Exp 6 Flashcards
It is used to detect the presence of specific ions in aqueous solutions
qualitative analysis
Basis for qualitative analysis is the fact that ____ will undergo specific chemical reactions with certain reagents to yield products that are observable
ions
________ is commonly accomplished by a system of analysis in which precipitation reactions play a major role
Qualitative detection of ions
Identify the group and sample of ions: produce insoluble chlorides so they can be precipitated using dilute HCl, while all other cations remains in the solution
Group I (Ag+, Pb+, Hg2+) cations
Identify the group and sample of ions: produce very insoluble sulfides so they can be precipitated by low amounts of sulfide ion; this can be achieved by adding acidic solutions of H2S
Group II (CU2+, Bi2+, Hg2+, As3+, Sb3+, Sn4+) cations
Identify the group and sample of ions: produce slightly soluble sulfides so they can be precipitated by adding basic H2S solution
Group III (Al3+, Cr3+, Fe3+, Zn2+ Ni2+, Co2+, Mn3+) cations
Identify the group and sample of ions: produce insoluble carbonates so they can be precipitated by adding carbonates once the ions of the first three groups have been removed
Group IV (Mg2+, Ca2+, Sr3+, Ba3+) cations
Identify the group and sample of ions: cations that cannot be precipitated with any of the above reagent
Group V (Na+, K+, NH4+) cations
In complete precipitation technique, what is the main method/process used?:
centrifuge
- Centrifuge the solution and add a drop of the reagent to the clear supernatant. Observe if more precipitate is formed.
- Centrifuge the solution and transfer the supernatant liquid to another test tube and add a drop of the reagent to this liquid. Centrifuge, and observe if additional precipitate has formed. If the additional precipitate is very small in quantity relative to the original precipitate, it may usually be discarded. If the quantity is large, it should be combined with the original precipitate.
Complete Precipitation
- Hold the test tube in the left hand in slanting position and carefully lower end of the dropper below the surface of the liquid so as not to disturb the precipitate. If the amount of liquid is large, remove it in several portions
To remove the supernatant liquid from the precipitate
- After removing the supernatant, add few drops of water or the appropriate washing solution to the precipitate and mix thoroughly. Centrifuge and remove the supernatant liquid. It is Important to see if all foreign ions have been removed.
Washing precipitate
Put the test tube in a beaker of warm water and allow it to remain there, stirring from time to time
To heat a solution below its boiling point
- Usually best to do it in an evaporating dish. It may be held using a crucible tong and is waved through the flame, or it may be put on a wire gauze (on an iron ring) which is suspended well above a small flame.
To evaporate a solution
Place a strip of litmus paper on a clean dry surface. Place a drop of solution to be tested on the litmus paper. Use clean stirring rod to do this. Read the color of the paper.
Test with pH and litmus paper
7 reagents used in exp 6
- 0.1 M AgNO3 (aq)
- 0.2 M Pb(NO3) (aq)
- 0.1 M Hg2(NO3)2 (aq)
- 6 M HCl (aq)
- 1 M K2CrO4 (aq)
- 6 M NH3 (aq)
- 6 M HNO3 (aq)
12 materials used in exp 6
- 3 centrifuge tubes
- 3 small test tubes
- glass stirring rod
- small 10-mL graduated cylinder
- water bath
- wire gauze
- tripod
- small watch glass
- dropper pipets
- blue litmus paper
- wash bottle filled with deionized water
- Bunsen burner
Exp 6 procedure:
1. Add ____ drops of each of the following aqueous solutions to separate centrifuge tubes: 0.1 M AgNO3, 0.2 M Pb(NO3)2 and 0.1 M Hg2(NO3)2. Then add ___ drops of 6 M HCI to each centrifuge tube. Record your observation.
20, 3
Exp 6 procedure:
2. Add one more drop of ___ to ensure that the precipitation is complete. If more precipitate is observed, then centrifuge the mixture again.
HCI
Exp 6 procedure:
3. ______the three mixtures to completely separate the solid precipitate from the solution. Make sure that you balance the centrifuge before you start. Balancing is done by placing a test tube containing water opposite to the test tube containing the precipitate in the centrifuge. The two test tubes should have approximately equal weights.
Centrifuge
Exp 6 procedure:
4. Decant the ______ into another small test tube and save the precipitate for further study. The supernatant solution may be discarded in the lab waste container.
supernatant liquid
Exp 6 procedure:
5. Prepare a water bath. Assemble a _____ and _____ to heat the water over the ______ until it is boiling. Add ___ mL of _____ to the centrifuge tubes containing the clean precipitate and place it in the hot water bath. Periodically stir the mixture in the test tube for ______
- tripod and wire gauze,
- Bunsen burner,
- 2.0 mL of deionized water,
- 3-4 minutes
Exp 6 procedures:
6. Remove the centrifuge tubes from the hot water bath and immediately ______. Carefully decant the supernatant liquid into another small test tube. Save both the precipitate and the supernatant solutions for further study.
centrifuge the mixture
Exp 6 procedures:
7. In the centrifuge tube containing the Pb2+, add ____ of ____. Observe the reaction.
3-4 drops of 1 M K2CrO4
Exp 6 procedures:
8. In the centrifuge tubes containing Hg22+ and Ag+, Add ____ drops of _____, to the precipitate and mix thoroughly with your stirring rod. Observe the reaction.
10-12 drops of 6 M NH3
Exp 6 procedures:
9. To the solution containing Ag+, slowly add _____, until the solution is acidic to litmus paper. The acidity can be tested by dipping a stirring rod into the solution and then touching it (with a drop of solution) to a piece of blue litmus paper resting on a clean, dry watch glass.
6 M HNO3
