μSv/hr
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On Conjugate Bases
On the topic of conjugate bases, I am wondering if the conjugate base may converted into its acid form. The only way I can conjecture this to happen
is if the base can accept a hydrogen ion. Does anyone have any more knowledge on this subject and a process for it?
[Edited on 2-4-2019 by μSv/hr]
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j_sum1
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I am not 100% sure what you are asking. But if I understand you correctly, the answer is yes.
Per the Bronsted-Lowry theory of acids, an acid is defined as a species capable of donating protons (H+) and a base is a species capable of
accepting protons.
Therefore a base may be converted to its conjugate acid by accepting a proton.
Note that under this theory, the terms acid and base are relative terms -- relative to what else is happening in the environment. They describe a
process rather than an absolute property. Therefore it is possible (common) for a species to be able to behave as both an acid or a base. For
example, the bicarbonate ion.
As an acid:
HCO3– = CO32– + H+
As a base:
HCO3–+ H+ = H2CO3
(This product is likely to decompose to water and carbon dioxide via an equilibrium reaction, but that is another topic.)
Nowhere in this is any indication of how acidic or basic a solution of say, sodium bicarbonate might be. You can speak of acids and bases and their
conjugates without even considering the pH. (It turns out that bicarbonate solutions have pH>7. Contrast with bisulfate solutions which have
pH<7.)
Maybe the most important example is the water molecule which also acts as both acid and base.
As an acid:
H2O = OH– + H+
As a base:
H2O+ H+ = H3O+
All of this is wrapped up in the meaning of the terms conjugate base and conjugate acid. I would be looking for a good textbook explanation or maybe
a Khan Academy video if this does not make sense. It does take a bit for it to sink in.
Additional note:
All of this relates to the Bronsted-Lowry definition of acids and bases -- which is a reasonably good model for most purposes.
The Lewis acid-base model redefines things in terms of electron movement rather than proton movement. It has the advantage of making sense of species
when they are not in an aqueous environment and also allows us to consider properties and behaviour outside the pH range of 1-14.
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μSv/hr
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On Conjugate Bases
Yes, that is what I was asking. Also, is there any direct process of adding these hydrogen ions(H*) to form the conjugate acid? Reading a textbook
proved unhelpful on this subject.
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DraconicAcid
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Just add an acid that is stronger than the conjugate acid of the base.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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μSv/hr
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As in stronger do you mean a lower ph such as adding hydrochloric, e.g., to salicylates, the conjugate base, and forming Salicylic Acid? From what I
have read, the hydrolysis of the conjugate base with an acid will form the conjugate acid. Such as, methyl salicylate hydrolized by HCL will form
methanol and salicylic acid. Is this correct?
[Edited on 2-4-2019 by μSv/hr]
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j_sum1
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Yep. Add an acid that is compatible with your system and the process you are trying to accomplish.
For example, if I was trying to add H+ to a solution of sodium bicarbonate, I would achieve this by adding a solution of hydrochloric acid:
HCl.
HCl dissociates (fully) in solution to form H+ ions and Cl– ions. These chloride ions are generally spectator ions, but the
H+ will be available to undertake the reaction that you want.
Another example.
Suppose I have a solution of borax which contains the (reasonably) soluble ion [B4O5(OH)4]2–. If I add
H+ in the form of HCl then I will see a precipitate of boric acid appear. (Boric acid is not soluble.)
[B4O5(OH)4]2– + 2H+ + 3H2O = 4H3BO3
DraconicAcid is correct here. The acid you add (HCl in this instance) must be a stronger acid than the one you are trying to produce. If not, the
H+ ions will not be available for the reaction
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DraconicAcid
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Quote: Originally posted by μSv/hr  | | As in stronger do you mean a lower ph such as adding hydrochloric, e.g., to salicylates, the conjugate base, and forming Salicylic Acid?
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I mean stronger as in having a higher Ka.
Hydrochloric acid is a strong acid in aqueous solution, and will convert any weak base into its conjugate acid (salicylate to salicylic acid, acetate
to acetic acid, phosphate to phosphoric acid, cyanide to hydrocyanic acid, oxalate to oxalic acid, etcetrate to etcetric acid). Acetic acid is a
weaker acid (Ka = 1.8 x 10-5), so it will convert, say cyanide to hydrocyanic acid, hydroxide to water, phenoxide to phenol and ammonia to ammonium,
but it will not convert oxalate to oxalic acid, formate to formic acid, sulphate to sulphuric acid, or chloride to hydrochloric acid.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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μSv/hr
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This has helped greatly. Your example with boric acid was helpful, for I have attempted its synthesis via hydrolysis. With this knowledge and
confirmation I shall be able to complete the process I am attempting.
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j_sum1
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No problems.
This is the basis for acid-base extraction which is a common practice in organic chemistry.
Often an organic species forms a soluble ion while the substance itself is insoluble or poorly soluble.
If it is an anion, addition of a strong acid will cause the free acid to precipitate.
If it is a cation, addition of a strong base will cause the free base to precipitate.
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