substitute for nitric acid?

While neutral and basic solutions of potassium ferricyanide are relatively safe, the addition of acid can release hydrogen cyanide. Even relatively weak acids can do this. There is a case where a man committed suicide by adding a teaspoon of potassium ferricyanide to a glass of lemonade. Another point to remember is that cyanide ions pass readily through the skin. So avoid skin contact with acidified solutions such as the bleach mentioned in the post.

Since modern papers are quite different from the old Defender papers the toners mentioned may no longer be very useful. This is a common problem when trying to use old formulas with modern emulsions.

Jerry;

I'm afraid that all of those stories about release of HCN from Potassium Ferricyanide are apocryphal. See here:

http://en.wikipedia.org/wiki/Potassium_ferricyanide

This argument has been carried out here and on Photo Net several times. You have to really beat on the Ferricyanide ion to release HCN. That is why it was considered so safe in color processes up until the 70s when it was found that due to Biological degradation or UV degradation in the effluent slowly released HCN.

You need acid and heat to really release HCN fast enough to harm yourself. So, some acidified Ferricyanide dropped on a hot hotplate will do the job.

Nevertheless, I still recommend extreme caution to avoid this problem from arising.

PE

I would just like to thank all that contributed to this thread for their frankly amazing level of knowledge. APUG can be an amazing resource.

I've noticed a slight amount of highlight bleaching with the B3 bleach. This is tested on Ilford Multigrade IV FB. By slight, I mean much less than a third of a stop on my Stouffer step wedge. I'm not doing any densitometry; just judging visually. And I agree with Ian Grant's statement that there will be an overall loss of density. The blacks are slightly but noticeably weaker as well. I guess that can be said for most sepia toners, I suspect. I'm not chasing absolute dmax, so it's not a massive concern.

I wouldn't use this bleach all the time, but it does produce a brilliant 'sunlit' tone in the highlights with the right toner combination, and I can see the potential for use in some images.

Photo Engineer said:

Jerry;

I'm afraid that all of those stories about release of HCN from Potassium Ferricyanide are apocryphal. See here:

....You have to really beat on the Ferricyanide ion to release HCN...

PE

Click to expand...

The Autotype formula for Carbro processing mixes ferricyanide and chromic acid - sounds like a fairly ferocious mixture to me, but I've not heard tales of HCN poisonings. Nontheless, exercising caution and never taking a cavalier attitude toward chems is always a good policy.

Usually, when an odd acid is called for in a formula, there is a reason. Here it is probably, as suggested, to control grain. The additional loss of silver by production of AgNO3 may have been intentional with the old style emulsions.

As for the more general question, sulfamic acid is often used as a solid alternative to sulfuric acid. It is fairly expensive, however. I have used it in a couple of situations where hauling around a bottle of concentrated acid was ill advised, and it is a good substitue. Ilfochrome uses sulfamic acid in its bleach to avoid liquid packaging problems. Sodium bisulfate can be used in many formulas, but it takes quite a lot of it to generate enough sulfuric acid. A mixture of sodium bisulfate and sodium chloride has been suggested (Anschell and Troop) as a substitute for small quantities of hydrochloric acid as well.

Just how much cyanide can be released from potassium ferricyanide depends on several factors, I quote from the following site www.cyanidecode.org/cyanide. One important point is "mildly acidic conditions." Another is how long the contact with an acidic environment lasts. I have personally noticed a definite odor of HCN from a ferricyanide solution that was a few days old. One needs to be careful with all chemicals but some require a bit more care.

"Cyanide forms complexes with gold, mercury, cobalt and iron that are very stable even under mildly acidic conditions. However, both ferro- and ferricyanides decompose to release free cyanide when exposed to direct ultraviolet light in aqueous solutions. This decomposition process is reversed in the dark. The stability of cyanide salts and complexes is pH dependent, and therefore, their potential environmental impacts and interactions (i.e. their acute or chronic effects, attenuation and re-release) can vary.

Metal cyanide complexes also form salt - type compounds with alkali or heavy metal cations, such as potassium ferrocyanide (K4Fe(CN)6) or copper ferrocyanide (Cu2[Fe(CN)6]), the solubility of which varies with the metal cyanide and the cation. Nearly all alkali salts of iron cyanides are very soluble, upon dissolution these double salts dissociate and the liberated metal cyanide complex can produce free cyanide. Heavy metal salts of iron cyanides form insoluble precipitates at certain pH levels."

The recipe given by the OP lacks necessary detail in how the solution can be prepared safely. The nitric acid should be well diluted before it is added to the ferricyanide solution to prevent a localized low pH which could cause problams.

Jerry;

My only response is that Ektparint C, E4 and C22 bleaches were used as mildly acidic solutions at 85 F for years with no problem.

I do urge, as always, that everyone err on the side of caution.

PE