chem 18.1 qualitative analysis
TRANSCRIPT
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Group # 10ESPINOSA, Elise Angela H.
MANUEL, Immary Alleivin R.
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SALT SAMPLE:Cd(CH3COO)2
LIQUID SAMPLE INCLUDES:Fe2+ (possibly oxidized to Fe3+)
and K+
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Soluble in Water
Dissolve a pinch of sample in water
Add a few drops of Sodium Carbonate
Precipitation? Proceed to Step 2.
No Precipitation? Proceed to Anion Preliminary
Tests.
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Insoluble in Water
Add 3mL of Sodium Carbonate to a small amount of test sample
Heat for 5 mins. Centrifuge and Decant. Use decantate for Anion Test.
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Add little Sulfuric Acid
to a small amount of unknown sample
Note all observations like gas and
product formation, change in color, etc.
If no change occurs, warm the test tube.
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ANIONS VISIBLE RESULT REACTION EQUATIONCarbonate Gas formation CO32- + 2H + CO2 + H20
Oxalate Gas formation C2O42- + 2H+ H2C2O4
Sulfide Rotten egg odor S2- + 2H+ H2S
Chloride Fumes in moist air Cl- + H2SO4 HCl + HSO4-
Bromide Reddish brown color Br- + H2SO4 HBr + HSO4-
Iodide Violet color; fumes in moist air I- + H2SO4 I2 + H2S H2O
Chromate Yellow to Orange change in color 2CrO42- + 2 H+ Cr2O72- + H2O
Nitrite Evolution of brown fumes NO2- + 4H+ 4NO + 2H2O + O2
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ANIONS VISIBLE RESULT REACTION EQUATION
Acetate Gas formation; vinegar-like smell
OAc- + H+ HOAc
Nitrite Evolution of brown fumes; strong odor
4NO3 + 4H2SO4 2SO42- + 4H2O + 4NO2
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Add Silver Nitrate.
Precipitation? Centrifuge and Decant (C&D).
Wash the residue with water. Add Nitric Acid.
C&D.
To the decantate,
add Ammoniaand Silver Nitrate.
Yellow ppt = phosphateRed ppt = chromate
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Add Ammoniauntil basic to
unknown sample. Ppt? C & D. Discard the residue.
Add Barium Chloride and
Calcium Chloride. No precipitate?
Sulfate, chromate, phosphate and oxalate are absent.
Yellow precipitate? Chromate might be
present. Add HCl until acidic. If
everything dissolved, sulfate is absent.
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Carbonate
Disregard if no effervescence occuredin Sulfuric Acid Test
Chromate &
Permanganate
Disregard if unknown sample is colorless
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Add Fe(NO3)3.
Formation of blood-red solution confirms thiocyanate.
CNS--
Fe3+ + CNS- FeCNS2+
FeCNS2+ is a complex ion and is responsible for the formation of the blood red solution
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I--
Acidify with HNO3. Add Fe(NO3)3 and CCl4. Shake.
Purple layer confirms iodide. If no iodide is present, proceed to bromide confirmatory
test.
If iodide is present, remove CCl4 layer and add new CCl4and Fe(NO3)3. Remove violet CCl4 layer. Repeat until no
iodide is present. Proceed to bromide confirmatory test.
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Br--
To the solution from Iodide test, add equal amount of HNO3. Heat then cool in
cold water.
Add CCl4. Shake thoroughly.
Yellowish/Reddish layer confirms bromide. Discard bromide layer. Proceed to Choride
test.
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Cl--
Acidify test solution from bromide test with HNO3.
Add water and AgNO3.
White precipitate confirms chloride.
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*Iodide, Bromide and Chloride are ionic while CCl4 is non-polar.
*Purple immiscible layer is formed for Iodide. *Yellowish brown immiscible layer is formed for
Bromide (due to oxidation: Br- Br2)*White precipitate is produced for Chloride because
AgCl is formed.
I-, Br -& Cl-
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S2-
Place a small amount of unknown sample in a small beaker. Add HCl.
Moisten a small piece of filter paper with lead acetate. Place it on the convex side of a watch glass. Cover the beaker with the watch glass.
Heat gently. If filter paper turns black/silvery, sulfide is present.
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S2-
(a) H+ + S2- H2S(b) Pb2+ + S2- PbS(s)
H2S reacts with lead acetate forming PbS which is responsible for the
black/silver color.
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CO32-
Prepare a glass medicine dropper with a drop of barium hydroxide suspended at
the tip.
Cover a test tube with unknown sample with the medicine droper. Warm but not boil.
Observe the droplet. If it becomes cloudy/white, carbonate is confirmed.
