Bleach is a substance that can whiten or decolourise other
substances or fabrics. Commercial bleach solutions are obtained by pumping
chlorine gas through cold, dilute, aqueous sodium hydroxide to form sodium
hypochlorite (NaOCl). Hypochlorite is the bleaching agent in bleach. Bleaches
can act as either oxidising agents or reducing agents.
Cl2 + 2OH- à OCl- + Cl- +
The OCl- ion oxidises chromophores in coloured
materials and is reduced to chloride and hydroxide ions.
OCl- + H2O + 2e- à Cl- + 2OH-
It acts by oxidising the “stain” molecules to less coloured
forms of the molecules. In the reaction chlorine is reduced from an oxidation
state of (+1) in OCl- to (-1) in Cl-.
The sodium hypochlorite (NaOCl) dissociates in water as
Na+(aq) + OCl-(aq)
In this experiment, the concentration of NaOCl in commercial
bleach would be determined by titration. (“A titration is a technique where a
solution of known concentration is used to determine the concentration of an
unknown solution.”) http://chemed.chem.purdue.edu/genchem/lab/techniques/titration/what.html
The concentration of sodium hypochlorite in a name-brand
bleach and a generic bleach would be compared. To determine the concentration
of sodium hypochlorite, the following method will be carried out.
Adding an excess of potassium iodide (KI) to an acidified
sample of diluted bleach will cause the following reaction:
OCl-(aq) + 2H+(aq)
+ 2I-(aq) à
I2(aq) + Cl-(aq) + H2O(l)
The produced product, I2(aq), will be yellow when
dilute and red-brown when concentrated. The amount of I2(aq) formed
by this reaction will then be measured and titrated with a standardised
solution of sodium thiosulfate, Na2S2O3:
2S2O32-(aq) + I2(aq)
From the volume of standardised Na2S2O3
solution used, the concentration of NaOCl (sodium hypochlorite) in the sample
can be calculated.
In this experiment, the concentration of the sodium
thiosulfate needs to be known accurately, because it will be used to
stoichiometrically determine the concentration of hypochlorite ion in our bleach,
however, it forms a hydrate with 5 waters of hydration and this means that it
cannot be used as a primary standard. (A primary standard is a material that
can be quantitatively used to determine the concentration of other substances
and a substance must possess certain characteristics to be regarded as a
primary standard such as being stable when solid and when in solution, must be
available in high state of purity etc.) https://www.thoughtco.com/definition-of-primary-standard-and-examples-605556
.The primary standard being used in this experiment is potassium iodate (KIO3).
It can be used to standardise the sodium thiosulfate in the following
Firstly, a known amount of KIO3 is reacted with
an excess of KI in and acidified solution. The reaction taking place is
represented as follows:
IO3-(aq) + 5I-(aq)
+ 6H+(aq) à
3I2(aq) + 3H2O(aq)
This reaction produces a known amount of iodine in solution.
The reason iodine was not just weighed and dissolved is because it is not
stable enough and sublimes at a fairly low temperature. This known iodine
concentration will be titrated against the unknown thiosulfate concentration in
the following reaction:
2S2O32-(aq) + I2(aq)