10% vs. 12.5% chlorine... price vs. quantity & waste

[bbrock]

Hi TFPers! Happy 4th weekend.

I usually purchase 12.5% from my local mom/pop pool store. Price is fair. I use the calculation to figure out the price per oz. of NaClO. I calculate it by hand and it is quick, but recently stumbled upon this website from someone's TFP thread if anyone cares to bookmark it: Chlorine Price Per Ounce Calculator


With a sale on 10% chlorine (Kem Tek) for $4.99 for 2 gals. at my local OSH, and even when not on sale, the 10% is cheaper per oz. of NaClO vs. the 12.5% pool store stuff. The sale at OSH makes the Kem Tek stuff $0.20/oz of NaClO. Normally at Lowes I can get it for $5.86/2 gal. of the 10% Kem Tek which is $0.23/oz of NaClO. The reg. price at my local pool store for the 12.5% is $17.98/4 gal. which is $0.28/oz NaClO. What I am getting at the pool store is HASA.

The problem I am having is that even though the Kem Tek is a great deal right now on sale, I will have to still buy more of it to produce the same desired effects. Thus, I will be buying it more, which means more trips, more gas, and more waste/recycling on the environment vs. the pool store stuff which is a closer drive for me, and the bottles are re-used vs. having to be recycled. Therefore, wouldn't the 12.5% be a better deal? Even if just looking specifically at the volume I will need to equal the 12.5% over just one summer period, wouldn't the 12.5% be the best choice?

Tx.

 

[BuckeyeChris]

if the 12.5 is fresh (like, was made within the week) and you are going to use it quickly, it would seem to be a better option. The problem is you can't buy a whole bunch and store it to reduce trips, as the higher the FC%, the faster it loses potency. So your # of trips calculation might end up a wash out. But otherwise, mileage and plastic waste in favor of the 12.5.

 

[bbrock]

Called my pool store right now. The guy told me the HASA is about 1 wk old. This definitely beats the best/freshest Kem Tek I can find at either OSH or Lowes.

 

[bbrock]

....

 

[BuckeyeChris]

Breakdown of bleach over time by storage temperature

Chem geek is saying the effect is not as drastic as I thought, provided it's stored in a cool place. See post #23.

 

[laprjns]

I think you should do the math first then make your decision. Assume that your pool chlorine demand is 2 to 3 ppm per day. Then using Pool Math, determine the amount of chlorine that you will need to add each day for each strength. Then calculate that out over a month ( weeks or 28 day) and you should end up with something like this.

	10.00%	12.50%
demand at 3 ppm per day	73 oz	58 oz
Jugs / month (4wks)	16	12.7
cost per month	$46.79	57.03
	10.00%	12.50%
demand at 2 ppm per day	49 oz	39 oz
Jugs / month	10.7	8.5
cost per month	$31.41	38.35

So you will need between 2 and 4 jugs more a month if you use the 10%, but you will be saving $7 to $10.

I found that i can buy a months demand of chlorine with not having to worry about strength degradation by following these three rules.
1) Buy it fresh, no more than two weeks old from a store that stores the product indoors.
2) Store it in a dark, cool place. In this case in my basement
3) use it within a month of purchase, so it will be used up within 6 weeks of it's born on date

 

[bbrock]

BuckeyeChris & laprjns, tx for the great replies. Laprjns, that analysis really helped.

Looking at the chlorine degradation link u posted Buckeyechris, especially post 23, I am shocked to learn how fast it degrades. I don't know how to take that info then. If I can buy a case, which is 4 gallons, of the 12.5% and let's say I use 39 oz per day. The case will last approx 13-14 days. If the 12.5% is stored in my garage and let's average that in the summer the temperature is 90°, in 2 weeks it degrades to 9.87%. Does this mean then that my chlorine additions should take the degradation into consideration? Or, should I add and then retest after I add, and then test again? If that is the case, I will be burning through my FAS-DPD powder just like that. The degradation article/post is great, it's just that I would not know how to incorporate that info into my chlorine additions.

 

[bbrock]

Laprjns, even though it looks like according to your analysis I would be saving some using the 10% chlorine vs. the 12.5%, the 12.5% HASA I get at the pool store is fresher. The freshest I've seen the 10% Kem-Tek at Lowes or OSH based on decoding the barcode is still usually about 1-2 mo. old. Therefore, this would have to be calculated into the analysis.

Additionally, I just get the feeling that the opaque bottles of the 12.5% vs. the transparent bottles of the 10% would hold up better to UV exposure & increased temperatures (or, at least UV). This may not matter that much as I don't store my chlorine where it is exposed to sunlight.

 

[chem geek]

Don't forget that nighttime temperatures are cooler so it won't be 90ºF for 24 hours. Even so, because the degradation rate varies non-linearly with the temperature, it will degrade faster than the average day/night temperature would indicate. If we assume for simplicity a 50/50 split of 12 hours at "Day" temp and 12 hours at "Night" temp, then we can calculate the effective average temperature from a rate perspective.

R2 = R1 * 2^{(DayTemp - NightTemp)/10}
AvgRate = (R1 + R2)/2 = R1 * (1 + 2^{(DayTemp - NightTemp)/10}) / 2
The question is what effective average temperature gives us this rate?
AvgRate = R1 * 2^{(EffAvgTemp - NightTemp)/10}
R1 * 2^{(EffAvgTemp - NightTemp)/10} = R1 * (1 + 2^{(DayTemp - NightTemp)/10}) / 2
2^{(EffAvgTemp - NightTemp)/10} = (1 + 2^{(DayTemp - NightTemp)/10}) / 2
(EffAvgTemp - NightTemp)/10 = (ln(1 + 2^{(DayTemp - NightTemp)/10}) - ln(2))/ln(2)
(EffAvgTemp - NightTemp)/10 = (ln(1 + 2^{(DayTemp - NightTemp)/10}) / ln(2) - 1
EffAvgTemp = 10 * ( (ln(1 + 2^{(DayTemp - NightTemp)/10}) / ln(2) - 1 ) + NightTemp
EffAvgTemp = 10 * ln(1 + 2^{(DayTemp - NightTemp)/10}) / ln(2) - 10 + NightTemp

So you add the following to the night temperature for the Day minus Night temperature difference indicated and then use that effective temperature in the table.

TempDiff . EffAvgTempAdder
..... 5 ............ 2.7
... 10 ............ 5.8
... 15 ............ 9.4
... 20 .......... 13.2
... 25 .......... 17.3
... 30 .......... 21.7
... 35 .......... 26.2
... 40 .......... 30.9

So in your case if the nighttime temperature were 70ºF, then the effective average temperature would be 70 + 13.2 or around 83ºF so you could use a rate in between 80ºF and 85ºF in the chlorine breakdown table.

 

[bbrock]

Tx Chem Geek. Always impressed at the extent of your knowledge.