Learn the Pragmatics of Perchloric Acid Chemistry Prior to Use

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This article amounts to a plea to analytical chemists, supervisors, and organizations who use perchloric acid to make the effort to understand its reaction chemistry, as an acid or salt, and the peculiarities of the numerous mixtures used in analytical sample digestion. If your organization uses standard methods of digestion via one of the many acid mixtures and temperatures, it behooves your organization to have at least one individual on site who understands a bit more than just the procedure. If there is an incident of some kind involving perchloric acid, be it a spill, splash, or worse, having a grasp of the real hazard presented before you is useful. It is possible to underreact or overreact to any given incident scenario.

I am not an analyst. My interest is to understand reactive chemical hazards and devise means for preventing the transition from hazard to danger. Whether someone uses perchloric acid or not makes no difference to me. I have no investment in perchloric acid. However, I’m greatly interested in users being informed.

Comments on Safety Training

Safety training is commonly executed as a result of company policy where documentation of satisfactory completion is collected and filed. For lab chemists this includes training sessions on chemical storage, fire safety, fire extinguisher training, hazardous waste practices and regulations, storm water regulations, company safety and health SOP training, building evacuation, general lab safety, and perhaps basic first aid.

Often safety training sessions are canned professional video presentations or a corporate home brew of PowerPoint slide shows followed by some Q&A and a quiz. It is what I refer to as infotainment. Attendees may watch a video with dramatized incidents while the voiceover describes what should have happened. This approach is not without merit or some success, but this passive approach may not be of lasting value. Furthermore, it is a very sketchy assumption that such passive training will result in proper decision making in an off-normal circumstance where hazard may transition to danger.

The military has solved this problem long ago by mastering the art of the drill. They realize that if you need people to respond in a particular way rapidly, they have to be trained and drilled. In times of peace, the military has the opportunity to train and drill to maintain operational readiness. This is one way to address the difficult problem of low probability, high consequence scenarios. Industry as a whole, however, may not inclined to offer a lot of free time to dedicate to training. Man-hours in drills subtract from productivity. In my opinion, much of industrial management suffers from a lack of imagination in this matter. Safety training and drills are cost overhead. But, what you lack in training hours may be made up for by effective mentoring.

We live in the age of OSHA regulations. Of importance to the process industry is Process Safety Management or PSM. The mission of OSHA is copied and pasted below.

With the Occupational Safety and Health Act of 1970, Congress created the Occupational Safety and Health Administration (OSHA) to assure safe and healthful working conditions for working men and women by setting and enforcing standards and by providing training, outreach, education and assistance.

The Wikipedia link below gives an excellent summary of OSHA regulations relating to the chemical process industry. PSM in 29 CFR §1910.119  titled Process safety management of highly hazardous chemicals, is a regulatory framework covering all aspects of safety management and threshold quantities (Appendix A) of highly hazardous materials. Whether your facility is operating at the PSM scale of operation or not, employers have a duty to assure a safe operating environment for their employees. In my view, PSM regulations frame a safety mindset and diligence that is useful outside of PSM reach. Given that a debilitating injury, fatality, explosion or major fire will bring the unblinking eye of regulators and possible litigation, sensible practices found in 29 CFR §1910.119 that are woven into your chemical safety SOPs are in the direction of goodness. Again, this is my view and should not be construed as legal advice. Your chemical safety plan is your responsibility alone.

Finally, a word to lab managers and supervisors. I cannot point to a ancient stone or a law of nature that commands that leaders be effective instructors and mentors. But I can throw an idea on the table which is that as a senior employee in a supervisory role, you have a moral obligation to your charges to make sure that they practice their art with diligence and in a safe manner.  The best way I know of is to train staff thoroughly in lab operations and have high expectations of your staff. Management by wandering around can be very effective in maintaining discipline and keeping tabs on your shop. Besides, you should be walking around and asking questions anyway.

HClO4 – The Meat and Potatoes

There is much to know about the chemistry of perchloric acid digestion beyond it’s renowned acidity and explosive potential. Appreciating the corrosivity and  close adherence to standard laboratory techniques are necessary but not always enough. One such circumstance begging for informed action is method development. In researching this topic I was a little surprised to find that many important details are buried in the primary literature. Worse, a few key references are downright difficult to obtain. By important details, I mean whatever information might help define the safe operating window for a given digestion, or, better put, under what circumstances might a digestion procedure transition from hazardous to dangerous.

