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Qualitative Analysis. Determines only the presence (or absence) of a substance Sometimes referred to as “Wet Chemistry” Involves the identification of ions present in solution. Qualitative Analysis.

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qualitative analysis
Qualitative Analysis
  • Determines only the presence (or absence) of a substance
  • Sometimes referred to as “Wet Chemistry”
  • Involves the identification of ions present in solution
qualitative analysis1
Qualitative Analysis
  • General idea is to separate ions into large groups first, then separate and identify within each group
  • Classic scheme has five groups
group 1
Group 1
  • Includes Ag+, Pb+2, and Hg2+2 ions
  • Separated out as insoluble chlorides by adding 6M HCl
group 11
Group 1
  • The precipitate is then separated from the remaining liquid by first using a centrifuge.
group 12
Group 1
  • Then, the liquid is extracted using a micropipette
group 13
Group 1
  • The Ksp’s for each chloride are:
    • PbCl2 = 1.7 x 10-5
    • AgCl = 1.8 x 10-10
    • Hg2Cl2 = 1.2 x 10-18
  • Being the most soluble, the lead(II) chloride will dissolve fairly readily in hot water.
  • Thus, we will add DI water and heat in a hot water bath to dissolve this compound.
group 14
Group 1
  • Once again, centrifuge and separate the liquid from the solid using the micropipette
  • The liquid will be tested for the lead ion by adding K2CrO4
group 15
Group 1
  • The solid precipitate that remains will be treated with 6M NH3 with formation of a black solid confirming mercury(I) is present.
group 16
Group 1
  • The liquid solution from the previous step is tested for silver ion by acidifying with HNO3. The formation of a white precipitate confirms this ion.
litmus tests
Litmus Tests
  • Throughout the schemes, you may be required to add acid until the solution becomes acidic or base until it becomes alkaline.
  • “Blue” litmus  pink = acidic
  • “Red” litmus  blue = basic (alkaline)
  • Correct method – use stirring rod – dunk in solution, then touch to paper.
group 2
Group 2
  • Separated first by precipitation as an acid-insoluble chloride.
  • Includes the ions: Hg+2 , Pb+2 , Bi+3 , Cu+2 , and Cd+2
  • Sulfide source is Thioacetamide.
group 3
Group 3
  • Separated first by precipitation as an alkaline-insoluble sulfide.
  • Includes the ions: Ni+2 , Co+2 , Mn+2 , Zn+2 , Al+3 , Cr+3 , and Fe+3.
  • Also uses Thioacetamide, but under basic (alkaline) conditions.
  • Group 3 also has individual confirmatory tests for each ion.
group 4
Group 4
  • Separated by precipitation as an insoluble carbonate.
  • Includes the ions Ba+2 , Ca+2 , and Sr+2.
  • Identification includes flame tests for each metal ion.
    • Ba = pale green
    • Ca = orange-red
    • Sr = crimson red
general unknown
General Unknown
  • Lab final will include a General Unknown
  • Will include one ion from EACH group
  • Groups 2 and 3 will be reduced to three ions
  • After precipitation of EACH group, you will need to save the liquid for the next group(s)
  • Any mistakes may mean that you have to start over from the very beginning!
  • You will have a total of three hours to complete both the written and general unknown sections
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