total and free chlorine

[chem geek]

Phil,

You are absolutely correct and all over on this forum (and some others) you will find that sort of information repeated. Specifically, the following are chemical facts that are not currently disclosed in the industry, not even in NSPF CPO nor APSP TECH courses (please ask for this information to be disclosed -- I have, but nothing has changed):

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.

Also, all sources of chlorine result in added salt because as the chlorine gets consumed/used, it turns into chloride. For every 10 ppm FC added by any source of chlorine, this will result in 8 ppm salt. In the case of bleach, chlorinating liquid and lithium hypochlorite, one also gets an additional 8 ppm salt added upon addition so the net total for those sources of chlorine is 16 ppm salt.

Some simple calculations show how quickly these side effects can build up. Even assuming a low 1 ppm FC per day chlorine usage (most residential pools have 1.5-2 ppm FC per day unless they have a pool cover in which case it can be < 1 ppm per day), use of only Trichlor will result in the CYA increasing by over 100 ppm in 6 months if there is no water dilution. With Dichlor, it's over 150 ppm. With Cal-Hypo, the CH increases by over 120 ppm. For all sources of chlorine, the salt level would increase by almost 150 ppm and with bleach, chlorinating liquid and lithium hypochlorite the salt would increase by double that amount (i.e. almost 300 ppm).

One really needs to look at this on a percentage basis since normal CYA levels should be less than normal CH levels which are less than normal salt levels. If I assume normal levels for these of 50, 300, 1000 respectively, then the increases in 6 months at 1 ppm FC per day are over 200% for CYA from Trichlor, over 300% for CYA from Dichlor, over 40% for CH from Cal-Hypo, and nearly 15% for salt from these sources (Cal-Hypo is a little more) and nearly 30% for salt from chlorinating liquid, bleach and lithium hypochlorite. Relative to a salt pool at 3000 ppm, the percentage increase for the salt is 1/3rd as much. So over time, clearly some water dilution is needed no matter what, but the salt is the most innocuous and rises more slowly. The higher CH can lead to scaling, especially in plaster pools where the water would normally already be saturated with calcium carbonate (i.e. have a saturation index of zero). The higher CYA makes the chlorine less effective leading to algae growth unless one uses a supplemental algaecide or phosphate remover (at extra cost and not always completely effective).

Chlorine alone can prevent algae growth, but it is the FC/CYA ratio that is relevant and proportional to the active chlorine (hypochlorous acid) concentration, not FC alone. There is much more about this and related topics at the Pool School. For more technical info on the chlorine/CYA relationship and scientific studies, see this post.

Welcome to the forum!

Richard

[allblue pools]

HELLO, ALL BLUE HERE I HAVE READ THE READINGS TAKEN ON??
OH WELL YOU SAY THE READINGS ARE
TH=TOTAL HARDNESS100
TC=TOTAL CHLORINE 10
FC=FREE CHLORINE10
PH=6.2
CA BELOW 0
THE KEY I SEE HERE IS THE PH LEVEL WHEN THE PH IS EXTREMELY LOW AS IT IS THE REST OF THE EQUATION WILL BE SKEWED. ADD 15 LBS OF BORAX AND BRING THE PH UP, THIS IN TURN WILL CHANGE THE ALKALINITY READINGS, THEN GET YOUR ALKALINITY TO 180 PPM, I DONOT SEE AN ALKALINITY READING ALKALINITY IS A MAJOR KEY TO ALGAE GROWTH. THIS SUMMER I KEPT HAVING YELLOW ALGAE AND HAD TO GO BACK TO THE BOOKS AND REALIZED MY ALKALINTY READINGS WERE TO LOW. I USE AQUUA CHECK YELLOW CAP TESTORS IT HAS THE NEEDED BASICS
FREE CHLORINE
PH OR ACID READING
ALKALINITY
CYNERIC ACID
WHEN THE ACID LEVEL OR PH IS SO LOW CHLORINE BURNS REALLY FAST, BUT RIGHTN NOW THE READINGS ARE SKEWED/OFF WITH SUCH A LOW PH. SO OTHER CHEMICALS YOU HAVE ADDED??
WATER CLARIFIERS, AMMONIA BASED ALGACIDES, PHOSPHATE REMOVERS, MINERAL ADDITIVES TO ENHANCE THE WATER. THE BEST CHEMS ARE CHLORINE AND ACID. CYNERIC ACID IS A MAJOR ROLE "PPOL SERVICE MANS NEWS "IN THE WATER CHEMISTRY BOOKS HAS PUT TO REST THAT STABIZER LOCK IS NON EXISTENT AND A MYTH ALL MY POOLS RUN AT 150 HERE IN THE ARIZ SUN. THE REASON THE CHEM READING ARE SKEWED IS IF THE CHLORINE IS ACTUALLY 10 AND THE P-H 6.2 THE PLASTER OR LINER IS GOING TO BE IN GREAT TROUBLE. DID YOU SAY THIS WAS A PLASTER OR LINER POOL AND WHAT STATE??
HOPR THIS HELPS ALAN

