On the one hand, if the plants are happy, you should be too
On the other hand, I would be bothered by this the same as you. They might be happy now, but maybe they could be even happier if the PH was in range?
In fact, I have the same issue. For me, its my water. I am on a well. The PPM's are low - 70-100ppm mostly. The PH starts out anywhere from 6.7 to 7.4 out of the tap - depending on how much rain we get. Every time I PH my water down, it climbs right back up.If I repeat the cycle two or three times durring a day, it will stay sort of stable, but it still climbs back up slowly over the next day or two.
If I draw water from my tap, and just let it sit, the PH will climb. If I shake it up or aerate it with an air stone, it shoots up super fast. So, basically any agitation or aeration accelerates the PH climb. So, if I am circulating or aerating my rez water or the water in buckets, it will make the PH climb even faster.
Im running an HPA system right now, and I am having to PH my water down to 4.9-5.0 in order to get the water coming from the nozzles to be in the 5.8 range. I can PH the rez down to 4.9, then fill the accumulator tank, and when I check the tank it is at 5.1. When I sample the water coming from the nozzles, it is at 5.8. The water in the tank stays fairly stable over time, because its not getting aerated.
Just circulating the water in the rez makes the PH go up slightly. The act of pumping it into the tank raises it a bit more, but shooting it out of a nozzle at hi pressure makes it shoot up a bunch.
My water has stuff in it that causes this process to be more annoying than with RO water, but any water can have this problem. It all comes down to CO2 and dissolved minerals..
Here is a quote from an article I read recently that explains it fairly well. Ive attached the full article as well if you are interested.
The role of CO2 in pool water #1
While some service techs go about their business taking care of various swimming pools,
and specifically after they lower pH by adding acid, they may ask themselves why the pH
of the water begins to rebound (rise back up again) afterwards. Also, they may wonder
why this pH rebound happens faster in some pools than in others.
The answer lies in the behavior of carbon dioxide in the water. Carbon dioxide (also
known as CO2 & carbonic acid) is formed when acid is added to swimming pool water,
and it is this compound that affects the changes in pH of pool water.
Carbon dioxide (CO2) is a common, essential compound in nature. It is found almost
everywhere, from what flowers and trees breathe in, to what humans and animals breathe
out; and to the bubbles in the soda you drink. In its warmer phase it is a gas, and in its
colder phase it becomes a solid – dry ice. Our atmosphere (the air we breathe) contains a
relatively small amount of gaseous carbon dioxide – only about 0.03% to 0.06% – which
is fortunate, since levels around 10% or higher would cause us all to lose consciousness!
Because it exists in the air, a slight amount of carbon dioxide can be absorbed by water.
In water, CO2 primarily exists as aqueous CO2 (gas in, but not fully reacted with the
water), but a small amount also combines with water to form carbonic acid: CO2 + H2O =
H2CO3, and the slightly acidic nature of this compound lowers the pH somewhat.
CO2 in Pool Water #2
Carbon dioxide plays an important role in the make up and balancing of pool water.
When dissolved in water, carbon dioxide has a direct effect on the water’s pH. The more
CO2 in the water, the lower the pH, and the less CO2, the higher the pH. Pool water with
no dissolved CO2 (but with a minimum alkalinity of 100 ppm) will have a pH of about
8.4 (as long as no other chemicals have been added). On the other hand, pool water that is
saturated with CO2 will have a pH down around 5.
Although CO2 can be introduced to water from the air, it is also produced in pool water
by simply adding acid. As we all know, when acid is added, both the alkalinity and the
pH are lowered. The alkalinity is lowered because, with normal pool water parameters,
the added acid reacts with bicarbonate alkalinity in the water, converting it to carbonic
acid – which is then no longer alkalinity.
For you who enjoy formulas, bicarbonate and acid form carbonic acid and chloride, or
HCO3 + HCl = H2CO3 + Cl, and then all but a fraction of a percent of the carbonic acid
Page 2 of 5
shifts to aqueous CO2: H2CO3 <==> CO2 (aq) + H2O. Depending on the amount of acid
added, a specific and calculatable amount of alkalinity is eliminated.
