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Thread: Rain = PH dropping

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    frogabog's Avatar
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    Rain = PH dropping

    I finally got enough rain to do a ph and alkalinity test on it. I know... we say it rains all the time here but the volume is actually low and most days we get about one or two tenths of an inch, even if it rains all day. Yesterday it was closer to a whole inch. Woot! Got me a whole bowl full of the stuff.

    PH for our rain is off the charts low. Bright freakin yellow. Like OTO chlorine bright yellow.

    Alkalinity is also low. One drop turned it red.

    PH in the pool before this rain yesterday was 7.7. After, a very clear 7.5.

    Fill water ph is 8.0, alkalinity is 40.

    Now I know why my ph kept dropping while all you's out there complain that it rises over time. The rain hadn't let up yet.


    Dangit! 8yo just had to get out of the pool because it's started raining again.
    Where kids swim in 54 degree water, turn blue, and giggle happily cuz they got a POOL!
    Year 3 BBB -15' x 48" Intex Metal Frame - Was using (2) 1000gph Intex cartridge filters (see Full time pumping Intex). 2012, converted to 1600gph and sand filter+SWG = Sand filter love affair!
    Don't waste time and energy looking for a better value on test kits, the TF100 is the best deal around. I did the looking and spent the extra money, but you don't have to make the same mistake. Just go here: TFTestkits. I use Pool Calculator for min/max, and shocking chlorine levels.

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    chiefwej's Avatar
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    Re: Rain = PH dropping

    It's called acid rain
    chiefwej
    Tucson, AZ
    16x36 rectangular (19k) Pebble Tec play pool/spa, Pentair Intelliflo VS 011018, Super II 2hp (spa), Aqua Rite T-15 SWG, Pro Grid 60 DE, Hayward H400 & Heliocol Solar heating, A&A infloor system, fill water w/high CH and TA, 50 ppm borates,TF-100 test kit

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    frogabog's Avatar
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    Re: Rain = PH dropping

    I guess I'll have to get the Hanna meter out now and get a more accurate number. Till then, I don't know for sure but I honestly don't think our rain is "acid rain" which would indicate a ph of ~5. Normal clean rain is always slightly acidic at 5.7 apparently (according to wiki).

    Acid rain was a big deal in the 70's and 80's in highly industrialized locations. Our rain comes from the ocean, I'm only 80 miles from the Pacific. Rain travels over forests and some homes before it reaches us, no industry.

    I'm sure that if it was acid rain, in my 40 years of living here someone would have mentioned it to me, or publicly. Portland is the "granola" capital of the world yano... the TV show Portlandia isn't THAT far off. This city turns perfectly useable car lanes into bike lanes regularly (GRRRR!) and I have been called "PIG" by bikers driving my suburban. If we had real acid rain, the bikers would be nice and peely and up in arms by now, eh?

    Need to get another bowl out there so I can Hanna it to be sure.
    Where kids swim in 54 degree water, turn blue, and giggle happily cuz they got a POOL!
    Year 3 BBB -15' x 48" Intex Metal Frame - Was using (2) 1000gph Intex cartridge filters (see Full time pumping Intex). 2012, converted to 1600gph and sand filter+SWG = Sand filter love affair!
    Don't waste time and energy looking for a better value on test kits, the TF100 is the best deal around. I did the looking and spent the extra money, but you don't have to make the same mistake. Just go here: TFTestkits. I use Pool Calculator for min/max, and shocking chlorine levels.

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    Re: Rain = PH dropping

    Rain should really have no TA as rain is the result of evaporation.

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    Re: Rain = PH dropping

    Maybe rain varies depending on location. I'm going to catch some of ours and see what we get. Didn't seem to lower my ph or TA after it raised my water level 3 inches. (math quizzes, that is 425 sq feet x 3 inches is what volume of water.). Maybe the rate matters we had downpours over two days.

