The hidden vein

[zozo]

Do you have some to be able to test?

I no longer do High-Tech anymore, I'm stuck in the old fashion low-tech... But tested it in the past in relatively soft water with Gh4 and Kh4. My tap water has Ph 8 to 8.5 in a depleted state. I worked with these parameters so the Ph1 drop gave me still a rather neutral Ph 7 with the addition of CO² I had pieces of this particular limestone in the tank and never measured a difference within the water change period.

Also used it in low-tech for very long periods same results... If it releases anything it's not enough to influence the available test results...

I'm not a geologist but I guess it might be because it is fossilised limestone and too hard and compacted to dissolve in water that quickly. But acid does...

 

[Tim Harrison]

I guess if @Hanuman intends to keep sensitive, or what could be calcifugous plants, or those that prefer low KH, then not taking chances is a good strategy. However, using 35% HCL for the acid test is pretty extreme. Geologist usually use dilute HCL about 5-10%. So rocks containing carbonate that usually aren’t particularly reactive to dilute HCL will probably react a lot more vigorously. Either way it’s perhaps misleading to use it to gauge which rocks are suitable for use in an injected tank.

 

[dw1305]

Hi all,

because you can pick it off the ground over there and it's a hard limestone,

We have the same in the UK, <"Carboniferous age limestone">, that is really hard. As a general all exposed old rocks are hard, because if they weren't they would have eroded away over geological time. Same with <"rounded cobbles">, only hard rock becomes rounded and any cobble isn't going to have much effect on water chemistry.

Easy enough to test in a jug of RO or DI water, and bubble some CO2 though it for a few days. I'd be willing to bet a substantial fistful of change that any rock that fizzes with an acid test is going to raise your water parameters with CO2 injection.

I think you are probably right, when you add CO2 you need to keep away from calcareous rocks.

The CO2 situation is slightly different from having naturally soft water, in that you are constantly adding an acid (the proton (H+)) from the dissolution of H2CO3, so you never reach equilibrium.

If you think about it like making <"fruit squash"> in a glass, where the limestone is the squash concentrate and in the non-CO2 situation you add water and achieve a suitable level of diluted squash and nothing changes. In the added CO2 case you are continually pouring water (CO2) into the glass and you can only keep your squash at the right dilution by continually adding more concentrate*.

When you turn the CO2 off (the tap) then the calcium carbonate (CaCO3) will reform, but only if <"you've reached saturation"> (about 17 dGH and 17 dKH at 420 ppm atmospheric CO2).

The practical application of this is that the only way you can retain low dGH and low dKH, if you have limestone rocks and add CO2. is by continually replacing the tank water with RO etc.

* You could substitute "Gin" for limestone and "Tonic" for water, if you don't like soft drinks?

cheers Darrel

 

[Andy Pierce]

Just to get all physical chemistry on it, whilst the non-equilibrium situation described above means eventually all the CaCO3 will dissolve, that doesn't necessarily mean it will happen on a timescale relevant to what you're doing. You could try the experiment of putting your rock in tank of water, measure the alkalinity of the water (e.g. using acid titration) at the start of the experiment, then bubble CO2 gas through the water under whatever you would consider real-world reasonable levels for a day (or a week), then measure the alkalinity again. If the rock is dissolving, your alkalinity will increase. Note that the presence of CO2 gas dissolved in water does not itself change alkalinity and does not affect alkalinity measurements so you don't have to worry about that.

 

[Cédric]

When you turn the CO2 off (the tap) then the calcium carbonate (CaCO3) will reform, but only if <"you've reached saturation"> (about 17 dGH and 17 dKH at 420 ppm atmospheric CO2).

Hi Darrel,
The pH of a pure water at equilibrium with atmospheric CO2, then at equilibrium with CaCO3 should be around 8,3 if I got it right. Does it means dKH and dGH will be both 17 in that case ?

 

[_Maq_]

You can reach pH=8.3 with much lower alkalinity, about 3 °dKH, I believe. At that point no free CO2 will be present in the water and only carbonates & bicarbonates will be present in the solution.

 

[Cédric]

You can reach pH=8.3 with much lower alkalinity, about 3 °dKH, I believe. At that point no free CO2 will be present in the water and only carbonates & bicarbonates will be present in the solution.

Hi Maq,
I got no notification for your answer. Thanks. I know pH can be reached with much lower alkalinity (i learned it on UKAPS) but I realize I failed to formulate my question clearly. I wanted to know if a water saturated with CaCO3 (dKH and dGH ~17) can go over pH 8,3.