pH Variances Over Time

[jameson_uk]

Sanata brought me a Seneye Reef which I figured would be a bit of a gimmick, something to play with and a cheap PAR meter.

When I first set it up the pH was a lot higher than I was expecting (about 8) when the API test had consistently being telling me the pH was about 7.4. The water company has pH in my area being between 7.1 and 8.4 with a mean of 7.4 (which I am guessing would be out the tap before anything has gassed off).

So you can see when I did the water change (lowest point is at 3pm on 1st and then pH rises until about 6pm on 2nd and starts to rise again about noon on 3rd; which is when the lights come on).

So this gets me thinking...

• Presumably the increase after the water change is just the CO2 added by the water company gassing off? (I am sure when I did a test by leaving tap water in a bucket with an air stone it did not change this much)
• Why does the pH start dropping at about 6pm? The lights are on until 8pm and as I can see on the 3rd, the pH goes up when the lights come on (presumably because plants start consuming the CO2)
• Would that swing of 0.8pH actually be dangerous to the inhabitants?

I did think that pH was one of those things that could be relatively accurately measured so not sure why I am getting the 0.4 difference between the API kit and the Seneye.

I am not going to chase pH or get too worked up about about this but I am interested in whether this looks sensible and whether I should be looking at changing my water change process to avoid this big shift?

 

[dw1305]

Hi all,

Presumably the increase after the water change is just the CO2 added by the water company gassing off?

They won't add any CO2, but because the water in the main is cold and under-pressure it can hold more CO2 than at room temperature.

Why does the pH start dropping at about 6pm? The lights are on until 8pm and as I can see on the 3rd, the pH goes up when the lights come on (presumably because plants start consuming the CO2)

The pH rise is both CO2 being consumed and oxygen being produced. Oxygen is a base. Possibly the plants are photosynthesising less after 6pm.

Would that swing of 0.8pH actually be dangerous to the inhabitants?

No.

In a planted tank how much the pH changes depends on the carbonate buffering of the water, if water is very strongly carbonate buffered it will resist pH change. As the dKH falls pH will become more variable.

cheers Darrel

 

[jameson_uk]

Hi all,They won't add any CO2, but because the water in the main is cold and under-pressure it can hold more CO2 than at room temperature.

Thanks, makes sense.

The pH rise is both CO2 being consumed and oxygen being produced. Oxygen is a base. Possibly the plants are photosynthesising less after 6pm.

Could this be down to natural light disappearing or is this just the equivalent of them getting tired and being done for the day? If the former could this mean my tubes aren't providing the ideal frequencies for plant growth? (This weekend I will have a play with the Seneye as it should produce a spectrum analysis and PAR readings for different parts of the tank so this might actually work and show what light is getting to the plants). If the latter could this suggest I might be able to drop the lighting duration without really impacting the plants?

In a planted tank how much the pH changes depends on the carbonate buffering of the water, if water is very strongly carbonate buffered it will resist pH change. As the dKH falls pH will become more variable.

Have just had a bit of a stupid moment questioning whether acidity would affect pH
I am guessing that the sinusoidal nature means that it is not a case of KH getting depleted (else it would be more of a constant stepped reduction?) but rather having a finite capacity and being able to free this capacity?So would a (sorry really bad) comparison be like a sponge under a dripping tap? this sponge would have a capacity and it would absorb the leaking water until it reached that capacity and then everything else would just end up on the floor, if I then fixed the drip and heated the sponge the water would evaporate giving it back capacity ?

When I do a water change I am adding water that has more CO2 than it will keep hold of in the tank but this equalises itself out so I will always see a reasonably large drop in pH which will even itself out after 24 hours. It will then continue with the sinusoidal flow between 7.6 and 7.8 until I next do a water change (assuming my KH holds out?) . This would suggest that there is little point adding liquid carbon after a water change and I should really add it the day after? (presumably the glutaraldehyde in Excel doesn't turn to acid like CO2 so doesn't affect pH?)

my KH has always been 5 whenever I have checked which I believe is low to middling?

 

[sciencefiction]

My water is similar. Ph out of the tap is 6.6 and after it outgasses, it goes to 7.4. The Kh is not affected by PH change caused by CO2 fluctuations. Therefore its easy to swing the ph due to injecting or outgassing CO2. The Kh is affected by nitrification. In a large water changed non-overstocked tank one wouldn't expect any Kh change at all...not that can be measured with the liquid tests...One does not want their Kh to fluctuate as fish are more sensitive to this type of change. Ph fluctuation without determining the reason for the change means nothing. But as you suggested, the Kh in terms of its connection to Ph, seems to be like a sponge...it can drop quite a bit before actually affecting the Ph.

