Originally posted by DerAlte
To the best of my knowledge, perchlorates CANNOT be produced by the electrolysis of chlorates using carbon as an anode.
I had two samples, produced in 1995. One, labeled Chlorate (call it ‘C’ for short) was produced by electrolysis of saturated NaCl using a carbon
anode. Part - the major part – was used by metathesis with KCl to produce KClO3.
The rest was then somewhat purified to get rid of as much NaCl as possible and subjected to further electrolysis, following a convincing sounding
procedure “guaranteed” to produce perchlorate (found somewhere on the WWW). I added KCl to the electrolyte at this point order to precipitate any
KClO4 so formed. I kept temperature much lower (by reducing current density and ohmic losses) than in the first case, since I assumed the process was
a purely electrochemical ion oxidation conversion, with a different cell configuration (wide anode/cathode separation). A precipitate was formed.
This was bottled as ‘KClO4 (impure)’ – call it sample ‘P’. The only use I ever made of this was to demonstrate a few pyrotechnical reactions
to my younger son. It seemed less reactive than the sample C.
When Dann called into question its composition, I sought methods available to me to determine this. I did not have conc. H2SO4 available nor Methylene
Blue.
Chlorates react with any strong acid to give either Cl2 or ClO2 or a mixture; perchlorates do not, due to the kinetic barrier which accounts for their
relatively poor oxidation performance, although the thermodynamics favor it over chlorates.
(1) A test of P with HCl gave off a gas that was either Cl2 or a mix with ClO2, by nose test. Conclusion: P contains Chlorate. This does not
definitively say it does not contain perchlorate, however.
(2) Saturated solutions were made of both samples and a drop placed on a microscope slide and allowed to evaporate slowly. Crystallization was
observed under X50 mag.
Result: C gave lozenge shaped crystals characteristic of KClO3, plus a few needles. Both of these forms are possible under the monoclinic
classification of KClO3, where sides may be unequal but parallel and only one angle is a right angle.
P also gave the same shapes in the main – only one crystal was orthorhombic, where all angles are right angles, but sides unequal (unlike cubic)
which is characteristic of KClO4. However, needles were also produced, and perchlorate can also be of the needle form (it is isomorphic with KMnO4,
which tends to make needles in my experience).
Conclusion: If there was any perchlorate in P, it was a small amount.
(3) 1.000 g (+- 0.005g) of each of C and P were dissolved in 33.3 mL (+-0.3mL) water at RT – both dissolved. This shows that less than 70% was
perchlorate. The solution was so chosen that pure chlorate would be just saturated at 0c. Both solutions were cooled for 3 hrs. in a water bath in a
refrigerator to slow the ice melting. One or two small crystals were then seen floating in C; none in P. The P solution was somewhat turbid at the
start and a fine white ppt. settled during cooling, but there was very little of it. Examination under magnification showed what looked like particles
of a fibre. Non –crystalline.
The conclusion from test 3, (solubility) indicates that not more than 0.25g in solution (<25%) could possibly be perchlorate. Taking account of the
common ion effect, even including activity coefficient (from CRC) reduces this to a maximum of 0.063 g (<6.3%).
Overall conclusion: if perchlorate was formed at all, it was probably less than 5%. I was merely oxidizing carbon! Moral: Don’t believe what you see
on the web, however convincing. And do some tests, always…
DerAlte
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