pH drop at night?

[TheUltimateNoob]

this may be a stupid question, but what exactly determines pH and why does it drop at night. sometimes i see people write something like "my pH is 8.3 during the day and 7.9 at night." why is that? intuition tells me that it should stay constant no matter what time of the day it is? please educate me. thanks.

 

[wrassefan]

I'm not the chemist but has to do with the lights out and carbon dioxide exchange. If you run a fuge with reverse light cycle, you don't get this drop.

 

[Moe_K]

Whew, chemistry is fun!

Short answer: photosynthesis pushes up your pH.
When there's no light, there's no p/syn, so the pH drops.

I'll leave it to one chemistry wonks to go into greater depth.

 

[Scuba_Dave]

I have a 75g fuge in the basement that is helping level off my pH swings. Once I finish the setup it should help. I was going from 7.6/7?? at nite to 8.4 during the day.
One note if anyone is using a Pinpoint pH monitor, as the battery sslowly gets weak, THE pH will rise!!
I had a pH of 8.8-9.0 & I freaked. Changed the battery & it went back to normal..

 

[TheUltimateNoob]

thanks for the info guys. all i really wanted was a short answer. i never knew that photosynthesis played a part in pH but i guess it makes sense. i just started doing the reverse light cycle thing so i guess that will do. thanks again.

 

[NateHanson]

Photosynthesis changes pH because it uses CO2, which acts as an acid in your tank (well, actually it is in equilibrium with another form, which is itself an acid, but you said you wanted the simple answer . . .)

 

[jacko]

the extent that your pH changes depends on the number of hours surfing internet reef-keeping boards.

Peter, the pH in our tanks is pretty solid, you might check yours and regulate it.

 

[TheUltimateNoob]

uh oh. i must be getting some huge pH swings then!

 

[montec24]

Good Article on ph and what makes it swing in either the April or May issue of Coral magazine. I just bought htem both at AA, very good magazine for those who haven't seen it.

 

[shearwater]

Peter, pH is the log of the concentration of hydroxyl ions in your water. Or something like that. I am reaching back to organic chem about 20 years ago. If you don't have photosynthesis and respiration (the opposite of photosyn), then the cycle wont' be apparent.

 

[NateHanson]

I think it's log of the reciprocal of hydroxyl ion concentration, but not bad at all for 20 years Peter (shearwater).

(pH goes down as [H+] goes up).

Nate

 

[shearwater]

Yeah! You're correct Nate. I knew there was something missing from my response!

Not bad either, for a young furniture designer/builder, Nate!

 

[NateHanson]

(and former microbiology/biochemistry graduate student at MIT)

 

[TheUltimateNoob]

yeah, way to show off nate. haha. j/k i'm actually pretty impressed.

 

[KevK]

Simple explanation CO2 becomes carbonic acid in the water, this lowers pH. When the lights are on, the plants and algea convert the CO2 into oxygen and carbon(sugar/food). This removes the CO2 raising the pH. When the lights go out it reverses. The plants take up O2 combine it with the sugar/food they made during the day and make more plant/algea. Some people light their refugium lights on at night and off during the day. Or drip Kalk at night to minimize the pH changes

 

[Matt L.]

Wow,

I didn't even have to jump in! Just one quick note, though. Its the hydrogen ion [H+] that pH technically is a (negative base-10 logarithm) measure of, not the hydroxyl ion [OH-] (although there is a relationship between the two!).

Yeah, what everybody says is pretty much correct. Plants only photosynthesize during the day. At night, they respire like you and I, producing CO2 instead of consuming it, as they do during photosynthesis. When you add CO2 to water, it drops the pH. When you remove it, it raises the pH. That's the over-simplified story,

Matt

 

[TheUltimateNoob]

it's all making some sense to me now. thanks.