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post #1 of 12 (permalink) Old 12-21-2005, 12:26 PM Thread Starter
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quick temperature and pH question

I'm wondering how much temperature influences pH...I've noticed that when I ran my tank at a cool 78 the pH never dipped below 6.4 with my co2 injection (from 7.0). But now the pH is around 6.0. Could it be that the higher temp (86-88) is affecting the pH? I'm not a chemist (did horrible in all of them except organic), so I'm a bit puzzled. I turned down the co2, but the main point is...Am I lowering the overall co2 concentration by doing so? hummmm

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post #2 of 12 (permalink) Old 12-21-2005, 01:14 PM
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So far as I know is that there is a correlation between solubility, dissolution and temperature. Higher temp usually means faster dissolution and higher solubility capacity. Lower temp translates otherwise. For instance if you make a saturated solution of sugar, filter only the solution and cool down the temperature, you will get back some of the sugar crystals from the solution.


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post #3 of 12 (permalink) Old 12-21-2005, 02:03 PM Thread Starter
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Quote:
Originally Posted by medicineman
So far as I know is that there is a correlation between solubility, dissolution and temperature. Higher temp usually means faster dissolution and higher solubility capacity. Lower temp translates otherwise. For instance if you make a saturated solution of sugar, filter only the solution and cool down the temperature, you will get back some of the sugar crystals from the solution.

That's what I was thinking too...so I reduced the amount of gas I'm pumping into the reactor. If this holds then the following should be true:

Lets say I'm pumping X amount of gas at 78 degrees to achieve 45 ppm co2.

If I STILL pump X amount of gas at 86 degrees then the co2 ppm should be really high?

So, if I reduce the amount of gas, to Y amount at 86 degrees, I should hit a corresponding pH (with kh) of 45 ppm?

I believe my sister (Santa) is getting me a pH meter that controls for temperature, but I'm just wondering how I can accurately measure it with a standard colormetric pH drop test kit...

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post #4 of 12 (permalink) Old 12-21-2005, 02:23 PM
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Gases are less soluble at higher temperature, unlike most (but not all) solids:

http://www.elmhurst.edu/~chm/vchembook/174temppres.html

But this is at equilibrium. If you're below the equilibrium concentration, which for CO2 in water is very high, much higher than occurs in aquariums, the temperature really shouldn't make any difference. The CO2 concentration in the tank is limited by the amount you're supplying from the source.
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post #5 of 12 (permalink) Old 12-21-2005, 03:20 PM Thread Starter
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Quote:
Originally Posted by wapfish
Gases are less soluble at higher temperature, unlike most (but not all) solids:

http://www.elmhurst.edu/~chm/vchembook/174temppres.html

But this is at equilibrium. If you're below the equilibrium concentration, which for CO2 in water is very high, much higher than occurs in aquariums, the temperature really shouldn't make any difference. The CO2 concentration in the tank is limited by the amount you're supplying from the source.

That's interesting...good link. But this still doesn't make sense to me that my pH has dropped from 6.4 to 6.0 with everything else constant except water temperature.

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post #6 of 12 (permalink) Old 12-21-2005, 03:24 PM Thread Starter
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Of course I just found this...

Quote:
Question - Despite consulting a variety of sources, I do not yet
have a satisfactory explanation as to why carbon dioxide is more soluble
in cold water than it is in warm water.
------------------------------------------------
Most gases follow this pattern. The reason is simple: at higher
temperatures, the gas molecules prefer to be in the gaseous state rather
than in the condensed state (dissolved in a liquid). The molecules are held
in the condensed state by their attractions to the solvent molecules. As
the temperature increases, the molecules move faster and with more kinetic
energy, overcoming the attractive forces. It is exactly the same principle
that makes liquids boil at high temperatures.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
Which still doesn't explain the drop in my pH. I suppose I can always increase the amount of co2 input back to where it was and just forget about it. Augh.

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post #7 of 12 (permalink) Old 12-21-2005, 03:37 PM
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Quote:
Originally Posted by Georgiadawgger
That's interesting...good link. But this still doesn't make sense to me that my pH has dropped from 6.4 to 6.0 with everything else constant except water temperature.
A simple possibility is that your plants just aren't growing as well at the higher temperature and aren't consuming as much CO2. This leaves more CO2 in solution and lowers the pH. I see a similar sort of thing on days I drop the lighting over my tank. On those days the pH goes down. When I raise the lighting, CO2 consumption goes up and the pH rises. How temp affects overall plant growth in a tank would probably be a function of the specific mix of plants present in the tank and their respective temperature optima.
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post #8 of 12 (permalink) Old 12-21-2005, 03:47 PM Thread Starter
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Quote:
Originally Posted by wapfish
A simple possibility is that your plants just aren't growing as well at the higher temperature and aren't consuming as much CO2. This leaves more CO2 in solution and lowers the pH. I see a similar sort of thing on days I drop the lighting over my tank. On those days the pH goes down. When I raise the lighting, CO2 consumption goes up and the pH rises. How temp affects overall plant growth in a tank would probably be a function of the specific mix of plants present in the tank and their respective temperature optima.

That's another good possibility. I have seen a decline in pearling and relative size of the plants' growth over the last couple of weeks. I suppose the next couple of weeks at a lower co2 level will dictate how things run. Of course, less gas, less pruning, less "pissing off the discus" would be a good thing for me time-wise

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post #9 of 12 (permalink) Old 12-21-2005, 04:21 PM
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Quote:
Originally Posted by Georgiadawgger
Of course, less gas, less pruning, less "pissing off the discus" would be a good thing for me time-wise
That's why I run my lights at less than max most of the time too. Sometimes less is more .
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post #10 of 12 (permalink) Old 12-21-2005, 06:11 PM
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Increasing the heat does slow down the photosynthetic activities of some plants. Another possibility is an increase in bacterial activity adding both to your CO2 and the acidity of the water. You're fish will also respire at a higher rate with the water being warmer.

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post #11 of 12 (permalink) Old 12-21-2005, 06:14 PM Thread Starter
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Quote:
Originally Posted by SCMurphy
Increasing the heat does slow down the photosynthetic activities of some plants. Another possibility is an increase in bacterial activity adding both to your CO2 and the acidity of the water. You're fish will also respire at a higher rate with the water being warmer.
Ah...that makes sense too...obvious increase in bioload and biological activity switching from 20+ tiny harlequin rasboras to 5 pig-like discus and 9 just-as-hungry cories.

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post #12 of 12 (permalink) Old 12-22-2005, 01:54 PM
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Most plants don't like 86-88F. In fact quite a few will wilt at that temp.
Some sites even sell 'warm water plants' for discus setups, using species that don't mind the higher temps.
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