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CO32-
CO2(g) + Ba2+(aq) BaCO3(s) + H2O
*Carbonates are soluble in acids, producing Carbonic Acid
*Addition of barium hydroxide reacts with carbonic acid, forming BaCO3, which
is the cloudy/white precipitate
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Add HCl until acidic then add excess HCl. Add BaCl2.
White precipitate confirms sulfate.
SO42-
Sulfates dont easily react but it reacts with Ba2+.Addition of BaCl2 reacts with sulfate forming Barium Sulfate
SO42-(aq) + Ba2+(aq) BaSO4(s)
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Add 1M HNO3 until acidic. Add 3M HNO3 until a layer forms.
Add fresh hydrogen peroxide. Shake once quickly.
Formation of blue colorconfirms presence of chromate.
CrO42-
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(a) 2CrO42- + 2H+ Cr2O72- + H2O(b) Cr2O72-(aq) + 4H2O2(aq) + 2H+(aq)
2CrO5(aq) + H2O
CrO42-
*Chromates react with acid and is converted to dichromate
*Dichromate reacts with H2O2 which causes the fading of color
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Add 16M HNO3 and ammonium molybdate. Warm in a water
bath.
Yellow precipitate confirms phosphate.
PO43-
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*Phosphates are generally stable although there are exceptions: FePO4, CrPO4, BiPO4,
Ca3(PO4)2 and AgPO4
*Yellow color is caused by the formation of ammonium phosphomolybdate
PO43-
H3PO4(aq) + 12 MoO42-(aq) + 3 NH4+(aq) + 22H+(aq) (NH4)3PO4 12 MoO3(s) + 12 H2O
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Add 6M HOAc and CaCl2. Precipitation? C&D.
Wash precipitate with water. Discard washing. Add water, KMnO4 and H2SO4.
If permanganate color fades in 30 seconds, oxalate is present.
C2O42-
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5C2O42-(aq) + 2MnO4-(aq) + 8H+(aq) 10CO2(g) + 2Mn2+(aq) + 4H2O
C2O42-
*Oxalates are soluble in acidic solutions*Addition of permanganate causes the
conversion of oxalate to CO2 gas* Fading of permanganate color is caused by
the CO2 gas
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Add 1:1 sulfuric acid to water solutionn. Vinegar-like odor? Acetate is present
No vinegar odor? Add 2 drops ethyl alcohol. Warm gently. Sniff cautiously.
Presence of sweet alcohol smell confirms acetate.
CH3COO--
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*Vinegar-like smell is due to the conversion of acetate to acetic acid upon the addition
of sulfuric acid.
CH3COO--
CH3COO- + H2SO4 CH3COOH + HSO4-
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Add concentrated sulfuric acid and a grain of ferrous sulfate. Wait for
a few minutes.
A brown ring formation confirms nitrate.
NO3--
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NO3--
(a) 3Fe2+(aq) + NO3-(aq) + 4H+(aq) 3Fe3+(aq) + NO(aq) + 2H2O(l)(b) Fe2+(aq) + NO(aq) Fe(NO)2+(aq)
*Nitrate oxidizes Fe2+ to Fe3+
*Nitrogen (IV) oxide gas is produced which is responsible for the brown ring formation
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Add 3M HCl.
A cream/whitish precipitated confirms thiosulfate.
S2O32-
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S2O32-
S2O32-(aq) + 2H+(aq) SO2(g) + S(s) + H2O
*Thiosulfate decomposes into sulfite and elemental sulfur
*Elemental sulfur, S , is the white precipitate formed
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Add 6M HOAc until acidic. Add K3Fe(CN)6. Let it stand for 1
minute.
Add 1 drop FeCl3. Deep red precipitate confirms nitrite.
NO2--
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NO2--
(a) HNO2(aq) + Fe2+(aq) + H+(aq) Fe3+(aq)+ NO(aq) + H2O
(b) NO(aq) + Fe2+(aq) Fe(NO)2+(aq)
*Nitrites are converted to nitrous acid whenever an acid reacts with nitrites
*Deep-red solution is caused by complex ion Fe(NO)2+
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Add water and sodium oxalate and sulfuric acid/
Color disappearance within 30 seconds confirms
presence of permanganate.
MnO4--
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5C2O42-(aq) + 2MnO4-(aq) + 8H+(aq) 10CO2(g) + 2Mn2+(aq) + 4H2O
MnO4-
*Similar to oxalate test*Formation of CO2 gas occurs which cause
the permanganate color to fade
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Qualitative Analysis = used in identifying the presence of a substance in an unknown solution
Different concepts and techniques have to be applied
Conclusions are made after every procedure performed
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Follow the procedure very carefully and accurately
Observe ALL possible reactions Observe CLEANLINESS at all times so as to prevent
contamination
Work efficiently and precisely in order to minimize human error.
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