E10
CONDUCTOMETRIC TITRATIONS
You are required to submit a report for this exercise. A report includes results, calculations and
questions.
PURPOSE
To use a conductivity cell to monitor the endpoint of titrations.
PROCEDURE
I. Determination of the acidity of vinegar
1.
Pipette an aliquot of brown vinegar (5 mL) to a volumetric flask (250 mL). Make up to mark
using distilled water and mix well.
2.
Pipette an aliquot of the diluted vinegar (10 mL) into a 150 mL beaker. Add distilled water (50
mL) by measuring cylinder to the beaker.
3.
Set up a magnetic stirrer and immerse the conductivity cell in the solution, ensuring that the
platinum electrodes of the conductivity cell are completely immersed. If not, add more water
(by measuring cylinder – note the volume).
4.
Titrate with standardised 0.01 M NaOH solution, taking conductance readings at 1 mL intervals.
Allow about 30 seconds equilibration time between each addition.
5.
Titrate another 10 mL aliquot of diluted vinegar, which has a 5 mL aliquot of standardised
0.01M ethanoic acid added to it as a recovery check.
II. Determination of the salt content of soy sauce
6.
Pipette 1 mL of soy sauce into a 100 mL volumetric flask. Make up to the mark with distilled
water and mix well.
7.
Pipette an 5 mL aliquot of the diluted soy sauce into a 150 mL beaker. Add by measuring
cylinder enough water to cover the electrode. Note the volume of water added.
8.
Set up the apparatus as before, again checking the solution level and adjusting accordingly.
9.
Titrate as above with standardised 0.01 M AgNO3.
CALCULATIONS
For each titration,
•
using a spreadsheet, correct the measured conductance, Gobs, for dilution effects, using the
equation in your theory notes.
•
plot Gcorr against titrant volume.
•
draw lines of best fit through the linear regions of the graph (neglect points in the vicinity of the
end point).
•
extrapolate the arms to their intersection and determine the endpoint volume.
•
using the endpoint volume, calculate the moles of titrant used in the titration (equal to moles of
ethanoic acid present)
•
calculate the moles of analyte in the titrated aliquot (both reactions are 1:1)
•
calculate the mass of analyte in the titrated aliquot
•
convert this to g/100 mL
•
apply the dilution factor to calculate the concentration of analyte in the original sample
For the vinegar recovery check, calculate the percentage recovery using the following formula:
% re cov ery = 100 x
(m2 − m1)
0.005 M
where m1 and m2 are the moles of ethanoic acid in the sample and recovery check titrations,
respectively, and M is the molarity of the ethanoic acid added as the recovery check.
QUESTIONS
1.
Compare your results to the expected values for ethanoic acid in vinegar and salt in soy sauce.
2.
Compare potentiometric and conductometric methods for endpoint detection.
Why is it not necessary to calibrate the conductivity cell before using it in a titration?
3.
4.
Explain the shape of the two titration curves.
E10. CONDUCTOMETRIC TITRATIONS
RESULTS SHEET
Vinegar ID
Brand
Barcode
Expiry date
Soy sauce ID
Brand
Barcode
Expiry date
Molarity standardised CH3COOH
Molarity NaOH titrant
Molarity AgNO3 titrant
Record the titration data in your logbook.
Tester’s signature ________________________________
Date _______________
Supervisor’s signature ______________________________
Date ______________