    I was curious though. Thanks for checking.
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    Diver's Avatar
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    Re: Rain = PH dropping

    Quote Originally Posted by 257WbyMag
    Rain should really have no TA as rain is the result of evaporation.
    i read somewhere on the forum that zero TA means PH of 4.3 or something like that. 4.3 would be extremely acidic. so i'm a bit confused now..
    26K In-ground Vinyl Pool | Hayward DE4820/SP0710XR50 filter | 1.125 HP Dual Speed Hayward Super Pump | Aquasol Solar

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    Re: Rain = PH dropping

    425sf x 3" is 425/4 cf or 107cf times 7.4gal per cf is near 750 gallons of 0 TA rain added to your pool.

    Edit - if pool overflowed you may have lost only rainwater from top.
    23,000 gallon in ground pool with rock waterfall and spillover spa, Aqualink control system, Polaris 380 cleaner, Purex Triton Clean&Clear Plus cartridge filter. Located in The Woodlands, Texas.

    Pool owner since Nov 2008, Trouble Free since April 2009. Happy to help when I can.

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    agent86's Avatar
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    Re: Rain = PH dropping

    Decided to check the pH of the rain water here near Houston. Two different samples today, both coming in at exactly 6.8...guess I need it to rain more often as much acid I have to continually add to my pool!!!
    Pool: 24k, 38' x 19' x 24' Free-form IG Gunite with 24' Infinity Edge and Cantilever coping, SGM Diamond Brite Super Blue Plaster,
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    Re: Rain = PH dropping

    Yes, you will notice in a normal wet winter that you may even have to increase TA due to so much rain. Then it stops raining and the cold dry wind evaporates a lot of water and you are refilling the pool so much that TA climbs and you are lugging jugs of MA again.
    23,000 gallon in ground pool with rock waterfall and spillover spa, Aqualink control system, Polaris 380 cleaner, Purex Triton Clean&Clear Plus cartridge filter. Located in The Woodlands, Texas.

    Pool owner since Nov 2008, Trouble Free since April 2009. Happy to help when I can.

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    Re: Rain = PH dropping

    Pure water has a pH of 7.0, but it does not have any appreciable amount of total alkalinity. There will be a very small amount measured due to how the test is conducted. Total alkalinity is the buffering capacity of the water. It still takes a little bit of acid to lower the pH, but the pH drops according to the change in concentration of hydrogen ions.

    The total alkalinity test titrates down to a pH of 4.5, so when the pH is 4.5, the indicator will show red immediately.

    Rainwater will have a pH of about 5 due to gasses, such as carbon dioxide, which dissolve into the water. Dissolved carbon dioxide will create a very small amount of bicarbonate in the water, but it does not change the TA.

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    frogabog's Avatar
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    Re: Rain = PH dropping

    So where does one find pure water?

    Not rain then correct?
    Where kids swim in 54 degree water, turn blue, and giggle happily cuz they got a POOL!
    Year 3 BBB -15' x 48" Intex Metal Frame - Was using (2) 1000gph Intex cartridge filters (see Full time pumping Intex). 2012, converted to 1600gph and sand filter+SWG = Sand filter love affair!
    Don't waste time and energy looking for a better value on test kits, the TF100 is the best deal around. I did the looking and spent the extra money, but you don't have to make the same mistake. Just go here: TFTestkits. I use Pool Calculator for min/max, and shocking chlorine levels.

  12. Back To Top    #12

    Re: Rain = PH dropping

    Quote Originally Posted by frogabog
    So where does one find pure water?

    Not rain then correct?
    It depends on how you define "pure". Distilled or deionized water might be considered pure for some purposes, while not for others. Rain is exposed to many chemicals and particulates in the air so it's not exactly pure.

