If Dichlor is 56% chlorine

Rangeball

Well-known member
May 25, 2007
785
Does that mean it's 44% stabilizer?

So 1# of dichlor would be 8.96 oz of chlorine and 7.04 oz of CYA?

My CYA test only goes as low as 30 ppm. I can still see the black dot. I'm thinking of bumping my CYA by 10 ppm, putting me either at 30 or a bit over, so at least I can monitor it more accurately.

If my above info is correct, and bleach calc says I need 1# 7oz for a 10ppm CYA increase, I'd need right at 3# of dichlor, right?
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
It doesn't work out that way for several reasons, but mostly it's because Dichlor has TWO chlorine atoms attached to the ring so it's one part CYA and two parts chlorine. Trichlor has THREE chlorine atoms attached to it so it's one part CYA and 3 parts chlorine. But the "ppm" values don't work out exactly because the ppm for chlorine is for the weight of chlorine gas which has TWO chlorine atoms. Another factor is that the most common form of Dichlor has two water molecules attached to it (dihydrate).

Dichlor•H2O is 255.97766 g/mole
CYA is 129.075 g/mole
Chlorine gas is 70.906 g/mole
Trichlor is 232.4103 g/mole
Dichlor anhydrous is 219.9471 g/mole

So, Dichlor (dihydrate) is 129.075/255.97766 = 50.4% CYA and 2*70.906/255.97766 = 55.4% "available" chlorine.
Trichlor is 129.075/232.4103 = 55.5% CYA and 3*70.906/232.4103 = 91.5% "available" chlorine.
Dichlor anhydrous is 129.075/219.9471 = 58.7% CYA and 2*70.906/219.9471 = 64.5% "available" chlorine

129.075 / (2*70.906) = 0.91
129.075 / (3*70.906) = 0.61

Anyway, the bottom line is that for every 1 ppm FC added by Dichlor, you get 0.9 ppm CYA while for every 1 ppm FC added by Trichlor you get 0.6 ppm CYA.

Richard
 

Rangeball

Well-known member
May 25, 2007
785
Thanks. I knew you had addressed this previously, but darned if I could find it :)

So, since the answers probably there but I'm mathematically challenged, is there anyway to say how many ounces of CYA is in 1# of dichlor?

If .9 ppm CYA for every 1 ppm FC, needed 10 ppm of CYA would mean a bit over 10 ppm FC, which for my pool is real close to the 3# of dichlor I mentioned in my first post.
 

JasonLion

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May 7, 2007
37,880
Silver Spring, MD
I calculate that you need just about twice as many pounds of dichlor to get a given amount of CYA compared to adding CYA directly. So 8 oz of CYA in 1 lb of dichlor. Of course that is adding quite a bit of chlorine at the same time. Also, if I am doing the math right the CYA is about three times as acidic as the dichlor for equal amounts of CYA added.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
To increase the CYA level by 10 ppm in 10,000 gallons, you need 13.35 ounces weight of Cyanuric Acid. This amount will drop the pH from 7.50 to 7.33 if the TA level is initially 100 ppm and the initial CYA level is 30 ppm.

It takes 26.48 ounces weight of Dichlor (dihydrate) to raise the CYA level by the same amount (so yes, the "twice as much Dichlor by weight as CYA" rule is great), but the chlorine level also increases by 11.0 ppm. The initial addition of Dichlor will lower the pH from 7.50 to 7.46, but the usage of the chlorine will continue to lower the pH so the net result is that the pH goes from 7.50 to 7.18 so Dichlor is quite a bit more acidic than CYA.

Richard
 

Rangeball

Well-known member
May 25, 2007
785
That actually wouldn't be too bad a deal for me. Lot's of evaporation lately, so I had to add higher alk fill water. PH has crept back up to 7.5ish.

Dichlor in this instance seems to be just the ticket :) I can add the 3 bags over a week or so in place of my normal bleach.