Quantifying Magnesium

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pluvialis

New member
I have a test kit for calcium hardness. The results are expressed in ppm CaCO3. I also have a total hardness test kit. The results are expressed in ppm CaCO3.

For the calcium hardeness test, I know to convert PPM of CaCO3 into PPM Ca all I do is multiply the result by 0.4 to get ppm Ca.

I assume that I can subtract my calcium hardness from the total hardness to quantify the magnesium I am unsure what to do with the Total hardness result. How do I convert the odd units so I can quantify magnesium?

Here are my current parameters.

Calcium hardness = 600 ppm as CaCO3
Total hardness = 2100 ppm as CaCO3

What is my magnesium concentration?
Do I simply 2100-600 = 1500 ppm as CaCO3

Now what do I need to do to copmpensate for the odd units? Is my magnesium 1500 ppm? Should I multiply 1500 by 0.4?

Should I multiply 2100 by 0.4? Do i multiply by some other number to compensate for the odd units in total hardness? As you can see I am lost here. Please some guidance with the math would be gretaly appreciated
 
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Well, "over the rainbow" after reading that, you just made me realize, its time to go to bed. :lol:

Seriously, Bertoni or Boomer will see this in the AM and get you an answer. It might help if you tell us what brand test kits you are using.

GOOD NIGHT :)
 
I drip kalk 24/7 and am using randy's 3 part. my levels remain low. I have about 6 corals that demand calcium in a 30 gallon tank. i have good corraline growth. I dont know why Ican t maintain higher calcium levels. is this considered a High calcium demand tank? I have no SPS. My alk is always low as well (100ppm) I dripped 1 liter of randy's magnesium supplement in thinking that was my problem. Still can not maintain adequate levels. Everything is thriving however
 
You can only deduct the calcium hardness from the total hardness if the total hardness kit exclude all other ions
As total hardness kits measure all divalent ions and for fresh water Calcium and magnesium are the major contributors then the assumption can be made that the total is Magnesium and Calcium. Unfortunately in salt water there are other divalent ions that create interference like Strontium and Iron and some of the other ions (Chloride, Suphate, Sodium etc. may creaste interference with some of the kits reagents if the kit is not specifically designed for salt water and unfortunately the kits you have do not particularly indicate any of the above.
In general for fresh water Total Hardness specified in mg/lt (ppm) of Calcium Carbonate and can be diefined as:
Hardness, mg equivalent/L CaCO3 = ([Ca, mg/l]*2.497) + ([Mg, mg/l]*4.116)
In other words if you are getting the Calcium portion as 600 ppm then Calcium equivalent will be 600/2.497 or 240 ppm
If Total Hardness is 2500 mg/lt the Magnesium equivalent would be: (2500-600) = 1900 mg/lt of calcium carbonate equivalent or 462 ppm magnesium. (1900/4.116)
Both readings are so low (specially if you do water changes) that I am inclined to belive those kits are not suitable for salt water
I would recommend getting some kits specifically designed to measure salt water nor to measure total alkalinity which is the other parameter you shall target for.
For Calcium, Seachem and Salifert can give you a good reference. For Alkalnity either a LaMotte or a Salifert kit do a good job.
BTW, try maintaining salinity at 1.025 to 1.027 sg to help keep all ions within their recommended parameters.
 
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From the description of the methods in the CHEMetrics kits it does not seem that they are suitable for salt water.

Hardness (calcium)
Reference: West, T. S., DSC, Ph.D., Complexometry with EDTA and Related Reagents, 3rd ed., p. 46, 164 (1969).
Originally described as water's capacity to precipitate soap, hardness is one of the most frequently determined qualities of water. It is a composite of the calcium, magnesium, strontium and barium concentrations in a sample. The current practice is to assume total hardness refers to the calcium and magnesium concentrations only.

Completely dehardened water, resulting from sodium zeolite or other suitable ion exchange treatment, is required for various processes-including power generation, printing and photo finishing, pulp and paper manufacturing, and food and beverage processing. Hard water can cause scale formation on heat exchange surfaces resulting in decreased heat transfer and equipment damage.

The Titrimetric Method. This method is specific for calcium hardness. The EGTA titrant in alkaline solution is employed with zincon indicator. Results are expressed as ppm (mg/L) CaCO3.

Shelf-life. 8 months. Although the reagent itself is stable, the end point indicator has a limited shelf-life. We recommend stocking quantities that will be used within 7 months.


