12.3 IdentificationofIons& Gases

• • For example, aqueous barium nitrate/chloride solution should not be acidified with sulfuric acid as this will form a white precipitate due to the sulfate ion in the acid
• Identification of Anions:

  • Acidification with aqueous silver nitrate and aqueous barium nitrate/chloride is done to remove ions that might give a false positive result
  • The most common ion that is removed is the carbonate ion
  • The choice of acid must be considered so that it does not influence the results
  • For example, aqueous silver nitrate solution should not be acidified with hydrochloric acid as this will form a white precipitate due to the chloride ion in the acid
• Identification of Cations:

  • Metal cations in aqueous solution can be identified by the color of the precipitate they form on addition of sodium hydroxide and ammonia
  • If only a small amount of NaOH is used, then normally the resulting metal hydroxide precipitates out of solution
  • In excess NaOH, some of the precipitates may redissolve
  • A few drops of NaOH is added at first and any color changes or precipitates formed are noted
  • Then the NaOH is added in excess and the reaction is observed again
  • The steps are then repeated for the test using ammonia solution
• Analyzing results:

  • The table contains the results for each of the cations included in the syllabus
  • If a precipitate is formed from either NaOH or aqueous ammonia, then it means that the hydroxide is insoluble in water
  • Zinc chloride, for example, reacts as such:

  ZnCl(aq) + 2NaOH(aq) → Zn(OH)(s) + 2NaCl(aq)

  • Calcium ions can be distinguished from Zinc and Aluminum as calcium hydroxide precipitate does not dissolve in excess NaOH but both zinc hydroxide and aluminum hydroxide do
  • Zinc ions can be distinguished
• Zinc hydroxide and aluminum hydroxide do not dissolve in excess NaOH
• Zinc ions can be distinguished from Aluminum ions as Zn(OH) dissolves in excess aqueous ammonia but Al(OH) does not
• Most transition metals produce hydroxides with distinctive colors
• The flame test is used to identify the metal cations by the color of the flame they produce
• Ions from different metals produce different colors
• Dip the loop of an unreactive metal wire such as nichrome or platinum in concentrated acid and then hold it in the blue flame of a Bunsen burner until there is no color change
• This is an important step as the test will only work if there is just one type of ion present
• Two or more ions means the colors will mix, making identification erroneous
• This cleans the wire loop and avoids contamination
• A small sample of the compound is placed on an unreactive metal wire loop such as nichrome or platinum
• Dip the loop into the solid sample/solution and place it in the edge of the blue Bunsen flame
• Avoid letting the wire get so hot that it glows red, otherwise, this can be confused with a flame color
• The color of the flame is observed and used to identify the metal ion present
• The sample needs to be heated strongly, so the Bunsen burner flame should be on a blue flame
• Identification of Gases
• Several tests for anions and cations produce gases which then need to be tested
• It is easy to confuse the tests for hydrogen and oxygen
• Try to remember that a lighted splint has an H for Hydrogen, while a glowing splint has an O for Oxygen