Buffering capacity of carbonate sands

[lduncan]

Someone requested my reasoning behind why i think that carbonate sands are limited in there ability to add to the buffering capacity of tanks.

So i'll post the maths chemistry and assumptions behind this in this thread.

[wasp]

The maths chemistry and assumptions may be interesting mental gymnastics.

But what actually happens in a real tank is what matters.

In my experience a tank with carbonate sand can easily drop to PH 7.8 at night ( lower than ideal ), if not buffered by means other than carbonate sand.

I've heard of cases where PH has dropped even lower than that.

[lduncan]

First we need to consider the relationship between carbonate (CO3-2), hydrogen carbonate (HCO3-), carbon dioxide (CO2) and water (H2O).

It's also important to consider phases:

(s) = solid

(aq) = aqueous (dissolved)

(g) = gas

The following chemical equilibrium are in process:

Dissociation of sand:

CaCO3 (s) <-----> Ca+2 (aq) + CO3-2 (aq)

Water dissociation:

H20 <----> H+ + OH-

Carbonate / Hydrogen carbonate equilibrium:

H+ (aq) + CO3-2 (aq) <----> HCO3- (aq)

Atmospheric / dissolved CO2 relationship

CO2 (g) <----> CO2 (aq)

CO2(aq) + H2O <----> H2CO3 (aq)

Dissociation of Carbonic acid:

H2CO3 (aq) <----> H+ (aq) + HCO3- (aq)

Equilibrium Equations

Solubility Product of calcium carbonate:

K = [Ca+2][CO3-2] = 6 x 10^(-9)

Dissolved CO2:

[CO2 (aq)] = K(CO2)P(CO2) where K(CO2) = 2 x 10^(-3)

Water:

K(w) = [H+][OH-] = 10^(-14)

Hydrogen carbonate / carbonic acid equilibrium:

K(a1) = [H+][HCO3-] / [H2CO3] = 4.45 x 10^(-7)

Hydrogen carbonate / carbonic acid equilibrium:

K(a2) = [H+][CO3-2] / [HCO3-] = 4.69 x 10^(-11)

[lduncan]

Those equation are reasonably complex to solve so just looking at the dominant process:

a0 = [H2CO3] / Fa = [H30+]^2 / { [H30+]^2 + K(a1)[H30+] + K(a1)K(a2) }

a1 = [HCO3-] / Fa = K(a1)[H30+] / { [H30+]^2 + K(a1)[H30+] + K(a1)K(a2) }

a2 = [CO3-2] / Fa = K(a1)K(a2) / { [H30+]^2 + K(a1)[H30+] + K(a1)K(a2) }

(It's damn hard writing equations in ASCII !)

So then:

CaCO3 (s)

initial: solid

change: -x

final: solid

Ca+2 (aq)

initial: 0

change: +x

final: x

CO3-2 (aq)

initial: 0

change: +x

final: x

So basically for x moles of CaCO3 dissolved, x moles of Ca+2 and x moles of CO3-2 are produced. But CO3-2 also exists as HCO3-.

These are still related through

K = [Ca+2](a2)(Fa)

From the list above:

K = (a2)(x^2) = 6 x 10^(-9)

x = sqrt{ 6 x 10^(-9) / a2}

where a2 = [CO3-2] / Fa = K(a1)K(a2) / { [H30+]^2 + K(a1)[H30+] + K(a1)K(a2) }

and [H3O+] = 10^(-pH)

Eg: for a pH of 8:

[H3O+] = 10^(-8 )

a2 = 4.45 x 10^(-7) x 4.69 x 10^(-11) / { 10^-16 + 4.45 x 10^(-7) x 10^(-8 ) + 4.45 x 10^(-7) x 4.69 x 10^(-11)} = 4.57 x 10^(-3)

x = sqrt { 6 x 10^(-9) / 4.57 x 10^(-3)} = 0.0011 mol / L

So solubility at pH 8 is 0.0011 mol / L == 91 ppm

Eg for a pH of 8.3:

[H3O+] = 10^(-8.3)

a2 = 0.009168

x= sqrt{0.000000654} = 0.00081 mol / L

So solubility at pH 8.3 is around 67 ppm

Remembering that a alkalinity of 10 dKH (around 3.5 meq/L) is equivalent to 178 ppm.

Layton

[lduncan]

The maths chemistry and assumptions may be interesting mental gymnastics.

But what actually happens in a real tank is what matters.

In my experience a tank with carbonate sand can easily drop to PH 7.8 at night ( lower than ideal ), if not buffered by means other than carbonate sand.

I've heard of cases where PH has dropped even lower than that.

Well there are two ways carbonate sand can dissolve. Purely chemically, or alternatively with the aid of bacteria. And the bacteria don't do it just for the fun of it. They use it, meaning other animals in the tank don't get it in the form of alkalinity.

So I think sand has little if any effect on buffering the water against pH changes.

Layton

[cracker]

The maths chemistry and assumptions may be interesting mental gymnastics.

But what actually happens in a real tank is what matters.

In my experience a tank with carbonate sand can easily drop to PH 7.8 at night ( lower than ideal ), if not buffered by means other than carbonate sand.

I've heard of cases where PH has dropped even lower than that.

raising you KH to 10 or 12 eliminates low ph readings. (adding bi-carb).

sorry about that ZEO users!!!

my tank sits on 8.2, 24 hours.

carbonate sand by itself will do nothing for buffering.

[lduncan]

raising you KH to 10 or 12 eliminates low ph readings. (adding bi-carb).

... depending on atmospheric CO2 levels around the tank.

Layton

[Sharkey]

my tank sits on 8.2, 24 hours.

sorry about that ZEO users!!!

