Activity of Bleach Lab

Bleach Lab
Activity of Bleach Lab
Name: ___________________________________________
Page 1 of 4
The traditional commercial bleaches such as ChloroxTM and PurexTM are solutions of sodium
hypochlorite, NaClO, in water. The hypochlorite ion, ClO-, is a fairly strong oxidizing agent. The bleach
removes stains by their chemical oxidation. In this experiment we will perform a quantitative analysis on
commercial bleach samples to determine the amount of the active ingredient hypochlorite ion they contain.
To do this, we will dissolve weighed samples, add an excess of a reagent, I-, which reacts with the
hypochlorite ions to form I3- with a standard solution of thiosulfate. The chemical reactions involved are
called oxidation-reduction reactions. A whole series of quantitative analyses are based upon the quantitative
reduction of a strong oxidizing agent by I- followed by titration of I3- with thiosulfate, S2O32-. Quantitative
methods of this particular type are called iodometric determinations.
The chemical reaction between hypochlorite and iodide ion can be written
ClO- + 3I- + 2H+ ----> Cl- + H2O + I3and the reaction taking place during the titration can be written as
I3- + 2S2O32- ----> 3I- + S4O62Thus two moles of thiosulfate ion are consumed by each mole of hypochlorite ion. (The product ion S4O62-,
is called dithionate.) One takes a known quantity of bleach solution and finds the number of moles of
thiosulfate it indirectly consumes through measuring the volume of a standard solution of thiosulfate
required to titrate the I3- is formed. Then the concentration of hypochlorite in the bleach can be calculated.
It is possible to prepare sodium thiosulfate solutions of accurately known concentration by weighing
out the anhydrous salt, but the titer of the thiosulfate solutions tends to change slowly with time making it
advisable to standardize the solution at the time it is used. Thus you will standardize your thiosulfate
solution using potassium iodate as the primary standard.
IO3- + 8I- + 6H+ ----> 3I3- + 3H2O
The triiodide (I3-) produced is then titrated with thiosulfate using the reaction above to determine the
concentration of the thiosulfate solution.
You will prepare a 0.1 M Na2S2O3 solution and standardize it against an accurately known amount of
KIO3, and then will be analyzed. The results for all the analyses for a given lab room and day (two sections)
will be collected and provided to you. You will then perform some statistical analysis of the results and
decide which of the two products provides the more bleach per buck.
Properties of Sodium Thiosulfate Solution
Sodium thiosulfate solutions properly prepared are quite stable, but there are two potential sources of
instability. First, in acid solutions, thiosulfate slowly disproportionates to sulfite and elemental sulfur to the
S2O32- + H+ ----> HSO3- + S
Since the deionized water coming out your tap is slightly acidic (as you will find out in the pH experiment)
because of dissolved carbon dioxide, this is potentially a problem. To make the thiosulfate solution to be as
stable as possible, one should boil the deionized water used to make the solution, thus driving out dissolved
CO2, and also add a small amount of sodium carbonate to insure the solution is slightly basic. The
concentration of your thiosulfate solution can change not only from the disproportionation reaction, but also
because some bacteria actually eat sodium thiosulfate! Sterilization of the reagents and glassware can
prevent this problem. Fortunately, because we’ll do all of our thiosulfate titrations in a short time we don’t
have to go to the trouble of insuring a very stable solution. During the preceding lab period you should dry
about 1 to 1.5 grams of KIO3 at 120˚ C for 1-2 hours.
1 The reaction produces more I3- by 6I- + O2 + 4H+u2I3- + 2H2O
Bleach Lab
Name: ___________________________________________
Page 2 of 4
Weigh out approximately 7.9 g of Na2S2O3 on a watch glass or weighing paper using one of the
rough balances in the main lab. Transfer into your clean 500 mL clear glass bottle and add approximately
500 mL of deionized water. Then insert the glass stopper and shake very thoroughly to obtain a solution of
uniform concentration.
The solution should be standardized on the day you titrate the bleach samples. Calculate before
coming to class the weight of KIO3 which will require 20 mL of 0.1 M Na2S2O3 solution to titrate the I3- that
it produces. This will turn out to be less than 0.1g of KIO3. Therefore in order to improve your precision,
weigh accurately a KIO3 sample that is within 15% of four times the weight for a single titration as
calculated above into your 100 mL volumetric flask. Dissolve and make up to the mark. Don’t forget to mix
thoroughly. You will titrate a 25.00 mL aliquot of this KIO3 solution withdrawn using your 25 mL
volumetric pipet. You need to have at hand 1g of KI for each sample, about 10 mL of dilute (1:4) sulfuric
acid, and about 5 mL of starch indicator solution.
Make sure that your buret is clean and drains without forming droplets on the walls. Rinse it three
times with small portions of your thiosulfate solution and then fill to a point on scale near the top. Read the
initial volume to 0.01 mL.
Put a 25.00 mL aliquot from your KIO3 solution into one of your 125 mL Erlenmeyer flasks and add
1g of KI to one of your KIO3 samples and swirl to dissolve. Then add 2-3 mL of the dilute sulfuric acid.
