Chapter 28: Q13P (page 790)

The following wet-ashing procedure was used to measure arsenic in organic soil samples by atomic absorption spectroscopy: A 0.1- to \({\bf{0}}.{\bf{5}} - \)g sample was heated in a \({\bf{150}} - {\bf{mL}}\) Teflon bomb in a microwave oven for \(2.5\;{\rm{min}}\) with \(3.5\;{\rm{mL}}\)of\(70\% \,\,\,{\rm{HN}}{{\rm{O}}_3}\). After the sample cooled, a mixture containing \(3.5\;{\rm{mL}}\)of \(70\% \,\,\,{\rm{HN}}{{\rm{O}}_3},1.5\;{\rm{mL}}\) of\(70\% \,\,{\rm{HCl}}{{\rm{O}}_4}\), and \(1.0\;{\rm{mL}}\) of \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)was added and the sample was reheated for three \({\bf{2}}.{\bf{5}} - {\bf{min}}\) intervals with 2 -min unheated periods in between. The final solution was diluted with \(0.2{\rm{M}}\,\,\,{\rm{HCl}}\)for analysis. Why was \({\rm{HCl}}{{\rm{O}}_4}\) not introduced until the second heating?

Short Answer

Expert verified

\({\rm{HCl}}{{\rm{O}}_4}\) is stronger than \({\rm{HN}}{{\rm{O}}_3}\) when considering the acidity so it can’t be introduced until the second heating to prevent possible explosion.

Step by step solution

Definition of Atomic Absorption Spectroscopy

The technique of infinitesimal immersion spectroscopy, or AAS, is used to determine the quantities of metallic rudiments in colorful accoutrements.
Electromagnetic wavelengths from a light source are used as a logical tool.
These wavelengths will be absorbed else by different elements.
It provides an image of the attention of a given element in the item or liquid being examined.

Determine that \({\rm{HCl}}{{\rm{O}}_4}\) not introduced until the second heating

First Teflon bomb was heated in a microwave oven for\(2.5\;{\rm{min}}\)with\(3.5\;{\rm{mL}}\) of \(70\% \,\,{\rm{HN}}{{\rm{O}}_3}\).
Then after the sample cooled down, we added a mixture that contains\(3.5\;{\rm{mL}}\) of \(70\% \,\,\,{\rm{HN}}{{\rm{O}}_3},1.5\;{\rm{mL}}\) of \(70\% \,\,\,HCl{O_4}\)and \(1.0\;{\rm{mL}}\) of \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)
\({\rm{HN}}{{\rm{O}}_3}\)was used first in order to oxidize compounds that are easy to oxidize, whereas \({\rm{HCl}}{{\rm{O}}_4}\)wasn’t because it is way stronger than\({\rm{HN}}{{\rm{O}}_3}\)when considering the acidity.
Also, it would prevent an explosion that would happen due to high and rapid temperature rise while adding the \({\rm{HCl}}{{\rm{O}}_4}\)