Ferricyanide solution - change of colour?

[pdeeh]

Earlier this afternoon I made up 100ml of Potassium ferricyanide solution at 7.5%, using deionized water from a freshly opened container.

(This was just to make a small amount of Farmer's reducer.)

The reducer worked fine, and I left the remains of the ferri stock (nb not mixed with hypo) in a graduate, intending to decant it for storage later.

About ½ an hour later, I noticed that the usual vivid orange of the solution had faded somewhat, and now about an hour later, the solution has turned milky and tan in colour.

I keep my graduates and other equipment clean and thoroughly rinse them after washing. I used separate disposable pipettes to take the ferri and hypo solutions from their separate mixing vessels to make up the reducer.

I can only assume that there is a contaminant present, and that I've been careless in some way, so I'll dump the ferri stock and clean everything thoroughly again before I do anything else ... but any clue as to what might cause such a reaction in ferricyanide solution?

 

[Gerald C Koch]

Ferricyanide solutions are unstable toward light but usually become green or blue. Next time immediately after mixing put the solution in a brown bottle for future use. This chemical will also decompose in solution after a period of time. Should the solution develop an odor of bitter almonds it should be carefully disposed of. This indicates the presence of free cyanide ion.

To get around the stability problem experienced darkroom workers usually mix up Farmer's reducer just before use. Measuring the amount of potassium ferricyanide based on the color of the solution. It should be a pale to light yellow color. You will see this mentioned in several books on printing. This is one case where the actual amount of a chemical is not that important. Varying the amount only effects the speed of the reduction and not its outcome.

 

[pdeeh]

Thank you Gerald.

I've mixed and used Farmer's reducer several times before without having seen this, and I followed the same procedures on this occasion as I have always done, which are essentially those laid out by AA in The Print Ch.6 and using the formula & mixing instructions from Appx. 1 of the same.

Hence my question about possible contamination.

As far as the production of cyanide gas, would that not only occur in the presence of either a strong acid or considerable heat?

 

[Gerald C Koch]

No light and air will do it too. I have had the ferricyanide solution for cyanotypes go bad on me. There was a definite smell of bitter almonds and the solution turned blue-green in color. The fact that your solution turned colorless puzzles me as a chemist.

 

[pdeeh]

Ok thanks. Always something new to learn

 

[georg16nik]

Whats the purity of your Potassium ferricyanide?
something went very wrong with this. ½ an hour later ? lol

 

[pdeeh]

I have no idea of the assay. It was purchased from silverprint, so there's no reason to suspect it is not of sufficient purity.
I've made up various solutions from the same container with no problems in the recent past.
I've no doubt I've introduced a contaminant at some stage in making up the solution today.

 

[grahamp]

Stirrers are a great source of contamination. Been there, trying to forget...

 

[pdeeh]

Damn, you mean I shouldn't use the pencil I stuck in my ear?