Determination of arsenic using spectroscopy

[PMOCH]

-hi everyone

I´m using a protocol for arsenic determination. It uses the bleaching of methylene blue for the action of arsine. The arsine is formed when arsenic compounds react with NaBH4. My problem es when I try to make a calibration curve I can´t see the bleaching while I increment the concentración of arsenic. According to this protocol, NaBH4 should be cold and prepared the same day of the experiment, done!!. But I get the same result every time I increase the concentration of arsenic. I´m using this article as a reference: https://pdfs.semanticscholar.org/10d1/b7f38475a6007628eb9c88a12bde4b037ea5.pdf

Is there something I´m doing wrong?

 

 

[John Cuthber]

It's impossible to know what is going wrong unless we know exactly what you are doing.

A few things spring to mind:

Have you done this before and has it worked?

Have you checked all the calculations for concentrations?

Are you able to independently check any of the solutions?

 

[PMOCH]

HI, yes, the first time I did it, worked!, it was in january. I double check the concentration, even I prepared new ones. About the solutions, I tested the reduction fo Methylene blue, using bacteria and milk, it worked!. I even write to the authors and they told me some advice, I followed all of them but I'm getting the same result.

I think the problem is within the reaction of arsine, when formed it should reduce methylene blue, but the reduction doesn't occur. Something is getting in the way. To avoid oxidation of MB, I made everything with almost nothing of light, total darkness in the lab. I look for information about arsine stability, I couldn't find anything usefull. I'm not giving up just yet. If you have some idea that might help is well received.