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TITRATIONS

What did one titration tell the other? 

Let's meet at the endpoint.

Why are chemists great for solving problems?  

They have all the solutions

Titration

A lab procedure by which the concentration of acidic or alkaline solutions are examined and determined

A strong acid titrated with a strong base will result in neutralization around pH 7

More complex titrations involve weak acids (or bases) titrated with strong base (or acid) and polyprotic acids

pH curves

Graphs that show continuous changes in pH as the titrant is added to the sample

The curves are drawn using the amount of titrant added as the x-axis and the pH as the y-axis

The midpoint of a nearly vertical portion of the line is known as the equivalence point

The number of equivalence points in a pH graph indicates the molar ratio between the acid and the base involved in the titration

There will be 1 equivalence point for each complete proton transfer that occurs

Example 1

Sketch the pH curves in a titration graph, demonstrating the change in pH when a weak acid is titrated with a strong base. In this case use CH3COOH titrated with NaOH

Because CH3COOH is a weak acid it acts as a buffer and does not give up its H+ as easily.

The pH at the endpoint is therefore slightly higher because more strong base is required.

Example 2

Sketch th pH diagram of a polyprotic base Na2CO3, titrated with a strong acid HCl. Use 25mL of 0.20mol/L Na2CO3 titrated with 0.20mol/L HCl for your data.

Example 3

Sketch the pH graph of the titration of polyprotic acid H3PO4 titrated with a strong base NaOH. In terms of experimental data, use 25mL of 0.12mol/L H3PO4 titrated with 0.12mol/L NaOH.

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INDICATORS

An indicator is a solution that changes color to signal that the equivalence point has been reached.

The point at which an indicator changes color is called its transition point.

Try to get the transition point near the equivalence point

Choosing an Indicator

Look at the curve to find the equivalence point

From the indicator list, choose an indicator with a range near the equivalence point

Acid-base indicators are a conjugate weak acid-weak base pair that have distinctly different colors when dissolved in water

Indicators are larger molecules (big enough to be noticed)

Example 6

A student finds that an unknown solution is colorless when mixed with phenolphthalein, red when mixed with chlorophenol red, and blue when in the presence of bromocresol green. She hypothesizes that the pH of the solution is 7.5. Do the indicators provide evidence to support her hypothesis?