The solution should immediately become dark red from the formation of I3-. Because I- (which is in excess)
is readily air-oxidized1 in strongly acidic solution, titrate immediately with the thiosulfate. You may proceed
with the addition of titrant very rapidly until the solution becomes a pale
yellow. When the solution is a very pale yellow, add 1-2 mL of the starch indicator solution. The solution
will immediately turn blue-black. Titrate to the disappearance of the blue color (probably about 1/2 mL
more of titrant will be required). Repeat with the other sample. The calculated concentrations of the
thiosulfate solution should agree within a few parts per thousand. We suggest that you verify that the ratio of
the volume thiosulfate to weight of KIO3 agree within a few parts per thousand before continuing. If not,
weigh out more KIO3, and do more standardization titrations. For the report, put a note in the
standardization section and the results on the back page of the report
Bleach is sold by volume so that in order to calculate the cost per mole of ClO- we need to measure
the sample by volume. Obtain about 30 mL of one of the two bleach solutions in the lab in a small beaker.
(Be sure to write down the cost/bottle and the volume of the bottle!) Immediately, using your 10 mL
volumetric pipet, pipet 10.00 mL of the bleach into a clean, but not necessarily dry 250 mL volumetric flask
to mark with deionized water, put in the stopper, and mix thoroughly by inverting the flask at least 10 times.
Using your 25 mL volumetric pipet, transfer 25.00 mL of the bleach solution to a 125 mL Erlenmeyer
flask. Fill your buret with thiosulfate solution and read the buret. Add 5 mL of glacial acetic acid
(WARNING: Glacial acetic acid burns the skin! If you get any on you wash it off with copious
amounts of tap water.), mix and then add 1g of potassium iodide. The solution will again immediately turn
dark reddish brown indicating the formation of I3-. Rapidly titrate (to minimize air oxidation) the solution as
in the thiosulfate titration above. Repeat. If the same amount of thiosulfate is required for both titrations
within 0.05 mL, go through the same procedure with the other bleach sample; otherwise repeat the titration
again until satisfactory reproducibility is achieved.
1 The reaction produces more I3- by 6I- + O2 + 4H+u2I3- + 2H2O
Bleach Lab
Name: ___________________________________________
Page 3 of 4
From the standardization titrations calculate the concentration of thiosulfate in moles/1 for each
standardization run. If your runs agree within a few parts per thousand use the average of the runs for the
thiosulfate concentration in the rest of the calculations. Otherwise you will have to make a decision about
which runs to keep and which to discard. Then calculate the number of moles of ClO- indirectly titrated for
each run and from that result the concentration of ClO- in the original bleach solution taking the dilutions
into account. Report your results on the computer in the lab your values obtained for the ClO- concentration
for each run on each bleach solution. There will be a sheet for each bleach solution. If you know that there
is good reason to believe that something went seriously wrong for a run, indicate that by putting an asterisk
on the run. You will be given the results of your section and another section on the same samples for the
statistical analysis asked for in the lab report.
Bleach Lab Procedure (Short Version)
Teacher Notes
Note: I need a little more time with the students for this lab: Start at 6:15: Sorry kiddos
Have them only draw a picture of the procedure (And some calcs) for their prelab)
Standardization of the Thiosulfate
Weigh out about 7.9 g of Na2S2O3 and dissolve in 500 mL of d.i. water. Shake thoroughly to get a
solution of uniform concentration
Calculate the weight of KIO3 which will require 20mL of 0.1 M Na2S2O3. [0.0713g]
Weigh out 4 times the mass calculated in 2 above A: Place this in a 100 mL volumetric and dissolve
Set up your burette rinsing it thoroughly with your thiosulfate solution.
Put a 25.00 mL aliquot from the KIO3 solution into a 125 mL erlynmeyer flask: add 1 g of KI, 2-3
mL of dilute sulfuric acid
Titrate immediately with the thiosulfate solution:
When the solution is pale yellow add 1-2 mL of starch indicator solution. It will then turn blueblack. Titrate until the blue goes away (Probably 1/2 mL more)
Repeat this and do it again.
Obtain 30 mL of the bleach and place this in a clean dry beaker
Use a 10mL volumetric pipet and place in a clean 250 mL volumetric flask. This is to dilute the
bleach so that it can be tested more easily.
Transfer 25 mL of the dilute bleach into a 125 mL Erlenmeyer flask
Add 5 mL of glacial acetic acid danger
Add 1 g of KI. The solution will turn reddish brown.
Rapidly titrate with the thiosulfate
Repeat until satisfactory reproducibility is achieved.
Bleach Lab Expectations
Formal Lab write-up
Procedure should have a series of pictures
Data and Calculations
1. Data in neat data tables:
2. Show the following calculations
 Amount of KIO3 required
1 The reaction produces more I3- by 6I- + O2 + 4H+u2I3- + 2H2O
Bleach Lab
Name: ___________________________________________
 Concentration of S2O32 Concentration of ClO- (Don't forget to multiply by the dilution factor)
Page 4 of 4
Error Discussion
List and discuss sources of error in a cause effect table
Discussion Questions:
1. Why was it necessary to dilute the bleach in order to titrate it?
2. Explain why it was necessary to use the thiosulfate ion to determine the concentration of the hypochlorite
ion. (Remember that the thiosulfate ion is not a part of the main reaction that involves the hypochlorite
Include the final answer which is both your answer and the class answer.
1 The reaction produces more I3- by 6I- + O2 + 4H+u2I3- + 2H2O