In a laboratory practice I have done a characterization of the $\ce{[Co(NH3)4CO3]NO3}$ and obtained the following spectra:

UV-vis spectrum

I don't know if the band between 300 nm and 400 nm can be due to the effect of the counteranion or if it is a d–d transition.

  • $\begingroup$ This UV spectrum is useless. Please see the y-axis. Where is there absorbance? An absorbance of 2 means almost 100% of the light has been absorbed. Your sample is too concentrated. Do the experiment again and dilute it till your absorbance is around 1. Collect two spectra, in the UV range (220 to 400 nm) and one in the Vis range (400 to 600 nm) with appropriate heights. $\endgroup$ – M. Farooq Dec 15 '20 at 19:49
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    $\begingroup$ @M.Farooq. The spectrum is surely not useless for all wavelengths greater than 300nm? (I have used spectrophotometers that obey the Beer-Lambert law up to absorbances of 2 (at 340nm) and those that markedly deviate from linearity at an absorbance of 1 (in the low uv region, for example)) $\endgroup$ – tomd Dec 15 '20 at 20:03
  • $\begingroup$ @M.Farooq Ok, I didn't know that. But anyways, is it possible to know the type of band placed between 300 and 400 nm? $\endgroup$ – Enric Ros Dec 15 '20 at 20:10
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    $\begingroup$ You will have to do little bit more controls. Are you using a quartz cuvet in the UV range? If not do not collect the UV data. If you suspect it is the counter ions, you will have to think about the color of the counter ions. Is carbonate ion coloress? Is nitrate ion colorless? And finally how pure is your preparation? What is the color of your complex. Do the Vis bands qualitatively match with the color of the complex? For example KMnO4 is purple, it absorbs in the green region. $\endgroup$ – M. Farooq Dec 15 '20 at 20:30
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    $\begingroup$ You could check the 300-400 nm band with soln of , say sodium nitrate, to see if its the counter ion. Check that you are using quartz cuvettes, otherwise uv absorption may be due to cuvette (or solvent), as a 'rule of thumb' ignore any absorption above approx 2.5 at any wavelength and below 220 nm anyway, most spectrophotometers do not work well there. $\endgroup$ – porphyrin Dec 16 '20 at 8:58

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