Cyanuric Acid, PH and TA Levels

Problems relating to pH and total alkalinity.
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Womble

Cyanuric Acid, PH and TA Levels

Postby Womble » Thu 31 Jan, 2008 17:37

Hi, I have had my pool now for 4 weeks and have managed to maintain it fairly well. Two days ago it poured down overnight and the PH level went from 7.7 to 8.4 and the TA from 115 to 140 I had also put 4L of chlorine in the pool as per the testers recommendations.

I have been told a few things by the place I buy chemicals and get my water tested that I want to run past people here. My pool is 24000L. three days ago as stated above I added 4L of Chlorine and 2 days later (day-before-yesterday) the Free Chlorine was only 0.84 my CYA is 32.

1. I was told that the CYA floats to the bottom of the pool when the water is stagnent and that I need to run my pool filter during the day to ensure that the CYA comes to the top of the pool and screens the chlorine from the suns harmful rays. Is this correct? I have been running my filter in the evenings/overnight, I usually have it off during the day.

2. I was told that adding chlorine to the pool raises the TA level, does it also raise the PH Level? If so does it make sense to add hdyrocloric acid and chlorine at the same time?

I am hypothising that the TA increased because I added 4L of Chlorine and the PH increased because it rained and the water was airated.

I added 1L of Hydrocloric Acid to the pool this morning and will test the water again this afternoon.

My readings 3 days ago were

CYA 32
Free Cl 0.06
pH 7.7
TA 110

Then I added 4L of Chlorine and it rained for about 3 hours quit heavily.

My readings yesterday morning were:

CYA 32
Free Cl 0.84
TDS 900 -- what is this?
pH 8.3
TA 140
CH 110
Langelier SI 0.5 -- what is this?

It also says Langelier SI Test Factor 18.0.

I have had no problems with the water clarity, the pool has consistently been crystal clear except for one day 2 weeks ago when my chlorine levels were 0.06 the water went a bit cloudy, according to the test results everything was fine just no chlorine ... so I added chlorine (4L) and the water went crystal clear again.

Thoughts/advice?


chem geek
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Postby chem geek » Fri 01 Feb, 2008 15:50

If I start with the numbers you gave and add 4 liters of 12.5% chlorinating liquid (you didn't say the strength of the chlorine), then that adds 20.8 ppm Free Chlorine (FC) and raises the pH from 7.7 to 8.5 and the TA from 115 to 130 which is roughly what you were seeing.

If you are using a DPD test for chlorine, the one where you measure the intensity of pink/red color, then this will bleach out above around 10 ppm FC so you may think you don't have any chlorine when you really do. If you used an OTO test, where you measure the intensity of yellow color, or the best test which is the FAS-DPD test where you add powder to turn pink/red and then count drops to get clear, then you would know if you really had chlorine in the pool.

When the chlorine levels is high, above 10 ppm FC especially, then one of the two indicators in the pH can bleach out making you think the pH is higher than it really is, though in this case a high pH is expected soon after you've added the chlorine. The pH should drop back down when the chlorine is used up.

If your chlorine measurement is accurate, then the large drop in such a short period of time probably means you've got algae trying to bloom or a lot of ammonia or organics that the chlorine is trying to oxidize. If this is the case, then more chlorine is needed and generally you want to maintain an FC level of 40% of the CYA level when shocking, adding more chlorine as needed until the overnight drop in FC is minimal (< 1 ppm FC drop). It will obviously drop during the day due to sunlight (UV breaks down chlorine, though CYA helps lessen this effect).

The part about CYA settling is not true. Once mixed in the water, it stays mixed. You can, of course, run your circulation pump during the day to see if that changes anything, but I doubt that it will. The main reason to run circulation during the day is that it is best to circulate the water when it is being used so that the chlorine is well mixed as it will get used up locally from sweat, etc. Of course, with vigorous splashing and swimming, the water near the surface should get mixed.

