Question

Decomposition of dithionite$\left(S_2{O}_4^{2-}\right)$.was studied by chromatography on an anion-exchange column eluted with 20mM tri-sodium 1, 3, 6-napthalenetrisulfonate in 90 vol%${\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{/}\mathbf{10}$

vol%${\mathbf{CH}}_{\mathbf{3}}\mathbf{CN}$with ultraviolet detection at 280 nm. A solution of sodium dithionite stored for 34 days in the absence of air gave five peaks identified as${\mathbf{SO}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}\mathbf{,}\mathbf{}{\mathbf{SO}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\mathbf{,}\mathbf{}{\mathbf{S}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}\mathbf{,}\mathbf{}{\mathbf{S}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\mathbf{}\mathbf{and}\mathbf{}{\mathbf{S}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{s}}^{\mathbf{2}\mathbf{-}}$All the peaks had a negative absorbance. Explain why.

Short answer

The explanation to the above question is:

Our analytic anions don’t absorb strongly at 280 nm, therefore the peaks have a negative absorbance.

Step-by-step solution

Case of indirect detection

This is a case of indirect detection where the eluent contains trisodium 1, 3, 6-napthalenetrisulfonate with ultraviolet detection at 280 nm, meaning it absorbs at 280 nm.

Explanation of peaks having negative absorbance

Charge balance determines that when anions emerge from the column – less trisodium 1, 3, 6-napthalenetrisulfonate should be emerging.

• Our analyte anions don’t absorb strongly at 280 nm, therefore the peaks have a negative absorbance.