 

[dw1305]

Hi all,

but rather having a finite capacity and being able to free this capacity?So would a (sorry really bad) comparison be like a sponge under a dripping tap? this sponge would have a capacity and it would absorb the leaking water until it reached that capacity and then everything else would just end up on the floor, if I then fixed the drip and heated the sponge the water would evaporate giving it back capacity ?

That is quite a good analogy, it is the carbonate (CO3) ~ bicarbonate (HCO3) ~ CO2 (as carbonic acid (H2CO3)) equilibrium that causes the changes in pH.

When you have ~400ppm of atmospheric CO2, and water with a reserve of carbonate hardness, then pH is stable at ~pH8. It doesn't matter whether you have 2dKH, or 20dKH, if the only source of acid (acids are H+ ion donors) is the dissolved CO2 (really the very small proportion of it that is H2CO3) then the pH will be ~pH8.

The dKH is a measure of the carbonate (CO3) reserve (the buffer), as you add H2CO3, it disassociates into H+ and HCO3-ions and the ratio of H+:OH- ions (this is pH) moves towards H+. If you have a buffer of CO3 it goes into solution as 2HCO3- and pH moves back to the equilibrium value.

When you add CO2 (either via CO2 addition or from respiration) it pushes the equilibrium towards H+ and the pH falls. When you stop adding CO2 (or it is removed more quickly by photosynthesis than it can be replenished by diffusion) the pH rises, and 2HCO3- ions will come out of solution as the least soluble carbonate compound (nearly always CaCO3).

The dKH can be thought of as the sponge, if you can add enough acid eventually you will reduce the pH of water, but if you started with 20dKH you have a really huge amount sponge to "soak".

This would suggest that there is little point adding liquid carbon after a water change and I should really add it the day after? (presumably the glutaraldehyde in Excel doesn't turn to acid like CO2 so doesn't affect pH?)

It doesn't tun into CO2, so it doesn't matter.

cheers Darrel

 

[jameson_uk]

It doesn't tun into CO2, so it doesn't matter.

But if the carbon levels (as CO2) are higher straight after the water change then the plants should have access to it? Given Excel is only meant to hang round for 24 hours adding this on top would just mean wasted carbon? (Or more likely massively fluctuating carbon levels)

Out of interest what would be the best thing to do if I did add pressurised CO2? Would it be better to lower levels following a water change or are the differences in the low tech much lower so it wouldn't make a big difference?

 

[dw1305]

Hi all,

But if the carbon levels (as CO2) are higher straight after the water change then the plants should have access to it?

Yes they will do, but the CO2 will out-gas pretty quickly (to reach equilibrium with atmospheric levels) as the water warms.

Out of interest what would be the best thing to do if I did add pressurised CO2? Would it be better to lower levels following a water change or are the differences in the low tech much lower so it wouldn't make a big difference?

I'm not a CO2 user, but I don't think it will make any difference.

cheers Darrel

 

[jameson_uk]

A week later and some odd measurements...

So you can see the water changes however there is (and seems to continues to be) a steady rise in pH.

I am just awaiting some pH calibration solution for another meter I have but the API liquid kit looks like 7.8 about now (Seneye is currently reporting 7.9) so is roughly in the right ballpark.

What might cause pH to rise like this (and not get particularly reset by a 50% water change)???

I did just test KH and that was 4 (normally 4 or 5) but does KH actually measure the capacity or how much is available to use? (eg. the size / absorbency of the sponge or the amount of water it could still take on before becoming saturated ?).

 

[dw1305]

Hi all,

What might cause pH to rise like this (and not get particularly reset by a 50% water change)???

It is probably just the plants have grown, so they are adding a bit more oxygen and removing a bit more CO2.

You don't need to worry about it.

I did just test KH and that was 4 (normally 4 or 5) but does KH actually measure the capacity or how much is available to use?

The test doesn't actually measure dKH at all, it is a semi quantitative titrimetric method using a pH indicator, and it adds acids until the alkalinity is exhausted, at that point the pH falls and the pH indicator changes colour.

cheers Darrel