Titrations

Titration is a way to determine the concentration of an unknown solution. This is done by adding a known concentration and volume of an acid to a known volume, but unknown concentration basic solution. This reaction can be reversed, so that bases can be added acids.

The “standard” or “known” solution is filled in a burette. It’s like narrow graduated cylinder. The standard solution is slowly added to a flask with an unknown concentration. As the two solutions are mixed the acid and the base are neutralized.

Acid + Base = ? As the Acid and Base react, a neutralization reaction occurs. What are the products going to be? A salt is an ionic compound made up of a cation from a base and an anion from an acid. Water will be produced, but what else can we expect to change?

pH does what? Example. There is an unknown concentration of an HCl and I want to titrate it with a strong base such as NaOH. What is in the burette? NaOH What is in the flask? HCl Where would the pH start? Around 1 What would happen to the pH? Increases!

H + + OH - → H 2 O Water is made by adding OH - to H + ions and causes the pH to change. How do you know how much to add? When are you done titrating? A titration is complete when the “equivalence point” is reached. This is the point when [H + ] = [OH - ]

Let’s model our reaction! Remember we are adding NaOH in the burette to HCl in the flask. When will we stop the titration? When they reach their “equivalence point”! Let’s pretend we could see the ions as they were added. What would it look like?

Base added to Acid: pH is low

Base added to Acid: B A pH is getting higher

Base added to Acid: B A pH is even higher

Base added to Acid: B A pH is even higher

Base added to Acid: B A pH is near 7

Base added to Acid: B A pH is 7 The titration stops here because there is equal concentrations of H+ and OH- ions in the flask at the bottom. Equivalence Point!

Problem So we can’t see the actual number of ions with our eyes. What are some other ways we could know when we reached our equivalent point? pH meters – Tell us the pH Values Indicators – Change colors in pH

Indicators Indicators are substances that change colors in the presence of acids or bases. Each indicator has their own pH ranges where they are effective: Bromophenol Blue – (3.0 – 4.6) Y to P Methyl Red - (4.4 – 6.2) R to Y Bromothymol Blue – (6.0 – 7.6) Y to B Phenolthalein – (8.3 – 10.0) W to F

Indicators Continued When using indicators, you keep adding your titrant (what’s in the burette) until you see the color change. The point where the color changes is called the “End Point”. Ideally, you want the “End Point” and the “Equivalent Point” to occur at the same time so choice of indicator is important!

Which indicator to use? pH mL OH- added 7 The dot represents the equivalence point in a titration. Which indicator(s) would work in our experiment so the end point would happen at the same time? Bromophenol Blue – (3.0 – 4.6)Methyl Red - (4.4 – 6.2) Bromothymol Blue – (6.0 – 7.6) Phenolthalein – (8.3 – 10.0)

Titration Curves You just saw a titration curve and understanding them is an important part of this lesson. Titration curves plot the change in pH (Y-axis) versus the amount of titrant used (X-axis) The equivalence point is always in the middle of the vertical line as it represents when the [H + ] = [OH - ] concentrations are equal.

Practice Question: What letter would represent the “Equivalent” Point? C! What letter would represent the “End” point? C as well b/c we want them to occur at the same time!

Strong Acid Titrated by Strong Base mL OH - added pH Notice slow change in pH at the beginning. As it approaches the equivalence point, there is a rapid change in pH due to small difference in H+ concentrations After equivalence point, slow change.

Weak Acid Titrated by Strong Base pH mL OH - added A greater change in pH in the beginning? Why? As it approaches the equivalence point, there is a less dramatic change in pH. After equivalence point, slow change.

Side by Side Comparison Equivalence point = 7 Equivalence point = 9

Different Curves for Different Combinations Remember that all of these could go in the opposite direction if we started with a base and titrated with an acid.