Nascent hydrogen challenge - Semantic Scholar

1 downloads 0 Views 65KB Size Report
May 17, 2008 - nineteenth century after English chemist James Marsh. (1794–1846) developed a sensitive method for exposing murders that seemed to have ...
Anal Bioanal Chem (2008) 391:1475–1476 DOI 10.1007/s00216-008-2143-4

ANALYTICAL CHALLENGE

Nascent hydrogen challenge Juris Meija & Alessandro D’Ulivo

Published online: 17 May 2008 # Crown Copyright in right of Canada 2008

We would like to invite you to participate in the Analytical Challenge, a series of puzzles to entertain and challenge our readers. This special ABC feature has established itself as a truly unique quiz series, with a new scientific puzzle published every other month. Readers can access the complete collection of published problems with their solutions on the Analytical and Bioanalytical Chemistry homepage at http://www.springer.com/abc. Test your knowledge and tease your wits in diverse areas of analytical and bioanalytical chemistry by viewing this collection. In the present challenge nascent hydrogen is the topic. And please note that there is a prize to be won (a Springer book of your choice up to a value of €75). Please read on…

Meet the nascent hydrogen challenge For a long time arsenic, in the form of its white oxide, was a highly favored choice of poison among the poor and rich of Europe. Easily and odorlessly incorporated into food and J. Meija (*) Institute for National Measurement Standards, National Research Council Canada, 1200 Montreal Rd., Ottawa, ON K1A 0R6, Canada e-mail: [email protected]

drinks, it was virtually untraceable in the body to any physician before the nineteenth century. No wonder As2O3 was dubbed the poudre de succession. The Borgia dynasty, led by Cesare Borgia (1475–1507) and his father Pope Alexander VI (1431–1503), assassinated numerous wealthy cardinals using their trademark poison “cantarella” [1]. It does not come as a surprise that by the church law they could retain the holdings and wealth of the deceased. Whether this was a papacy at its best or worst is for others to decide, but the word poison is now largely associated with the name of Borgia in the minds of many readers of history. Arsenic-based career development came to an end in the nineteenth century after English chemist James Marsh (1794–1846) developed a sensitive method for exposing murders that seemed to have gone unnoticed far too long. In 1836 he published a method that could detect as little as 20 μg arsenic [2]. The arsenic was converted into its volatile hydride which was then burnt. When a piece of cold porcelain was held in the flame, a silvery-black arsenic mirror was deposited. The Marsh test became a household name in September of 1840 when evidence from forensic toxicology was successfully introduced into the courtroom at the widely publicized poisoning trial of the young Charles Lafarge in Tulle, France. From this trial onwards the Marsh test was performed widely and has now been replaced by more sophisticated techniques, such as atomic spectroscopy and x-ray fluorescence analysis.

The challenge A. D’Ulivo Institute for Chemical and Physical Processes, National Research Council, Via G. Moruzzi 1, Pisa 56124, Italy

Arsenic forms a volatile hydride, AsH3, when reduced with agents such as zinc or sodium borohydride in acidic medium. Modern analytical methods utilize this chemistry

1476

Anal Bioanal Chem (2008) 391:1475–1476

to determine trace amounts of arsenic, and metal hydride generation is now among the most widely used methods for trace metal determination [3]. British theologian and natural philosopher Joseph Priestley (1733–1804) was one of the first to recognize the ‘nascent hydrogen’ and various theories involving atomic hydrogen have witnessed popularity ever since. The mechanism of volatile metal hydride generation is among these. Many textbook explanations of the (semi)metal hydride generation mechanism invoke the formation of ‘nascent hydrogen’ during the acid decomposition of borohydride [4]: ½BH4  þ3H2 O þ Hþ ! H3 BO3 þ 8H

ð1Þ

The atomic (nascent) hydrogen is considered to be responsible for the derivatization of the element to the hydride. In the absence of metal ions, however, the unreacted atomic hydrogen forms molecular hydrogen, which is one of the final products of acid-catalyzed hydrolysis of [BH4]–: 8H ! 4H2

ð2Þ

The validity of the nascent hydrogen theory can be inspected by the use of deuterium-labeled reagents. For example, hydrolysis of borohydride can be performed either with deuterated borohydride or in heavy water and deuterated acid. Such experiments have been performed and for over half a century it has been known that decomposition of K[BD4] in 3 M H2SO4 leads to formation of HD as the main gaseous product with only traces of both D2 and H2 present [5]. Similar results have been reported for the hydrolysis of Na[BH4] in D2O, where again the gaseous product is almost pure HD [6]. The readers are asked to evaluate the results of these classical experiments and infer whether or not the produc-

tion of HD contradicts the formation of nascent hydrogen (Eq.(1)) during acid-catalyzed hydrolysis of tetrahydroborate. In addition, it is known that [BH4]– and [BD4]– do not exchange their protons with D2O or H2O in aqueous solutions.

We invite our readers to participate in the Analytical Challenge by solving the puzzle above. Please send the correct solution to [email protected] by August 20, 2008. Make sure you enter “Nascent hydrogen challenge” in the subject line of your e-mail. The winner will be notified by e-mail and his/her name will be published on the ‘Analytical and Bioanalytical Chemistry’ website at http://www.springer.com/abc and in the Journal. Readers will find the solution and a short explanation on the ‘Analytical and Bioanalytical Chemistry’ website after August 20, 2008, and in the Journal (Issue 392/5). The next Analytical Challenge will be published in Issue 392/1, September 2008. If you have enjoyed solving this Challenge you are invited to try the previous puzzles on the ‘Analytical and Bioanalytical Chemistry’ website.

References 1. Bond J (1927) In the pillory: the tale of the Borgia Pope. Washington DC 2. Marsh J (1836) Edinburgh New Philos J 21:229–236 3. Sturgeon RE, Mester Z (2003) Appl Spectrosc 56:202A–213A 4. Robbins WB, Caruso JA (1979) Anal Chem 51:889A–899A 5. Mesmer RE, Jolly WL (1962) J Am Chem Soc 84:2039–2042 6. Jolly WL, Mesmer RE (1961) J Am Chem Soc 83:4470–4471