grantfair2001 wrote:
> Thanks for the additional info, Adam. What kind of methyl orange are
> you using - aqueous? What percent, 0.1% or....?
>
> (MO seems to be more readily available than bromophenyl blue and not
> too pricey). What is the shelf life of the stuff?
>
don't know about shelf life, its from my chemistry set that I got
when I was a kid (~ 20 years ago) and its works fine.
The indicator solution I use is about 0.1% I think. Not
important, just enough to look nice and colorful.
As far as I know, the different indicators are used depending on
pH you want them to change at. For HCl and NaOH titration I think
almost any can be used because the pH change near the end point
is very dramatic as it passes neutral. Its because NaOH is a
strong base and HCl is a strong acid, which means they ionize
completely in water which means there is no free NaOH or HCl
molecules in the solution instead they exist almost entirly of as
ions, Na+, OH- or H+ and Cl-.
In aquious solutions the concentrations (in moles per liter) of
H+ and OH- ions are always at equilibrium according to
[H+] ∗ [OH-] = 10E-14
where [H+] is concentration of H+ ions in moles per liter.
[OH-] is concentration of OH- ions in moles per liter.
so if you add H+ ions to water then there automatically has to be
less OH- ions for the above equation to balance.
now pH is defined as -log10 ([H+])
in pure water [H+] must equal [OH-] and so
[H+] = [OH-] = 10E-7
pH of pure water then is equal to -log10(10E-7) = 7
[H+] of 10E-7 is a small number so if you add tiny bit of HCl to
pure water then [H+] goes sky high and so pH drops down a lot.
The same goes for NaOH and water ,except [OH-] goes sky high
which means [H+] must get a lot smaller than 10E-7 to keep the
equation balanced. Hope this makes some sense.
here is some stuff on indicators
http://ecommerce.hach.com/stores/hach/pdfs/wah/eng_pdf/ex_pHindicators.pdfNotice in figure 1 showing the dramatic pH change
If you really interested then I'd recommend getting hold of a
secondhand copy of a good 1st year chemistry book and learn about
solution equilibrium.
Adam
> I haven't figured out how to access usenet groups with my current
> setup, so MO seems the easiest way to go.
>
use
http://groups.google.comas a web based NNTP browser and for posting.