Have you ever changed your chemical supply (brand, purity etc.) ? Assuming you are preparing the ascorbic acid and Ammonium molybdate(VI) tetrahydrate solutions fresh and using the same chemical/reagent quality, it seems that a redox reaction between above-mentioned reagents is being altered randomly. This may be due to the ascorbic acid instability. As you know very strong antioxidant nature of the molecule is the mainstay of the labile nature of the ascorbic acid. Stabilizing the conditions for this molecule (such as using amber vials/bottles, storing the solution in cold temp and nitrogen purged non-oxidising environment in a short period, preparing the solution fresh) is the key for being succesfull when it is in use for redox based-reaction. I am not sure whether this answer is helpful or not..

Hi İsmail Emir Akyildiz ,

I am using the same bottle of chemicals I used before, so the purity should be the same. What I could do is order new ones.

The strange yellow coloring is happening when adding the ammonium molybdate so I guess the problem is situated around that chemical

Best,

Jorik

Hi Jorik,

As you say the inconsistency in your colorimetric protocol keeps appearing in a random manner.

Sometimes these puzzling problems have simple causes. Here is a thought... are the glass tubes properly washed? Sometimes glass material lying in the lab is used by everyone!... and not everyone takes the time to properly clean the material colleagues use.

For that specific protocol, I have my own glass material, I wash it and I keep it stored separately (to avoid cross-contamination issues).

Hope this helps,

Ana

Hi Ana Reis ,

That is indeed a valid point. I did clean them myself but maybe not good enough. What kind of washing procedure do you suggest?. (And could this really have such a big influence?).

Best,

Jorik

Hi Jorik Waeterschoot

I use a acid bath (HNO3, 10%) to wash all glass material, and leave it overnight.

Hope this helps

Ana

Ana Reis I have been trying the HNO3 bath and the result were still not as expected. A short additional question does your solution also turns a bit yellow at a certain point? Mine sometime turn already yellow when I add the ammonium molybdate and sometime it only happens when I add the ascorbic acid solution.