N4. ANALYSIS OF AN IMPURE SAMPLE OF SODIUM HYDROXIDE
PURPOSE: To determine the level of carbonate (as sodium carbonate) impurity in a sample
of sodium hydroxide
PROCEDURE
I. Double Indicator Method
1. Weigh accurately the entire contents of the sample container (about 1g) into a 250 mL
beaker and add about 150mL of purified water, mix to dissolve the sample. Quantitatively
transfer to a 250 mL volumetric flask and make up to the mark.
2. Pipette 25 mL aliquots of the sample into conical flasks (in triplicate).
3. Add 1-2 drops of phenolphthalein (PP) and titrate to endpoint with standardised 0.1M
HCl. Record the volume on the burette as the volume to PP endpoint. Do not discard this
solution or refill the burette!
4. Add 2-3 drops of screened methyl orange (SMO) to the same solution and continue
titrating to endpoint. Record the final volume on the burette as the result to SMO
endpoint.
II. Barium Chloride Modification
5. Pipette fresh 25 mL aliquots of the sample into conical flasks (in triplicate).
Carry out steps 6 and 7 one flask at a time!
6. Add 10 mL of 10% BaCl2 solution and mix thoroughly by swirling
7. Immediately titrate the remaining NaOH using standardised HCl to phenolphthalein
endpoint
CALCULATIONS
If available use the EXCEL spreadsheet.
DISCUSSION
• Explain :
- how the “double indicator” method works
- how the BaCl2 modification works
- why it is more accurate than the “double indicator” method
• Explain the calculations
• Explain why precipitate of barium carbonate does not interfere with the titration in Part II
QUESTIONS
1. Why is solid NaOH susceptible to contamination by carbonate? Write an equation to
illustrate your answer.
2. Describe the use of a special mixed indicator in accurately detecting the first titration
endpoint in this mixture.
REFERENCE
Vogel’s Textbook of Quantitative Inorganic Analysis