Colored Water

Colored water contains either inorganic ions or organic compounds. To be able to know the source of color in water an analysis needs to be performed in the lab. Ionic species can be detected using simple photometric tests for example using Hach Lange DR Spectrophotometer. Also ionic chromatography can be used to detect ionic species. Organic compounds (can be natural or man made) can be detected and analyzed using Infra Red spectroscopy, NMR (Nuclear Magentic Resonance) Spectroscopy, HPLC or Mass Spectroscopy.

Below I am listing some transition mental ionic compounds with typical colors.

• Sc - scandium salts, such as the chloride, ScCl 3, are colourless and are not typical of transition metals
• Ti - titanium(III) chloride, TiCl 3 , is purple
• V - vanadium(III) chloride, VCl 3 , is green
• Cr - chromium(III) sulphate, Cr 2 (SO4) 3 , is dark green ( chromate(VI) salts are yellow, dichromate(VI) salts are orange )
• Mn - manganese compound -  potassium manganate(VII), KMnO4 , is purple ( manganese(II) salts eg MnCl 2 are pale pink )
• Fe - iron(III) chloride, FeCl 3 , is yellow-orange-brown.
  • Iron(II) compounds are usually light green and iron(III) compounds orange/brown.
• Co - cobalt sulphate, CoSO4 , is pinkish
• Ni - nickel chloride, NiCl2 , is green
• Cu - copper(II) sulphate, CuSO4 , is blue.
  • Most common copper compounds are blue in their crystals or solution and sometimes green.
  • The blue aqueous copper ion, Cu 2+ (aq) , actually has a more complicated structure:
    • * [Cu(H2O)6] 2+ (aq) and when excess ammonia solution is added,
    • after the initial gelatinous blue copper(II) hydroxide precipitate is formed, Cu(OH)2, it dissolves to form the deep royal blue ion:
      * [Cu(H2O)2(NH3)4] 2+ (aq) .
    • * are called complex ions and when coloured are typical of transition metal chemistry.
  • Copper(II) oxide, CuO , black insoluble solid, readily dissolving in acids to give soluble blue salts e.g.
    • copper(II) sulphate, CuSO4 , from dilute sulphuric acid,
    • copper(II) nitrate, Cu(NO3)2 , from dilute nitric acid
    • and greeny-blue copper(II) chloride, CuCl 2 , from dilute hydrochloric acid.
  • Copper(II) hydroxide, Cu(OH) 2 , blue gelatinous precipitate formed when alkali added to copper salt solutions.
  • Copper(II) carbonate, CuCO 3 , is turquoise-green insoluble solid, readily dissolving in acids, evolving carbon dioxide, to give soluble blue salts (see above)
  • Copper's valency or combining power is usually two e.g. compounds containing the Cu 2+ ion.
    • However there are copper(I) compounds where the valency is one.
    • This v ariable valency, hence compounds of the same elements, but with different formulae, is typical of transition metal compounds e.g.
    • copper(I) oxide, Cu 2 O, an insoluble red-brown solid (CuO is black),
    • or copper(I) sulphate, Cu 2 SO 4 , a white solid.
• Zn - zinc salts such as zinc sulphate, ZnSO 4 , are usually colourless and are not typical of transition metals.
• Lead tetroxide , also called minium , red lead or triplumbic tetroxide , is a bright red or orange crystalline or amorphous pigment . Chemically, red lead is lead tetroxide , Pb3O4. Red lead is virtually insoluble in water and in alcohol. However, it is soluble in hydrochloric acid present in the stomach, and is therefore toxic when ingested. It also dissolves in glacial acetic acid and a diluted mixture of nitric acid and hydrogen peroxide.
  
• Mercury(II) oxide, also called mercuric oxide or simply mercury oxide, has a formula of HgO. It has a red or orange color. Mercury(II) oxide is a solid at room temperature and pressure. The mineral form montroydite is very rarely found. Mercury oxide is a toxic substance which can be absorbed into the body by inhalation of its aerosol, through the skin and by ingestion. The substance is irritating to the eyes, the skin and the respiratory tract and may have effects on the kidneys, resulting in kidney impairment. In the food chain important to humans, bioaccumulation takes place, specifically in aquatic organisms. The substance is banned as a pesticide in the EU. Its solubility in water is 0.0053 g/100 mL (at 25 °C) and 0.0395 g/100 mL (at 100 °C)

Compiled by Rami E. Kremesti M.Sc.