Qualitative tests on anions: Cl, Br, I, SO42-, PO43-, CO32-, NO3-

Determination of Common Anions

Introduction. To establish the presence of a compound in an unknown mixture, you need to run a reaction with clear visual effect. The best kind of effect is the change of color of the solution or formation of the colored precipitate. The formation of colorless (white) precipitate is a little more difficult to detect. The formation of gas (some bulbs inside the solution) can be a possible option as a visual effect.
In this laboratory you wll learn how to make simple but reliable tests on several anions which are among the most common in our environment.

Qualitative tests on anions: Cl, Br, I, SO₄^2-, PO₄^3-, CO₃^2-, NO₃^-.
Preliminary notes: no only one of Cl, Br, I is to be expected in an unknown. Otherwise the problem is too complex for CHE112.

Preliminary tests.
1. To the 5-10 drops of the unknown, add some HNO₃ + AgNO₃.
Formation of the precipitate indicates the presence of at least one of halogenide anions (Cl, Br, I).
Ag⁺+ Cl^- ® AgCl or AgNO₃ + NaCl ® AgCl + NaNO₃
Ag⁺+ Br^- ® AgBr or AgNO₃ + KBr ® AgBr + KNO₃
Ag⁺+ I^- ® AgI or AgNO₃ + NaI ® AgI + NaNO₃
AgCl is white, AgBr is slightly yellowish, and AgI is light yellow:

Cl Br I

2. To the 5-10 drops of the unknown, add some BaCl₂. The formation of the white precipitate indicates the presence of at least one of SO₄^2-, PO₄^3-, CO₃^2-.
Ba²⁺ + SO₄^2- ® BaSO₄
Ba²⁺ + PO₄^3- ® Ba₃(PO₄)₂
Ba²⁺ + CO₃^2- ® BaCO₃
Determination of individual ions:
Br and I: To the 5-10 drops of the unknown, add some aqueous Cl₂ , some organic solvent (toluene, chloroform, etc) and shake. Br₂ give yellow to orange color while I₂ is purple. An excess of aqueous Cl₂will cause I₂ oxidation to colorless iodate ion thus masking iodine.
2 I^- + Cl₂ ® I₂ + 2 Cl^-
2 Br^- + Cl₂ ® Br₂ + 2 Cl^-

I₂ Br₂

Cl In this case (CHE112) Cl^- is detected by the preliminary test 1 only.
Ag⁺+ Cl^- ® AgCl

CO₃^2- . To the 5-10 drops of the unknown, add some acid (HCl, for example). Formation of the gaseous CO₂ is a good indication of carbonate.
CO₃^2- + 2 H⁺ ® CO₂ ( ) + H₂O

SO₄^2- . To the 5-10 drops of the unknown, add 1-2 mL of HCl solution and some BaCl₂. Formation of the white precipitate indicates the presence of SO₄^2-.
Ba²⁺ + SO₄^2- ® BaSO₄
Phosphate and carbonate do not react in the presence of HCl.

PO₄^3-: To the 3-5drops of the unknown, add some nitric acid and ammonium molibdate solution. Warm the solution gently. Formation of the yellow precipitate indicates the presence of PO₄^3-.
PO₄^3-+12 MoO₄^2- + 24H⁺ + 3NH₄⁺ ® (NH₄)₃[PMo₁₂O₄₀] + 12H₂O

(NH₄)₃[PMo₁₂O₄₀]

NO₃^-: To a one(!) drop of unknown, slowly add some diphenylamine solution in concentrated sulfuric acid (caution!). A deep blue color at the spot of contact of these two solutions indicates nitrate.

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OH^- in solution can be indicated simply by pH sensitive indicator paper strip.