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Author: Subject: "Brown" prussian blue
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[*] posted on 20-3-2023 at 09:46
"Brown" prussian blue

I made (think) for the first time Potassium ferrocyanide.

As the only reference I found was from wiki: "Can be prepared by heating a nitrogen source, such as potassium nitrate, with iron and carbon. The product is purified via recrystallization from water."

As I did not find any ratio of mixing put a 1/3rd of every reagent (potassium nitrate, with iron and carbon) by "sight" in a small stainless cup (mixed), heated from bellow, and suddenly it ignited with heavy white fumming. (luckily it was done outside).

After 10 seconds fumes (as flames) subsided and a hard mass of molten "something" was glued to the cup. (it was black with green / yellow spots)

Crushed (it was difficult), tried to grind in a mortar, and then put into a beaker with water.
The crude solution was orange/brown (like iron oxide). Tried to use a teflon
magnet stirrer, but a lot of iron fillings were attached to it.

Filtered and a yellow solution was optained.

to test if Potassium ferrocyanide was made:

take 10 ml sample from it and added some drops of concentrated ferric chloride. Some brown/black clumps where precipitated - no blue/dark blue. Thought that possible ferric chloride was very concentated.

so I prepared a new diluted solution of ferric chloride and a tested in 10ml of the yellow solution. The solution became dark green and in time, orange.

The third test was with an old (green almosr becoming orange) FeCl2,
prussian blue was obtained, but as filtered, the residue in the filter paper in 10 minutes became orange again. there was a fine oxide layer., when removed, there was the prussian blue. Next day mostly became orange/brown.

1) is there a ratio of component mixing to get most Potassium ferrocyanide? (will Stoichiometry works? - or for example, Carbon needs to be twice because it volatilizes a lot)

2) currently the solution is in cristallizing procees, but from what I think is heavily contaminated with Iron / Iron oxide.
a) will it crystallize?
b) Should the "excess" of Iron be removed? How?, without affecting the Potassium ferrocyanide
c) can Iron fillings be substituted with Iron Oxide?


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[*] posted on 20-3-2023 at 21:40

It sounds like your solution is contaminated with potassium carbonate. Potassium carbonate would react with iron salts to produce various brown iron carbonates and hydroxides. If it is contaminated, the solution probably bubbled when you added ferric chloride. If so, you can remove the potassium carbonate by crystallising the potassium ferrocyanide. At room temperature, potassium carbonate is much more soluble than potassium ferrocyanide.

If you want to make Prussian blue from the solution, you can remove the potassium carbonate beforehand by adding calcium chloride. This will precipitate calcium carbonate, which can be filtered out, and leave potassium chloride in solution. You probably don't want to do this if you want to extract the potassium ferrocyanide, since potassium chloride has a fairly close solubility to potassium ferrocyanide.

You probably don't have any excess iron in your solution, since any excess iron would have been filtered out as iron oxide. The iron that you observe is probably coming from your ferric/ferrous chloride.
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