Illustrated Guide to Home Chemistry Experiments

340 DIY Science: Illustrated Guide to Home Chemistry Experiments

SBSTITUTIU oNS ANd modIfICATIoNS

• Make the anion sample solution by adding a small
  amount (about 1/8 teaspoon) of each of the following
  chemicals to a small beaker that contains about
  20 mL of distilled water: potassium nitrate, sodium
  sulfite, sodium carbonate, sodium chloride, potassium
  bromide, potassium iodide, sodium sulfate, sodium
  phosphate. Swirl until all of the solids dissolve. If
  necessary, heat the solution gently or add a bit more
  distilled water.


• You may substitute salts with the same anions; for
  example, ammonium nitrate for potassium nitrate or
  copper sulfate for sodium sulfate.

CUTIOA nS

Nitric acid, sulfuric acid, and ammonium hydroxide are

corrosive. Potassium permanganate is a strong oxidizer.

Silver nitrate is an oxidizer, a corrosive, and stains skin,

clothing, and other organic materials. (Stains can removed

with a solution of sodium thiosulfate.) Barium hydroxide

and barium nitrate are toxic, and barium nitrate is a strong

oxidizer. Read the MSDS for each chemical before you use

it. Wear splash goggles, gloves, and protective clothing.

z

but not the bromide or iodide salt. A 15 M ammonia solution

dissolves silver bromide, but not silver iodide. (Actually, the

halides are not dissolved, but instead form soluble complexes

with the ammonium ions.)

Sulfate (So 4 2–)
Adding barium nitrate to an acidic solution that contains
sulfate ions causes white barium sulfate to precipitate.

phosphate (po 4 3–)
Phosphate ions in solution with nitric acid are precipitated
by the addition of ammonium molybdate, which reacts with
the phosphate ions to form the insoluble complex salt
(NH 4 ) 3 PO 4 · 12MoO 3.

There are dozens of other common anions, each of which
has a corresponding test. For example, thiocyanate (SCN–)
behaves as a pseudo-halide anion, with characteristic reactions
similar to the chloride, bromide, and iodide ions. The presence
of thiocyanate ions can be confirmed by adding Fe3+ (ferric)
ions, which combine with thiocyanate ions to form the blood-
red ferrithiocyanate [Fe(SCN)(H 2 O) 5 ]2+ complex. Similarly,
the presence of the hexacyanoferrate(III) ([Fe(CN) 6 ]3−) anion
(usually called ferricyanide) can be confirmed by adding a
solution that contains Fe2+ (ferrous) ions, which combine with
ferricyanide ions to form a characteristic dark-blue precipitate
of the pigment Prussian Blue.

1. Transfer about 5 mL of the sample to a clean test tube.


2. Acidify the sample by adding two or three drops of 3 M
   sulfuric acid and swirling or stirring to mix the solution.


3. Add about 2.0 g of anhydrous iron(II) sulfate (or the
   equivalent mass of hydrated ferrous sulfate) to the test
   tube and swirl until the solid dissolves. If necessary, heat
   the solution gently.


4. Hold or clamp the test tube at a 45° angle and slowly add
   about 2 mL of concentrated sulfuric acid, allowing it to
   run down the inside of the tube. Do not swirl or stir the
   liquid. The goal is to allow the concentrated sulfuric acid
   to form a separate layer at the bottom of the tube.

After a few minutes, examine the phase interface between the

sulfuric acid and aqueous layers carefully. If nitrate ion is present

in the sample, a hazy brown to brownish-purple ring appears at

the interface.

PI: RTA TfoSTINGE R NITRATE IoNS

PII: RTA TSTINGfoE R oTHER ANIoNS

1. If you have not already done so, put on your splash
   goggles, gloves, and protective clothing.


2. Fill a large clean test tube about one quarter full of the
   sample solution.


3. Add an equal volume of 6 M nitric acid.


4. Quickly and carefully sniff to detect the odor of sulfur
   dioxide. Confirm the presence of sulfur dioxide with a
   drop of 0.02 M potassium permanganate on the tip of
   a stirring rod. (Don’t add it to the test tube; just expose
   the drop on the tip of the stirring rod to the gas inside
   the test tube.) Sulfur dioxide reacts with potassium
   permanganate, turning the violet solution colorless and
   indicating that sulfite ions are present in the sample.


5. Confirm the presence of carbon dioxide with a drop
   of saturated barium hydroxide on the tip of a stirring
   rod. Carbon dioxide reacts with barium hydroxide to
   form insoluble barium carbonate, which turns the clear

This lab has two parts. In Part I, we’ll test a portion of the
anion sample solution for the presence of nitrate ions.
In Part II, we’ll test a second portion of the anion sample
solution for the presence of sulfite, carbonate, chloride,
bromide, iodide, sulfate, and phosphate ions. If you have
not already done so, put on your splash goggles, gloves, and
protective clothing.

PROCEDURE