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DSRM
science forum beginner

Joined: 24 Mar 2006
Posts: 4

Posted: Fri Mar 24, 2006 4:32 pm    Post subject: Metals digestion acid concentration

I was reading the EPA Methods for metals digestion. (I run an AA
furnace.) Now, the method says to take your sample (say, 50 ml) and add
3 ml of conc HNO3... and then another 3 ml of conc HNO3... and then a
little more... "so that the final dilution contains 0.5% (v/v) HNO3."

That's over 6 ml of acid. What would the final volume need to be to
have 0.5% (v/v) HNO3? Wouldn't that be something like 1L? Am I missing
something?

Thanks.
Farooq W
science forum Guru Wannabe

Joined: 25 Mar 2005
Posts: 240

Posted: Fri Mar 24, 2006 5:33 pm    Post subject: Re: Metals digestion acid concentration

msalguod@yahoo.com wrote:
 Quote: Somebody please enlighten me here: I was reading the EPA Methods for metals digestion. (I run an AA furnace.) Now, the method says to take your sample (say, 50 ml) and add 3 ml of conc HNO3... and then another 3 ml of conc HNO3... and then a little more... "so that the final dilution contains 0.5% (v/v) HNO3." That's over 6 ml of acid. What would the final volume need to be to have 0.5% (v/v) HNO3? Wouldn't that be something like 1L? Am I missing something? Thanks.

Is your method same as http://www.uga.edu/~sisbl/epaman4.html, which
mentions the same expression. I assume this is same.

I think it safe to ignore the volume of (3 mL + 3 mL) HNO3, since this
is already evaporated to near dryness in the given method. Suppose you
started with a 50 mL sample, added 3 mL HNO3, evaporated to near
dryness and again added the same volume and evaporated to near dryness,
again. Now let us assume you want to make up the sample to some volume
say in a 25 mL volumetric flask depending on the metal ion
concentration, you would add about ~ 0.12 mL of conc. HNO3 to the 25 mL
volumetric flask so that "the final dilution contain 0.5 % (v/v) HNO3".

The complete sentence written there is "If the sample is to be analyzed
by the furnace procedure, substitute distilled HNO3 for 1:1 HCl so that
the final dilution contains 0.5% (v/v) HNO3. "

Basically this is most likely being done to avoid formation of volatile
chlorides of certain metals in the graphite furnace. Nitrates nicely
decompose to metal oxides.
DSRM
science forum beginner

Joined: 24 Mar 2006
Posts: 4

Posted: Fri Mar 24, 2006 7:00 pm    Post subject: Re: Metals digestion acid concentration

Oh, okay. I thought the 3+3 ml would simply concentrate further instead
of evaporating, since it doesn't go dry.

Thank you for the explanation.
Farooq W
science forum Guru Wannabe

Joined: 25 Mar 2005
Posts: 240

Posted: Sat Mar 25, 2006 5:46 pm    Post subject: Re: Metals digestion acid concentration

msalguod@yahoo.com wrote:
 Quote: Oh, okay. I thought the 3+3 ml would simply concentrate further instead of evaporating, since it doesn't go dry. Thank you for the explanation.

Nitric acid has a lower boiling point than water, perhaps around 80 oC
and heating further decomposes the acid into nitrogen oxides. I assume
it would be safe to ignore the previously added volumes.
waterpollutionlaboratory@
science forum beginner

Joined: 05 Dec 2005
Posts: 6

 Posted: Fri Apr 07, 2006 1:36 pm    Post subject: Re: Metals digestion acid concentration I generally use a 200 ml sample an 10 mls of acid an that works fine.

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