Im a little confused ? How do you get your tank to sit at 8.2 24 hours a day?

:-?

[cracker]

High KH keeps the PH system solidly stable.

A KH of 6 to 7 is what most people keep their tanks at because thats what people have told them they should do.

High KH does two things.

1: PH stays EXTREMELY stable.

2: The water column has LOADS of bicarbonate for rapid coral growth.

(So long as calcium and MG are present at correct levels).

You can safely run your KH at up to 14 if necessary, but stay away from a KH of around 18. This reading and higher can "Undisolve" or precipitate calcium out of the water column. in other words the calcium turns back into powder and becomes useless. Corals cannot use it whilst in this condition.

If you run a KH of 14 to 16 continuously, the PH will stay so stable it will stick like poos to a blanket!!

12KH is a bit more realistic though I feel, still much more stable than 6 or 7.

[jetskisteve]

A KH of 6 to 7 is what most people keep their tanks at because thats what people have told them they should do

Where did you get that info??

mine sits at 10.5

[reef]

I think running kh about 8 is a waste of time. NSW is about 7-8 so why have it much higher. Keep kh too high then it is hard to maintain calcium levels . KH and Calcium will balance each other.

In terms of ph I don’t think there is a magic number to achieve, mine ranges from 7.8-8.20.

[Ira]

I aim for 8.5 kh.

[cracker]

Where did you get that info??

mine sits at 10.5

NSW parameters...maybe 6 - 8 then.

[cracker]

I think running kh about 8 is a waste of time. NSW is about 7-8 so why have it much higher. Keep kh too high then it is hard to maintain calcium levels . KH and Calcium will balance each other.

In terms of ph I don’t think there is a magic number to achieve, mine ranges from 7.8-8.20.

At 10-12KH, I have no issues with maintaining calcium....

I think it will only be an issue maybe, regarding Reactor use.

Calcium and Carbonates are only affected by incorrect Magnesium levels.

Magnesium needs to be right in order to maintain the other two.

[lduncan]

... and CO2 levels.

[chimera]

Where did you get that info??

mine sits at 10.5

a bit high for zeovit i thought steve?

[reef]

At 10-12KH, I have no issues with maintaining calcium....

What is the benefit to maintain kh so high??

[cracker]

a bit high for zeovit i thought steve?

good point Chim....

do you run ZEO steve?

[cracker]

High KH does two things.

1: PH stays EXTREMELY stable.

2: The water column has LOADS of bicarbonate for rapid coral growth.

(So long as calcium and MG are present at correct levels).

Above.

Exactly the same for swimming pools. You need heaps of buffer otherwise the ph can swing more easy causing sore eyes etc.

Remember I dont use a Reactor, and I only dose weekly with MG, Ca and Bicarb.

By the end of the week my KH is down to 8 or 9.

If I didnt run at 11 or 12 it would be too low by the end of the week.

[reef]

Maybe a high KH could stress fish or damage the slim coat of the fish. could be part of the whit spot problem.

High KH does two things.

1: PH stays EXTREMELY stable.

2: The water column has LOADS of bicarbonate for rapid coral growth.

(So long as calcium and MG are present at correct levels).

SO why does ph have to be extremely stable??

I still cant see how you can maintain a calcium level of 450ppm with a Kh of 12. maybe that is why your KH is comming down so quick as it is not balanced?

I am sure Layton can find a balanced equation to show this.

[puttputt]

Over the last 4-5 weeks I've been settling my calcium reactor in and tuning the drip and bubble rates. I've had Kh swinging from 8-12 although its settled down nicely to 9.0, with calcium at 440.

I went overseas for a week when things hadn't settled down and Kh was 12 when i got back. During this time calcium (I presume) precipitated out onto my pumps, rocks, bottom glass, etc. made a bit of a mess as it wasn't a nice white, but a tan/brown colour, but definitely a gritty sandy solid. Also lost a couple of acros at this stage with STN, but may be more to the fluctuations than the high Kh.

Now that its settled down, definitely some growth starting to show (rapid growth on Montis) and finally getting coraline growth, although again, it doeslike the precipitate that formed.

[reef]

good points puttputt.

Corals can only use so much carbonate anyway so adding too much is a waste also your tank chemistry gets out of balance.

[wasp]

I am sure Layton can find a balanced equation to show this.

I'm sure he would too but he is conspicious by his absense today.

[lduncan]

Maybe a high KH could stress fish or damage the slim coat of the fish. could be part of the whit spot problem.

SO why does ph have to be extremely stable??

I still cant see how you can maintain a calcium level of 450ppm with a Kh of 12.

Maybe the same way corals do. Running a "high" pH may do it. It can be done. There is more of a link between calcium and magnesium levels though.

maybe that is why your KH is comming down so quick as it is not balanced?

I am sure Layton can find a balanced equation to show this.

A 4 dKH drop in a week isn't that much. Mine uses around that amount daily.

From the calculations earlier, calcium carbonate is already supersaturated in sea water, so is prone to precipitating out at any time. Magnesium and phosphorous inhibit it by poisoning carbonate crystals to avoid a catastrophic chain reaction occurring.

Of course the higher the calcium and alkalinity levels, the higher probability that these inhibitors can't keep up with their poisoning, and result in precipitation being easier to trigger.

Equations don't really show much when the solution is supersaturated as it is in seawater. It is possible to maintain both high alk and high calcium, but it does make catastrophic precipitation easier to trigger.

I'm sure he would too but he is conspicious by his absense today.

Jes, I don't post for a few hours and someone get suspecious . Just too busy working . Got to pay for the house some how, especially after not working (for money) as uni was taking up my life for the last 6 months or so.

Layton

[wasp]

Thought that'd flush you out! :lol:

suspecious? Si Amigo