Hardness (total)
References: APHA Standard Methods, 20th ed., p. 2-37, method 2340 C (1998).
USEPA Methods for Chemical Analysis of Water and Wastes, method 130 (1983).
For a discussion of hardness, see Hardness (calcium).

This method is applicable to drinking, surface, and brine waters

The EDTA titrant is employed in alkaline solution with a calmagite indicator. This method determines the combined calcium and magnesium concentration of a sample. If no magnesium is present, the end point of the titration normally appears sluggish. However, the reagent has been specially formulated to ensure a sharp end point regardless of the presence of magnesium. Results are expressed as ppm (mg/L) CaCO3.
 
jd on the ball :D

Although Chemeterics makes good kits in many ways they do not suit us. This is a ampule comparator test kit. You have a series of ampules that you compare your reading against. These difference between the ampules is way to large. The increments are in 20 or 50 or 100 ppm and that is way to large for us.

You need a drop kit such as those jd suggested.

jd

Unfortunately in salt water there are other divalent ions that create interference like Strontium and Iron and some of the other ions (Chloride, Suphate, Sodium etc. may creaste interference with some of the kits reagents if the kit is not specifically designed for salt water and unfortunately the kits you have do not particularly indicate any of the above.

These other ions have little to do with titration procedures/ par the small amount of Sr. These kits are the same for FW as SW. It is more of a matter of their scaling. The kits that Randy and I use are HACH and there is no distinctions between FW and SW. You can also dilute the sample as Randy does but not with this Chemetrics.
 
Thanks for all the info. Too bad, Those kits were very easy to use and offered a clear endpoint to a color blind person like myself.

The total hardness kit indicates it can be used for brine waters. Does that include Seawater?

So u guys recommend using Hach for testing Mg , Ca and Alk?
 
First, let me make an error correction. I could not get the page up you posted and assumed in was a comparator test. I have some bad internal networking issue with this new PC. I finally got it up to read. I was assuming you look at the color of your sample and then compare it to a number of other sealed color stds. This is not the case. Still it is an issue, based on how they have it work. The increments will be to large as I posted above.

On your Ca++ kit it only goes to 1000 ppm CaCO3 = 400Ca ++,

1.) Not high enough
2.) The scale unit at that level is 500 or 1,000, as CaCO3. So how is one suppose to determine between 600 and 700 ppm CaCO3. This is because this kit is set more to FW and scaling conditions like in boilers, where the 500 or 1,000 is not much of an issue.

In other kits you just add the titrate until a color change takes place, just like you got with your kit , the same sharp end-point. The number of drops you added then fits a formula giving hardness. These other kits also have a limit to scaling or range they all do. HACH, for example, has scaling in 20 ppm increments as CaCO3 = 8 ppm Ca++ and your is somewhere between 200-400 ppm Ca++.


So u guys recommend using Hach for testing Mg , Ca and Alk?


That depends on you :D A reall good HACH unit is $150 and tests to within 0.04 - 4 ppm Ca++ :eek2: Other HACH's are not as pricey and of course not as accurate. These and LaMotte, par the $150 HACH unit, will will have to be set to do dilution stds, which may be a pain in the ash for you ;) I would suggest something more to and set up more to our hobby, likey like Salifert.
 
<a href=showthread.php?s=&postid=9575899#post9575899 target=_blank>Originally posted</a> by Boomer
On your Ca++ kit it only goes to 1000 ppm CaCO3 = 400Ca ++,

1.) Not high enough
2.) The scale unit at that level is 500 or 1,000, as CaCO3. So how is one suppose to determine between 600 and 700 ppm CaCO3. This is because this kit is set more to FW and scaling conditions like in boilers, where the 500 or 1,000 is not much of an issue.
Click to expand...

The scale is in 10 ppm increments up to 160ppm caco3 then 200,250,300, 350,400,500,700,1000 ppm CaCO3. I think the scale is adequate for aquarium needs. I think a little error is Ok if you are aware of the possibility of error. The main question is if the reagents work for seawater. Im waiting for a reply from Chemetrics, I ll let you know what they say. Thanks for all the help. Hopefully I can salvage some usefulness from these test kits. i tried to buy salifert but everyone seems to be out of stock of the Mg and some vendors are out of stock of Calcium and Mg.

I looked into the hach digital titrator , very attractive. It may be an overkill for aquarium needs but its nice to have that confidence. One could purchase the alk reagent with digital titrator for $180 and then buy reagents for everything else for about $20 each. The digital titrator can be used for all the tests. Its not really digital but it counts the drops for you and measures very small drops accurately. Since I am colorblind, I could use pH as an endpoint with magnetic stirrer.
I think it is worth the price (especially for the colorfully challenged) if there would be no inteference from other ions. Hach offers potassium cyanide to eliminate inerference from metals when using their Total Hrdness and calcium tests. I also read somethign about using CDTA as a titrant instead of EDTA to eliminate inerference .

