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PH rise in water cup

Hanuman

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Hello,

I would like to have the chemical explanation of why water taken out of a tank, in say a cup, will have its PH rise and be > to the PH of the water in the tank. This is assuming the following:
  • The tank has some aquasoil that buffers PH.
  • Co2 is not being injected at the moment water is taken out of the tank, and obviously no Co2 is being injected in the tank in the meantime and during PH test times.

The explanation I have is that the soil is acidifying the water and hence when removed from the tank, water is no longer in contact with the soil. This said this seem a very simplistic and perhaps incomplete/erroneous explanation. A more scientifc explanation would be appreciated.

Thank you.
 
Last edited:

ian_m

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How are you measuring pH ?

If using a pH probe, you may be suffering electrical interference in the tank and none in a cup leading to different pH probe readings ?
 

Hanuman

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Testing with a PH pen.

I did the test. Last night I took some water from the tank and left it to rest in a glass. This morning before Co2 started being injected in the tank I tests both the water in the tank and the water in the glass. The tank water was at PH 6.4 and the glass water was at PH 6.7.
 

Wookii

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Testing with a PH pen.

I did the test. Last night I took some water from the tank and left it to rest in a glass. This morning before Co2 started being injected in the tank I tests both the water in the tank and the water in the glass. The tank water was at PH 6.4 and the glass water was at PH 6.7.

If it is a CO2 injected tank, it takes a long time for the CO2 to off-gas from the tank water. Unless you have very vigorous surface agitation, or use airstones outside the photoperiod, the tank likely never gets back down to atmospheric levels of CO2 before the next injection cycle starts (plus the plants output CO2 when the lights are off).

This can often be seen when using a drop checker in tank - when you change the fluid after maintenance, it is always a strong blue colour, but you'll likely never see it go back to blue again once in the tank, often only ever going back to very dark green before the next CO2 injection period starts.

When you store some tank water in a cup, it will likely reach atmospheric equilibrium much quicker due to the higher surface area to volume, low overall volume, and the fact that nothing is generating CO2 within that container.
 

Zeus.

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How are you measuring pH ?

If using a pH probe, you may be suffering electrical interference in the tank and none in a cup leading to different pH probe readings ?

Yes , best done when lights off if comparing to glass of water. My 'in tank' pH probe jumped 0.1pH when lights came on.

If it is a CO2 injected tank, it takes a long time for the CO2 to off-gas from the tank water. Unless you have very vigorous surface agitation, or use airstones outside the photoperiod, the tank likely never gets back down to atmospheric levels of CO2 before the next injection cycle starts (plus the plants output CO2 when the lights are off).

This can often be seen when using a drop checker in tank - when you change the fluid after maintenance, it is always a strong blue colour, but you'll likely never see it go back to blue again once in the tank, often only ever going back to very dark green before the next CO2 injection period starts.

When you store some tank water in a cup, it will likely reach atmospheric equilibrium much quicker due to the higher surface area to volume, low overall volume, and the fact that nothing is generating CO2 within that container.

Yes.

This can often be seen when using a drop checker in tank - when you change the fluid after maintenance, it is always a strong blue colour, but you'll likely never see it go back to blue again once in the tank, often only ever going back to very dark green before the next CO2 injection period starts.

Yes, My two tanks with CO2 injection are at different ends of this spectrum, the DC in the 500l goes blue, the DC in 50l is always green, but tanks have different DC fitted as well the one in the 50L is slower in changing colour.

I always wait 24hrs for the glass of tank water to completely degas of CO2, then used that pH reading as the baseline pH of tank
 

dw1305

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Hi all,
The tank water was at PH 6.4 and the glass water was at PH 6.7.
I'd agree with @Wookii & @Zeus. and I suspect it is just the difference between the CO2 : O2 ratio in tank and glass. In the glass the water is 100% saturated with dissolved oxygen and at equilibrium with atmospheric CO2, in the tank there is either less oxygen or more CO2.

That is a pretty small pH difference.

cheers Darrel
 

Hanuman

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Ok here is the deal. The reason I asked is because recently I read the following in some facebook group:
CO2 is on 7.30 - 16.30, last night I took a cup of water from the aquarium and measured pH. Tank and cup pH was 6.6. 10 hours later aquarium pH was 6.7 so increased only 0.1 overnight. In the water cup pH was 7.3 and it further increased to 7.6 pH.

So I tested myself but it's nowhere near what that guy is experiencing. So I wander what is going on there.

Sounds like the guy forgot to turn off his Co2 😂
 
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Zeus.

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So I wander what is going on there

maybe the guy had a lower rate of loss of CO2 from tank due to lower surface agitation or lower turnover rate or tank has increased CO2 production in tank due to plants or decaying plant/livestock.
 

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