Table of colors characteristic for selected ions
Table shows solutions color characteristic to given ions (cations or anions) such as Cu2+ (blue) or MNO4- (purple).

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# Solution color

 Ion Solution color Notice $Ag^{+}$ colorless - $Mn^{2+}$ light pink - $Au^{3+}$ yellow or orange depending on the combination dissolved in water $Mn^{3+}$ red - $Cd^{2+}$ colorless - $MnO_4^{-}$ purple - $MnO_4^{2-}$ green - $MnO_4^{3-}$ blue - $Ce^{3+}$ colorless - $Ce^{4+}$ yellow - $Co^{2+}$ pink - $Co^{3+}$ orange - $Mo^{3+}$ yellow - $MoO^{2+}$ pink - $Cr^{2+}$ blue - $Cr_2O_7^{2-}$ orange - $CrO_4^{2-}$ yellow - $Cr^{3+}$ purple - $Ni^{2+}$ green - $Pd^{2+}$ yellow or red depending on the combination dissolved in water $Pt^{2+}$ yellow or red depending on the combination dissolved in water $Cu^{2+}$ blue - $Cu^{3+}$ yellow - $Pt^{4+}$ yellow or red depending on the combination dissolved in water $Re^{3+}$ red - $Fe^{2+}$ pale green - $FeO_4^{2-}$ red-purple - $ReO_4^{-}$ colorless - $Rh^{3+}$ yellow or red depending on the combination dissolved in water $Rh^{4+}$ green - $Hg^{2+}$ colorless - $Hg_2^{2+}$ colorless - $V^{2+}$ purple - $V^{3+}$ cyan - $Ir^{3+}$ yellow-green - $Ir^{4+}$ yellow, red or purple depending on the combination dissolved in water $VO^{2+}$ blue - $Zn^{2+}$ colorless - $La^{3+}$ colorless -

# Some facts

• The presence of ions (cations or anions) of some metals results in a characteristic coloration of the solution.
• The change in the solution color results from selective (i.e. only within selected wavelength range) absorption of light during its contact with the solution. For example, if we remove the range related with green color from the white light (i.e. the whole spectrum), we obtain a pink color.
• The observed color of the solution may differ not only depending on the content of a given ion, but also on its concentration. For example, a concentrated solution of potassium permanganate (KMnO4) is almost black, while at low concentration, we notice a pale pink color.
• The ability of some ions to absorb light of a specific wavelength can be used to measure the concentration of a given ion in the solution (i.e. to perform so-called quantitative analysis). The basis for this measurement is the Lambert-Beer Law:
$A = kcl$
where:
• A - absorbance,
• k - proportionality coefficient,
• c - concentration of ion,
• l - the thickness of the absorbing layer (the way it overcomes the radiation passing through the solution).
From the point of view of quantitative analysis, the key point is that the amount of absorbed light depends linearly on the ion concentration.

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