CO2 in Pool Water #3
After adding acid to the pool water, the pH goes down (at first) because of the effect on
pH of the increasing amount of CO2 (aq) produced by the acid. Shortly afterwards, the pH
begins to rebound and eventually can return to its original level. This is due to the
equilibrium relationship between the amount of CO2 in the water and the amount of CO2
in the air above the water. This is known as Henry’s Law. Since the acid addition forms
more CO2 (aq) in the water than is dictated by the equilibrium, most of the CO2 (aq) created
by the addition of acid will then begin to release and off-gas into the atmosphere – which
will gradually raise the pH level but not the alkalinity.
It may be instructive at this point to mention that the “natural” level of CO2 in balanced
pool water after it has had sufficient time to reach equilibrium with the atmosphere is
from about 0.5 ppm to 2.0 ppm and the pH will be around 8.0 to 8.3. This pH range is
dependent on total alkalinity.
Since it is dissolved CO2 in water that keeps calcium soluble in water, we want to
maintain pool water with just enough CO2 to keep the pH down in the mid to high 7
range. Too much CO2 in water creates low pH conditions that are aggressive to pool
plaster, and no CO2 creates high pH conditions that are scale forming to pool surfaces. Of
course, etching and scaling are something service techs are always trying to prevent from
happening.
Page 3 of 5
maintain pH and alkalinity. But, because the content of the CO2 being injected in the
water is higher than the natural level (equilibrium, or 8.2-8.3), most of it will off-gas over
time. Therefore, a continual addition of CO2 will be needed to keep the pH from rising.
Although this may be done manually, it is much easier by dosing the CO2 with a pH
controller.
CO2 in Pool Water #5
Why the pH rebounds.
While some of the following information has already been mentioned, let’s specifically
address the question raised at the beginning of this update series. Why does the pH
eventually rise after acid initially makes it drop? Again, the answer lies in the fact that
whenever the pH is below 8.2-8.3, there is generally more CO2 in water than its natural
equilibrium level with the atmosphere. Because of this, the extra CO2 will off-gas into
the atmosphere over time. The pH rises because CO2 is off-gassing from the pool water.
Unless checked, the pH will continue to rise until the CO2 reaches its equilibrium or
about 8.3. The more alkalinity (over 80 ppm), the stronger pull to a pH of 8.3. This is
one of the difficulties that pool service techs have to deal with. Generally, there is no
getting around this.
But there are exceptions to the above rule. For instance, a common but occasionally
unrecognized factor which interferes with CO2 off-gassing or absorption is a pool cover.
This particular situation will be cover in our next update. Incidentally, this entire update
discussion on carbon dioxide was published by Service Industry News in their Sept. 15,
2004 issue.
CO2 in Pool Water #6
Page 4 of 5
water, and will not be able to off-gas. Therefore, the pH will probably remain unchanged
(low) until either other chemicals are added, or the cover is removed.
When non-permeable covers are used, pH needs to be watched carefully. Ideally, the
cover should be removed for enough time to allow gases to equilibrate (perhaps 6 to 8
hours, twice a week for residential pools). When this is not an option, careful control of
pH using acids and bases must be maintained. How will you know if the pool contains
the right amount of CO2? The pH will be balanced.
CO2 in Pool Water #7 – How Long for pH change?
So how long does it take for CO2 to off-gas after adding acid? Or, why does the
“rebound” happen faster in some pools than others, or faster one time than another in the
same pool? CO2 off gassing, and its relative speed, are dependent on several factors,
which include water temperature, circulation, total alkalinity, ratio of water volume to
air-exposed surface, and atmospheric pressure. These factors affect both the rate and the
percentage of either off-gassing or absorption.
Factors which accelerate off-gassing (and speed pH rebound) include higher water
temperatures, increased circulation, and especially increased aeration. Factors which
inhibit off-gassing (thus slowing down pH rebound) include decreased aeration,
decreased circulation, and low temperature. Factors which promote absorption of CO2
(which drops pH) include high total alkalinity, a pH above 8.4, and low water
temperature.