[chem geek]

WHEN THE ACID LEVEL OR PH IS SO LOW CHLORINE BURNS REALLY FAST, BUT RIGHTN NOW THE READINGS ARE SKEWED/OFF WITH SUCH A LOW PH. SO OTHER CHEMICALS YOU HAVE ADDED??
WATER CLARIFIERS, AMMONIA BASED ALGACIDES, PHOSPHATE REMOVERS, MINERAL ADDITIVES TO ENHANCE THE WATER. THE BEST CHEMS ARE CHLORINE AND ACID. CYNERIC ACID IS A MAJOR ROLE "PPOL SERVICE MANS NEWS "IN THE WATER CHEMISTRY BOOKS HAS PUT TO REST THAT STABIZER LOCK IS NON EXISTENT AND A MYTH ALL MY POOLS RUN AT 150 HERE IN THE ARIZ SUN. THE REASON THE CHEM READING ARE SKEWED IS IF THE CHLORINE IS ACTUALLY 10 AND THE P-H 6.2 THE PLASTER OR LINER IS GOING TO BE IN GREAT TROUBLE. DID YOU SAY THIS WAS A PLASTER OR LINER POOL AND WHAT STATE??

Alan,

Please do not use all caps.

You are right that the pH was too low and others suggested raising it, but that has nothing to do with the algae growth. It is true that the rate of creation of chlorine gas will be about 100 times higher at a pH of 6.2 compared to 7.5, but there is still about 10 times as much gaseous hypochlorous acid at equilibrium than chlorine gas itself so the rate of chlorine loss isn't as great as would first appear. Also, in the water, at a pH of 6.2 there will be more hypochlorous acid than hypochlorite ion and the former takes longer to break down from sunlight than the latter. There are many Trichlor pools that are improperly managed and have their pH get too low, but they don't burn through chlorine because of the low pH though they do often develop algae from the high CYA without a corresponding high FC and the algae growth can consume a lot of chlorine (even nascent growth that is not yet visible).

Chlorine "lock" doesn't exist as is, but there IS a relationship between chlorine (specifically hypochlorous acid) and Cyanuric Acid (CYA) and has been known for decades. The equilibrium relationship was definitively determined in 1974 as described in this paper. You say that all of your pools in Arizona have 150 ppm CYA, but what FC reading do you keep in such pools? Are you adding chlorine weekly and if so what level do you raise the FC to and what level is the FC when you return the following week? Also, do you use any supplemental algaecides or phosphate removers in such pools?

There are thousands upon thousands of pool users (over 20,000 at The PoolForum, over 10,000 at Trouble Free Pool and hundreds if not thousands on this forum) who have demonstrated that indeed not having sufficient FC relative to CYA can result in algae growth and that algae can be prevented using chlorine alone by having the FC/CYA ratio at a certain minimum level. There is a pool service in Arizona, California, Nevada and Texas that maintains their pools with 100 ppm CYA and raises (in the hottest sunniest areas) the FC to 14 ppm where it drops to around 4 ppm when they return the following week. They use only chlorine with no need for supplemental algaecides. Is that similar to what you are doing?

Richard

[zetdah]

you may want to something about the PH as this can be a contributing factor.what temoerature is your pool and what chemicals are you using