The major supplier of perchloric acid and perchlorate salts in the USA is GFS Chemicals in Powell, OH. The founder of this company, G. Frederick Smith was, and remains posthumously through his writings, a top authority on the properties of this acid and numerous perchlorate salts as the result of his many decades of research. Laboratory quantities of perchloric acid can be had from GFS and the usual group of research chemical suppliers.

It is easy to find MSDS data and exemplar laboratory safety guides on your browser detailing sensible storage and use policy. Several found in google-space stand out in my opinion as comprehensive perchloric acid safety documents and SOP’s; UC Berkeley; Boston University; MIT; Harvard; British Columbia Code for Mines to name a few. Again, this is my opinion- form your own.  If your perchloric acid “policy” is limited to an MSDS document and perhaps a few safety statements found in a procedure, then I would urge someone in your organization to take it upon themselves to dig in a little deeper. Generate SOPs for all aspects of the perchloric acid life cycle in your facility.

There are many accounts of incidents with perchloric acid that should convince even the most refractory skeptic of the potential for a violent release of energy. There is a perchloric acid incident that stands out as an example of the dangers of a chemical ignorance.  It happened February 20, 1947, when a large and violent explosion killed 17 people and led the city of Los Angeles to specifically bar the use of perchloric acid (1)  through numerous sections of it’s zoning code.

The most common laboratory use of perchloric acid is in the analytical digestion of samples containing a matrix of organic matter, sludge, tissue, biomass or organic chemicals. There are a great many lab procedures to be found by an internet search including Chemical Abstracts (CAS), the AOAC Official Methods of Analysis manual, and ASTM relating to HClO4.  Numerous policy and prudent practices documents can be downloaded from well established institutions that outline some very sensible policies regarding the storage, use, and disposal of HClO4.  One particularly good source for sample digestion methods across the periodic table is from Inorganic Ventures. Kudos to Dr. Paul Gaines and this company for the quality of their products and their willingness to share their expertise in trace element analysis.

A search of Chemical Abstracts will turn up many research papers giving digestion procedures in the experimental section. However, it is not often made clear how the workers came upon their particular digestion conditions other than from a reference in an earlier procedure. This is because these papers are about the use and not about the chemistry of digestion. Most of the procedure writers will have done their diligence and provide warning about hazards. What may be omitted within papers that use the HClO4 procedure are the boundaries of safe operation and how the reactivity may vary with concentration and temperature.

For greater detail one must look elsewhere and well back into the 20th century. Much useful information on HClO4 and its salts is to be found in papers from the 1930’s thru the 1970’s.  Because of their energetic properties, the propellant and explosives folks usually expand on energetic materials including perchlorates, and yes, they go into some great and admirable detail (2). However these sources tend to be thermochemical in nature and perhaps not a lot of immediate help to a bench chemist.

Unlike many other reagents in the laboratory, perchloric acid can have a downside with immediate negative safety consequences. In particular, if one is aiming to develop a digestion procedure for a new type of sample, say, something with a mixed organic/inorganic matrix or certain heteroatoms compounds with nitrogen or sulfur, it behooves the chemist to take a serious interest in rooting out information about the safe operating boundaries of perchloric acid and what kinds of materials may be problematic. A perchloric acid MSDS will inform you of potential safety hazards, hazard classifications, etc., but a well researched and validated procedure can go far towards keeping you out of trouble. I would recommend that at least one person at your organization be more thoroughly educated in the chemistry of perchloric acid digestion, or wet ashing as it is called. Unlike some other strong acids, contact with organics may have immediate explosive consequences. And by explosive I mean violent, deafening, shrapnel-blasting detonations. Hazardous contact can include contact of hot concentrated acid on paper, on sample material, or even contact of perchloric acid vapor on a gloved hand passing through fumes.

