# Last edited on 2019-02-16 01:53:11 by jstolfi

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Introduction

Iron(III) hydroxide Fe(OH)3 is an unstable or non-existent compound.
In nature and in the lab, one usually obtains instead the oxyhydroxide
FeO(OH)(H2O)n, an insoluble brown or ocher solid.

Anhydrous FeO(OH) occurs in several forms. The main ones are alpha
(goethite), beta (akaganeite), gamma (lepidocrocite), and delta
(feroxyhyte).

The beta form in nature contains chloride anions replacing a small
fraction of the OH anions. When FeO(OH) is precipitated in the lab
from chloride-containing solutions, one obtains the beta form.
However, the chloride anions allegedly can be replaced by hydroxils by
washing with base.

Goals and methods

In this experiment we will make some FeO(OH) from copper chloride
common chemicals: iron(III) chloride FeCl3 and sodium hydroxide NaOH

The plan exploits the fact that FeCl3, NaOH, and NaCl are all quite 
soluble in water, whereas FeO(OH) is exceedingly insoluble.  Therefore, 
the plan begins with mixing stoichometric solutions of FeCl3 and NaOH
and filtering out the precipitate.

Dissolving FeCl3 in water should in fact already produce some amount
of FeO(OH) and HCl by hydrolysis.  But we can ignore that in the 
computations.  

As described above, the precipitate will likely contain some chloride
ions.  So we will have to wash and re-filter the precipitate with 
NaOH solution to hopefully get rid of it.

The result may be hydrated.  We need to de-hydrate it, but with care 
to avoid decomposing it by 2 FeO(OH) → Fe2O3 + H2O 

The reagents and the other products

Iron(III) chloride or ferric chloride is sold in electronics stores as
an etchant for printed circuit boards, since it dissolves copper like
nitric acid but without the fumes. The reaction is 2 FeCl3 + Cu → 2
FeCl2 + CuCl2.

Sodium hydroxide, commonly called caustic soda, has many uses,
including drain cleaning and soap making. Beware that some commercial
drain cleaning products contain other salts like sodium carbonate or
sodium chloride, colorants, and aluminium chips to create a fizz when
it is addedto water. So, one must be careful to get the "99% pure"
product.

Hydrates

To get the right amounts of the reagents, and check the mass of the
result, we must take into account the fact that the solid forms of these
substances contain some water, loosely bound to the metal ion and/or
trapped in the crystal structure.  

Iron(III) can be purchased in supposedly anhydrous form.

Sodium hydroxide is extremely hygroscopic and forms many hydrates, but
the "99% pure" version may be the monohydrate. They could be
distinguished by their melting points: 65 C for the monohydrate 318 C
for the anhydrous product.

Reagent amounts

We arbitrarily choose to make 50 g of FeO(OH). That will be enough for
other experiments that we plan to make later on. From that we can
compute how much of each reagent we need.

First, we convert the mass of FeO(OH) to mols of the formula FeO(OH):

  [ Recipe ]

To make one mol of FeO(OH) we need one mol of FeCl3 and 3 mols of NaOH.
Converting back to grams, we get:

 [ Recipe ]

That will make also 3 mols of NaCl.

Water amount

We will use 20% solutions by weight of both FeCl3 and NaOH.

Preparing the solutions

We will assume that we have anhydrous FeCl3.  If it is partly hydrated,
we will end up with less than the expected amount of FeO(OH).

 [ Recipe ]

Likewise we assume that we have anhydrous NaOH.

 [ Recipe ]

Reaction

The two solutions will be mixed in a large beaker.

On mixing, a brown-ocher precipitate of FeO(OH)(H2O)n is formed immediately.

Filtering and washing

The mixture is then filtered through some coarse paper.

We resuspend the precipitate in ?? of water containing about 5% NaOH and 
heat to near boiling for a while, then filter again.  We repeat this
procedure twice.

Drying and dehydration

After the final washing, we dry out the precipitate and keep it heated in air.
The product decomposes around 270 C, but must be heated to 130 C to 
release hydration water. So let's try to keep it at 140 C (in oil?)

Testing

To test the product we can heat some at about 300 C, which should turn it into 
Fe2O3.