360 DIY Science: Illustrated Guide to Home Chemistry Experiments
LABORATORY 2 0.2:
qUANTITATIvE ANALySIS of CHLoRINE BLEACH By REdox TITRATIoN
Chlorine laundry bleach is an aqueous solution
of sodium hypochlorite (NaOCl). Chlorine
bleach is produced by running an electric
current through a solution of sodium chloride.
The electric current oxidizes Cl– ions to Cl^02
gas and reduces H+ ions to H^02 gas, increasing
the concentration of OH– ions. In effect, this
electrolysis can be thought of as producing
an aqueous solution of sodium hydroxide and
chlorine, which immediately react to form
sodium hypochlorite.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance and weighing papers£ Erlenmeyer flask, 125 mL£ volumetric flask, 100 mL£ graduated cylinder, 10 mL£ graduated cylinder, 100 mL£ pipette, mohr or serological, 2.00 mL£ burette, 50 mL£ dropper or Beral pipette£ funnel£ ring stand£ burette clamp£ chlorine bleach (100 mL)£ potassium iodide (~2 g per titration)£ acetic acid, glacial (~5 mL per titration)£ sodium thiosulfate, 0.1 m (~60 mL per titration)£ starch indicator (see Substitutions and
modifications)In solution, sodium hypochlorite dissociates into sodium ions
and hypochlorite (OCl–) ions. The bleaching action of chlorine
bleach is entirely due to the presence of hypochlorite ion, which
is a strong oxidizer. The hypochlorite ion reacts with the colored
unsaturated organic compounds that make up stains, oxidizing
them to colorless saturated organic compounds.
Inexpensive no-name chlorine bleaches usually contain nothing
but 5.25% sodium hypochlorite by mass. (That is, 100.00 g of
bleach solution contains 5.25 g of sodium hypochlorite and 94.75 g
of water.) Name-brand bleaches are more expensive and often
contain scents and brighteners. Name-brand bleaches labeled
“extra strength,” “ultra,” or something similar contain slightly
higher concentrations of sodium hypochlorite, typically about
6%. Because the hypochlorite ion is the only active ingredient in
any chlorine bleach, the sodium hypochlorite content determines
bleaching effectiveness.
In this lab session, we’ll use redox titration to quantitatively
determine hypochlorite in a chlorine bleach sample. We use a
common quantitative analysis procedure called back titration,
in which the concentration of an unknown sample is determined
by reacting it with an excess of one reagent to produce a product.
That product is then back titrated with a second reagent of known
concentration until the endpoint is reached. Back titration is used
for many reasons, including for samples that are insoluble in water
or that contain impurities that interfere with forward titration. But
back titration is most commonly used when the endpoint is more
apparent by titrating back than by titrating forward.
Rather than attempt to determine hypochlorite concentration by
forward titration, we’ll first react the hypochlorite sample in an
acetic acid solution that contains excess iodide ions, which are
colorless in aqueous solution. The hypochlorite ions oxidize the
iodide ions, forming elemental iodine, which is brown in aqueous
solution. We’ll then back-titrate with a standardized solution of
sodium thiosulfate (Na 2 S 2 O 3 ), which reduces elemental iodine to
iodide ions. Here are the balanced redox equations for these two
reactions:oCl–(aq) + 2 CH 3 CooH^0 + 2 I–(aq)
→ Cl–(aq) + 2 CH 3 Coo– + I^02 (aq) + H 2 o^0 (l)2 S 2 o 3 2–(aq) + I^02 (aq) → 2 I–(aq) + S 4 o 6 2–(aq)As we add sodium thiosulfate titrant to the solution of iodine,
the brown color of aqueous iodine becomes fainter and fainter.
Because there is no abrupt color change, it is very difficult to
judge the endpoint of this titration directly. Fortunately, even at