There are some particularly comprehensive and broadly informative publications covering perchloric acid chemistry. A more recent work by John Long (3) of GFS is particularly insightful in regard to drawing a line between perchlorate salts and perchloric acid. The 1960 publication Perchlorates: Their properties, manufacture, and uses by J.C. Schumacher (4) contains an informative chapter (Ch 11) on perchloric acid safety. Perhaps the most useful reference is a book available from GFS (5) or Amazon titled Perchloric Acid and Perchlorates, by A.A. Schilt. The 2nd edition in particular contains a great many useful references.

On heating at ambient pressure, aqueous perchloric acid will concentrate by distillation to a constant boiling azeotrope of 72.5 % HClO4 and water. At this composition its number of waters of hydration is slightly greater than two. In the climb from ca 160 °C to a bp of 203 °C at 1 atm, the 72.5 % acid will transition from being “just” a hot super acid to a super acid and a potent oxidizer.

In the gas phase, this acid can decompose via a radical pathway leading to the evolution of Cl2, O2, H2O either abruptly or after an time interval (6). Note that when something quite hot abruptly decomposes to a greater number of moles of gaseous products, there can be plenty of potential for destructive pressure effects.

For the uninitiated, HClO4 is a “super” mineral acid capable of complete dissociation in aqueous concentrations up to about 4 molar (7). The dissociated form in water is H3O+ ClO4-, or oxonium perchlorate. This is normal Brønsted acid behavior in water, but three things set this acid apart from others, even nitric acid: i) due to the extremely weak coordinating ability of the perchlorate anion, the acid proton is extraordinarily mobile and reactive; ii) at room temperature the anhydrous acid will at some point spontaneously explode; and iii) in concentrated aqueous form at elevated temperatures, say > 160 ºC, the acid becomes an increasingly potent oxidizer with temperature.

The perchlorate anion has a central chlorine atom, formally +7, that sits in a tetrahedral array of four O2- anions to make it anionic. On average the negative charge is spread over the surface of the symmetric anion making the negative charge diffuse with the enthalpy of formation unfavorable to close ion pairing. The perchlorate anion is only weakly attracted to a given cation like H3O+ or oligomers and as such, allows the H3O+ (or larger clusters) to reside in a solvent shell unencumbered by tight ion pairing, depending on the nature of the solvent. Perchlorate salts can have very high water solubility and, in the case of magnesium perchlorate, serve as an excellent desiccant. One exception to the high solubility of perchlorates is potassium perchlorate at only 1.5 g per 100 mL H2O at 25 °C.

1. “Explosion at O’Connor Electro-Plating Corp.” LA Times, http://framework.latimes.com/2012/02/20/explosion-at-oconnor-electro-plating-corp/#/0       Site viewed on 12/22/16.
2. Perchlorates: A review of their thermal decomposition and combustion, with an appendix on perchloric acid, G.S. Pearson; Rocket Propulsion Establishment; October 1968.   http://www.dtic.mil/dtic/tr/fulltext/u2/857556.pdf
3. Perchlorate Safety: Reconciling Inorganic and Organic Guidelines, J.R. Long; Chemical Health and Safety, 2002, 9(3), 12-18. http://dx.doi.org/10.1016/S1074-9098(02)00294-0
   
4. Perchlorates: Their properties, manufacture, and uses; J.C. Schumacher, editor; Reinhold Publishing, 1960. See Chapter 11, “Safety Precautions in Handling Perchlorates”, E. Levens, 187-222. Download pdf: Do not try to correct the misspelling in the url. https://archive.org/details/pwechloratesthei001740mbp?q=perchloric+acid+and+perchlorates+schilt
5. Perchloric Acid and Perchlorates, Second Edition A.A. Schilt and L.C. McBride, 2003 https://www.gfschemicals.com/statics/productdetails/PERCHLORIC_ACID_AND_PERCHLORATES_496.html
6. Thermal Decomposition of Perchloric Acid, Gilbert and Jacobs, Combustion and Flame, 1971, 17, 343-353. DOI: 10.1016/S0010-2180(71)80056-1
7. Perchloric Acid and Its Salts- Very Powerful Catalysts in Organic Chemistry, Dalpozzo, Bartoli, Sambri, Melchiorre;  Chem. Rev, 2010, 110(6), 3501-3551.  DOI: 10.1021/cr9003488