Adding a hypochlorite source of chlorine (chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite) will raise both the pH and TA and then when the chlorine gets used up either from breaking down from sunlight or from oxidizing ammonia and organics, the pH and TA will drop back down to where they started. Other factors can have the pH rise with no change in TA independent of this, in particular outgassing of carbon dioxide which happens more at higher TA and lower pH levels as seen in this chart and also happens more with aeration such as from waterfalls, spillovers, fountains, and SWG and ozonator systems (the former adds hydrogen gas bubbles, the latter adds air along with ozone). If your pH is already on the high side, then add acid to lower the pH first and then add the chlorine (never at the same time in concentrated form -- wait for the acid to mix and always pour chemicals slowly over a return flow at the deep end), but if the pH is near 7.5 then you don't need to lower the pH for normal chlorine addition though may lower it for shocking (i.e. for large amounts of chlorine addition).

The aeration of water from the raindrops will tend to make the pH rise with no change in TA.

TDS is total dissolved solids and is mostly salt plus calcium and bicarbonate. Don't worry about it.

The Langelier saturation index says how saturated your water is with calcium carbonate which you want if you have a plaster pool or exposed tile with grout. Once your pH is back in line, this number will be closer to 0. You might need some more calcium hardness at some point, but only if you get your total alkalinity lowered, but one thing at a time -- first deal with the chlorine demand (after making sure you aren't just bleaching out a DPD chlorine test).

It doesn't take very long for low chlorine levels to have algae start to grow. After a day it will start consuming chlorine and turn the water dull and after two or three days it turns cloudy and then eventually green if you don't add chlorine. Once it gets started, it takes a constant level of FC to kill it, usually adding chlorine at least twice a day if not more, especially early on. Once it's gone, then maintaining a minimum FC level no lower than 7.5% of the CYA level (usually targeting around 11.5% of the CYA level for manual dosing) will keep it away.

Richard
Womble

Back to the pool man

Postby Womble » Sat 02 Feb, 2008 02:31

Wow, thanks for the great response Richard, really appreciate it.

Ok to test your DPB theory I went back to the pool guy this morning (they do free tests, drop chemicals into a row of test tubes and then stick each test tube in a hole in a "colour reader" which then updates the computer and spits out the results), so I went back this morning and confirmed that they use the DPD Test.

So I had them test everything again, the chlorine was 0.07, so to determine if my chlorine levels were above 10ppm and the DPD test was "bleaching out" I came home and added 2.5L of Chlorine, (12.5 - 15% Sodium Hypochlorite Solution is on the bottle) I poured this infront of the filter return jet and waited 1.5 hours (left the pump running) back to the pool shop and they tested my Free Chlorine at 8.0.

Now if I am correct this means that the Free Chlorine Test they are using is working correctly and my chlorine levels are not bleaching out this test.

So where should I keep my FC levels and for how long should I keep them there (assuming I have algae trying to bloom) and what is my indicator for lowering my levels to 11.5% of my CYA.

And what does 11.5% of CYA mean?

Thanks heaps.
chem geek
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Postby chem geek » Sat 02 Feb, 2008 03:55

With your CYA at 32 ppm (assuming that's getting measured correctly), then raise the FC to 12 ppm (about 40% of the CYA) and keep it there until three things occur: 1) the water is crystal clear, 2) there is minimal overnight loss in FC (< 1 ppm FC loss), 3) you measure minimal Combined Chlorine (CC <= 0.5 ppm). Your water will be free of algae when these three criteria are met, but it will take your keeping the FC high until that occurs. It will take more chlorine initially as it will be killing more algae at first, but should take less and less until the criteria are met.

The 11.5% of CYA refers to the desired FC level which at 32 ppm CYA means 0.115 * 32 = 3.7 ppm FC as your target after your done getting rid of the algae. Don't ever let the FC drop below 7.5% of the CYA, or 0.075 * 32 = 2.4 ppm FC so add chlorine to get to whatever you need so that it doesn't every drop below 2.4. If you find you are losing too much chlorine to sunlight, you can raise your CYA level a bit, perhaps to 50 or 60 ppm but probably not above that.

Richard
Womble

FAS-DPD

Postby Womble » Sun 03 Feb, 2008 20:02

OK,

2/Feb 2pm added approximately 2.5L of 12.5-15% Sodium Hypochlorite in front of return jet

2/Feb 2:30pm took water sample for testing 8.1ppm of Free Chlorine

3/Feb 12pm took water sample for testing 8.0ppm

4/Feb 10am took water sample for testing 0.3ppm

Yesterday (3/Feb) my CYA was 46 today it was 70 - I think their CYA test is faulty ... although I did ad some CYA about 4 days ago to try and lift the levels

Water is crystal clear, pool looks great but it is chewing up the chlorine.

