View Full Version : So you want to understand reef chemistry (part 1 - alkalinity)

10/20/1999, 10:29 PM
Read another post here where someone mentioned the importance of alkalinity over all other parameters.

So let's hear it from all of you. (Actually, I am just too lazy to do research. :)


10/20/1999, 10:58 PM
"Alkalinity- refers to the capacity to buffer against drops in pH. The greater the alkalinity the greater the ability to prevent rapid pH swings." TRA vol 1,page 513.Further reading on pages 227-228. Because we have closed systems that tend to produce acids which deplete calcium and alkalinity we're subject to pH swings and inhibited coral growth.[paraphrased from Natural Reef Aquariums by Tullock] Hope I didn't misunderstand. Free research FWIW Agu

10/20/1999, 11:18 PM
Since I'm the one who wrote that, I guess I'm supposed to start. (Ooops. Agu beat me!)

Alkalinity is often confused with Alkaline which refers to the property of having a pH above 7.0. Alkalinity is more of a measure of a tank's ability to maintain a stable pH in the face of acidifying processes. The terms "buffering capacity" and "hardness", or more precisely "carbonate hardness" come into play here too. There are many components of total Alkalinity (Calcium, Magnesium, Sodium, and other cations and their associated salts formed with Borates, Carbonates, etc. etc.) Carbonate (CO3) is one we are particularly interested in because of its function in calcification of coral skeleton. We tend to introduce Carbonate as calcium carbonate (CaCO3), sodium bicarbonate (NaHCO3), or other carbonate or bicarbonate salts. Measuring total hardness (as opposed to carbonate hardness) is a relatively useless endeavor as we would be measuring everything mentioned above including calcium, which would give results that couldn't be easily interpreted.

Alright, carbonate hardness is "where it's at", now we need to measure it. Carbonate hardness (KH) is often expressed as German Degrees (dKH) or miliequivalents per liter (meq/l). Normal sea water is around (I think I'm accurate here...doing this without hard facts in front of me...) 7 to 9dKH or 2.5 to 3.0 meq/l.

So we need to test for this stuff (KH) because it is depleted by life processes which all tend toward acidity, and it is often rapidly depleted by corals and algaes that utilize it for calcification.

Someone else take it from here....

[This message has been edited by Kirbster (edited 10-21-1999).]

[This message has been edited by Kirbster (edited 10-21-1999).]

10/20/1999, 11:41 PM
Kirbster, You're close,TRA quotes meq/l at 2.5 to 3.5, Tullock goes up to 5.0. The real issue is if the alkalinity can't buffer the changes in pH ,it's too low. My tanks run at 2.5/2.8 and the pH stays stable so it's not an issue for me.

10/21/1999, 07:43 AM
I am storing this on my hard drive. I thank you for the explanation :)

10/21/1999, 08:38 AM
The following is some information I dug out of a chemistry book. I posted this to another thread, but there was no interest in it. I thought I would add it here since it fits. Please reply if you find this type of info useful or interesting. If anyone wants, I will post the information I found about a possible chemical(read non-biological) use for Iodine in the reef aquarium.

Alkalinity and Acidity

Alkalinity is defined as the capacity of a natural water specimen to react with H+ to reach pH 4.5. To a good approximation, alkalinity is determined by (OH)-, (CO3)2-, and (HCO3)-:

alkalinity ~= [(OH)-]- + 2[(CO3)2-] + [(HCO3)-]

Alkalinity is normally expressed as mmol H+ needed to bring 1L of water to pH 4.5.

Acidity of natural waters refers to the total acid content that can be titrated to pH 8.3 with NaOH. The acidity is expressed as mmol OH- needed to bring 1L of water to pH 8.3.

Water Hardness

Hardness refers to the total concentration of alkaline earth ions in water. Since the concentrations of Ca2+ and Mg2+ are usually much greater than conectrations of the other alkaline earth ions, harness can be equated to [Ca2+] + [Mg2+]. Hardness is commonly expressed as the equivalent number of milligrams of CaCO3 per liter. Thus if [Ca2+] + [Mg2+] = 1mM, we would say that the hardness is 100 mg CaCO3 per liter because 100mg CaCO3 = 1 mmol CaCO3. Water whose is less than 60mg CaCO3 per liter is said to be "soft".

Individual Hardness refers to the individual concentrations of each alkaline earth ion.

