Hi everyone,
I am trying to make 200 mM sodium orthovanadate (Na3VO4). According to the protocol I have, I am suppose to adjust pH to 10 with either NaOH or HCl --> solution turns yellow --> Boil for 10 min or until solution turns clear --> cool down --> repeat this 3-5 times until solution doesn't turn yellow with NaOH or HCl anymore.
My problem is that the Na3VO4 solution keeps turning yellow even after 5X I add HCl. It will eventually turn clear when I boil but it will keep turn yellow when I add HCl. The protocol says I need to repeat Boiling-adjusting pH with HCl until the color doesn't change anymore. Could anyone who had the same problem please give me a feedback on this issue? Thank you!
Thank you Päch and Hosseini. You are right. It looks like Vanadate inevitably turn yellow when acid is added. I was able to adjust its pH with lower concentrated HCl/NaOH without changing its color as you suggested. Thank you for your tips and links to more detailed protocols!
Hi,
I'm trying to prepare 10 mL of 100 mM solution of sodium orthovanadate but the solution is at pH 9.8 (so apporx. = 10) from the beginning therefore i don't need to adjust it... even if i add NaOH i don't really need to boil it because it does not turn yellow so i'm not sure this is well prepared... any advice?
Hi Celia,
Try add NaOH first to make it 10 then add drop of HCl, it should turn yellow. Then, comes the boiling step.
@Zahraa al-isawi, Yes that was my plan although i haven't tried it yet. Thanks a lot for your advice!
And if you want to calculate the precise concentration of a dilute solution (pH 10.5) after boiling and cooling down, use the molar extinction coefficient (eM = 3,550 at 260 nm using quartz UV cuvette). The prepared 200mM for example may not be the same after finishing the activation process.
The reason for boiling is to get the monomer vanadate instead of the decavanadate.
Yellow colour observed before boiling is due to decavanadate. At pH 10, decavanadate will slowly depolymerize over several hours to the colourless monovanadate. This process is accelerated by boiling.
The molecular weight used for the calculating the molarity is that for the monomer not the decavanadate which is 183.91
after a long time boilin also the color is not changing . what to do now?
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