Chemical clues: The arsenic in Napoleon\'s hair was detected using a technique c

Question

Chemical clues: The arsenic in Napoleon's hair was detected using a technique called neutron activation. When As-75 is bombarded with high energy neutrons, it is converted t the radioactive As-76 isotope. The energy of the rays emitted by the radioactive isotope is characteristic of arsenic, and the intensity of the rays establishes how much arsenic is present in a sample. With this technique, as little as 5 ng (5 x 10 g) of arsenic can bie detected in 1 g of mata (A) Write symbols for the two isotopes of As, showing mass number and atomic number. (B) Name two advantages of analyzing the arsenic content by neutron activation instead of a chemical analysis. 2. Arsenic is not an essential element for the human body. (A) Based on its position in the periodic table suggest a reason for its toxicity. (B) In addition to hair, where else might one look for the accumulation of the element if arsenic poisoning is suspected? The Marsh test for arsenic involves the following steps: (a) The generation of hydrogen gas when sulfuric acid is added to zinc. (b) The reaction of hydrogen with As( oxide to produce arsine. (c) Conversion of arsine to arsenic by heating. Write equations representing these steps and identify the type of the reaction in each step

Explanation / Answer

Answer 1:

A. The two isotopes of As are of atomic mass 75 and 76. Both have 33 protons. So, the symbols are

7533As and 7633As

B. Advantages:

i. As arsenic is a non-essential element for human body and even a small amount of it is very toxic, most of the chemical methods donot have the senitivity to determine such minute quantities.

ii. The process is non destructive. So, the sample does not has to be only bombarded with neutrons which is non-invasive in nature.

3 a:

The Marsh test treats the sample with sulfuric acid and arsenic-free zinc. Even if there are minute amounts of arsenic present, the zinc reduces the trivalent arsenic (As3+ ).

Oxidation: Zn Zn2+ + 2 e

Reduction: As2O3 + 12 e + 6 H+ 2 As3 + 3 H2O

Overall, we have this reaction:

As2O3 + 6 Zn + 6 H+ 2 As3 + 6 Zn2+ + 3 H2O

In an acidic medium, As3 is protonated to form arsine gas (AsH3), so adding sulphuric acid (H2SO4) to each side of the equation we get:

As2O3 + 6 Zn + 6 H+ + 6 H2SO4 -------> 2 As3 + 6 H2SO4 + 6 Zn2+ + 3 H2O

As the As3 combines with the H+ to form arsine:

As2O3 + 6 Zn + 6 H+ + 6 H2SO4 ------> 2 AsH3 + 6 ZnSO4 + 3 H2O + 6 H+

By eliminating the common ions:

As2O3 + 6 Zn + 6 H2SO4 ---------> 2 AsH3 + 6 ZnSO4 + 3 H2O

Arsine is regarded as stable at rom temperature since its decomposition into As and H2 takes place very slowly unless a temperature of 230°C is reached

2 AsH3 ---------> 2 As + 3 H2

This is a decomposition reaction as arsine is converted to As and hydrogen.

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