Re:Alkalinity and Phosphates

[Paul Krombholz <krombhol at teclink_net>]

  I have a LaMotte CO2 kit, which uses phenolphthalein as the color 
indicator and  a syringe injector to titrate with dilute sodium 
hydroxide solution until a faint pink color persists.  The best way 
to detect the faint pink color reliably is to have an untreated 
sample of the tank water to use as a comparison.  When the sample you 
are titrating looks just a bit pinker than the other sample, then you 
stop.  The syringe injector is a bit tricky because, if you apply 
slowly increasing pressure on it, the plunger may suddenly jump and 
squirt so much in that you go past the end point.  I think it works 
better just to tap it, putting in small squirts, rather than to try 
to move it slowly.

The end point(around pH 8.2) where you get the first persisting pink 
color is arbitrary.  The choice of this end point does not mean that 
there is no free CO2 when the pH is higher.  No matter what the pH, 
there is always some free CO2 as long as you have some of the other 
forms of CO2 as a result of the equilibria between the various forms 
of CO2 in water: free CO2, carbonic acid (H2CO3) bicarbonate (HCO3-), 
and carbonate (CO3--).  The phenolphthalein end point is chosen 
because it is in the vicinity of the CO2 concentration where free CO2 
becomes significant for aquatic plants.

Aquatic plants vary widely in their ability to extract CO2 out of the 
water.  Those that can extract CO2 from the bicarbonate ion can get 
lots of CO2 at levels where the test kit would say no free CO2. 
Plants that can develop calcium carbonate deposits on the leaves in 
bright light have this ability.  Many popular aquarium plants, 
however, do not have this ability and can only extract free CO2.

I would like to propose a different point from which we measure 
"useful" CO2 in the aquarium.  Instead of the phenolphthalein end 
point, I propose the CO2 level when a sample of the aquarium water is 
in equilibrium with the air.  The amount of CO2 in air is around 
0.03% or around 0.65 milligrams per liter.  That is not very much. 
About the only aquatic plants that can grow well in water in 
equilibrium with air would be plants with the ability to take up 
bicarbonate and which also have thin, finely divided leaves providing 
a large surface area for uptake.  They would also need to have very 
bright light and to be in hard water so that there would be a 
reservoir of bicarbonate to draw upon.  In aquariums fish and 
decaying organic matter raise the CO2 level higher, but it is well 
known that additional CO2 beyond the levels ordinarily found in 
planted fish tanks greatly speeds up growth.

For these reasons I believe that the CO2 content in water at 
equilibrium with the atmosphere is a good starting point from which 
to measure the amount of "useful" CO2 in your tank.  You definitely 
want to have more than that.  Also, there is an additional huge 
advantage because, by subtracting the reading you get with tank water 
that has equilibrated with the air from the reading you get with 
water immediately from the tank, you eliminate all the effects of 
other acids, buffers, etc, in the water, such as phosphate, organic 
acids, and so on.  When you let the water stand in an open container, 
CO2 equilibrates, but the other acids and buffers, not being 
volatile, remain unchanged.

Here is how you do it.  Let me give an example of what I did over the 
last two days.  I took a sample of water from my guppy tank and let 
it sit in the vial provided by the LaMotte kit for 12 hours 
overnight.  I added the phenolphthalein and titrated to a faint pink 
color with the sodium hydroxide solution.  I got a reading of 7 
ppm.CO2.  I then took a sample from the tank and titrated that, 
getting a reading of 12 ppm CO2.  The difference is 5 ppm CO2.  That 
is what I would consider "useful" CO2.  My guppy tank does not get 
any CO2 additions, and so, that is what I would expect, given that 
the tank is getting somewhat crowded.  Just to be sure that the 12 
hour time is sufficient for the sample in the vial to become 
equilibrated, I took another sample and let it sit 24 hours.  I got 
the same reading of 7 ppm CO2.






-- 
Paul Krombholz in chilly central Mississippi, where we might see a 
piece of sleet or two on Sunday.