Qualitative Salt Analysis Cheatsheet

Posted on Sep 11, 2024 in Chemistry

Anions

Test mentioned next to group is the preliminary test; the ones mentioned under an anion are confirmatory tests for it.

Group I (Dilute H2SO4 Group)

CO32- (Carbonate), SO32- (Sulphite), S2- (Sulphide), NO2– (Nitrite)

Preliminary Test: Salt + Dilute H2SO4

  1. No reaction: Group I anion not present. Continue to Group II.
  2. Carbonate: Colourless and odourless gas (CO2)
    1. WE1 + MgSO4 = White precipitate
  3. Sulphite: Colourless gas with pungent smell
    1. WE + BaCl2 (aq) = White precipitate soluble in dilute HCl
    2. WE + Acidified K2Cr2O72 = Solution3 turns green
    3. WE + Acidified KMnO4 = Pink colour of KMnO4 is discharged
  4. Sulphide: Colourless gas with a smell of rotten eggs (H2S); turns lead acetate paper black
    1. Sodium Nitroprusside Test: WE + Sodium Nitroprusside4 = Purple/violet colour
    2. Lead Acetate Test: WE + Lead Acetate5 (aq) = Black precipitate
  5. Nitrite: Pungent light brown gas
    1. Starch-Iodide Test: WE + Dilute H2SO4 (or dilute acetic acid); boil, then add solid KI + fresh starch solution = Deep blue colouration

1 Water Extract: Pinch of salt + water

2 Potassium Dichromate

3 Solution

4 Na2[Fe(CN)5NO]

5 Pb(CH3COO)2

Group II (Concentrated H2SO4 Group)

Cl– (Chloride), Br– (Bromide), I– (Iodide), NO3– (Nitrate), CH3COO– (Acetate), C2O42- (Oxalate)

Preliminary Test: Salt + Concentrated H2SO4

  1. No reaction: Group II anion not present. Continue to Group III.
  2. Chloride: Colourless white pungent fumes (HCl); intensify when glass rod dipped in NH4OH is brought near the mouth of the test tube
    1. Silver Nitrate Test: WE + AgNO3 = White precipitate soluble in NH4OH
    2. Chromyl Chloride Test: Salt + Solid K2Cr2O7 + 2-3 drops Concentrated H2SO4 = Orange/red fumes of Chromyl Chloride6
      1. Vapours + NaOH (aq) = Yellow solution
      2. Yellow solution + Acetic acid + Lead acetate solution = Yellow precipitate
  3. Bromide: Reddish-brown vapour
    1. Silver Nitrate Test: WE + AgNO3 (aq) = Yellow precipitate partially soluble in NH4OH
    2. Organic Layer Test: WE + CCl4 + 1 drop Concentrated HNO3 = Upper layer aqueous; lower layer organic of orange/brown colour (bromine is soluble in non-polar solvent)
  4. Iodide: Violet vapours
    1. Silver Nitrate Test: WE + AgNO3 (aq) = Yellow precipitate insoluble in NH4OH
    2. Organic Layer Test: WE + CCl4 + 1 drop Concentrated HNO3 = Upper layer aqueous; lower layer organic of violet colour (iodine is soluble in non-polar solvent)
  5. Nitrate: Brown fumes with pungent smell, which intensify on adding paper pellets (may need heating)
    1. Brown Ring Test: WE + Freshly prepared FeSO4 solution + 1 drop Concentrated HNO3 added along the side of the test tube = Brown ring formed at the junction of solution and acid
    2. Potassium Permanganate Test: Salt + Dilute H2SO4 + heat; then add 2-3 drops KMnO4 solution = Pink colour of KMnO4 is discharged
  6. Acetate: Pungent vapour with vinegar-like smell
    1. Ester Test: Salt + Concentrated H2SO4 + ethanol = Fruity smell of ester
    2. Ferric Chloride Test: WE + FeCl3 (aq) = Brick red colour
      1. Add dilute HCl = Red colour disappears
      2. Add water and boil = Reddish-brown precipitate
  7. Oxalate: Colourless gas with effervescence (CO + CO2)
    1. Calcium Chloride Test: WE + Acetic acid + CaCl2 (aq) + boil = White precipitate; precipitate dissolves when dilute HNO3 is added and warmed

6 CrO2Cl2

7 (NH4)2MoO4

Group III Anions (Special Group)

SO42- (Sulphate), PO43- (Phosphate)

No Group Reagent

1. Sulphate

  1. Barium Chloride Test: WE + BaCl2 (aq) = White precipitate insoluble in concentrated HCl
  2. Lead Acetate Test: WE + Lead acetate (aq) + Acetic acid = White precipitate soluble in CH3COONH4 (ammonium acetate)

2. Phosphate

  1. Ammonium Molybdate Test: WE + Dilute HNO3 + Ammonium Molybdate7 + boil = Crystalline canary yellow precipitate

Cations

● Group reagent is mentioned next to group cations.

● Test mentioned next to cation is the preliminary; ones under it are confirmatory tests for it.

● When sulphate is detected, Ba2+, Ca2+, Pb2+, and Sr2+ are not present as sulphates of these radicals are insoluble.

● When phosphate is detected, cations of Group III and later are absent.

