Cyanide Level Testing with Copperas

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aga

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Destruction of cyanide wastes by any method is clearly a good thing to do before disposal.

Copperas appears to be a common chemical available to refiners, and luckily can also be used as an indicator for the presence of cyanate ions.

This can be used as a cyanide indicator because it reacts in a complex way with cyanates to produce Iron(II,III) hexacyanoferrate(II,III) (a.k.a. Prussian Blue) which is, er, Blue, and even appears Black if the concentration is high enough.

Commercial testing kits are definitely recommended to get an accurate measure of the cyanate ions in your waste. The EPA limit is stated to be 0.2ppm in this reference :-
http://www.freedrinkingwater.com/water-contamination/cyanide-contaminants-removal-water.htm

0.65g of iron(II)sulphate (a.k.a. Copperas) was dissolved in 25ml of distilled water to make a 0.1[M] copperas testing solution (approx 26,000 ppm) and was put into a dropper bottle.

0.82g of potassium hexacyanoferrate(III) (a.k.a. Potassium ferricyanide) was dissolved in 25ml of distilled water to make a 0.1[M] cyanate solution, which is approx 33,000ppm

This cyanate solution was diluted down in a binary fashion, halving the concentration each time.

Three drops of each concentration from 33,0001 to 2ppm was put into a labelled well on a reaction plate, and one drop of the testing solution was added.

33kto2ppm.jpg
The blue colour appeared instantly on addition of the copperas testing solution and did not change over time.

Given the extremly low 0.2ppm permissible limit, this test can only be used as a 'No-Go' test for wastes, as the appearance of any blue at all means that the cyanide concentration is 10x1 the limit.

On the other hand, this test could be of use if a minimum cyanate concentration is required for a process, such as leeching.


Note:-
1 As the prussian blue compound can be written as Fe7(CN)18, it requires 3x K3Fe(CN)6, so the ppm values could be out by a factor of three.
Not exactly sure about that (yet) as i'm just an amateur chemist, not a trained pro.
 
Nice test, is it working in basic solution?

I started to write this post to warn about mixing copperas and cyanide solutions as it is common to prepare it as an acidic solution and there could be left over acid from making it for those making it themselves. But doing some background research I found this article.
https://www.saimm.co.za/Journal/v092n01p017.pdf
where it is stated :
The precipitate from the reaction between cyanide and ferrous sulphate consists mainly of insoluble Prussian blue, Fe4(Fe(CN)6)3,and is shown to be unstable in the presence of alkaline solution, since it decomposes rapidly at pH values above 7 to form ferrocyanide, (Fe(CN)64-), in solution and various insoluble iron oxides.
Sounds like the test could have a problem in basic solution.

Göran
 
I have no idea at all.

I'll try it again with a base and an acid to see what it does at 14 and 1 pH.

Tentatively i'll speculate it will behave different at a High (basic|alkali) pH than a low (acidic) pH.

Edit:

Very small and calculated quantities only.

The risk of DEADLY HCN gas is not something to be ignored, if you want to live.
(cyanates/ides release HCN gas if the acidity is high = low pH)
 
I don't think testing in acidic conditions is necessary. It will turn cyanide into HCN and no one should use acids with cyanide solutions. High pH on the other hand is interesting since most cyanide leaches are run around pH 11.

I assume that you are running the tests in your fume hood?

Göran
 
Hi Goran

The levels of HCN produced from a few drops of acidified 2ppm CN based solution wouldn't present a health problem if done in a vented area. You could probably work the maths through and determine the maximum quantity of HCN that could be generated if you liked and it would fall way short of any accepted hazardous level.
 
g_axelsson said:
Nice test, is it working in basic solution?
No, it doesn't work very well at all :(

acidbase.JPG
With concentrated acids the colours are vivid, nitric giving a greener hue than sulphuric or hydrochloric.

In a basic solution the blue colour does not appear at all, just some fine brown particles can be seen.

If the solution is tested in neutral conditions, the blue appears, then is quickly destroyed when NaOH is added.

While this test makes pretty colours, it is useless in the context of PM recovery/refining, because it does not work at high pH levels, and is not sensitive enough to detect cyanide down to the 0.2ppm permitted level.
 
It might not work as a definite test for cyanide, but it shows a good way of removing the bulk cyanide from a solution.
According to http://www.sgs.com/-/media/global/documents/flyers-and-leaflets/sgs-min-wa017-cyanide-destruction-en-11.pdf it has a practical limit and you can reduce the amount of cyanide down to 5 mg/l by precipitating out ferro ferricyanide and ferri ferrocyanide (not sure they are different compounds).

Nice try though, now we all know. 8)

Göran
 
g_axelsson said:
It might not work as a definite test for cyanide, but it shows a good way of removing the bulk cyanide from a solution.
I definitely saw some ferrocyanide blues when doing cyanide destruction with bleach, too--that's part of why I knew it was working. Since we rarely work with clean solutions in our practical applications, the basemetal impurities in our spent solutions also react with the cyanates as they are formed.
 
Cyanide test papers which are a convenient way to test for CN- require the acidification of the sample with a drop of sulfuric acid. The papers are only sensitive to HCN-, which is why we keep the solutions used for leaching alkaline, to prevent forming the toxic gas.
 
I think you need to add peroxide to the copperas to ensure that can form if there's free cyanide. Though this assumes you aren't using peroxide as an accelerant and there's no undissolved gold since Hydrogen peroxide is an intermediate according to NurdRage's cyanide synthesis video. Granted, you can just add pool chlorine to the cyanide solution and draw a sample to a vial with a couple drops of acetone and see if it turns the solution cloudy. the cloudy solution indicates excess chlorine and it should be retested after a few days and if it still turns cloudy it is safe to dispose of assuming cyanide is the major toxin.[youtube]http://www.youtube.com/watch?v=15ZkhhJcXXk[/youtube] 3:00 in. Hope this helps :) the attachment gives a visual of the cloudiness that you should look forScreen Shot 2017-03-31 at 11.52.28 AM.png
 
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