WOW!!! What a chemistry quandry.
Narkon,
Carbon dioxide does not react with air. I assume Eric mistyped. I think he meant to say that kalkwasser reacts with carbon dioxide in air. This produces highly insoluble (at least in fresh water) calcium cabonate (limestone) This is the kind of chemistry the hill country was built on, literally.
Now for my views on the subject at hand, Calcium Addition. I am likely under-informed on this subject, so please forgive any misstatements I might make.
There are three primary means of calcium addition practiced in modern reef chemistry (other methods are available but not commonly practiced). They are lime dosing (kalkwasser), calcium reactors, and calcium chloride addition. I will discuss them separately, and add a bunch of my useless opinions.
1. Kalkwasser
Kalkwasser is calcium oxide (pickling lime) dissolved in fresh water. The calcium oxide is hydrolized to form calcium hydroxide upon dissolution. A saturated solution of kalkwasser will have an initial pH of about 12.0. This is very basic and will damage your marine fauna if added too quickly. Additionally, the calcium hydroxide will react with carbon dioxide in air to form the insoluble calcium carbonate. This insoluble compound regularly causes problems with dosing pumps and can also cause line plugging.
Often times we try to make up a settled (saturated) solution of kalkwasser and then use it for top-off. This is very problematic in that it will inevitably see small amounts of carbon dioxide and create the dreaded limestone. Some try to get around this by using a kalkwasser reactor. This does resolve some of the challenges associated with the external mix and settle method.
Another option is kalkwasser slurry addition. This is a method I have practiced for a long time (over a year). The up side to this method is that the kalkwasser is made up at the time of use and very little insoluble calcium carbonate results. The downsides include concerns over pH swings and the task of daily additions. I found that with careful addition (slowly over several minutes per 1/4 teaspoon), I could affectively add up to 3/4 of a teaspoon of calcium oxide dissolved in water while only realizing a 0.2 positive pH deviation in my 55 gallon reef. With my new 180 gallon tank, I am building a kalkwasser reactor to minimize the routine nature of this process.
A definite plus for this method is that the addition of kalkwasser not only supports calcium requirements but also helps to maintain the tanks alkylinity. This is due to the addition of hydroxide ions associated with the calcium hydroxide. When the calcium hydroxide reacts with carbon dioxide to form calcium carbonate, the hydroxide is available to neutralize an acidic compounds in the water. This supports the tanks alkylinity.
2. Calcium Reactors
The use of calcium reactors for calcium addition is likely the most natural and most convenient means of calcium addition. As has been pointed out earlier in this thread, it is also the most costly up front investment for adding calcium to a reef tank.
This process is based on dissolving a natural aragonite substrate using salt water that has a low pH. The pH is reduced by the addition of carbon dioxide. In solution, carbon dioxide forms carbonic acid following a reaction with water. (Check the pH of a carbonated drink sometime. No wonder I spend so much time at the dentist) This carbonic acid reduces the pH of the salt water. With reduced pH, the aragonite is dissolved to form calcium carbonate in a salt water solution. The limit of calcium dissolution is regulated by the saturation point at any given pH.
This process requires pH monitoring of the tank and automatic shut-off functions to prevent dangerous pH dips. This means of calcium addition mimics well the natural process of calcium dissolution that occurs in the ocean.
3. Calcium Chloride Addition
The final method discussed here is the addition of calcium chloride. The addition of calcium chloride is quick, relatively cheap, and very easy. So why doesn't everyone do it? Firstly, it requires the addition of another compound to support alkylinity. The addition of calcium chloride will reduce a tanks alkylinity sharply and this mandates the addition of a buffering solution. As stated by Eric (Reefsaver) this buffering can be accomplished by using sodium hydroxide. Alternately, a standard buffering solution such as a sodium carbonate/sodium bicarbonate can also be used. Given a fairly stable calcium uptake rate, simple experimentation can reveal the proper proportions of these two components to add.
The two-part calcium addition systems are typically based on this type of chemistry. The upside to two-part systems is the premeasured ratios of the two parts. The downside is the price. There are numerous internet references to the use of cheap homemade calcium chloride systems with secondary buffering addition based on baking soda and washing powder mixtures. If anyone chooses to use sodium hydroxide in place of a traditional reef buffer, please keep in mind the very hazardous nature of liquid caustic systems.
I hope this doesn't just confuse everyone even more. I think I just confused myself. That's why I love this hobby, because it's not easy.
Later,
Trey