Hun Ji Choi jc53885
TA=Kyle(Section#51230)
Experiment #8
In this experiment, the redox reactions were used to quatitatively determine the oxidizing
capacity of the household bleach. The titration of bleach with thiosulfate was conducted n this
experiment, while the data collected from the procudeures were used to determine the percent mass
of the unkown bleach. Also, balancing and combining two redox reactions were practiced to
determine the overall reaction occurred during the titration.
There are two oxidizing agent salts in the common household chlorine bleaches, NaOCl
[sodium salt] and Ca(OCl)2 [calcium salt]. To determine oxidiizng capacity of bleach, two
oxidation-reduction reactions were performed. When KI(aq) was added to the bleach sample, the
iodide ions are oxidized to iodine (one election loss). In the equation 2, the thiosulfate ions were
added to the iodine solution, reducing iodin back to iodide ions (one election gain).The overall
stoichiometry of this experiment was HOCl+2S2O3(-2)+H resulting in Cl- + S4O6(-2) + H2O(l).
From this equation, it can be stated that the two moles of thiosulfate ion react with every mole of
hyopchlorite. Initially, excess of iodide ions (KI) were added to the bleach solution. After the
acidification of the solution by adding sulfuric acid, the iodide ions are oxidized to iodine. When the
color of the solution turned yellow, starch was added as an indicator. The solution was dark-blueblack colored when the indicator was added. It is then titrated with sodium thiosulfate untill the
solution becomes clear or reached the endpoint. Just before the starch was added, I3- form of iodine
predominated because I3- results in yellow colored solution. I2- form of iodine predominated the
solutin after the endpoint had reached.
There were two steps during the titration experiment that needed special attention: addition
of starch and addition of sulfuric acid. It was imperative to add starch when the solution expressed
yellow color. If it was added prior to the color change of the solution, the formation of black-blue
complex could not be easily reversed. This makes the solution diffucult to and detect reach the
endpoint. The reaction happens very fast in the acid solution, therefore after the addtion of sulfuric
acid, titration must begin immediately or the production of I2 will pass its solubility limits
producting iodine precipitates. One of the potential problem associated with the iodine formation
during titration was clumping of iondines. When this occurs, the titration must be discontinued
because clumped iodines are difficult to re-dissolve in the solution. When the solid iodine forms in
the solution, it will lower the volume of sodium thiolsulfate needed to reach completion of the
titration, influencing the percent mass and oxidizing capacity of the bleach sample.
Oxidizing capacity is the ability of a substance to oxidize measured by redox potentials. In
this experiemnt, it is calculated by using the percent mass of the soidum hypochlorite in the
unkown bleach sample. The strong acid hypochlorite ion was converted to hypochlorous acid due to
Ji Hun Choi jc53885
TA=Kyle(Section#51230)
Experiment #8
the oxidation process or iodine acting as reducing agent. By measuring the volume of the titrant
needed to complete the titration, the moles of NaClO could be calculated by using matrix
conversions. Moreover, the calculated moles could also be converted to grams of NaClO. The
percentage by mass of NaClO could then be easily found. Since percentage mass is equal to the
oxidizing capacity of the bleach, no more calculations are needed.
The oxidizing capacity of the unkonwn bleach 4 was found out to be 2.6% with the standard
deviation of .14.
There are several possible errors that may have affected the experimental result. The
accuracy of analytic balance was low, due to high chances of contamination or interruptions. Also
sources of human error could have influenced the data. For instance, measuring increments, titration
skills, following procedures etc. The main weakness that this experiment had was figuring out how
yellow the solution was before the addition of starch. Since observing yellow solution was rather
qualitative data, it was difficult for individuals to figure out when to add stach indicator.