Bedlasky - 26-11-2019 at 00:14
Hi.
I tried to determinate nitrate spectrophotometrically as 4-nitro-2,6-dimethylphenol. I had standard contain nitrate and nitrite and I tried mask
influence of nitrite with amidosulfonic acid. I add some amidosulfonic acid in to the vial with concentrated sulfuric/phosphoric acid mixture and add
1ml of standard. I heated it on hot water bath for few minutes, than add 2,6-dimethylphenol and after another 15 minutes I meassured concentration.
But concentration of nitrate was two times higher - so masking doesn't work. Any suggestions why?
Boffis - 26-11-2019 at 07:06
How much is "some"? Are you using a published procedure or a home-brewed technique? Also this is not a masking reaction, it is a removal reaction of
the nitrite since the latter is destroyed. It sounds like you have a lot of nitrite in your solutions so are you adding enough sulphamic acid? Have
you used a nitrite-free standard to calibrate the equipment? Or is that what you are trying to do now?
What reading do you get with a nitrite-free standard?
Pumukli - 26-11-2019 at 08:53
In the summer once I determined the purity of my amidosulfonic acid by the volumetric measurement of released N2 from excess sodium nitrite.
The reaction was quantitative and quite quick even in 10% acetic acid as "solvent", without any heating! (In other words: the reaction makes a good N2
generator if someone is looking for one.)
So your sulfuric/phosphoric acid boiling was probably an overkill and may lead to some side reactions. Maybe the nitrite was partly oxidized by the
hot sulfuric acid, or the reaction took a different route in the strong, hot acid.
Bedlasky - 26-11-2019 at 09:02
With nitrite free standard it was ok. But we have in work samples contain both - nitrite and nitrate - so we tried it with nitrite-nitrate standard if
would be possible determine nitrate spectrophotometrically. Standard contained 0,25mg/l of nitrate and 0,25mg/l of nitrite in 2%H2SO4 (which
corresponds to the nature of the samples). Standard was 20x diluted and pH adjusted to 3,2. We followed instructions in manual from producer of tests
(test = vial with acid mixture + 2,6-dimethylphenol reagent). He says that for removing nitrite we must add tip of spatula of amidosulfonic acid.
Amidosulfonic acid isn't in acid mixture very soluble, after additon of 1ml of 20x diluted standard it dissolved much better. On the bottom of vial
stayed few crystals of amidosulfonic acid but most of it dissolved.