Group 0

NH4+ (Ammonium)

No Group Reagent

  1. Sodium Hydroxide Test: Salt + NaOH = Pungent smelling gas; gives white fumes when a glass rod dipped in concentrated HCl is brought near the mouth of the test tube
  2. Nessler’s Reagent8 Test: OS9 + NaOH + Nessler’s reagent = Brown/yellow precipitate

8 K2HgI4

9 Original Solution: Salt + Acid + Water

Group I

Pb2+ (Lead)

Group Reagent: Dilute HCl

OS + Dilute HCl = White precipitate; add water, boil, and divide into three parts

  1. Leave OS undisturbed = White crystals formed on cooling
  2. Potassium Iodide Test: OS + KI = Yellow precipitate
  3. Potassium Chromate Test: OS + K2CrO4 = Yellow precipitate

Group II

Cu2+ (Copper)

Group Reagent: Dilute HCl + H2S

OS + Dilute HCl + H2S = Black precipitate

  1. Throw off extra solution, retain precipitate, and dissolve in a few drops of concentrated HNO3 = Bluish-green solution, precipitate dissolves; divide into two parts
    1. Part 1 + Excess NH4OH = Blue coloured solution
    2. Potassium Ferrocyanide Test: Part 2 + Acetic acid + K4[Fe(CN)6] = Reddish-brown/chocolate coloured precipitate (Note – this test is difficult to get)

Group III

Fe2+, Fe3+ (Iron), Al3+ (Aluminium)

Group Reagent: Solid NH4Cl + Excess NH4OH

1. Fe2+ / Fe3+

Note – Ferrous salts are green in colour, ferric salts are brown in colour.

  1. If ferrous salt has been given, convert to ferric: OS + Concentrated HNO3 + heat = Brown precipitate; then do reaction with group reagent
  2. Brown precipitate + HCl; then divide into two parts
    1. Potassium Ferrocyanide Test: Part 1 + K4[Fe(CN)6] = Blue precipitate/colour
    2. Potassium Thiocyanate Test: Part 2 + KCNS = Blood red colour

2. Al3+

Gelatinous white precipitate

  1. Blue Lake Test: Retain precipitate + Dilute HCl = Clear solution
    1. Solution + Blue litmus and NH4OH (drop-by-drop) = Blue colour layer (‘lake’) floats over colourless solution

Group IV

Co2+ (Cobalt), Ni2+ (Nickel), Mn2+ (Manganese), Zn2+ (Zinc)

Group Reagent: Solid NH4Cl + Excess NH4OH + Pass H2S gas

1. Co2+ / Ni2+

Black precipitate; dissolve precipitate in aqua regia10 and evaporate solution to dryness to get residue

a) Co2+

Blue residue; turns pink/purple when dissolved in water; divide into two parts

  1. Part 1 + Dilute acetic acid + KNO2 + warm = Yellow precipitate
  2. Part 2 + Ether (1 mL) + Solid NH4CNS11 = Blue colour in ether
b) Ni2+

Yellow residue; turns green when dissolved in water; divide into two parts

  1. DMG12 Test: Part 1 + Excess NH4OH + DMG = Bright red precipitate
  2. Part 2 + NaOH + Bromine water + boil = Black precipitate

10 Aqua regia: 3 parts concentrated HCl + 1 part concentrated HNO3

11 Ammonium sulphocyanide

2. Mn2+

Buff/skin colour precipitate; divide into two parts

  1. Part 1 + Dilute HCl + boil off H2S + NaOH = White precipitate; which turns black/brown on adding bromine water
  2. Lead Dioxide Test: Part 2 + PbO2 + Concentrated HNO3 + boil = After cooling; pink colouration

3. Zn2+

Greyish-white precipitate; divide into two parts

  1. Part 1 + Excess NaOH = White precipitate dissolves
  2. Potassium Ferrocyanide Test: Part 2 + K4[Fe(CN)6] = White/bluish-white precipitate

Group V

Ba2+ (Barium), Sr2+ (Strontium), Ca2+ (Calcium)

Group Reagent: (NH4)2CO3 + NH4Cl + NH4OH

OS + (NH4)2CO3 + NH4Cl + NH4OH = White precipitate; add dilute acetic acid, divide solution into three parts and test for following IN ORDER

  1. Ba2+: Part 1 + Excess K2CrO413 (aq) = Yellow precipitate
  2. Sr2+: Part 2 + (NH4)2SO4 (aq) = White precipitate
  3. Ca2+: Part 3 + (NH4)2C2O414 (aq) + NH4OH (only if nothing appears at first) = White precipitate
  4. Flame Test: Take salt and make a paste by mixing with concentrated HCl. Take paste on the tip of a glass rod/platinum wire, and put in Bunsen burner flame
    1. Ba2+: Green flame
    2. Sr2+: Crimson red flame
    3. Ca2+: Brick red flame

12 Dimethyl glyoxime reagent

13 Potassium chromate

14 Ammonium oxalate

Group VI

Mg2+ (Magnesium)

No Group Reagent

OS + NH4Cl + Excess NH4OH + Ammonium phosphate = White precipitate

Coloured Salts

ColourInference
BlueCupric salts
GreenHydrated nickel salts
Rose redCobalt salts, HgI2
Light greenFerrous salts
Yellow/brownFerric salts
Green/blueHydrated copper salts
Deep blueAnhydrous cobalt salts
Pale pinkManganese salts
Dark green/purpleChromic salts

Miscellaneous Notes

● With Br–, most labs only have NH4+.

● With Ca2+, most labs only have Cl–.

● Easier way to do flame test: Use test tube holder as tongs, and pick up a sizeable chunk of salt with it. Put one drop of concentrated HCl, and put in Bunsen burner flame.

● Do flame test first; if you’re lucky you’ll get your cation early. Most students spend up a lot of time doing cation tests.

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