[CFMatt]

Hi all, I have been doing a chemistry project for school and I chose to do an analysis of how the concentration of bleach is affected over time as a result of exposure to air.

My hypothesis was that over time the concentration would decrease~ (due to the distinct smell given off bleach I assumed the evaporation of it over time would cause the OCl- to decrease in concentration)

I did 6 sets of titrations over 2 months and the average titres decreased from 32ml~ to 19ml~
n(OCl)=0.001398mol
v(OCl)=avg 32ml on first titration and avg 19ml on last titration
c=n/v - so initially the concentration was 0.001398mol/0.032=0.0437molL-1
and finished at 0.001398/0.019=0.0736molL-1

This increase in the concentration of the OCl suprised me and I was wondering if I have made a bad mistake somewhere along the way or if there are some explanations that can help me understand why what has happened, has happened .

note: The bleach was left with the lid off in a container in a dark space under a bench in a classroom, I did not measure or record the conditions of the classroom over the time so I am unsure of whether the temperature or humidity played a large role in this or not.

Thanks in advance, Matt

[discodermolide]

All you are doing is making the solution more concentrated by evaporation of the water.

[Arkcon]

What was your titration?  Did you do a red-ox titration against the oxidizing power of hypochlorite?  What reaction did you use?  Or did you do a pH titration against base?  If so, then discodermolide: is dead on -- hypochlorite may lose chlorine over time, but NaOH remains.  Can you salvage this experiment buy paying attention to the total volume at each time point to compensate for water evaporation?  That is, it might be getting stronger because you're losing water faster -- but each sample is more concentrated.

[AWK]

I did 6 sets of titrations over 2 months and the average titres decreased from 32ml~ to 19ml~
n(OCl)=0.001398mol
v(OCl)=avg 32ml on first titration and avg 19ml on last titration
c=n/v - so initially the concentration was 0.001398mol/0.032=0.0437molL-1
and finished at 0.001398/0.019=0.0736molL-1

Wrong calculations. You should use equation c₁V₁=c2V2
hence c1=c2V₂/V₁

You calculate a concentration of titrant of volume 32 ... to 19 ml needed for titration of constant amount of bleach.

[CFMatt]

I titrated against na2s2o3. I had 5ml of 10% KI and 5ml of dil. h2so4 with my bleach solution, I titrated untill it went from brown to a pale yellow then added starch solution which turned it a dark blue/black, I finished the titration when the solution became colourless.
So with c1v1=c2v2
so initially: 0.001398molL-1*0.03235L=0.0000452253
so 0.0000452253/0.01927L= 0.00235molL-1 = my final concentration of bleach. which is an increase? should I just be talking about the gradual evaporation of water which will increase the concentration of OCl- in the solution?

also - what is that type of titration I did called? o.o

[AWK]

Please, show clearly:
volumes of bleach, volumes and concentration of thiosulfate used.
Write down reaction and balance it.

[CFMatt]

20ml of bleach w/ 5ml dil. h2so4 and 5ml 10% ki
n(s2o3)=0.002802mol
c(s2o3)=0.144molL-1
v(s2o3)=
1. 0.03235L
2. 0.03237L
3. 0.0273L
4. 0.0233L
5. 0.0219L
6. 0.0193L

OCl- +2H+ + 2S2O32- → Cl- + H2O + S4O62-

[CFMatt]

C(ocl)           ?                             C(s2o3) 0.144molL-1
V(ocl)         0.02L                       V(s2o3) 0.03235L               
N(ocl)       0.0014mol               N(s2o3) 0.02802mol

so if my volume of my ocl is constant but the volume of my s2o3 is changing how do I work out the concentration of the ocl as it has changed?

[CFMatt]

           v(s2o3)   c(s2o3)n(s2o3)   n(ocl)                  v(ocl)   c(ocl)
titration 1   0.03235   0.144   0.0046584    0.0023292           0.02   0.11646
titration 2   0.03237   0.144   0.00466128    0.00233064   0.02   0.116532
titration 3   0.02733   0.144   0.00393552    0.00196776   0.02   0.098388
titration 4   0.023283   0.144   0.003352752 0.001676376   0.02   0.0838188
titration 5   0.0219   0.144   0.0031536    0.0015768           0.02   0.07884
titration 6   0.01927   0.144   0.00277488    0.00138744   0.02   0.069372

is this right...? so then the concentration of ocl is actually decreasing >.<

[AWK]

OK.
Lower volume of titrant means lower concentration of bleach.
Bleach is slowly decomposed by carbon dioxide from air.

[Babcock_Hall]

Bleach decomposition follows second order kinetics.  http://www.nrcresearchpress.com/doi/pdf/10.1139/v56-068  Heavy metals are said to catalyze the decomposition.