Insoluable carbonates are converted to soluable bicarbonates by excess carbon dioxide:

CaCO3(s) + CO2 + H2O --> Ca(HCO3)2(aq)

Heating converts bicarbonate to carbonate(driving off CO2) and causes CaCO3 to precipitate. The fraction of hardness due to Ca(HCO3)2(aq) is called temporay hardness because this calcium is lost (by precipitation of CaCO3) upon heating. Hardness arising from other salts (mainly dissolved CaSO4 is called permanent hardness, because it is not removed by heating.

Alkalinity is excess of the Ca2+ and Mg2+ content is called "risidual sodium carbonate."

Quantitative Chemical Analysis (Third Edition)
Daniel C. Harris
ISBN 0-71672170-8

10/21/1999, 09:18 AM

Why don't you start a new thread about the iodine. I'm curious.


Larry M
10/21/1999, 01:28 PM
My knowledge regarding reef chemistry is very basic compared to most people, but these are two rules I live by:
1) If you get a strange reading, confirm it before doing anything. Test kits, even electronic monitors can lie.
2) If you do get into trouble, try a good sized water change (20-25%) first, before using more additives, buffers, whatever. Sometimes that's all that's needed to restore the balance.

Larry M

Visit Reef Central's Home Site at:
www.reefcentral.com (http://www.reefcentral.com)

10/21/1999, 09:53 PM
I'm speaking from experience and with minimal knowledge of chemistry here so take it from there. The ways to maintain acceptable alk IMO are 1]Don't overstock your tank 2] don't overfeed 3]do regular water changes. Salt and RO water are alot cheaper in the long run than a shelf full of chemicals and test kits.

10/21/1999, 11:32 PM
You guys are great! :) Now how about a little discussion on the alternatives to maintain alkalinity, their relative costs and benefits and how each one works?

Keep up the great work!!! :) :) :)

Larry M
10/24/1999, 06:58 PM
Anyone else for this one?

Larry M

Visit Reef Central's Home Site at:
www.reefcentral.com (http://www.reefcentral.com)

10/25/1999, 08:13 AM
I'll give it a stab. As stated above:

alkalinity ~= [(OH)-]- + 2[(CO3)2-] + [(HCO3)-]

or in other words, alkalinity is approximately equal to the hydroxide concention (OH-) + the carbonate concentration (CO3)2- + the bicarbonate concentration (HCO3)- of the solution. The hydroxide concentration is reflected by the pH (pH is really a representative of H+ concentration of the solution, but that concentration is proportional to the OH- conectration.) of the solution. Once you have the hydroxide concentration (or pH) where you want it, the carbonate and bicarbonate ions are what will keep it there. The carbonate and bicarbonate ions are used by the system in chemical reactions. They are still in the system, but have just altered their forms. The alkalinity can be maintained by chemical additions.

There are two main methods to do this.

1. Kalkwasser (lime water) or calcium hydroxide Ca(OH)2. When calcium hydroxide is dissolved in water, it breaks down into Ca2+ ions and OH- ions. The Ca2+ ions are used by corals, and other calcium loving organisms. The (OH-) ions convert dissolved CO2 into carbonate and bicarbonate ions.

2. "Buffer" compound. The basic buffer compunds add sodium carbonate [Na2CO3] and sodium bicarbonate [NaHCO3]. This type of product is available from many different manufacturer's. Each manufacturer may or may not add other compounds to "help" your reef. Beware, and always read the label. If you only want to raise your alkalinity, then make sure that the product you are interested in only contains the two elements listed above. Often times, the "buffer" compund will be a part of a two part solution to help maintain calcium and alkalinity.

3. Calcium reactor. This is a device that uses CO2 to lower the pH of the tank water, making it more acidic. This acidic solution is used to dissolve Calcium carbonate. In a nutshell, it gives the tank calcium, carbonate, and bicarbonate ions. (You can see some of the formulas above. I am not sure how many of these are desired.)

The above solutions are designed to keep alkalinity levels stable. At the same time, they are working to add calcium (#2 requires a two part additive) to the system. They all have their relative benefits, and short comings. I personally use the two part additive, on my small tank. I don't want the hassle of dosing Kalk, and the calcium reactor seems like overkill for me. I am planning a new tank, that will use a calcium reactor. I think that the reactor will require less daily work which should lend to more enjoyment time. I am not qualified to compare their realtive costs. Anyone else?

How's that?


P.S. I realized I didn't answer the question, and that is why I edited the post.

[This message has been edited by ignatz (edited 10-25-1999).]