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The reaction of $\ce{FeCl3}$ with $\ce{CH3COO-}$ is

$$\ce{3Fe^{+3} +6CH3COO- +H2O<=> [Fe3(OH)2(CH3COO)6]^{+} +2H^{+}}$$

Recently I read that a solution of $\ce{FeCl3}$ hydrolyses very quickly to form $\ce{Fe(OH)3 +3H^{+}}$ as described by the following equilibrium:

$$\ce{FeCl3(aq) +3H2O <=> Fe(OH)3(s) +3H^{+}} \tag{1}\label{eqn:1}$$

with hydrolysis constant $K_\mathrm{h}$. I found a $K_\mathrm{sp}$ value of $\ce{Fe(OH)3}$ of $\pu{4E-38}$, so the equilibrium

$$\ce{Fe(OH)3 <=>Fe^{+3} +3OH-}$$

has $K_\mathrm{sp} =\pu{4E-38}$ and after a bit of rearrangement I got $K_\mathrm{h}$ of $(\ref{eqn:1})$ as $\frac{K_\mathrm w^{3}}{K\mathrm{sp}}$.

Hence at $\pu{25 ^\circ C}$, since the ionic product of water is $10^{-14}$, calculation gives $K_\mathrm{h}= \pu{2.5E-5}$. If we maintain neutral pH (ie. $10^{-7}$) we then get $\alpha$ (dissociation constant) for $(\ref{eqn:1})$ as almost 1.0.

How is this possible? If this is true then how can $\ce{Fe^3+}$ ions be in solution for complex formation even if $\ce{CH3COO-}$ ion is not hydrolysed to acetic acid because of such low concentration of $\ce{H^+}$ in solution?

Buck Thorn
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davacd
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1 Answers1

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Theory guides and experiment decides (I. M. Kolthoff). Sometimes thermodynamics predicts a reaction but it does not occur very fast because its kinetics is slow. If you dissolve iron (III) chloride in pure denionized water, assuming it is from a fresh bottle, it will take hours to form a precipitate of iron hydroxides. Therefore, in order to perform this qualitative test, iron (III) chloride should be freshly made in water. Ten or 15 minutes won't make a difference.

As the linked answer here is slightly incorrect. The main reason to use neutral pH iron (III) chloride is that the carboxylic acid to be tested should be present as carboxylate ion in order to chelate iron (III) rather readily (chemical kinetics).

Same goes for ethylenediaminetetraacetic acid, is a powerful chelating agent. However its chelating power is not that great at low pH. It will also not dissolve in water until you add a few drops of a strong base. Then it is converted into ethylenediaminetetraacetate ion, and this anion can rapidly chelate with many metallic ions.

AChem
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  • thankyou so much for the answer sir, just one thing ,can you confirm if i am understanding this correctly? so acc to you this making soln neutral say by adding few drops of K2CO3 will prevent acetic acid formation thus will enable fe^+3 ions present in soln to effectively form a chelating complex(end product of my eq 1) much faster than it would take for fe+3 ions to hydrolyse thus making the test feasable to perform – davacd Jun 11 '21 at 03:19
  • If you add carbonate ion to iron (III) soln, you would see immediate ppt. So, carbonate is out of the question. – AChem Jun 11 '21 at 04:27
  • so what do we use to keep the ph of soln neutral – davacd Jun 11 '21 at 04:31
  • ow so acc to you freshly made fecl3 is a neutral solution hence no need to add anything else – davacd Jun 11 '21 at 04:34
  • i was thinking of something like a buffer to mentain ph if test is to be performed after an hour or so – davacd Jun 11 '21 at 04:35
  • Buffer will have ions which will interfere with analysis! – AChem Jun 11 '21 at 05:39
  • I am sorry but I read this somewhere that FeCl3 is neutralized by adding na2co3 then its used to perform the test to maintain neutrality.Your argument seems correct that it would catalyse the formation of fe(oh)3 giving red brown ppt and would interfere with test so i suposse freshly made soln will do right' – davacd Jun 11 '21 at 06:30