I have been unable to find anyone in New Zealand who sells a FAS-DPD Test kit, they seem to be as rare as hens teeth, I have found test kits that will do FC and TC but they are done with colour matching and dropping some tablets in the water, no one seems to have the titration type tests here. I will need to order one in from the US (Amazon sell them). Unless I am able to buy the chemicals used from some other location? Are the chemicals used in the FAS-DPD test common chemicals that I could buy at some other sort of place?

In summary I have been unable to measure the CC level.

Cheers.

-Rowan
chem geek
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Postby chem geek » Sun 03 Feb, 2008 21:34

Rowan,

Sorry to hear that you can't get a FAS-DPD chlorine test easily. You can use a combination of OTO and DPD tests -- the OTO as a quick check since it won't bleach out with high chlorine levels and the DPD as a check for normal chlorine levels up to around 5 ppm. Just skip the Combined Chlorine (CC) checking for now.

As long as you pool is eating up chlorine, you need to add more. Even when it's clear, all the algae is not dead. At some point, the chlorine usage will decline so keep track of that overnight usage. Keep the FC level up and all will be fine soon, probably within a day, maybe two.

If you are going to be ordering from the U.S., you can get a Taylor K-2006 at a good price here or you can get a TF100 test kit from tftestkits(dot)com here and E-mail them to see if they'll ship and how much it will cost. The TF100 has 36% more volume of reagents so the price per number of tests is similar to the K-2006 test kit link. Both kits are excellent (the TF100 uses Taylor reagents, but is packaged with more intelligent quantities of them).

Richard
Womble

TF100 Kit

Postby Womble » Sun 03 Feb, 2008 22:11

Thanks Richard. I have ordered the TF100 kit, it looks like the best test kit I have seen so far, I found the Taylors and Aquachek kits (Aquachek also did Peroxymonosulphate) but the TF100 seems like the best I have come across!

I have been reading and learning a lot about pool chemistry and have been reading about non-chlorine shock treatment to breakdown my CC levels and any other organic waste.

Could I add say 1kg of "Ultra Shock" which is a product sold in NZ, made in Australia that contains 49.5% of Potassium Peroxymonosulphate - don't know what the remaining 50.5% of the product is because it dosn't say! The label suggests using 250gm for my pool (25000L).

In the meantime I will keep my FC levels up until the test kit arrives so I can see what is happening.

Thanks.
Womble

Does not Kill Algae

Postby Womble » Sun 03 Feb, 2008 23:30

Ok, looks like I have answered my question I found this on another website.

POTASSIUM PEROXYMONOSULFATE

The active ingredient and chemical name of a non- chlorine shock treatment or non-chlorine oxidizer. Does not kill bacteria or algae but it will oxidize or destroy ammonia, nitrogen and swimmer waste. It has a low pH, and it does not increase chlorine or bromine levels the way that super chlorination does, so water may be entered in 15 minutes after addition. It will also reactivate bromine to its killing form, hypobromous acid.
chem geek
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Postby chem geek » Mon 04 Feb, 2008 00:33

For an outdoor pool exposed to sunlight, you shouldn't need to use any non-chlorine shock nor even need to shock using chlorine. If you maintain the proper chlorine level, then the pool will be continually shocked in terms of oxidizing most organics and sunlight will help break some down as well. You can use non-chlorine shock if you want to -- it won't hurt -- but it's not necessary unless the bather load in the pool is high and you notice that the chlorine isn't able to keep up with oxidizing organics (for example, that you measure Combined Chlorine frequently).

The 49.5% Potassium Monopersulfate (MPS) is normal since MPS is a triple salt with Potassium Sulfate and Potassium Bisulfate. In fact, most MPS in the U.S. (from Dupont Oxone and branded by a variety of manufacturers/distributors) is only 43% pure since there is usually 3% Potassium Peroxydifulate and 2% Magnesium Carbonate. So the formulation you have is more pure and probably less irritating (the peroxydisulfate can be irritating).

Richard

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