[lint139]

Hello everyone,

I run a 20,000 litre closed loop recirculating aquaponic system (fish produce waste, bacteria converts waste into plant usable essential elements).
I rely on heterotrophic bacteria (mostly N. sonomas, N. bacter and N. spiria) for the bio-filtration. (The bacteria mostly convert fish toxic NH4/NH3 into NO2 then plant usable NO3 which is also non-toxic to the fish).

My question is: How realistic is it that potassium or sodium ferrocyanide (used as an anti-caking agent in KCl salt) could dissociate into either hydrogen cyanide, or free cyanide in an aqueous environment where the pH stays between 6.0 and 8.2, and temperatures stay between 10C and 33C ?

My whole operation is dependent on a healthy and thriving bacterial population, and I would be loathe to do anything which may jeopardize the well being of said bacteria.

Potassium and or sodium ferro-cyanide seem like very stable compounds, but my knowledge of such things is rather limited. Any pointers as to under what conditions such dissociation would occur would be most helpful.

Thank you in advance.

[curiouscat]

My question is: How realistic is it that potassium or sodium ferrocyanide (used as an anti-caking agent in KCl salt) could dissociate into either hydrogen cyanide, or free cyanide in an aqueous environment where the pH stays between 6.0 and 8.2, and temperatures stay between 10C and 33C ?

What quantity of KCl do you typically add? Anti-caking agents are in small amounts: How much of  ferrocynides are in there, do you know? I'd think ~0.5% or so? Devil's in the details.

As an aside, those additives are on the GRAS list I recall; so in small amounts deemed safe even in edible products I think. 6.0 is indeed an acidic pH but not sure how acidic it needs to be for the tight bound CN to split and make HCN.

[lint139]

Hi curiouscat,

The KCl is listed as 99.3% pure. Their MSDS mentions a possible 0.5% NaCl content, but makes no mention of any anti-caking agent, or their amounts used. So I'm assuming either 0.3% or 0.7% potassium/sodium ferro-cyanide content.
I wrote the company and they only confirmed that they do in fact use the said ferro-cyanides, but made no mention of amount used on a per weight basis. (Since on the bag they just list "anti-caking agent" with no clue as to which of the 4 common agents they used, nor in which amounts. I was rather hoping they were going to respond that they use magnesium carbonate, but no such luck).

I would be adding the KCl to the tune of between 300 mg/ L and 500 mg/ L (300-500ppm) perhaps once or twice a year.

Yes, I'm aware of the GRAS listing, as well as some of the other beneficial uses of potassium/sodium ferro-cyanide, but there are many things (some quite common) which are safe for human consumption, which would damage or even kill either fish or bacteria (or both). So, I prefer to be rather cautious until I can somehow become better informed on the matter. 

[Borek]

I would be adding the KCl to the tune of between 300 mg/ L and 500 mg/ L (300-500ppm) perhaps once or twice a year.

With less than 1% of substances other than KCl your solution is at most 3 ppm of hexacyanoferrate (or whatever you prefer to call it ) - or 8×10^-6 M. At pH 6.00 looks like most of the hexacyanoferrate is dissociated - my equilibrium calculations suggest [HCN] = 5×10^-5M and [CN^-] = 3×10^-8 M. But I doubt these numbers, as they are calculated assuming iron was not oxidized to Fe(III) - which is quite unlikely in the solution that is in the contact with atmospheric oxygen.

[lint139]

Hi Borek,

Yes, it seems that most folks on this side of the blue marble are for some reason ignoring the IUPAC recommendation and still calling it ferro-cyanide. (Things like Fe2+ and Fe3+ still get written in that way as well...instead of Fe(II) and Fe(III)...I have no preference whatsoever, heck, I'll even call it 'Prussian Blue' if it'll help me figure out if the stuff will harm the bacteria/bio-filter that I worked hard to establish

So what would the hexacyanoferrate dissociate to? (Yes, the system water is pretty aerobic, and I doubt there are too many organic acids in there that might help chelate the Fe(II). Even if there were, my guess is that time, pH and O2 would do there 'thing' and the Fe(II) would redox to Fe(III) petty quickly anyways...

I really appreciate your (and everyone's) help in this matter Borek.
 

[curiouscat]

If you did want to play super-safe you could try treating the KCl somehow to get rid of the offending agent.

Although how, I don't know. Especially at the ~0.5% levels which are pretty low.  The H2O-solubilities are also quite similar.   Maybe one of the chemistry pros on the site might have an idea.

[lint139]

Yes, I've thought about dissolving the KCl in a 220 litre barrel and adjusting the water pH with HCl to a nice low pH of 6 or so (something I do anyways before adding top-up water to the system) and aerating it for a number of days. That was my big plan "B".

I have no idea if that would actually help any, or just provide me an illusion of feeling better. Since I'd be "doing something" to potentially off-gas any hydrogen cyanide that may or may not be formed. Never mind that the entire endevour could potentially be utterly pointless But alas, that was the best I could come up with for pre-treating the KCl before adding it to the system...

[curiouscat]

What's interesting is that KCl has a MP of 700°C whereas the ferrocyanide only 70°C. That's a substantial difference. Wonder if you could exploit it?

Although at  0.5 % levels it is bound to be hard. Things will adhere, adsorb etc.

You could even think of baking to 400°C at which temp. the ferrocyanide apparently decomposes. That's nicer; since gaseous products ought to be easier to liberate away.

Just brainstorming here....

OTOH if it is some other cynide they decided to add you'd have to redo the logic.

[lint139]

Nope, the two most popular anti-caking agents for NaCl and KCl on the planet currently are the K or Na ferro-cyanides...It appears that CaCO3 and MgCO3 were used in the past. The manufacturer confirmed that they only use the two ferro-cyanides in question (both CaCO3 or MgCO3 would have been great...and are already used in the aquaponic industry to add as water buffering agents...for every mg of NH4 they oxidize to NO3, they use up about 4.5mg of carbonate alkalinity...which needs to be replaced 'regularly')

Thanks for the tip on the MP's curiouscat. I'll have to keep that tip on the backburner (pun intended

[Borek]

Other thing that comes to mind is that CN^-/HCN itself is probably not stable and reacts with water/oxygen/whatever. That would be in a way similar to hydrolysis of nitriles.