Calculating Aragonite Solubility State

[StrikerKratos]

Hi, I'm trying to calculate aragonite solubility state.

Let's say I know my temperature, pressure, pH, alkalinity, and calcium, where do I go from there?

Thanks!

[StrikerKratos]

edit: Aragonite saturation state, not aragonite solubility state.

[bertoni]

There are a lot of factors in calcium carbonate saturation limits:

http://reefkeeping.com/issues/2006-06/rhf/index.php#6
https://www.advancedaquarist.com/2002/7/chemistry

Are you trying to compute the solubility in a reef tank as a practical method of supplementation, or just working on the theory of chemistry?

[StrikerKratos]

Just working on a theory. I have a small experimental tank set up. In said tank I have a very tight control over CO2 and thus pH. It is supplemented via a CO2 tank and diffusor, or removed via chaeto. With this system, I can fine tune the pH to basically anything I want. CO2 dosing is absolutely necessary for this setup otherwise the chaeto becomes carbon limited and stops growing, I've seen the pH break 9.

I see a lot of talk about Ω in regards to ocean acidification, and reefs ideally seem to be around 4-5 if I'm not mistaken? Without being able to calculate Ω, I have no idea what my changes in pH or alkalinity are doing to this number.

[bertoni]

The little bit I understand of calcium carbonate supersaturation theory is from this article:

http://reefkeeping.com/issues/2005-07/rhf/index.php#12

I am not sure that it's practical for us to try to compute Ω in a complex system, since even the organic content of the water can have some effect.

[StrikerKratos]

From the article

(4) K = [Ca++]x[CO3--]

Where K is a value determined by multiplying the concentration of calcium by the concentration of carbonate. When K = Ksp* (the solubility product constant in seawater at any given temperature, pressure and salinity, which can be looked up in a book), then the solution is said to be exactly saturated (equation 5).

(5) Ksp* = [Ca++]x[CO3--] (at saturation)

This relationship is often quantified using the supersaturation parameter, which is symbolized as Ω:

(6) Ω = ([Ca++]x[CO3--] )/ Ksp*

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I can't find anything about calculating "Ksp" or finding a table for it. Searching google for "Ksp ocean, "Ksp saltwater", "K ocean", etc returns no relevant results.

[bertoni]

You are correct, that is the formula for Ω, but it's not enough to predict precipitation, which I am beginning to understand is not your primary concern. You'd need to look up that number in a book, or you could compute the constant from one or more of the parameter sets in the article. For example, Ω = 1 when alkalinity is 2.5 meq/L and calcium is 410 ppm, although he doesn't give the temperature. The Merck Index might have what you want. A college library might have a copy you could use.

[StrikerKratos]

I ended up doing that and determined my current Ω is somewhere around 5.2, which is a good enough estimation for my needs. I think the standard for seawater is temperature of 25 °C, salinity of 35 g/kg and 1 atm pressure, which my tank matches.

If I decide to change my pH or alk, at least now I have an estimate to where to move the other if I don't also want a big swing in Ω. This isn't a number I'm chasing, nor is pH, but they're numbers I can easily control via my phone so I figured it would be good practice to learn what those changes mean.

If I run across the formula or table, or number for standard seawater, I'll update and post it here.

[bertoni]

I'd be interested in any information you find. Good luck!