PAGB Etch, Saturated aqueous picric acid with copper chloride 2

[Coctyle]

I have been using saturated aqueous picric acid with sodium dodecylbenzene sulfonate as the wetting agent to reveal prior austenite grain boundaries in heat treated steel. I have found that the addition of copper chloride is helpful (as described in Vander Voort's Metallography Principles and Practice, with the substitution of the dodecyl salt for the tridecyl salt that is no longer easily available).

MY question for anyone familiar with this etchant, should I be using copper(II) chloride dihydrate or anhydrous copper(II) chloride? Does it make any difference? I have been using the dihydrate, but I am going to order some fresh supplies and want to get the best stuff for the application.

 

[EdStainless]

Where are you located?
Just curious, I want to make sure that I don't go there.
I have seen one Picric Acid explosion in my life and it was deliberate. I don't need to see another.

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P.E. Metallurgy, Plymouth Tube

 

[Coctyle]

Has anyone on this forum ever heard of an accidental picric acid explosion in a metallurgical laboratory? I have not.

Aqueous picric acid is perfectly safe. Moist picric acid crystals are perfectly safe. It has to dry out completely to be sensitive. It is ridiculous to restrict the use of picric acid if the most basic laboratory precautions are followed.

I have a jar of wet solid picric that is probably 20 years old. I am careful about the cap and threads. I occasionally spray a minimal amount of distilled water on the inside to wash down anything stuck to the sides and of course I keep the cap on unless I am in the immediate process of removing some to make picral or Vilella's reagent

For aqueous etchants, such as is the subject of this post, I use premixed saturated aqueous picric acid from Ricca Chemical. It was delivered by a normal package carrier (UPS or FedEx). Maybe it would become dangerous if I left the cap off for weeks, but that would not be in keeping with basic laboratory precautions.

 

[EdStainless]

Search the OSHA site, there are numerous accidents related people opening containers of Picric.
The one that I was was a bottle from a lab. There was still water in the seal, but you could see that it had dried out at one time.

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P.E. Metallurgy, Plymouth Tube

 

[Coctyle]

How was it deliberately made to explode? Was it originally in solution, or just wet crystals?

Maybe I shouldn't have said it is ridiculous to restrict it, but it is used in numerous etchants that are important to me. I have a little residual annoyance from working for an employer that would not allow it. If you (or anyone else) have any tips for handling it as safely as possible, I would be happy to hear them.

 

[dbooker630]

I had similar annoyances years ago when I began using picric at my employer. We established a safe handling plan which included wiping the jar and lid threads clean after each opening. I also kept only a small quantity with very small amounts of crystalline material on hand if at all. I also used the premixed from Ricca. I have never seen an explosion either in 30+ years and I think safe handling has something to do with it, but others in my organization were sufficiently spooked to plead with me to find another way to get the job done...

 

[EdStainless]

I have also used it is premix, and have no issues at all with that approach.



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P.E. Metallurgy, Plymouth Tube

 

[berkshire]

As alluded to here but not spelled out, the danger is the solution forming anhydrous crystals.
Which are unstable?
Modern safety precautions recommend storing picric acid wet. Dry picric acid is relatively sensitive to shock and friction, so laboratories that use it store it in bottles under a layer of water, rendering it safe. Glass or plastic bottles are required, as picric acid can easily form metal picrate salts that are even more sensitive and hazardous than the acid itself. Industrially, picric acid is especially hazardous because it is volatile and slowly sublimes even at room temperature. Over time, the buildup of picrates on exposed metal surfaces can constitute a grave hazard


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