2. Why Water Solutions?
Why do chemists prefer to conduct reactions in
water solution?
−
Water is a “universal solvent”
−
A true solution is homogeneous
−
Every mL solution mmoles = any other mL mmoles
Water is a non-conductor
No “noise” obscuring conductivity titrations
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Water is the product of most acid-base reactions
−
Many interesting salts not soluble in water
Enables precipitation reactions
3. Types of Titration
Acid-base titration
−
Oxidation-reduction titration
−
Usually one or the other has a color change
Precipitation titration
−
Requires a weak acid indicator for equivalence pt.
Difficult: equivalence point hard to spot (no ppt)
Conductimetric titration
−
Relies on the different size of the ions reacted and
in excess
4. Acid-Base Titration
Indicator is one color at pH below the equivalence
point and changes color close to equivalence point.
Problem: Titrating an unknown molarity of HCl
(32.20 mL) with 0.0520 M NaOH reaches a color
change at 25.04 mL base. What is molarity of acid?
−
Strong acid and strong base—what is equiv pH?
−
Ma x Va = Mb x Vb so Ma = Mb x Vb
Va
Ma = (0.0520 M) x 25.04 mL = 0.0404 M acid
32.20 mL
−
5. Why Indicators Work If Chosen Properly
If the indicator's
pKa is close to the
equivalence pH,
then the rapid
change in pH
around the equiv pt
makes the indicator
change colors like a
stop light
Methyl orange changes colors just
short of the equivalence pH of 7 when
using strong acid and strong base
Titrations.info
So methyl orange would be a decent indicator if you
titrate base into acid. Would it work if you titrated acid
into base?
6. Why Indicators Work If Chosen Properly
If the indicator's
pKa is close to the
equivalence pH,
then the rapid
change in pH
around the equiv pt
makes the indicator
change colors like a
stop light
Methyl orange changes colors just
short of the equivalence pH of 7 when
using strong acid and strong base
Titrations.info
So methyl orange would be a decent indicator if you
titrate base into acid. Would it work if you titrated acid
into base?