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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 Analysis.

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Qualitative Analysis

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  1. 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

  2. Qualitative Analysis • General idea is to separate ions into large groups first, then separate and identify within each group • Classic scheme has five groups

  3. Group 1 • Includes Ag+, Pb+2, and Hg2+2 ions • Separated out as insoluble chlorides by adding 6M HCl

  4. Group 1 • The precipitate is then separated from the remaining liquid by first using a centrifuge.

  5. Group 1 • Then, the liquid is extracted using a micropipette

  6. 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.

  7. 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

  8. Group 1 • The solid precipitate that remains will be treated with 6M NH3 with formation of a black solid confirming mercury(I) is present.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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

  14. 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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