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Stupid question from someone who used to know these things....
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TheAndroid
science forum beginner


Joined: 13 Jul 2006
Posts: 2

PostPosted: Thu Jul 13, 2006 8:38 pm    Post subject: Stupid question from someone who used to know these things.... Reply with quote

Can someone explain to me why most examples of titration use
phenolthyaline (sp?) as the indicator. In a related question, why is
the titration considered complete when the pH reaches 10 (the point the
pink color appears)? Why isn't the endpoint at 7 (balanced pH)? Are the
formula's relating to the processes calibrated for a pH of 10? I used
to know this stuff.
In case its important, I'm trying to work through creating an
electropolishing bath without having to acquire laboratory grade
chemicals.

Thanks.
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Bob111
science forum Guru Wannabe


Joined: 13 Jan 2006
Posts: 115

PostPosted: Fri Jul 14, 2006 2:27 am    Post subject: Re: Stupid question from someone who used to know these things.... Reply with quote

On 13 Jul 2006 13:38:57 -0700, "TheAndroid" <a_w_abate@yahoo.com>
wrote:

Quote:
Can someone explain to me why most examples of titration use
phenolthyaline (sp?)

Let's guess that you mean phenolphthalein (pronounced just like it
looks, preferably without spitting!).

Quote:
as the indicator.

If by examples, you mean the first case that is introduced in a
textbook... it's basically tradition.

If you mean, are most titrations done with this indicator... not
really; it's used when it is ok to use it.


Quote:
In a related question, why is
the titration considered complete when the pH reaches 10 (the point the
pink color appears)?

For phenolphthalein, the color change is more at 8.5-9. But the point
of your question still stands.


Quote:
Why isn't the endpoint at 7 (balanced pH)?

That's not really true in general. It holds only for a certain class
of titrations, strong acid and strong base. Other titrations may have
endpoints at most any pH, depending on the case.


Quote:
Are the
formula's relating to the processes calibrated for a pH of 10?

Ok, the main point, I suspect. Consider the case of a strong acid,
strong base titration. Theoretical end point is 7, indicator end point
with phenolphthalein is 9. It is ok, because the titration curve is so
steep between 7 and 9 that there is negligible error.

One can also use standards (reference samples) as a check.


An indicator must be chosen so that its end point is near the
theoretical end point of the specific kind of titration at hand.


bob


Quote:
I used
to know this stuff.
In case its important, I'm trying to work through creating an
electropolishing bath without having to acquire laboratory grade
chemicals.

Thanks.
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Lloyd Parker
science forum Guru


Joined: 08 May 2005
Posts: 657

PostPosted: Fri Jul 14, 2006 1:13 pm    Post subject: Re: Stupid question from someone who used to know these things.... Reply with quote

In article <1152823137.409841.53930@h48g2000cwc.googlegroups.com>,
"TheAndroid" <a_w_abate@yahoo.com> wrote:
Quote:
Can someone explain to me why most examples of titration use
phenolthyaline (sp?) as the indicator.

It has the easiest to see color change, going from no color to pink.

Quote:
In a related question, why is
the titration considered complete when the pH reaches 10 (the point the
pink color appears)?

If you look at a titration curve, the pH changes so rapidly around the
equivalence point, the difference between the vol. for pH 7 and around 9.5
where PHTH changes is typically less than a drop.

However, if you're titrating, say, a base with an acid, you don't use PHTH --
it's very hard to see when the pink color disappears. We use bromthymol blue
in our gen chem lab when the students titrate NaOH with oxalic acid. We do
use PHTH when they titrate vinegar (acetic acid) with NaOH.

Quote:
Why isn't the endpoint at 7 (balanced pH)? Are the
formula's relating to the processes calibrated for a pH of 10? I used
to know this stuff.
In case its important, I'm trying to work through creating an
electropolishing bath without having to acquire laboratory grade
chemicals.

Thanks.
Back to top
TheAndroid
science forum beginner


Joined: 13 Jul 2006
Posts: 2

PostPosted: Fri Jul 14, 2006 2:51 pm    Post subject: Re: Stupid question from someone who used to know these things.... Reply with quote

Bob,
Thank you for your response. I think I have a handle on it now
except for one small detail: The calculations are not related to the
endpoint pH, just the fact that the endpoint was reached? If so, then
the indicator is simply showing that fine line, not a specific pH
target?

For reference:
Bob wrote:
Quote:

Are the
formula's relating to the processes calibrated for a pH of 10?

Ok, the main point, I suspect. Consider the case of a strong acid,
strong base titration. Theoretical end point is 7, indicator end point
with phenolphthalein is 9. It is ok, because the titration curve is so
steep between 7 and 9 that there is negligible error.
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James Copeland
science forum beginner


Joined: 25 Jun 2005
Posts: 33

PostPosted: Fri Jul 14, 2006 3:50 pm    Post subject: Re: Stupid question from someone who used to know these things.... Reply with quote

"TheAndroid" <a_w_abate@yahoo.com> wrote in message
news:1152888673.955737.189020@m73g2000cwd.googlegroups.com...
Quote:
Bob,
Thank you for your response. I think I have a handle on it now
except for one small detail: The calculations are not related to the
endpoint pH, just the fact that the endpoint was reached? If so, then
the indicator is simply showing that fine line, not a specific pH
target?

