I did an acid-base extraction experiment. First extracted a solid mixture containing benzoic acid, p-nitroaniline, and naphthalene into three separate solutions:

  1. NaOH extract
  2. HCl extract
    3.organic with the neutral compound.

Then the HCl and NaOH extracts were neutralized and underwent vacuum filtration to isolate crude p-nitroaniline and crude benzoic acid, while the organic solution underwent rotary evaporation to get crude naphthalene.

Crude benzoic acid was purified by recrystallization using hot filtration, and then benzoic acid crystals were isolated by suction filtration. The melting range of the benzoic was relatively pure.

How can I improve this experiment? My recovery of benzoic acid was less than 40%

And as for sources of error, extraction processes always lose some compound during each step. What are other probable sources of error?

  • 1
    $\begingroup$ Do you know the recoveries of the other 2 compounds? $\endgroup$
    – airhuff
    Commented Mar 1, 2017 at 3:54
  • $\begingroup$ What was % recovery for crude p-nitroaniline and crude benzoic acid? What was your final % recovery for p-nitroaniline and naphthalene? // Zone refining $\endgroup$
    – MaxW
    Commented Mar 1, 2017 at 3:54
  • $\begingroup$ How many NaOH extracts did you use? How many back extractions did you do? What was the back extraction solvent? Did you use a single component solvent for recrystallization? Please provide more experimental details. $\endgroup$
    – Zhe
    Commented Jun 30, 2017 at 20:02
  • $\begingroup$ Jason, what organic solvent did you use to extract the aqueous solution. Have you tried using different organic solvents? $\endgroup$
    – user68891
    Commented Oct 14, 2018 at 2:09

1 Answer 1


I can only assume that you we following some recipe for the purification of the benzoic acid. Frankly if I were doing this I would have done it differently.

Wikipedia shows that the solubility of benzoic acid is 1.7 g/L (0 °C) and 56.31 g/L (100 °C) in water. So I'd added benzoic acid to a test tube and put in a boiling water bath. I'd added just enough water to get the benzoic acid to dissolve and then let the liquid cool in air. When down to room temp, I'd put it into an ice bath. After 10 minutes in ice bath I'd pour off water. I'd wash with a few drops of ice cold water. I'd test for purity and if not good enough I'd repeat.

0.34 g of crude benzoic acid should dissolve in about 6 ml of boiling water.

Each water cycle should have an efficiency of close to (56.31 - 1.7)/56.31 = 97%

  • $\begingroup$ Jason I don't understand your comment. If purity wasn't an issue why were you recrystallizing the benzoic acid? // (1) Did you "invent" hot filtration purification method? How much water did you use? // If you dissolve 0.34 g in 5 ml of boiling water then a small mass won't dissolve. But the benzoic acid that doesn't dissolve doesn't really get purified much. $\endgroup$
    – MaxW
    Commented Mar 1, 2017 at 5:24
  • $\begingroup$ @Jason - How much water did you add? $\endgroup$
    – MaxW
    Commented Mar 1, 2017 at 8:10
  • $\begingroup$ @Jason - Did you "invent" the hot filtration purification method or were you following the experimental procedure given? $\endgroup$
    – MaxW
    Commented Mar 1, 2017 at 8:11

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