Dear Pier and Wembo,
Thanks for the clear demystification of the
"Phosphoric acid folklore". I refer to it as folklore because I found
that if you ask 7 labs how they prepare their acid, you'll get 7 different
answers. Some of those will inevitably describe the acid as anhydrous or
water-free, which as Pier plainly pointed out, there is no such state of
phosphoric acid. Since there seems to be much variation in acid prep
technique (even though we'll all continue to cite Coplen, 1983, Thanks Ty!),
we are probably all using slightly different reaction conditions. If
that is true, then our isotopic fractionation factors must be
inaccurate. If that is true, then isn't our (the isotope community) data
comparable by using a good calibration traceable to NBS19?
Much thanks,
Georges
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Georges L.
Paradis
ICPMS Lab Manager
Department of Earth Science, Marine Science
Institute
University of California Santa Barbara
Santa Barbara, CA
93106
805-893-7182
[log in to unmask]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
----- Original Message -----
Sent: Monday, September 18, 2006 1:32
AM
Subject: Re: [ISOGEOCHEM] preparing
100% phosphoric acid
Wembo and others,
Just a note on the
phosphoric acid discussion. What Wembo states, to have 100 percent-water
free acid is not completely true. By extracting water (either by heating or
by adding P2O5) the acid polymerizes and liberates water – and with
polymerization viscosity increases. Even if getting more than 100 percent
acid (some name the acid with 1.93 density 103 percent acid) by continued
polymerization water is formed.
By adding water in priciple the acid
de-polymerize. Probably a reason why the concentrated acid is highly
hygroscopic.
In fact phosphoric acid has different ‘forms’, such as
orthophosphoric acid (H3PO4), pyrophosphoric acid (H4P2O7), and
polyphopsphoric acid (chain and ring shaped polymerized acid; highly
concentrated acid may crystallize).
One also can gently heat the acid
while connected to a low-vacuum line. No back-hydration of the acid by
air-humidity can occur then (if simply heated in open-air) and also eventual
dissolved gases are better removed.
But the original procedure (McCrea,
1950), as part of the calibration of the carbonate acid digestion method and
of the isotopic fractionation(s) related to this method, was by adding P2O5,
a spatula-tip of CrO3 (acid turns yellow and after heating into green) and
about 3 ml of H2O2 is added after a 7 hours heating.
The CrO3-H2O2
procedure was included because the phosphoric acid in older days probably
contained some traces of organic matter (was less pure as it is
nowadays).
Using lower density acid in automates (for viscosity reasons)
has a risk to deviate from the original McCrea (1950) calibration and
related isotopic fractionations, and thus from the proper scale. In any
case, the less dense acid contains more water, increasing possibilities of
isotopic exchange with the oxygen in the produced CO2.
Wasn’t it Harmon
Craig in the early days of existence of the isogeochem list who warned
against deviating from the original calibrated procedure...(must still be in
the archives...)?
Some thoughts.
Best
wishes,
Pier.
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From: Wenbo Yang <[log in to unmask]>
Reply-To:
Stable Isotope Geochemistry <[log in to unmask]>
Date:
Sun, 17 Sep 2006 17:29:38 -0700
To:
<[log in to unmask]>
Subject: Re: [ISOGEOCHEM]
preparing 100% phosphoric acid
Yes, the easiest way to make
"100%" H3PO4 is to just simply heat the 85% H3PO4 on a hot plate to remove
(or evaporate) the 15% H2O in fume hood. But the alternative way is by
chemical reaction: P2O5 + 3H2O = 2H3PO4, so that you can get the "100%"
H3PO4 or the water-free H3PO4 acid, too.
-----Original Message-----
From: Stable Isotope
Geochemistry [mailto:[log in to unmask]]
On Behalf Of millionh@yahoo,com
Sent: Sunday,
September 17, 2006 9:56 AM
To:
[log in to unmask]
Subject: [ISOGEOCHEM] preparing
100% phosphoric acid
I know this topic have been on this
site few months ago. I have some questions and suggestion on the
topic, how to make 100 % phosphoric acid. I have been following the
same procedure Pier described on this site, that is following
Bowen's (1966) and Coplen et al. (1983). (i.e., adding P2O5 to 85%
orthophosphoric acid and heating at 200
oC)
But, the On-line lab manual for
Stanford by Mucciarone uses very simple method for his Kiel device.
Just heat the ACS grade 85% commercial Phosphoric acid to get
ride of the 15% water out of it. Does anyone see any problem with
this method? The specific gravity he is using is 1.89 to 1.90
which is similar to the method most are
using.
WHY do we need to add P2O5 to the
85 % orthophosphoric acid??
Thanks
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