The typical blue colour of the famous Delft pottery originates from the absorption of red and green — Physical Chemistry — Thermodynamics Chemistry Question
Delft Blue and Vitamin B 12
The typical blue colour of the famous Delft pottery originates from the absorption of red and green light by Co2+ ions which are incorporated in the thin layer of glaze on the pottery. [VISUAL: Delft Blue Pottery Vase] The glaze is made by mixing cobalt salt with the glass forming components, e. g. silicate, borate and sodium. Upon heating the thin glass layer is formed which contains Co2+ ions. The Co2+ ion is a transition metal ion which has a partially filled 3d orbital. The colour of the 3d transition metal ions is caused by electronic transition between lower and higher energy 3 d orbitals which are split by the crystal field.
The transitions between 3d orbitals are not very strong. The molar absorption coefficient for Co2+ in the green and red is about 20 dm3 mol–1 cm–1. For an intense blue colour about 99 % of the red and green light needs to be absorbed.
In the body trace amounts of Co ions are present mainly incorporated in vitamin B12. The total amount of Co in a human body of 70 kg is about 3 mg. In 1964 Dorothy Crowfoot-Hodgkin received the Nobel prize for the structure determination of this vitamin. The structure is shown below.
[VISUAL: Vitamin B12 Structure]
The oxidation state of Co can vary. Common oxidation states are II and III, but in the vitamin B12 also Co+ is possible.
What is the full electron configuration of Co2+? (Atomic number of cobalt = 27)
Model Answer
1s2 2s2 2p6 3s2 3p6 3d7
Sketch the shape of the 3d orbitals. Draw also the x, y, and z axes.
Model Answer
[VISUAL] (The solution diagram displays the shapes and coordinate orientations of the five 3d orbitals: dyz, dxy, dxz, dz2, and dx2-y2, with shaded and unshaded lobes to depict wave function phases.)
Calculate the concentration of Co2+ in the glaze if the thickness of the glaze layer is 1 mm. (Hint: Use the Lambert-Beer law.)
Model Answer
If 90 % of the light is absorbed, the transmission T is 0.1 (10 % transmitted). Fill out: A = – log I / I0 = ε c l
– log 0.1 = 20 × c × 0.1
c = 1 / (20 × 0.1) = 0.5 mol dm–3
Arrange the Co ions in order of increasing ionic radii for the three different oxidation states.
Model Answer
r(Co3+) < r(Co2+) < r(Co+)
For which oxidation state of Co ions (I, II, III) do expect to see a signal in an EPR spectrum (Electron Paramagnetic Resonance). Assume a high-spin configuration in all oxidation states.
Co+ Yes / No
Co2+ Yes / No
Co3+ Yes / No
Model Answer
All three oxidation states have unpaired d-electrons (d6, d7 and d8) in the high spin configuration and thus for all three oxidation states an EPR spectrum can be measured. The correct answer in all cases is „yes“.
Calculate how many Co ions are present in a human body of 70 kg. (The relative atomic mass of Co = 58.93.)
Model Answer
\frac{3 \times 10^{-3} \text{ g}}{58.93 \text{ g mol}^{-1}} \times 6.022 \times 10^{23} \text{ mol}^{-1} = 3 \times 10^{19} \text{ Co ions}
What is the coordination number of cobalt in the vitamin B12 complex.
Model Answer
Six