Question

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

\({\rm{HCl}}{{\rm{O}}_4}\) 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 \({\rm{HCl}}{{\rm{O}}_4}\)