testing bleach strength

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iam4iam

iam4iam

LifeTime Supporter
May 5, 2012
257
Johnson City, TN
I wondered if 6% bleach equated to 6%=0.06=60,000 PPM of FC in straight bleach. Pool Calculator and a little math seemed to confirm this, so I decided to test to see if my bleach had lost some potency. (I bought 10 cases of 3x182 oz from Sam's Club over a month ago and the cases were sitting in the sun for a few days before I realized the stupidity of that.) I didn't suspect the bleach might be bad until today. Most of the time when I add bleach I don't bother re-checking FC level after adding, but just wait until it's probably time to add more based on typical FC loss and theoretical FC gain from the addition of bleach. I did a pre- and post-bleach addition FC check tonight (took average of 3 samples each time since I was testing what should have been an increase of 2.2 ppm according to Pool Calc, and I know from experience each test precision is only about +/- 0.5 PPM. Increase only came out to be 1.7 (Remember the precision is because I was averaging.) Anyway, there are too many variables there to conclude anything, so I decided to actually test the bleach. Here's what I did:

Combined 1 ml of 6% bleach with 6 liters of chlorine free water, which should be 10 PPM FC according to Pool Calculator's confirmation of the first sentence in the previous paragraph. Since starting to write this post, I have confirmed that my test worked, since I got 11 PPM (10% error is okay with me in this case!). But I still have the question which led me to create this post. After creating the theoretical FC = 10 solution (and stirring), I tried measuring by DPD titration, and the water "blinked" pink and went back to clear when adding the titrating powder. So I decided to give the K-1000 Cl tester a try. 5 drops of the reagent made the water orange (very close to the pH=7.5 color on the pH test). I assumed this meant a very high FC concentration. About an hour later (after beginning this post which was originally intended to ask if my test should theoretically work, and if so, what I might be doing wrong, but it turns out all I did wrong was not waiting long enough to test it. Apparently the chemical reaction H2O + NaClO --> ... (Richard could finish this if I've even got it started correctly) had not finished. So whatever form of Cl the tests read (HClO?--I think I remember that being the "working" form of Cl from previous reading in the Deep End) hadn't formed yet.

I'm sure others have conducted a similar bleach test to the one described above, but it was really neat to see the theory work out experimentally! (By the way, in case you're wondering, the water in the mixture was filtered, basically chlorine-free. Aquasana rocks!)

Now that all is said and done, I might be sorry I asked, but why did the K-1000 test turn orange and why did the DPD test "blink" pink?
 
It only takes a very short time for bleach to mix into water. You do need to do some mixing, but no waiting is required. If you dilute bleach 10,000 to 1 with chlorine free water the resulting sample will have the same number of ppm of chlorine that the original had in % by weight. Likewise, your 1 ml of 6% in 6 liters give 10 ppm math is correct. Just make sure you are doing the dilution with chlorine free water! It is also rather difficult to do this dilution with even close to 10% precision.

The OTO test (drops turn the sample yellow compared to a color chart) will turn orange when FC is around 10 to 30. Light orange at the low end of that range and a darker burnt orange at the high end of that range.

The DPD test (drops turn the sample shades of red compared to a color chart) will go red then clear when FC is above about 10. The indicator bleaches out at high FC levels.

The FAS-DPD test (powder turns the sample pink, then drops to clear) will blink pink and then go clear if the FC level is very very high, typically 30+. Adding more DPD powder until the sample turns pink will allow you to complete the test (up to an FC level of about 50 or 60, above that it is hopeless).
 
Thanks. Based on your reply, then, I guess even though I mixed thoroughly, for some reason, the chlorine had not yet dispersed evenly, since what happened with initial OTO test and FAS-DPD test both indicated FC of 30+, while an hour later FAS-DPD indicated FC = 11 PPM (only 10% error)! OTO test was still yellow, a little darker than the 5 PPM color, but I wouldn't call it orange, on final test at 11 PPM.
 
Most likely you have a factor of two or three error in the dilution, which resulted in a sample with too high an FC level. The FC level then fell rapidly while the sample was sitting around, it will do that in small containers, and just happened to come out "correct" after an hour.
 
Thanks for the quick responses, even though this rat trail is hardly urgent. Still, I remain perplexed. I really think my dilution was rather accurate--definitely not more than 20% error. I was using a 5 ml syringe for the 1 ml of bleach and a 3 liter container which I filled twice for the 6 liters of water. It would take a minimum of 100-200% error to be off by what you are describing. (I would have had to have mistaken 2-3 ml for for 1 ml!) Besides, why would FC deplete from 30+ to 11 in an hour just because the container is (relatively) small? It wasn't under a sunlamp. :? The water was about 1/3 of the way up a 5 gal. bucket, so the ratio of volume to surface was probably very similar to that of most above ground pools.
 
The surface area to volume ratio is higher in a smaller container and you didn't have any Cyanuric Acid in the water lowering the active chlorine level so hypochlorous acid (and some chlorine gas) can outgas much more rapidly.

