Rain = PH dropping

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frogabog

frogabog

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Jul 16, 2010
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Portland, Oregon
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. :rant:
 
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.
 
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.
 
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.
 
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 said:
So where does one find pure water?

Not rain then correct?
Click to expand...
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.
 
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.
 
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 :)
 
Diver said:
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..
Click to expand...

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.
 
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.
 
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