Experiment 2: Preparation and Standardization of 1 N Sodium Hydroxide Flashcards
(26 cards)
Reagents
- Sodium hydroxide
- Barium hydroxide
- Potassium biphthalate
- Phenolphthalein TS
principle
alkalimetric analysis
More than 1 equiv, 40.00 g, of sodium hydroxide is weighed out, since it is _________ and if only 1 equiv were used the resulting solution would be below normal strength.
- hygroscopic
The solution is treated with __________ to precipitate any carbonate formed through the action of carbon dioxide on the sodium hydroxide.
The solution is preserved in a tightly stoppered bottle fitted with a ________ to protect it from the carbon dioxide of the air.
- barium hydroxide
- soda-lime tube
Solutions which contain __________ are not suitable for titration with __________ as indicator, but when ___________ is used, the results are the same as if all the sodium present were combined as hydroxide.
- carbonate
- phenolphthalein
- methyl orange
________ or _________ solutions can be standardized by use of reagent ___________ as the _______ standard.
- Sodium hydroxide or potassium hydroxide
- potassium biphthalate; primary standard
used to remove carbonate impurities
Barium hydroxide (Ba(OH)₂)
primary standard
Potassium biphthalate (KHC₈H₄O₄)
- Stable, high purity
- monoprotic acid salt
secondary standard
Sodium Hydroxide (NaOH)
- Hygroscopic
- deliquescent, strong base
Titrant, standardized using a primary standard
Sodium Hydroxide (NaOH)
indicator
Phenolphthalein TS
Used for standardizing NaOH solution
Potassium Biphthalate (KHP)
endpoint and its pH
Phenolphthalein
- colorless to pink at pH ~ 8.2–10
A strong base that absorbs CO₂ and moisture, making it unsuitable as a primary standard, hence must be standardized against a stable primary standard.
Sodium Hydroxide
Serves as an accurately weighable, stable solid acid salt. One mole donates one equivalent of hydrogen ion (acts as a monoprotic acid).
Potassium Biphthalate
Reacts with dissolved CO₂ to precipitate carbonates (as BaCO₃), improving accuracy and shelf life of the NaOH solution
Barium Hydroxide
NaOH solution is a…
secondary standard solution (its exact concentration is determined by standardization).
KHP solution is a…
primary standard solution—prepared by dissolving an exact amount of a highly pure substance in a known volume of solvent.
example of acid-base titration using alkali as titrant and monoprotic acid salt as analyte.
Titration setup
Why is sodium hydroxide considered a secondary standard?
Because NaOH is hygroscopic and absorbs CO₂ from air, its concentration changes over time and cannot be accurately weighed for direct use.
Explain the role of barium hydroxide in the preparation of NaOH solution.
It reacts with carbonate ions (CO₃²⁻) formed from CO₂ contamination, precipitating them as BaCO₃, thus purifying the NaOH solution.
What makes potassium biphthalate a good primary standard?
It is pure, stable, non-hygroscopic, and has a known molar mass. It reacts in a 1:1 molar ratio with NaOH and is easily dried and weighed accurately.
Why must potassium biphthalate be dried at 120°C for 2 hours before use?
To remove moisture and ensure accurate mass measurement, ensuring it acts as a true primary standard.
Why must the water used be free from carbon dioxide?
CO₂ reacts with NaOH to form Na₂CO₃, which alters the concentration of hydroxide ions and introduces titration error.
