Nitric Acid as Eluent Additive

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11 posts Page 1 of 1
Hello dear community,

I have already experimented with HBF4 as an additive in my LC system and have had great success with it. The pH is adjusted to 2 and I get great separation for my compounds (preparative HPLC).
Now I was wondering if its possible to use nitric acid (HNO3) as an eluent additive and not damage the system? I know that it is corrosive but so is HBF4 and I havent experienced any signs of degradation so far.
I know its VERY unusual but in my case it is essential to use this uncommon acids.
Nitric acid is typically used as a passivation agent for stainless steel, so you shouldn't see any corrosion issues there. In fact, here's a nice resource on nitric corrosion of stainless:

https://bssa.org.uk/bssa_articles/selec ... acid-hno3/

The purple line is where corrosion of 0.1 mm/year starts. If you're using it as an additive, I doubt you're going to concentrations high enough (nor temps high enough) to cause such corrosion.

Check the other components of your system to make sure they're compatible with nitric. I think PEEK parts typically advise against high concentrations of nitric acid, but are likely fine with lower concentrations:

https://www.vici-jour.com/support/resistance.php

Vici gives PEEK an A for compatibility at 20% nitric, but a C at 100%. So concentration matters.
Just remember, it's triprotic, so make sure you choose a MP pH that's not right near a pKa...
Thanks,
DR
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DR wrote:
it's triprotic

??? HNO3
The only other thing to consider is if nitric will degrade your analyte of interest. Nitric can be very active against organic molecules.
The past is there to guide us into the future, not to dwell in.
I use sulfuric acid. I have never heard about using nitric acid other than as a passivating reagent (warm 10% citric acid works well for that as well). I just add enough sulfuric acid to acidify the water to pH 2.5. It gave me a better baseline at 210nm vs 20mM (H3PO4/KH2PO4 buffer).
According to the sources I found nitric acid absorbs in the UV range peak 250nm so it wouldn't work well for HPLC-UV work.
MSCHemist wrote:
nitric acid absorbs in the UV range peak 250nm so it wouldn't work well for HPLC-UV work.

I saw a method with indirect UV detection.
Anybody reading this should also be made aware that if you mix nitric acid at sufficiently high concentrations with organic solvents, the result may explode some hours later, without much warning.

This happened in a lab not far from mine, when someone used nitric acid at sufficiently high concentration to passivate a system, and pumped it into a waste pot that already contained isopropanol, which had been used as an intermediate solvent in changing the pumps over. It caused personal injury, as well as trashing a lab, so it's a genuine hazard to be aware of.

The other thing to note is that with the rise of cheap MS detectors, don't consider doing this in any instrument with a mass spec attached. It will oxidise the spray chamber horrendously even at very low concentrations, and may do damage further downstream in the mass spec.
I saw an Agilent seminar recently and they recommend 0.5% phosphoric acid in 10%water 90%acetonitrile 0.1ml/min overnight for passivation. 10% citric acid also works well. Much safer than nitric acid.

I used 6N nitric acid once and it worked but it wrecked the cigarette filter on the purge valve of my 1100.
vmu wrote:
DR wrote:
it's triprotic

??? HNO3

brain-fart.
Thanks,
DR
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