Trichlor vs Dichlor

itabb

Well-known member
Jun 22, 2007
71
Atlanta, GA
#1
I went to home depot today, to see if they carried Dichlor. Leslie's carries dichlor at $36 for 8lbs. Home Depot does not carry Dichlor. Instead, you can buy trichlor tabs, 3", 1", and trichlor "shock" granuals which includes fun additives such as Boron (borax?) and some clarifier. The 1" 5lb pack of trichlor is $22.50.

Since cyanuric acid clearly isn't doing it for me in my pool, I have decided to add CYA through stabilized chlorine. I have yet to see someone cause themselves problems by adding CYA, but I see people getting into trouble all the time with stabilized chlorine. So stabilized chlorine it is. So do I use trichlor or dichlor. I had always thought of using dichlor because it puts in .9ppm CYA for each 1ppm FC. However, if I play with the spreadsheet, I find I actually get more CYA for 80oz of trichlor than I do for 80oz of dichlor, and a heck of a lot more FC. So for $22.50, I should get 40FC and 22CYA from trichlor. And with 64oz of CYA for $20, I should get 32ppm CYA, but I actually got 10ppm CYA. It seems to me that trichlor is the best deal I can find (though pure CYA would be better if I could find a reliable source).

Now, Home Depot does sell CYA at 80oz for $18, I have just learned. It is certainly not Leslies, but the packaging does not even attempt to guess at how much ppm CYA I would get for any amount added. All it says is to add some if you are below 40ppm CYA and retest. I have this funny feeling that the purity of 100% cyanuric acid has a lot of variability. However, attached to chlorine in the form of dichlor or trichlor, it seems quite reliably potent. I'll know for sure as I start using my trichlor tabs.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
#2
Trichlor contains more chlorine by weight and less CYA by weight than "pure" (anhydrous) Dichlor. But the chlorine makes the Trichlor weigh more (per molecule) so the net effect is that in 10,000 gallons 1 pound of Trichlor gives 11.0 ppm FC and 6.7 ppm CYA while 1 pound of Dichlor gives 6.6 ppm FC and 6.0 ppm CYA. HOWEVER, this is for Dichlor that is DIHYDRATE. Part of the inefficiency of dichlor is that you are buying water with it (in "dry" form as it is a hydrated crystal with the Dichlor). If you were able to find ANHYDROUS Dichlor, then one pound would give 7.7 ppm FC and 7.0 ppm CYA. So with anhydrous Dichlor, you get a little more CYA per pound than with Trichlor. But anhydrous Dichlor is far less common -- most is the dihydrate form.

It is unfortunate that you ran into inconsistent CYA concentration. Most people have reported reasonable correlations of amounts with expected CYA level. But at low CYA levels the +/- 10 ppm CYA (or worse) accuracy of the test really makes a big difference percentage-wise.

While you are comparing CYA choices, you should also take a look at Natural Chemistry's Instant Pool Water Conditioner found here. It is the monosodium salt of CYA in a slurry so it dissolves quickly. They claim that one gallon adds 40 ppm CYA to 10,000 gallons so that implies 53 ounces weight of CYA in a gallon. I found it for $30.30 here. You would need 43.3 ounces weight of Trichlor for this same amount of CYA so that's ($22.50/(5*16))*43.3 = $12.18 so this "instant" CYA product is more expensive (and is only CYA with no chlorine).

Generally, Trichlor is the least expensive source of chlorine and is also the least expensive source of CYA except for pure CYA which you found out may not be pure. It is very acidic so you also have to factor in the amount of base you will need to add to compensate (or alkalinity if you just aerate to raise pH). One pound of Trichlor, after the chlorine gets used up, will require 53 ounces weight of Borax or 27 ounces weight of soda ash / washing soda / pH Up or 11 ounces weight of caustic soda / lye.

It takes 8.9 ounces weight of CYA to be equivalent to one pound of Trichlor (for CYA level) and that requires 11 ounces weight of Borax or 6 onces weight of soda ash / washing soda / pH Up or 2.3 ounces weight of caustic soda lye.

The Instant Pool Water Conditioner will take a very small amount of acid to restore pH.