That is essentially correct. For some examples: A solution of the strong
base NaOH is often standardized against a solution of potassium acid
phthalate (KHP) (a weak acid) of known concentration. The final product is a
solution of sodium cations and weakly BASIC phthalate anions. Therefore,
phenolphthalein, which changes color at about pH 9 (mildly basic) is
appropriate for the indicator.
One the other hand, a solution of the strong acid HCl is often standardized
against a solution of tris(hydroxymethyl)aminomethane (TRIS or THAM) of
known concentration. The final product is a soluton of weakly ACIDIC
tris(hydroxymethyl)aminomethane hydrochloride. Therefore, methyl red, which
changes color at about pH 5 (mildly acidic) is appropriate for the
indicator.
Jim C.

For reference:
Quote:
Bob wrote:

Are the
formula's relating to the processes calibrated for a pH of 10?

Ok, the main point, I suspect. Consider the case of a strong acid,
strong base titration. Theoretical end point is 7, indicator end point
with phenolphthalein is 9. It is ok, because the titration curve is so
steep between 7 and 9 that there is negligible error.
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Marvin
science forum Guru Wannabe


Joined: 24 Mar 2005
Posts: 224

PostPosted: Fri Jul 14, 2006 3:52 pm    Post subject: Re: Stupid question from someone who used to know these things.... Reply with quote

TheAndroid wrote:
Quote:
Bob,
Thank you for your response. I think I have a handle on it now
except for one small detail: The calculations are not related to the
endpoint pH, just the fact that the endpoint was reached? If so, then
the indicator is simply showing that fine line, not a specific pH
target?

The comment was about a particular, common, textbook type of
titration. There are cases where the pH change point for
the indicator matters. One is the analysis of a mixture of
sodium carbonate and sodium bicarbonate by titration with a
strong acid. The first end point comnes when the carbonate
in the mixture is changed to bicarbonate. The second
enpoint is when that bicarbonate and the bicarbonate in the
original mixture become weakly ionized carbonic acid. One
would use two indicators, selected for their pH points and
foe a first color change that is easily distinguised from
the second. The procedure is still simple to carry out, but
the planning is a bit harder. Using sa pH meter instead of
indicators helps.

Quote:

For reference:
Bob wrote:

Are the
formula's relating to the processes calibrated for a pH of 10?

Ok, the main point, I suspect. Consider the case of a strong acid,
strong base titration. Theoretical end point is 7, indicator end point
with phenolphthalein is 9. It is ok, because the titration curve is so
steep between 7 and 9 that there is negligible error.


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lucasea@sbcglobal.net
science forum beginner


Joined: 30 Jun 2006
Posts: 8

PostPosted: Fri Jul 14, 2006 4:07 pm    Post subject: Re: Stupid question from someone who used to know these things.... Reply with quote

"TheAndroid" <a_w_abate@yahoo.com> wrote in message
news:1152888673.955737.189020@m73g2000cwd.googlegroups.com...
Quote:
Bob,
Thank you for your response. I think I have a handle on it now
except for one small detail: The calculations are not related to the
endpoint pH, just the fact that the endpoint was reached? If so, then
the indicator is simply showing that fine line, not a specific pH
target?

For reference:
Bob wrote:

Are the
formula's relating to the processes calibrated for a pH of 10?

Ok, the main point, I suspect. Consider the case of a strong acid,
strong base titration. Theoretical end point is 7, indicator end point
with phenolphthalein is 9. It is ok, because the titration curve is so
steep between 7 and 9 that there is negligible error.

If you look at a plot of pH vs. titrant volume, the curve is very steep near
the endpoint--i.e., the amount of extra volume added to get from pH 7 to the
pink phenolphthalein endpoint (pH 9 or so) is *very* small. Phenolphthalein
is exceptionally useful as an indicator because of the very sharp and easily
detectable colorless-->pink transition. With other indicators, the change
for example from yellow --> blue is much less obvious because the yellow
obscures the very first (faint) traces of blue color. It's not easy to see
the difference between yellow and very slightly greenish-yellow--so you have
to add excess titrant to get enough blue that the color is noticeably
different than just the yellow. (And yellow-->blue is a relatively easy
one. Try yellow-->organge sometime!) Thus, it's a compromise of a very
small error due to non-pH 7 endpoint for phenolphthalein vs. the extra
volume you add while you try to figure out if you actually have a color
change with other indicators.

What I say is only true for strong acid/strong base titrations, which are
the ones where phenolphthalein is most useful. Any titrations involving a
weak acid, a weak base, or both, need to use an indicator whose endpoint and
color change are better optimized for the particular titration being
undertaken...or for very careful work, you suck it up and buy a pH meter and
do a pH titration.

Eric Lucas
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