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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: mia1530 on May 25, 2015, 05:10:13 PM

Title: Thesis method not working - cadmium detecton using Alizarin red s dye
Post by: mia1530 on May 25, 2015, 05:10:13 PM
Hi guys!
So I'm working on a method for cadmium detection using a dye Alizarin red s, but the method is not giving me the expected results (a linear increase in absorbance with increasing concentration of cadmium)

The pKa of alizarin red S is 4.5, and I have been carrying the method out in a 10ml solution, of which 1ml is 0.05M sulfuric acid (as per source paper) I have increased the acid volume in the sample, to no avail. The method also stated that it was a 1:2 stochiometry of dye:cadmium.

I think I'm working at a pH above 4.5, so my dye should be deprotenating sufficiently for it to bind to the cadmium, but I am not sure, and do not have access to the lab to  test the pH of the solution. But beyond this, I cannot think of any reason why the method does not work. I'm a little worried that a step was left out of the method, as I need to be able to explain why I think it may not have worked for my final write up.

Does anyone have any ideas? I have attached the structure according to the source paper.

Any help would really be appreciated!
Title: Re: Thesis method not working - cadmium detecton using Alizarin red s dye
Post by: Corribus on May 25, 2015, 06:13:38 PM
Quote from: mia1530 on May 25, 2015, 05:10:13 PM
I think I'm working at a pH above 4.5,
Never good to assume. On what basis do you think this?

You don't need pH paper to get a good estimate. If you have 10 mL of solution and 1 mL is 0.05 M H2SO4, what is the pH? Just for a quick estimate, ignore the second deprotonation.
Title: Re: Thesis method not working - cadmium detecton using Alizarin red s dye
Post by: kriggy on May 26, 2015, 03:03:58 PM
Seems weird to work in acidic medium when you need to deprotonate those OH groups
Title: Re: Thesis method not working - cadmium detecton using Alizarin red s dye
Post by: AWK on May 27, 2015, 05:03:28 AM
I do not know the exact Mia1530 source, but report in Journal of Chemical Engineering IEB
Vol. ChE. 25, No. 1, December 2010 pages 1-12 contains an evident printing error that causes 100 times lower concentration of Alizarin Red S (should be 1.39x10-1 M instead of 1.39x10-3 M.
Article in public domain:
https://www.google.pl/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCgQFjAA&url=http%3A%2F%2Fwww.banglajol.info%2Findex.php%2FJCE%2Farticle%2Fdownload%2F7233%2F5452&ei=LYRlVdDyEcKssgH1gIKICg&usg=AFQjCNEqqAWyrG9i7hU6SIdoeDzl3d2tsQ&sig2=yu8GQLfoSsomJHXHhDxbgw&bvm=bv.93990622,d.bGg