I believe that "odd" in my case was the fact that I
collected the water at the end of the day, after hours
of heavy photosynthesis by the seaweed. This made the
pH high and the contribution of OH- to the alcalinity
became higher. However, the total alcalinity didn't
change so much. This confounded my mind at first.
Thanks again,
Matheus
--- Steve Nelson <[log in to unmask]> escreveu:
> actually, it brought up a good discussion. we
> titrate. only when you
> have really odd water composition does anything
> other than DIC
> contribute substantively to the alkalinity.
>
> good luck.
>
> On Apr 24, 2007, at 4:53 PM, Matheus Carvalho wrote:
>
> > Antony and Steve, thanks again for the comments! I
> > have been following the protocol of Thermo's
> > consultant and they don't mention shaking, but
> leaving
> > the acid solution resting for at least 18 hours.
> > However, since the advice is to shake, I will do
> this
> > for the next samples I plan to measure (maybe
> > tonight).
> > Regarding alkalinity, yes, yesterday I realized
> that
> > it is not the same of DIC, I only needed to read
> Stum
> > & Morgan :). I then calculated total carbon using
> the
> > simplified approach and got some nice results. I
> am
> > really embarassed to have brought this up here! :)
> > Thanks again!
> >
> > Matheus C. Carvalho
> > PhD student
> > Kitasato University - School of Fishery Sciences
> > Japan
> >
> > --- Steve Nelson <[log in to unmask]>
> escreveu:
> >
> >> We shake for a minute and then put the vials in
> an
> >> ultrasonic bath for
> >> another minute.
> >>
> >>
> >> On Apr 24, 2007, at 7:27 AM, Anthony Aufdenkampe
> >> wrote:
> >>
> >>> Matheus,
> >>> I'll echo everyone's advice to vigorously shake
> a
> >> sample after
> >>> acidification. The gas exchange is quite slow
> and
> >> depends on air-water
> >>> interface surface area and turbulence in the
> >> water. Therefore,
> >>> reaching
> >>> equilibrium between water and headspace in a
> small
> >> vial will take
> >>> hours (up
> >>> to 24 hrs) in the absense of shaking.
> >>> For my post-doctoral work with river waters I
> >> manually equilibrated
> >>> 30 mL of water with 30 mL of headspace in a 60
> mL
> >> syringe immediately
> >>> prior
> >>> to gas injection into a flow through IR detector
> >> system (I'm working on
> >>> publishing this). I found that after acidifying
> I
> >> had to very, very
> >>> vigoursly shake the sample for a full 15 seconds
> >> to get reproducible
> >>> results
> >>> (i.e. complete gas equilibrium). This is much
> >> longer than one would
> >>> naturally shake and I found it best to keep my
> eye
> >> on a watch to make
> >>> sure I
> >>> didn't undershake. On larger samples, we found
> >> that it took minutes of
> >>> shaking to come to equilibrium. Using this
> >> method, my reproducibility
> >>> was
> >>> typically <3%.
> >>> For this reason, when I finally start playing
> >> with a
> >>> Gasbench-DeltaPlus DIC method, I definitely
> intend
> >> to do all the
> >>> acidification and gas equilibration manually
> prior
> >> to loading vials
> >>> into the
> >>> autosampler.
> >>> Last, it might be worth reminding you, given
> your
> >> comment below,
> >>> that alkalinity and DIC are not equilvalent and
> >> you should not expect a
> >>> tight fit between the two. In fact the two are
> >> mathmatically
> >>> independent
> >>> and if you measure both well you can calculate
> the
> >> other carbonate
> >>> species
> >>> (and also pH).
> >>>
> >>> Anthony
> >>>
> >>>
> >>
> >
>
_____________________________________________________________________
> >>> Anthony K. Aufdenkampe, Ph.D.
> >>> Assistant Research Scientist - Isotope & Organic
> >> Geochemistry
> >>> Stroud Water Research Center
> >>> 970 Spencer Road
> >>> Avondale, PA 19311
> >>> Tel: 610-268-2153 ext. 263
> >>> Fax 610-268-0490
> >>>
> http://www.stroudcenter.org/about/aufdenkampe.htm
> >>>
> >>>
> >>> -----Original Message-----
> >
> >
> >
> > Matheus C. Carvalho
> > PhD student
> > Kitasato University - School of Fishery Sciences
> > Japan
> >
> > __________________________________________________
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> >
> >
> --
>
*************************************************************
> Steve Nelson
> Professor
> Dept. of Geological Sciences
> S-389 ESC
> Brigham Young University
> Provo, UT 84602
>
> 801-422-8688
>
> "Guns don't kill people, Dick Cheney kills people."
> George W. Bush: "Decider in Chief"
> "Iraq burns. We shop." Bob Herbert
>
**************************************************************
> Disclaimer for the feeble-minded: It should be
> painfully obvious, but
> the identification of my affiliation with Brigham
> Young University does
> not imply that the University endorses any political
> or otherwise
> partisan content of this message.
>
Matheus C. Carvalho
PhD student
Kitasato University - School of Fishery Sciences
Japan
__________________________________________________
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