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    FlyAU98's Avatar
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    Re: Rain = PH dropping

    I know this is an old thread...but I just tested tonight's Ft. Lauderdale rain...light orange...very close to 6.8 pH

    I suspected all the rain we've had lately was behind my climbing pH...I guess not...
    15.5K gal, concrete, 1HP AO Smith pump, Hayward C1200 cartridge filter, Zodiac T5 Duo auto cleaner, 8 panel solar heater controlled by Hayward Aqua Solar GL-235

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    Re: Rain = PH dropping

    http://jfhtm350.webs.com/pool/FillWaterCalculator.html
    Use this and try several different inches of fill water then try several different pHs in the right column.

    4" of acid rain (5.3pH) would only drop me 0.2 max if it didnt aerate the water to raise it some. If the water didnt mix, then you would have 5.3ph on the top 4" of water if one pulled a pH sample from the top of the water.

    I suppose your TA level would have alot to do with the rain drops aerating the water. Higher TA = pH rise, lower TA pH steady or dropping.
    18,000 Gallon Aboveground. Sand Filter with 2HP 2 speed Pump.

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    FlyAU98's Avatar
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    Re: Rain = PH dropping

    Here are my test results from yesterday, after some pretty constant rain for a day or so.
    FC: 8.5
    CC: 0
    pH: 7.9 (gustimate, just slightly darker than the 7.8 pink)
    TA: 80
    CH: 275

    FC was a little higher than normal due to a Chlorine "spill" while adding from a completely full 2.5 gallon jug.
    pH was 0.2 or 0.1 higher than where I normally see it, though I've tried to keep it closer to 7.6
    TA dropped by 10 after 3 weeks of consistently being at 90. (I did this test twice to confirm after I messed up the count on the first round of the CH testing)

    The pool looks and feels better than ever. I actually did this test because the water looked and felt (even on the eyes underwater) better than it has and I wanted to see where everything was.

    Historically with the pH around 7.8 I always felt a little itchy. The water has been very clear, but it really looks even better now.

    Perhaps the aeration from the rain raised the pH, despite the slightly acidic rain?

    Either way, I'm happy...I just don't know what to think about a proper pH level now
    15.5K gal, concrete, 1HP AO Smith pump, Hayward C1200 cartridge filter, Zodiac T5 Duo auto cleaner, 8 panel solar heater controlled by Hayward Aqua Solar GL-235

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    Re: Rain = PH dropping

    Quote Originally Posted by Diver View Post
    i read somewhere on the forum that zero TA means PH of 4.3 or something like that. 4.3 would be extremely acidic. so i'm a bit confused now..
    It would be around 4.5 if the sample immediately turned red, but that was not the case as it took one drop to turn red. That just means the TA is <= 10 ppm, but the pH could be anything at that point and such pH would be unstable as a relatively small amount of acid or base would move it. That also means that the pH won't have a large effect when added to other water when that other water has a reasonable amount of TA as is the case for most pools.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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    Re: Rain = PH dropping

    The aeration from the raindrops hitting the waters surface usually has the pH rise from the greater amount of carbon dioxide outgassing. This effect is usually larger than the effect of the rain's pH on the pool pH. This is not only because the amount of rain water volume is usually only a fraction of the pool's water volume, but also because the rain's TA is usually very low so it's pH has little capacity to moving the pH of other water that is higher in TA.

    Pure water has a pH of 7.0, but carbon dioxide in the air dissolves into evaporated water so most rain water is naturally acidic and according to the EPA it is around a pH of 5.6. Acid rain is lower in pH with the most acidic rain being 4.3. If I assume that a pool has a pH of 7.5 and TA of 80 ppm, then even if 25% of the pool water were replaced with rain water, so that's 13.5" of rain if the pool were 4.5 foot average depth, the pH would drop only to 7.43. Now if this were acid rain at pH 4.3, then the pH would drop to 7.24. This all assumes that there is no aeration from the raindrops outgassing carbon dioxide from the pool.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: Rain = PH dropping

    We had massive storms here last night and for the fun of it, I tested the water that wound up in the dog's bowl outside. pH tested to 7.4.
    Central TX / 12K GAL / IG / SWG / DE filter / Pentair SVRS & 3 swim jet pumps / Spa bench and jets / Single body of water / No heater.

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