Klarinet Archive - Posting 000457.txt from 1995/04

From: Michiya Nishino <mnishino@-----.EDU>
Subj: Re: Re Roger Shilcock's reference to phenophthalein
Date: Wed, 26 Apr 1995 13:22:12 -0400

Dan Leeson wrote:

> Roger, I found your comment unclear. First I don't know what the
> substance is that you suggested I try saliva on. Second, I don't
> know what is supposed to happen if I did try your suggested action.
> And third, I would not know how to try the experiment that you
> suggested.
>
> Would you run that one by us again with some greater detail, please?

Phenolphthalein is an acid-base indicator used in acid-base titrations
(when a strong acid is titrated with a strong base). It is a
pinkish-purple color in basic solutions and colorless in acidic solutions
(ie, colorless from pH range 1 to 8, pink/purple from pH range 10 to
13). The way indicators work is shown below:

H+ = hydrogen ion
In- = Indicator (ionized form)

HIn --> H+ + In-
<--

HIn is Color 1
In- is Color 2

The molecular form of the indicator HIn is one color, and the ionized
form In- is another color. Thus, when the equilibrium shown above is
disturbed by removal of the H+, for example, (by adding excess base), the
equilibrium shifts toward the products (right side) and the color of the
solution changes from color 1 to color 2.

I guess a way to determine the basicity of your saliva is to fill a buret
with it (quite a task!) and to titrate it into a flask containing KHP
(potassium acid phthalate--a primary standard often used to determine the
concentration of bases), distilled water, and phenolphthalein.

Regards,
Michiya Nishino
mnishino@-----.edu

   
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