Determine the Concentration of an Analyte
- One of the most important analytical techniques is titration
- Titrations include
- Acid-base titrations
- Redox titrations
- The most important glass equipment used in a titration is a burette
- Titrations are used to determine the concentration of a chemical compound in an unknown sample
- The unknown sample is called the analyte
- The concentration of analyte is determined using a solution of known concentration that is able to react with the analyte
- The solution of known concentration is called titrant
- The burette is filled with the titrant, while the analyte is placed in a conical flask under the burette
- The endpoint or equivalence point occurs when the moles of titrant have reacted completely with the moles of analyte
- In most titrations indicator is placed in the analyte solution
- The indicator is a substance that will change color near the endpoint of the titration
Common titration setup
Titration setup showing the titrant and the analyte. It is a good practice to fill the burette to 0.0 cm3 as shown in the diagram
How to perform a titration?
- Measure a known volume of analyte and place it into a conical flask
- Fill the burette with titrant. Remember that the titrant must be a solution of known concentration
- If necessary, add few drops of indicator in the conical flask
- Open the tap on the burette carefully and add the analyte portion by portion to the conical flask
- After each portion the conical flask must be swirled
- As you start getting near the endpoint, the portions should be added dropwise
- Close the burette tap when one drop has caused a permanent color change
- Record the volume of titrant used for the equivalence point. This volume will be used to determine the concentration of analyte
Determining the concentration of analyte
- One of the most assessed calculations in AP Chemistry regarding titrations is determining the amount of analyte
- The steps for this calculation are summarized in the following diagram
Steps for determining the concentration of analyte
General steps that must be followed to determine the concentration of analyte using the volume recorded from the equivalence point of the titration
Worked example
A 30.0 mL HCl sample of unknown concentration was titrated using NaOH 0.150 M. A permanent color change was reached when adding 25.0 mL of titrant. Determine the concentration of the analyte. The balanced chemical equation is given below
HCl + NaOH → NaCl + H2O
Answer:
Step 1: Analyze the statement and set up the steps that you are going to use to solve the problem. The initial quantity is always given by the statement. Remember to start with the reactant or product that you have the most information about
Since, volume and molarity are given for sodium hydroxide (titrant), it must be the starting point
Step 2: Calculate the moles of NaOH by transforming the volume of the equivalence point into L and using the molarity equation
Rearranging the equation,
n of solute = M × V of solution in liters
n of solute = 0.150 M × 0.0250 L
n of solute = 0.00375 mol of NaOH
Step 3: Calculate the moles of HCl that reacted using the ratio from the chemical equation
Since the ratio is 1:1 the moles of HCl must be the same
Step 4: Calculate the molarity of the HCl solution using the volume of the sample in L
M = 0.125 M
Step 5: Write the answer with the appropriate number of significant figures
Since all the quantities from the statement are given with 3 significant figures. The answer must be written with 3 significant figures
The concentration of the unknown HCl sample is 0.125 M