Titration

Using water solutions to make analytical work faster, easier and more

accurate

Mpcfaculty.net

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

Water is the product of most acid-base reactions Many interesting salts not soluble in water

Enables precipitation reactions

Types of Titration

Acid-base titration Requires a weak acid indicator for equivalence pt.

Oxidation-reduction titration Usually one or the other has a color change

Precipitation titration Difficult: equivalence point hard to spot (no ppt)

Conductimetric titration Relies on the different size of the ions reacted and

in excess

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

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 Titrations.info

Methyl orange changes colors just short of the equivalence pH of 7 when using strong acid and strong base

So methyl orange would be a decent indicator if you titrate base into acid. Would it work if you titrated acid into base?