Titration Basics
What is a Titration?
A titration is a laboratory technique used to find the concentration of an unknown solution by reacting it with a solution of known concentration. The volume needed to complete the reaction is then used to calculate the unknown concentration. An indicator is often used to show when the reaction is complete through a colour change.
Materials
Materials:
Aim
To determine the concentration of ethanoic acid in white vinegar by titrating it with a standardised sodium hydroxide (NaOH) solution.
Standardising NaOH
The moles of oxalic acid were calculated using n=cVn=cV, then stoichiometry was used to find the moles of NaOH. Finally, c=n/Vc=n/V was used to determine that the NaOH concentration was 0.091 M.
Results
Our experimental concentration was 10.74%, compared with the accepted value of 8% for Coles Double Strength Vinegar. Therefore, our result was 2.74 percentage points higher than the accepted value.
Chemical Process
CH3COOH+NaOH→CH3COONa+H2O
Ethanoic acid reacts with sodium hydroxide in a neutralisation reaction, producing sodium ethanoate and water.
Method
1. Preparation of standardised NaOH: The NaOH solution was standardised in a previous experiment using a primary standard to accurately determine its concentration.
2. Using a 25 mL pipette and pipette filler, transfer 25.00 mL of white vinegar into a 250 mL volumetric flask.
3. Add deionised water to the volumetric flask until the solution reaches the calibration mark, then stopper and invert several times to mix thoroughly.
4. Rinse the burette with a small amount of NaOH solution, then fill it using a small funnel. Remove the funnel and record the initial burette reading.
5. Using a 25 mL pipette, transfer 25.00 mL of the diluted vinegar into a conical flask.
6. Add a few drops of phenolphthalein indicator to the conical flask and place it on a white tile.
7. Slowly add NaOH from the burette while swirling the flask continuously.
8. Near the endpoint, add NaOH dropwise until a faint pink colour remains.
9. Record the final burette reading and calculate the titre.
10. Repeat the titration until concordant titres are obtained, then use these to calculate the mean titre.
Hypothesis
The ethanoic acid in the vinegar will react with NaOH in a 1:1 mole ratio. The concentration of ethanoic acid can therefore be calculated from the volume and known concentration of NaOH required to reach the endpoint.
Moles of NaOH
Using the average titre of 23.6 mL and the NaOH concentration of 0.091 M, calculate the moles of NaOH used.
n=cV
n=(0.091)(23.6×10^−3)
n=0.00215 mol
Accuracy & Precision
Accuracy describes how close our result is to the accepted value, while precision describes how close repeated measurements are to each other. Our titre results were reasonably precise if they were close together, but our final concentration of 10.74% was not very accurate because it differed from the accepted value of 8%.
Equivalence Point vs Endpoint
The equivalence point is when the ethanoic acid and NaOH have reacted in exactly the correct amounts. The endpoint is when the indicator changes colour, showing that the equivalence point has been reached. NaOH should be added slowly near the endpoint to avoid adding too much, which could make the titre inaccurate.
Independent Variable
Volume of standardised NaOH added.
Moles of Ethanoic Acid
CH3COOH+NaOH→CH3COONa+H2O
The mole ratio is 1:1
n(CH3COOH)=0.00215 mol
Chemical Theory
The hypothesis was partially supported. The reaction between ethanoic acid and NaOH followed the expected 1:1 mole ratio, consistent with the balanced equation:
CH3COOH+NaOH→CH3COONa+H2O
However, the experimental concentration was higher than the accepted value, suggesting that experimental errors affected the result.
The experiment followed the Green Chemistry principle of preventing waste by using small quantities of chemicals. It also followed safer solvents and auxiliaries by using deionised water instead of more hazardous solvents. These practices reduced the environmental impact of the experiment while still allowing the titration to be performed effectively.
Dependent Variable
Volume of NaOH required to reach the endpoint (titre).
Concentration of CH3COOH
c=n/v
c=(0.00215)/(0.02)
c=0.1074M
Errors, Limitations & Improvements.
Possible limitations included judging the endpoint by eye, incorrectly reading the burette, and the limited number of trials. The experiment could be improved by adding NaOH dropwise near the endpoint, reading the burette at eye level, performing more trials and ensuring all equipment is used correctly.
Final Concentration
Our experimental value of 10.74% was higher than the accepted value of 8%, giving a difference of 2.74 percentage points. This suggests that there may have been experimental errors or limitations that caused the concentration to be overestimated.
Repeatability & Reproducibility
Repeatability is the ability to obtain similar results when the same method is repeated under the same conditions. Reproducibility is the ability to obtain similar results when the experiment is performed by different people or under different conditions. Our concordant titres indicate good repeatability, but more trials and results from other groups would be needed to properly assess reproducibility.