming this is roughly the method you are using:

http://www.outreach.canterbury.ac.nz/ch ... iodine.pdf

An immediate question that comes to mind is, "How do you know when you are near the endpoint of the titration if you don't have any indicator? There won't be any color change at that point, correct?"

Any chemists on the board with a better answer to this?

Well, the solution is also colored before you add the starch (from the iodine)... apparently you monitor that color change to tell when to add the starch.


From http://infohost.nmt.edu/~jaltig/RedoxTitration.pdf
"The endpoint of the titration is detected by using a starch indicator, where the disappearance of the blue starchiodine color indicates the end of the titration. The starch indicator is added to the solution near the end of the titration, at the point when the dilute iodine imparts a pale yellow color to the solution. There are two reasons why the indicator is not added at the beginning of the titration when the iodine concentration is high. First, a diffuse endpoint would result from the slow dissociation of the starch-iodine complex if a large amount of iodine were absorbed in the starch. Second, iodometric titrations are carried out in strongly acid media, a situation that promotes the reaction between oxidizing agents and iodide. Unfortunately starch has a tendency to hydrolyze in acid media."

Interesting.

zzzzdoc wrote:Interesting.

Yes, it is. I vaguely remember doing this experiment in high school, but I did not remember or understand why the starch was added near the end. Learn something new everyday. (Especially hanging out here )

Louise

Hi all!

Another website to look at is:

http://en.wikipedia.org/wiki/Ascorbic_acid - the wikipedia entry for ascorbic acid (Vitamin C), look at the discussions of end points..

Many other websites are generated by inputting:

"starch indicator end point titration" to answers.com

Notes:

1. The exact wording of your input to answers.com, or any other search engine, may have a big impact on your result. So, some experimentation may be needed.
2. Beware of sites that are just trying to sell you something. In answers.com, these are supposed to be segregated under "Sponsored Links." But, they occasionally leak into "Web Results."

thanks louise and alan.Louise that sounds like a good answer! thank you:)