 |
|
|
|||
|
| ||||
|
|
|
|
|
|
|
|||
|
| ||||
|
|
|
A Homemade Magnesium Supplement
There are three main sources of magnesium that aquarists have used to supplement magnesium in reef aquaria: commercial supplements, magnesium chloride and magnesium sulfate. There are a couple of commercial magnesium supplements marketed for aquarium use that claim to provide magnesium along with anions in proportions resembling the anionic composition of natural seawater. Many people, deterred by the price of these supplements, have endeavored to make their own magnesium supplements. Typically, they use either pure magnesium chloride or magnesium sulfate.Magnesium chloride is available in several grades, with varying amounts of water of hydration. The most commonly encountered forms are magnesium chloride hexahydrate and anhydrous magnesium chloride. Most “snow melt” grades of magnesium chloride are probably nearly anhydrous. The physical and handling properties of dry magnesium chloride are superior to highly hydrated forms of this, to the joy of people putting it into spreading equipment. Anhydrous magnesium chloride is 25.53 percent magnesium by mass. The balance is made of chloride ions.
Magnesium chloride hexahydrate has so much water that the magnesium is very nearly in solution already. It is astonishingly soluble and rather a mess to handle. One of its more unpleasant tricks is to re-crystallize in the storage container, forming a solid mass. It often requires considerable persuasion, pounding, chipping and, finally, cursing to get it out. Magnesium chloride hexahydrate is only 11.995 percent magnesium by mass. The rest is chloride ions and water. Nevertheless, I suggest that people use food grade or higher magnesium chloride for their reef systems. Pure grades of anhydrous magnesium chloride tend to be more expensive per unit mass of magnesium than the hexahydrate and “snow melt” grades of magnesium chloride are not guaranteed to be of adequate purity for aquarium use.
Magnesium sulfate heptahydrate, commonly called Epsom salts, is readily available in most drug stores. The Epsom salts you find there will be United States Pharmacopoeia grade and pure enough for human use. The purity and ready availability of this compound has led some people to advocate its use as a magnesium supplement for aquaria.
Although it may be fine for a small one-time adjustment, use of pure Epsom salts as a magnesium supplement will lead to an increase in the sulfate concentration of the water in the aquarium. The percentage change in sulfate concentration can be substantial. For example, if one wanted to correct the water in an aquarium from half-normal to normal seawater concentration of magnesium by using Epsom salts, it would double the sulfate concentration of the water. Like magnesium chloride hexahydrate, Epsom salts contain rather less magnesium per unit weight than you might suspect — only 9.891 percent.
For people who are avoiding the use of commercial magnesium aquarium supplements for price reasons, I’d like to suggest an alternative supplement that you can make at home, and which will cause less ionic disruption than using either pure magnesium chloride or pure magnesium sulfate. Natural seawater (35 parts per thousand salinity) has 550 millimoles per liter chloride ion and 28 millimoles per liter sulfate ions. Magnesium is accompanied by two moles of chloride per magnesium in magnesium chloride, and one mole of sulfate per magnesium in magnesium sulfate. Working through the math, it turns out that if one took 10 moles of magnesium chloride and mixed it with one mole of magnesium sulfate, one would wind up with a mixture that had almost exactly the chloride/sulfate ratio of natural seawater.
Many of you won’t have good balances at home, but making approximately a 10:1 mixture of these two salts should be fairly simple. Because their densities are fairly close to each other (1.67 grams per cubic centimeter for Epsom salts and 1.56 grams per cubic centimeter for magnesium chloride hexahydrate) one should be able to get fairly close by simply using 10 volumes of magnesium chloride hexahydrate and one volume of Epsom salts. Use of this supplement will not appreciably disrupt the ionic ratios of the major two anions in seawater.
HOME Table Of Contents BIOCHEMISTRY OF AQUARIA
©Copyright, Aquarium Frontiers and Fancy
Publications Inc. All rights reserved.