Chemlab: Chemistry 3/5

Qualitative Analysis of Anions


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Testing for Carbonate Ion, CO32-
Image 1
Baking soda is sodium hydrogen carbonate or sodium bicarbonate. It reacts with acid the same way that carbonate ion does.

Image 2 Image 2

H+ (aq) + HCO3- (aq) H2O (l) + CO2 (g)

Image 4
Chalk is calcium carbonate, CaCO3.

2 H+ (aq) + CO32- (aq)
H2O (l) + CO2 (g)

Image holder
Reaction with Ba2+ ion is a test for CO2 gas.

CO2 (g) + Ba2+ (aq) + H2O (l)
BaCO3 (s) + 2 H+ (aq)

The barium carbonate precipitate is visible in the drop of Ba(OH)2 solution, running down the side of the test tube.
Testing for Chloride Ion, CI-
Image 6
The source of chloride ion in this experiment is NaCl, sodium chloride or table salt.
Image 2 Image 8

Cl- (aq) + H+ (aq) HCl (g)
Image 9
To test for HCl gas, put wet pH paper in the test tube, as the gas is evolved. HCl is an acid and should change the color of the pH paper accordingly.
Image 11 Image 12

Another test for chloride ion is reaction with silver nitrate, AgNO3. Nitric acid is added to aid the precipitation.
Image 13
Cl- (aq) + Ag+ (aq) AgCl (s)
Image 14
The precipitation of AgCl may be less obvious for tap water, but is chloride ion present?
Testing for Sulfate, SO42-
Image 15
The sulfate in this experiment is supplied by magnesium sulfate, MgSO4 7H2O, or Epsom Salts.
Image 2
Sulfate anion plus sulfuric acid

SO42-(aq) + H+ (aq) ??
Image 11 Image 20

Add nitric acid (HNO3) and barium chloride (BaCl2)

Image 21
SO42- (aq) + Ba2+ (aq)
BaSO4 (s)
Testing for Iodide, I-
Image 22
Sodium iodide, NaI, is the source of iodide anion for this experiment.
Image 24
Reaction with bleach involves three steps. The brown color shows the presence of I3- ions.

Hypochlorite ion yields chlorine:
OCl- (aq) + Cl- (aq) + H2O
Cl2 (aq) + 2 OH-

Chlorine reacts with iodide anion:
Cl2 (aq) + 2 I- (aq)
I2 (aq) + 2 Cl- (aq)

Triiodide ion is formed:
I2 (aq) + I- (aq) I3- (aq)

Image 27
Starch reacts with iodine and iodide to form a characteristic blue/black complex. A corn starch packing peanut is shown here.
Image 11 Image 12

Silver ion reacts with iodide to form silver iodide, AgI.

Image 28
Ag+ (aq) + I- (aq) AgI (s)

Image 29 Image 30

Reaction of iodide with sulfuric acid produces hydrogen sulfide gas and brown triiodide solution in a series of reactions:
I- (aq) + H2SO4 (aq) HI (aq) + HSO4- (aq)
8 HI + H2SO4 (aq) H2S (g) + 4 I2 (aq) + 4 H2O
I2 (aq) + I- (aq) I3- (aq)

Image 31
Again, starch is used to confirm the presence of iodine and iodide.
Unknown Ion
Image 26
Chose any unknown from the box and record your unknown number in your lab notebook. Test your sample to identify it as containing iodide, chloride, sulfate, or carbonate. Make sure you do a complete set of confirmation tests on your sample and record your observations.

You can practice the analysis on the web, using the Qualitative Analysis of Anions Applet

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