GATE 2026 CY – Question 25

Inorganic Chemistry · Instrumental Methods of Analysis: UV-visible, fluorescence and FT-IR spectrophotometry, NMR and ESR spectroscopy, mass spectrometry, atomic absorption spectroscopy, Mössbauer spectroscopy (Fe and Sn) and X- ray crystallography. Electroanalytical methods – cyclic voltammetry, ion-selective electrodes. Thermo-analytical methods: TGA, DTA and DSC. · 1 mark · Multiple select

The correct statement(s) about Mössbauer spectroscopy of iron compounds is(are)

57Co is used as a source
Their Mössbauer spectra are obtained using γ-ray with resonance energy of 14.4 keV
K2[Fe(CN)5NO] shows large quadrupole splitting
Isomer shift of FeSO4∙7H2O is smaller than that of K3[Fe(CN)6]

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Correct answer: (A) 57Co is used as a source; (B) Their Mössbauer spectra are obtained using γ-ray with resonance energy of 14.4 keV; (C) K2[Fe(CN)5NO] shows large quadrupole splitting

Explanation

Check each statement about iron Mössbauer spectroscopy.

**A. $^{57}$Co is used as a source.** True. $^{57}\text{Co}$ decays by electron capture to an excited state of $^{57}\text{Fe}$, which emits the γ-ray. ✓
**B. The γ-rays have an energy of 14.4 keV.** True. The 14.4 keV transition is the resonant one used for $^{57}\text{Fe}$. ✓
**C. $\text{K}_2[\text{Fe(CN)}_5\text{NO}]$ shows a large quadrupole splitting.** True. The Fe site (nitroprusside) has a strongly non-spherical electric-field gradient, which gives a large quadrupole splitting. ✓
**D. The isomer shift of $\text{FeSO}_4\cdot7\text{H}_2\text{O}$ is smaller than that of $\text{K}_3[\text{Fe(CN)}_6]$.** False. High-spin Fe(II) sulfate has a much **larger** isomer shift (about 1.2 mm/s) than low-spin Fe(III) ferricyanide (about −0.1 mm/s).

Answer **A, B and C**.