2nd recipie continued
Part 3: The Magnesium Sulfate
Dissolve a 64 ounce container of Epsom salts (about 8 cups) in enough purified fresh water to make 1 gallon total volume. This solution is added much less frequently than the other two parts. Each time you finish adding a gallon of both parts of the Recipe #1, add 305 mL (1 ¼ cups) of this stock solution. You can add it all at once or over time as you choose, depending on the aquarium size and set up. Add it to a high flow area, preferably in a sump. In a very small aquarium, or one without a sump, I'd suggest adding it slowly; especially the first time you do so to make sure that corals don't get blasted with locally high concentrations of magnesium, sulfate, or any impurities in your Epsom Salts. The first time that you add it, you might add a small portion and make sure there isn't any problem before proceeding to add the remainder. This solution contains about 47,000 ppm magnesium and 187,000 ppm sulfate.
The calculation rational is shown below for recipe 1. The rational for recipe 2 is the same, except everything is divided by 2 and there is no baking of the baking soda. This section is provided for those who want to know how the recipe is devised, are concerned that there might be an error, or who might want to change it slightly. It is not necessary to read the following section if all that you want to do it use it.
The Dowflake material is supposed to contain 77-80% calcium chloride. From the Dow Flake web site, it has a bulk density of 0.82 - 0.96 g/dry mL, or 194 - 227 grams/level measuring cup. We will assume that it is 78.5% calcium chloride by weight and weighs 200 grams per level measuring cup. Since calcium comprises 36% of calcium chloride, by weight, each cup contains 200 x 0.785 x 0.36 = 56.5 grams of calcium.
Consequently, dissolving 2 ½ cups (500 g) of Dowflake per gallon = 141 grams of calcium per gallon, or 37,300 mg/L. You final concentration will vary with how much moisture was actually in the calcium chloride, and how well it packed in your measuring cup. A concentration of 37,300 ppm calcium is equivalent to 0.93 molar.
When calcification takes place, there are 2 moles of alkalinity lost for every 1 mole of calcium. So we need to match the calcium above with 1.86 molar baking soda (sodium bicarbonate) equivalents (before or after baking, the baking doesn't change the alkalinity). As I measure it, Arm & Hammer baking soda weighs about 264 grams per level measuring cup. Since sodium bicarbonate has a molecular weight of 84 g/mole, we need to dissolve 1.86 x 84 = 156 grams/L, or about 594 grams (2 ¼ level measuring cups) of baking soda per gallon. Note that it doesn't matter how many grams the 594 grams of baking soda turns into on baking. All you are doing is changing the amount of carbon dioxide in the baking soda:
More or less baking will only alter the pH rise on addition to the aquarium. However, substantial under baking may make it impossible to attain full dissolution of the solid material in the recipe as sodium bicarbonate is less soluble than sodium carbonate (and is why Recipe#2 is more dilute).
When you add 1 gallon of each of these additives, there will be a residue of ions remaining after calcification. These are mostly sodium and chloride, and the amounts of those two added are equal in numbers (e.g., moles), but slightly different in weight-based concentrations like ppm since they do not weigh the same.
After adding 594 grams of baking soda (1 gallon of Recipe #1), we will have added 163 grams of sodium. In natural seawater, magnesium is present at about 12.0% of the sodium concentration (by weight). In order to match the magnesium additions to the sodium additions, and leave them in a natural ratio, we then need to add 12% of 163 grams, or 19.5 grams of magnesium for every gallon of the two-part additive that we add.
After adding 500 grams of Dowflake Calcium Chloride (1 gallon of Recipe #1), we will have added 250 grams of chloride. In natural seawater, magnesium is present at about 6.7% of the chloride concentration (by weight). In order to match the magnesium additions to the chloride additions, and leave them in a natural ratio, we then need to add 6.7% of 250 grams, or 16.5 grams of magnesium for every gallon of the two-part additive that we add.
Additionally, we may want to account for magnesium that is actually incorporated into the coral skeletons. For this calculation, I have assumed that the amount of magnesium incorporated is about 6.5% of the calcium level (by weight), or about 2.5% of the skeleton by weight. In the course of adding this gallon of both parts of the two part supplement, we added 141 grams of calcium, so we need to add 0.065 x 141 = 9 grams of magnesium to account for this deposition.