A 16'x32'x4.5' pool has a surface to volume ratio of 16*32/(16*32*4.5) = 0.22 feet-1 while a 5 gallon bucket with 11" diameter and 6 liters of water so 366 cubic inches so 366/(pi*(11/2)2) = 3.85" water height has a surface to volume ratio of 0.26 inch-1 or 3.1 feet-1 which is 3.1/0.22 or 14 times higher. An FC of 30 ppm added via bleach to buffered water (assuming TA of 80 ppm and initial 7.7 pH in the tap water) has the pH at around 8.3 so that the active chlorine is roughly the same as 8.8 ppm FC at pH 7.5 (with no CYA) compared to a pool with an FC around 10% of the CYA level for an active chlorine level the same as with 0.1 ppm FC and no CYA. So that's a factor of 88 difference.

So between the surface area to volume ratio difference and the hypochlorous acid concentration difference, there's a factor of 1232 so if the outgassing rate were linearly related to these factors, then it's a much more rapid loss where 1 hour in the bucket would be equivalent to around 51 days in a pool (not exposed to sunlight and with similar air circulation, so say an indoor pool).

Another factor is that the bucket may not have been perfectly clean in which case the chlorine would react with any nitrogenous organics in it (or possibly with the bucket material itself, though if a typical plastic than probably not). In fact, if you didn't thoroughly clean the bucket and then thoroughly rinse it, then contamination is the most likely cause for the chlorine depletion. To eliminate the outgassing loss, you'd have to use a container with minimal headspace (air above the liquid) and cover it airtight.
 
Thanks, Richard. I was hoping you would chime in eventually!

As stated in my previous reply, I am quite certain that my error in creating the initial solution was not so large as to create an actual 30 ppm FC, when the theoretical (intended) concentration was 10 PPM. What would FC = 10 in the bucket make the ratio of HClO concentration of the bucket vs. typical pool? (I can handle the math, but not the chemistry, like the pH factor, for example. Also, I didn't think CYA would be relevant, since there's no sunlight, but I guess CYA also effects outgassing rate since only unattached HClO can gas out, right?)

Speaking of the math, the surface to volume ratios you calculated are actually relatively close. I realize that feet-1 and in-1 are not the same algebraically, but in2/in3=ft2/ft3 since it's a ratio of area to volume. ft2/ft2 and in2/in2 cancel out algebraically, but you lose the concept of the units of the ratio. In other words "square in. per cubic in." is the same conceptually as "square ft. per cubic ft." It's the ratio of area of one face of a cube to the cube's volume, which is equal regardless of the size of the cube.

I think I'm going to conduct another trial and compare FC right after mixing to FC after 1 hour (and then maybe 2 hours just to get more data). How long should I wait to be confident that FC concentration will be relatively equal throughout the water in the bucket?
 
As Jason notes, the surface area to volume ratio is NOT unitless so when comparing that ratio in two situations you have to use the same units for comparison.

As for 10 ppm FC diluted in water similar to as I had described the filtered water before (remember I assumed it was buffered water, not distilled), the pH goes to around 8.0 and has a roughly 5.0 ppm FC equivalent instead of the 8.8 that I used so the factor is 50 instead of 88. So when combined with the surface area to volume ratio effect, it's still a factor of 700 which is substantial.

The high active chlorine level outgassing faster is particularly noticeable in whirlpool baths that have chlorine without CYA and that have hot water and aeration. Chlorine levels can drop rapidly in hours even with no bather load.

I think in your situation IF there was really a rapid drop within an hour, it was more likely due to organic contamination in the container. You can, of course, repeat the experiments now that you know some of the factors that affect chlorine loss.

One more possible factor is that if you used filtered water from your tap water and if your tap water is chloraminated, then the resulting water will have ammonia in it. Nevertheless, that would account for only 2-3 ppm FC loss if the chloramination (original monochloramine) amount was the typical 1-1.5 ppm. You can check your municipal water districts water quality report.
 

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A public admission of having a mental block (pun intended) which was causing me to be stuck inside the unit cube box. It was dark in there, and now see the light. Thank you the enlightenment.

While this explains why chlorine loss could be much more rapid in a small container, I'm still puzzled by the fact that my initial reading (after thorough stirring, but no waiting) was 3 times higher than the theoretical concentration, when I know the mixture was not off by that scale factor. I'm going to end up running another trial, and this time I'll take more measurements to watch the rate of FC loss. I think I'll wait until I get the new bottle of R-0871 and use just 3 liters of water for a theoretical concentration of 20 PPM. Another alteration I'll make is that I'll add the 1 ml of bleach to half of the water and then add the other half of the water, which might help disperse the chlorine. (It has just occurred to me when I added the 1 ml of bleach to the top of the 6 liters of water, even rigorous stirring might not have sufficiently mixed the chlorine. Is this a possible explanation for what is still puzzling me?)
 
iam4iam said:
I'm still puzzled by the fact that my initial reading ... was 3 times higher than the theoretical concentration
Click to expand...
That is puzzling. I do know that making large mistakes on dilutions like that one is much much more common than you seem to think, but still a 3 times error is more than even I would expect.
 
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