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1. Experimental Methods
DAY 1
An approximately 0.1M solution of NaOH was prepared by weighing out 2g of NaOH
and dissolving it in 500ml of water. The solution was transferred to a storage bottle.
∗ Potentiometric Titration
The pH electrode was calibrated as detailed in the pH meter instruction manual using
buffers of pH 4, 7 and 10. The acid mixture – approximately 0.05M HCl and 0.1M
CH3COOH – provided was pipetted (25ml) and placed in a clean, dry 150ml beaker. The
solution was stirred with a stirring rod and then the pH electrode lowered into the
solution. Three (3) drops of screened methylorange was added and the pH and initial
burette reading recorded. From the burette, approximately 1ml of NaOH was added and
the burette reading recorded. The mixture was stirred and the pH of the solution recorded
when the pH meter stabilised. The titration was continued in this way - adding
approximately 1ml aliquots of NaOH, noting the burette readings and pH values after
each addition – until just passing the first equivalence point by about 2ml. Three (3)
drops of phenolphthalein indicator was added and the titration continued to about 5ml
beyond the second equivalence point. The titration data was plotted on a pH vs. titrant
volume graph and the end points determined.
∗ Standardisation of NaOH
Three (3) 0.3g portions of dried AR potassium hydrogen phthalate was weighed out and
placed in three 250ml conical flasks. These were dissolved in 50ml of distilled water and
three (3) drops of phenolphthalein indicator added then the whole was titrated with
NaOH to a light pink colour.
DAY 2
The pH electrode was recalibrated as done in the previous weeks and the pH found for
buffers of pH 4, 7 and 10.
∗ Potentiometric titration
The apparatus was set up as before and the same amount of components added to the
solution prior to the equivalence point of the titration was added. Upon approaching the
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2. first equivalence point or at pH 2.0, the addition of NaOH was switched to a drop-wise
addition. The pH and burette reading was recorded after each drop. This was done up to
about 2ml beyond the equivalence point or pH 3.8. The addition of NaOH was then
returned to 1ml aliquots until approaching the second equivalence point at pH 5.8. The
drop-wise addition was done up to a pH of 12.22 and following this five additional 1ml
portions were added.
∗ Standardisation of NaOH
The standardisation of NaOH was carried out in a similar manner as in the previous week
and the necessary calculations done.
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