sample preparation in water analysis

[Ay]

Hello everybody,

I have some questions about ionic chromatography.
In bibliography and books, I read that it is necessary to use HNO3 to
acidify water samples using ionic chromatography.
I understand that it improves conservation of samples but I don't
understand well how it works chemically. Can you explain me in
details ?

In european standards, acidification with HNO3 is used only for cation
analysis not for anion analysis. I don't understand why we need to
acidify for cation and not for anion. Especially that I'm using a HSO4
eluent (in cation chromatography). In anion chromatography, i'm
working with KOH eluent.

At what time is it better to acidify? At sampling or only in
laboratory?
In which concentration/amount have I to acidify my samples?
When I have only one sample for anion and cation analysis, does it
matter if the sample is acidified for the anion analysis? I think it
does because we have more NO3- in solution. Then, in this case, how to
proceed ?

I really want to understand every step of analysis, that's why I'm
asking all these questions.
Thank you for your help !

[Samite Alchemist]

A little late to the party...

What method are you following? I do metals, and HNO3 is the
preservative for keeping metal cations in solution, since most metals
form insoluble hydroxides. I've never heard of preserving anions with
nitric acid. Must not be looking at nitrate/nitrite.

Samite Alchemist

[Ay]

Post by Samite Alchemist
A little late to the party...
What method are you following?  I do metals, and HNO3 is the
preservative for keeping metal cations in solution, since most metals
form insoluble hydroxides.  I've never heard of preserving anions with
nitric acid.  Must not be looking at nitrate/nitrite.
Samite Alchemist

Hello Samite Alchemist,

I'm working on water samples, and looking for current ions (Na+, K+, Mg
++, Ca++...) anf I read with HNO3 I prevent samples degradation. Maybe
it's the same for these cations in keeping them in solution...
But I don't know exactly the method to use HNO3 in my samples... What
quantity? when acidify (at sampling or just before analysis... in my
case, I most of time receive samples not acidified..)
Thank you again for your help..

[JJ beans]

2% nitric is common - if you do anion chromatography watch out the
nitrate will saturate your column and detector. Nitrate salts are
completly ionizable and its just a matter of preserving samples over
time.And yes by acidifying your sample carbon dioxide absorbtion from
the atmosphere is eliminated.

Post by Ay

Post by Samite Alchemist
A little late to the party...
What method are you following?  I do metals, and HNO3 is the
preservative for keeping metal cations in solution, since most metals
form insoluble hydroxides.  I've never heard of preserving anions with
nitric acid.  Must not be looking at nitrate/nitrite.
Samite Alchemist

Hello Samite Alchemist,
I'm working on water samples, and looking for current ions (Na+, K+, Mg
++, Ca++...) anf I read with HNO3 I prevent samples degradation. Maybe
it's the same for these cations in keeping them in solution...
But I don't know exactly the method to use HNO3 in my samples... What
quantity? when acidify (at sampling or just before analysis... in my
case, I most of time receive samples not acidified..)
Thank you again for your help..