one for the chemists..

nobby said:

i have found out the correct mix doses for all but the magnesium chloride, i know how much i need to add to bring me up to nsw, but i dont know if there is a formula for how strong the dosing bottle should be, i dont want it to be dangerously strong or stupidly weak...

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Hey Nobby,

On the Magnesium, the biggest problem is that there are few organisms that truly consume magnesium only, and that Magnesium is one of the conservative elements of seawater, i.e., that the concentration doesent change relative to the Chlorinity of the solution or its proportion to other elements in the mix. With that in mind, making a series of water changes will restore the proportionality of the mix. However, should you want to adjust the Magnesium (due to skewing of the conservative mix with other additives in a large system), a solution that delivers Magnesium in the amounts you desire would be based on what your total volume of the water column is, and how much and how often you'd want to add magnesium. Sent me a note on your volume and your mixing skills, and I'll send you a formula based on how much you want to add at a time within the limits of the MgCL2 solubility. Prolly best to go with MgSO4, a it doesn't skew chlorality of the solution, and if Mg is truly off, most likely, so will Sulfate.

JMO

nobby said:

... the column of water is 250 uk gallons, regularity of dosing is only going to be dependant on water tests, i am using dolomite rock atm in my reactor, but the mag level does drop on occasion, so i have been dosing with things like kent's liquid magnesium, but the cost of the stuff is horrendous,i have managed to get some MgCl2.6(H20) as a crystal but i dont know how strong the actual dosing bottle should be made to...

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Just read some of the info at the Kent Marine site of which the part we want is:

Description: Tech•M Magnesium supplement is a ionically balanced product which will safely raise the magnesium level in any marine system without upsetting the ionic balance of the system. Tech•M is very concentrated, containing over 70,000 ppm of cationic magnesium, and is nitrate, phosphate and gluconate free!

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If the product is 70,000 PPM Magnesium (not all that concentrated), it would be a 7 gm/100ml solution of Magnesium ions, or a solution of 27.41% MgCl2 (anhydrous MgCl2 is 25.53% magnesium). As we would have difficulty handling anhydrous MgCl2 (VERY hygroscopic!!!) we can use the hexahydrate salt, with a molecular weight of 203.30 gms per mole. The ratio of the weights of the anhydrous salt to the hexahydrate is l : 2.135 , so that if we placed 58.52 gms of the hexahydrate salt in an appropriate volumetric flask and added sufficient water to make the total volume 100 ml, we would have a solution that would supply 7 gms of Magnesium ions for each 100 ml dose, a close approximation of the Kent product in terms of its Magnesium content. Of course, we wouldn't have all the other little additives in their solution, but THAT to me is a bonus.

In nobby's tank, 250 UK gallons is a volume of 1135 liters, or 300 gal US. If our goal is 1300mg/l of Mg, and you measured 1200 mg/l AT 35 PPT SALINITY (very important to have salinity adjusted), then you would have a deficit of 113 GMS of Magnesium, requiring 16.21 liters of the magnesium solution we just made... quite a bit, even in relation to the size of this system.

If this system has been supplemented for a long period with a 2 part additive, then the magnesium is being crowded out by the increasing amounts of sodium (from the sodium bicarbonate in the alk supplement) and the chloride (from the CaCl2 in the calcium part) present in equimolar amounts of the ca(equimolar x 2) and alk supplied. Craig Bingman suggests such systems have a 50% water change every 12 months to correct for the increasing amounts of sodium and chloride left behind when corals uptake the calcium and bicarbonate/carbonate from the water column. With this taking place, increasing sodium and chloride reduce the amount of Magnesium in the water column be increasing their relative proportions in the salinity specific gravity mix. This requires more water to maintain the salinity at 1.026 (because there are now more ions/solutes in the water column). The Magnesium is removed as we remove "salt" from our system as the salinity increases with a fixed desired volume. Salinity increases over time again, and we remove more "salt" by taking out a gallon of the seawater again, and so on... eventually with some of the computer modeling that Craig has done, the water column can become primarily Sodium and Chloride above all other ions. With such a relatively large volume, it may behoove you to have your conservative proportions evaluated, then decide whether it is better to do one big water change, or try to reestablish those proportions with additives.

HMMMMMmmmmmm.....

JMO, but prolly better to start some big water changes...

Well...

Doug1 said:

...Is there more or can this go to the archives?

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Just a few random thoughts on the subject of Mg supplementation in closed systems.

For those of you who are making your own Mg supplements due to the cost of the professional products, there is a much less expensive alternative that you can make that results in 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. Calculating the ratio of the anions of these two salts (how convenient, Sulfate and Chloride are the two major anions in seawater...), we can demonstrate that mixing 10 moles of magnesium chloride with one mole of magnesium sulfate will result in a mixture that had almost exactly the chloride/sulfate ratio of natural seawater!!!

As most of you won't have good balances at home, measuring these two substances can be accomplished for aquaria use by using 10 volumes of the chloride to 1 volume of the sulfate (espon salts), as the densities of these two salts is about the same (1.67 grams per cubic centimeter for Epsom salts and 1.56 grams per cubic centimeter for magnesium chloride hexahydrate). Use of this supplement will not appreciably disrupt the ionic ratios of the major two anions in seawater, which is my major bone of contention with the use of Magnesium supplements in general.

Addressing the issue on shifting salinities with B-ionic and similar 2 part additives is the subject of part 1 and the corresponding Part 2 of Craig Bingman's article in the Aquarium Frontiers archive.

I could prolly spend a great deal of time on this subject, but suffice it to say that most of the Magnesium shifts we see are due to incorrect salinities or mixing our ASW with one of the mixes that cuts costs by cutting the Mg content (It is NOT IO). In closed systems, there can be some precipitation of Magnesium as the Hydroxide, but very little (with limewater, this only occurs in the direct vicinity of the area of addition to the water column, if at all.) Some hermatypic organisms (primarily coralline algae) and sclerite-forming soft corals do make a "dirty" calcite or aragonite, especially the corallines, of which some spp. may incorporate up to 30% of their calcium deposits as Magnesium Aragonite (heh, if you will ). Fortunately, these sinks account for very little of the total Magnesium in the water column (think in terms of how much accreation actually occurs with coralline...). Very old systems with large deposits of coralline may see this, but regular water changes over time will have supplied all the magnesium that is needed to supplement the growth and development of the coralline.

OK, unless there are extra questions, I am done.