Titration Flashcards
What is the goal of the Titration Process?
The goal: determine the concentration of our acid!
known volume but unknown concentration
The strategy of finding concentration of acid
If we add a basic solution (calcium hydroxide, for example) to the flask, a neutralization reaction will occur:
If we determine the amount of base required to compeltely react with the acid, the we can use stoichemetry (mole ratios) to determine **the amount of acid **that was in our flask!
How do we know we have added enough base?
This is where pH comes in! What pH will the solution have when all the acid has been reacted?
Answer: The pH will be 7 (neutral) when all of the acid is gone!
Only for strong acid – strong base reactions
Know as the equivalence point
Where neutralization is completed
moles of base = moles of acid
How do we know when we reach the equivalence point?
The easiest way is to use a pH probe
But pH probes are expensive (we don’t have those here!)
The most common method is to add a special chemical to our solution that changes colour (ideally) at the equivalence point
This special chemical is called an acid-base indicator
Acid-Base Indicators
Acid–base indicators do not change colour at a specific pH value; they change colour over a pH range
Methyl red (red-acid) (yellow-base)(4.4 to 6.2)
Bromothymol blue (Yellow-acid) (blue-base)(6.0 to 7.6)
Phenolphthalein (colourless) (pink) (8.0 to 9.6)
We’ve estimated the equivalence point…now what?
Use the volume of base added and it’s known concentration to calculate the amount (moles) of base added
Next, use the mole ratios from the chemical equation to calculate amount (moles) of acid that must have been present
Finally, determine the concentration of the acid
this process is called TITRATION
The solution with the known concentration is called the titrant
In our example, it was the base (calcium hydroxide)
The solution with the unknown concentration is called the analyte
In our example, it was the acid (hydrochloric acid)
Equivalence point vs. Endpoint
The endpoint is the the point during a titration when there is a sudden change in colour of the acid-base indicator
We use the endpoint as an estimate of the equivalence point
(The true, exact equivalence point is not knowable)
This is why it is advisable to do multiple trials of a titration and use the average endpoint volume
Some practical details
The titrant in the burette doesn’t necessarily need to start at 0.0 mL
As long as we record the initial and final volume, we can subtract to find the volume of titrant added to reach the endpoint
The endpoint has been reached when a single drop of titrant results in a colour change that does not fade when the mixture is swirled
Steps of a titation calculation
- volume of titrant (NaOH) added (mL)
- use c and V of NaOH to find amount (moles)
- Write balanced chemical equation for the nuetralization reaction
- moles of base to acid
- use n and V of acid to find concentration
Titrations require standard solutions
The concentration of the titrant must be precisely known if titrations are to be accurate
As a result, standard solutions are typically used as the titrant
However, standard solutions can be difficult to prepare (if a pure solid form of the compound is difficult to obtain) and potentially unstable
For example, sodium hydroxide solutions react with carbon dioxide in the air to form carbonate compounds
This decreases the sodium hydroxide concentration in solution!
As a result, the concentration of our standard solution (titrant) may need to be confirmed by using a primary standard as the analyte in a preliminary titration
Standardizing the standard!
A primary standard is composed of a chemical that is highly pure and chemically stable – it can be used to prepare an analyte with a precise concentration
Examples:
Potassium hydrogen phthalate, KHC8H4O4 (abbreviated to KHP) – used to standardize bases
Sodium carbonate, Na2CO3 – used to standardize acids
