Testing strength of liquid chlorine

[ktdave]

The stoichiometry of this is hurting my brain. I've got some 10% (originally) liquid chlorine in the garage that I'd like to know the "current" strength of. I've seen how to test this on here somewhere before, but I can't find it.

TIA

 

[JasonLion]

If you do a 9999 to 1 dilution with chlorine free water the resulting concentration in ppm will equal the original concentration in percent. There are various ways to do the 9999 to 1 dilution, none of them particuarly easy to get right without some lab equipment. One thing you can try is 10 ml of bleach into a 1 liter soda bottle and then dilute that again 10 ml to 1 liter. Most people have a test kit vial which can measure 10 ml and 1 liter soda bottles are not that difficult to come by.

 

[ktdave]

Thanks Jason!

 

[gonefishin]

I've only tried it three times before (at the same sitting with three different bleach suppliers) and found that my mixing method needs improvement. I took two samples from each bucket at a minimum of 10 minutes apart. I found that none of these second samples backed up the first measurement.

Because I couldn't repeat the measurement results I scraped all the measurements. I'll be trying again to get something that repeatable...and I'll be paying a bit more attention to my mixing methods.

Check out the post in this thread by CarlD.

let us know what you come up with.
thanks,
dan


oh...you can also get 1ml syringes from your pharmacy.

 

[ktdave]

I tried what Jason suggested (somewhat), but I halved the volumes. I used a 0.5 liter (2cup) pyrex measuring cup and one of those syringes from the Target pharmacy with 1 ml increments. I measured 0.5 liter water filtered with a PUR filter and added 5 ml 10% liquid chlorine. I mixed well, then collected a 10 ml sample for analysis with the TF100 using FAS/DPD titration. When the DPD powder was placed in the vial, no color change. Tried several times and just kept getting a clear, colorless sample.

May have to be much more accurate at measuring than I have capability of as Jason suggested with lab equipment.

 

[ktdave]

Ahhh! Just read the Carl D post from pool forum that gonefishin supplied. I should have used ten times the water, so 5 liters! Guess I'll have to try again.

 

[CRG_80cc]

ktdave

You’ll get better accuracy if you do two 1:100 serial dilutions of your sample (still gives 1:10,000 dilution).

If you can measure 100mls, do your dilutions this way:

Measure 100mls of water and place in a cup.
Using your 1ml syringe, remove 1 ml of water
Add 1ml of your 10% Bleach solution and mix well.
This is your first 1:100 dilution

To another cup, add 100mls of water.
Using your 1ml syringe, remove 1 ml of water
Add 1ml of your 1:100 dilution solution from the above step and mix well.
You now have your 1:10,000 dilution needed for your test

 

[gonefishin]

ktdave

You’ll get better accuracy if you do two 1:100 serial dilutions of your sample (still gives 1:10,000 dilution).

If you can measure 100mls, do your dilutions this way:

Measure 100mls of water and place in a cup.
Using your 1ml syringe, remove 1 ml of water
Add 1ml of your 10% Bleach solution and mix well.
This is your first 1:100 dilution

To another cup, add 100mls of water.
Using your 1ml syringe, remove 1 ml of water
Add 1ml of your 1:100 dilution solution from the above step and mix well.
You now have your 1:10,000 dilution needed for your test

Thanks CRG 80cc

I've tried this method a couple times now...and I've come up with nice repeatable results.

thanks!
dan

 

[gonefishin]

I bought another 5gallons of chlorine to test the rate of deterioration that I get during more moderate temperatures. When I picked the chlorine up they said the truck had just dropped off a new load and the percentage was near 18%. I plan to test once a week on Saturdays.



Sept 24 - 18% (stated from the water treatment store)
all other results were tested by myself using the two 1:100 serial dilution method described by CRG_80cc.
Sept 29 - 15%



dan

 

[CRG_80cc]

Thanks CRG 80cc

I've tried this method a couple times now...and I've come up with nice repeatable results.

thanks!
dan

Hi Dan

Your welcome. Just wanted to pass on how we would do the dilution in the lab when testing something. Alot less hassle than trying to measure 5 liters and better reproducability as you have seen.

 

[waste]

Oh man, another scientist Thankfully I just bought a bottle of aspirin and a 12 pack :lol:

I truly appreciate the info you guys share here, but it takes a little time and repetition for it to sink in to my head

 

[chem geek]

Sept 24 - 18% (stated from the water treatment store)
Sept 29 - 15%

This link shows the variability in half-life of chlorine as a function of concentration and temperature. I have something similar to this put into my spreadsheet. You show that the concentration is reduced by a factor of 15/18 = .83 over 5 days. This implies a half-life derived from 0.83 = e^-k*5 or k = -ln(0.83)/5 = 0.037 so t_1/2 = -ln(0.5/0.037) = 18.6 days, but this is an average from the 18% to 15% concentration.

Using my spreadsheet, that would imply around 95F which does not sound like moderate temperatures to me so the rate of loss you are seeing is higher than predicted. I would bet that the initial concentration you received was not really 18%, but was already lower, possibly 16% since that would imply a temperature of 83F and would also make things very sensitive to error in measurement.

Next time, I would measure the concentration when you first get the chlorine (perhaps twice, to ensure repeatability) and then measure again after 1 week, 2 weeks, 1 month. That should give you a better of idea of the true life of your chlorine source. If the loss is truly as fast as you are seeing, then the chlorine may be contaminated with heavy metal ions that catalyze the bleach decomposition reactions.

I performed a similar sort of experiment with my 12.5% chlorinating liquid from my pool store a while ago and it showed a slow decline consistent with what was expected. The lower 12.5% lasts longer than the higher concentrations you are getting and initially measured very close to 12.5% (usually it's slightly higher at around 12.8%), but I find that storing it in my pool shed with temperatures of roughly 60F at night to 85F in the day results in a drop over 1 month (30 days) down to around 11.5% which implies a temperature of 75F which is about right as a weighted average. This is also consistent with the expected FC rise when I add the chlorine to the pool on a regular basis.

Richard

 

[Ohm_Boy]

I think I'm confused... should that state that the concentration was reduced TO 15/18? That's reduced BY 3/18s, isn't it? I'm sure I'm missing something, but I may need a pointer.

 

[gonefishin]

Hi chemgeek. I do suspect that the starting point wasn't 18% when I started, which is why I noted that I didn't take a measurement. My plan was to get a measurement as soon as I got the chlorine home...but with three kids under 5 and work...it just didn't work out.

:?

dan

 

[chem geek]

I think I'm confused... should that state that the concentration was reduced TO 15/18? That's reduced BY 3/18s, isn't it? I'm sure I'm missing something, but I may need a pointer.

You are right. My terminology wasn't very good. It was reduced to 15/18ths of its original value.