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A/B Extraction Theory

  1. Apr 14, 2008 #1
    I understand the basics of acid base reactions but I don't understand how you determine which compounds in a mixture will react to the acid or base and convert into a salt. Does there have to be a large pH difference? For example say the compound I want to isolate has a pH of 11 but two other impurities in there have pH's of 6 and 8. If I was to add an acid with a pH of 2 I know that will convert the compound into a salt but how do I know if its going to convert the 2 impurities into a salt or not.

    Is it only compounds with a pH in the range of 4-8 that will remain unchanged by the acid or base? I'm having trouble understanding how A/B extractions can be used effectively to isolate compounds seeing as all chemicals are acids and bases just some are stronger than others.

    Also what steps are involved in A/B extractions? Lets say my compound has a pH of 11 and its called lembodine. To begin with I dissolve the mixture into a non polar solvent such as diethyl ether and do a wash with a polar solvent such acetone to lose all the inorganic compounds. Next I add conc. sulfuric acid and convert my compound into lembodine sulfate. Assuming the sulfuric acid has a pH of about 2 how do I know which impurities it will protonate and which it will leave untouched so I can wash out?

    Anyhow lets say I add acetone to the acidified mixture so my lembodine sulfate will dissolve in it then I filter out the diethyl ether layer and add more ether to make sure I wash out any organic impurities which were not converted to salts. After this step I'm lost. Everyone says you should rebasify the mixture and do another wash with a polar solvent to further remove polar impurities. I don't understand this. Seeing as I already did a wash with a polar solvent in the beginning whats the point of doing it a second time?

    Anyway following everyones advice lets say I add some conc. sodium hydroxide to basify the mixture which will deprotonate my compound and convert it back into lembodine base. After I do another acetone wash lets say I'm left with some impurities which converted back into organic bases with my lembodine. Is there now no way I can remove these basic impurities with acid base extractions alone and should I resort to another method?

    Are acid base reactions only really a first step to purifying a compound which should be followed by methods such as recrystallization to insure the compound is properly isolated?
  2. jcsd
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