Is all this really necessary? Im not sure I want to use cyanide to ensure accurate results for an aquarium. I agree Boomer, salifert is the easy way out but who has Mg, Ca and Alk in stock?


Randy speaks about alkalinity test kits here
http://www.advancedaquarist.com/issues/feb2002/chemistry.htm

This is taken from the article........"Well, the Hach kit was designed for use in fresh water where the pKa of the bicarbonate is much higher than in seawater, and in that situation, it is appropriate. In seawater, however, it is marginal. My tank water took 3.4 meq/L to get down to pH = 5.03, and then an additional 0.4 meq/L to get down to pH 4.00. Consequently, this kit (and others with a similar dye mix) may be missing out on 10% of the alkalinity simply because it isn’t titrating low enough. This difference obviously isn’t significant to most reef keepers, but is something to keep in mind when doing such things as comparing test kits to standards (in seawater) or to each other."


I have tested Natural sea water using chemetrics alk test and got accurate results for NSW (7 dKH).


In light of this , I think the chemetrics test for Alakalinity is fine. I base this on my observations and not chemistry. I hope no one can prove me wrong cos this hobby is already expensive with out buying the wrong test kits. Good thing they are free for me. Calcium and total hardness may be a different story and could explain my strange results.
 
The scale is in 10 ppm increments up to 160ppm caco3 then 200,250,300, 350,400,500,700,1000 ppm CaCO3. Ithink the scale is adequate for aquarium needs.

How is it adequate for your needs ? You need to be around 1000 ppm and the scale increments at the high range are either 700 or 1000. It would be great if it did it in 10 ppm increments all the way to 1000 ppm but it does not and that is the issue. Your sample when tested, will for example, be either 400ppm Ca++ or 280 ppm Ca++. There is no in between like 350 ppm. This is the issue and seawater is 400-425. If the scale went to 1200, then your choice is 400 or 480. If we tell you to keep the Ca++ at around 425 how are you going to know where you are ? All you will know is you are somewhere between 400 and 460 and that is know where good enough for a reef tank. I have had Chemetrics kits in the past and gave some to Randy. They are nice kits when used for the appropriate parameter in question



looked into the hach digital titrator , very attractive

Yes, it is . That is what I used for years and ended up giving it to Randy. I had many DT water parameters for it.

Randy speaks about alkalinity test kits here

Yes but that is NOT Water Hardness as in Total Hardness or Calcium Hardness that is Harndess as in Alkalinity Hardness and they are not the same. But I'm sure you know that.

The HACH Alk can be compensate for that, as Randy has pointed out . The pKa1 of seawater is 5.85 (Millero) although others use the NIST std of 6.0 for NSW and FW it is 6.34. So, this can be corrected for.

I think the chemetrics test for Alakalinity is fine

Yes, it is fine but you have not asked or brought it up till now. Most of the Lamotte are also OK
 
From Boomer...........
Although Chemeterics makes good kits in many ways they do not suit us.

Whoa! Thanks, I think ?, For the abrupt reply. The reason I stated that the chemetrics alakalinity test is fine for our needs is because it was stated that chemetrics test were no good for this hobby. To just shoot down a company like that across the board is not fair to Chemetrics and to other colorblind people that could benefit from a clear endpoint.

In my book, between 700 and 1000 is about 850, Take into account the exponential scale and your fine. IMO if Im around 350-450 Ca++ . Im happy with that cos trying to balance the Ca with alkalinity is too time consuming. If one believes that the difference between 400 and 450 ppm Ca is a big deal than your taking all this way to seriously. This is what happens when you give a chemist a reef tank. HAHHAA
Please dont take offense by that, its just a friendly stab if u will. I have worked with countless chemist and understand their frame of mind when dealling with this type of thing.

Most people dont even test the parameters and have thriving reefs.

The inherent error in all these test, especially hobby grade test like salifert can not distinguish between 400 and 450 anyway. Take one box of salifert and get 400, Take another salifert Ca test and get 450. Which is correct? Im not trying to convince people to run out and by chemetrics test kits. Perhaps Im just trying to convince myself :)Im not saying salifert is rubbish, Im trying to buy some myself.

Thanks for taking the your time boomer to help me. You are a true asset to the reef chemistry forum on RC . I was worried when Randy left but it is clear that you have an excellent understanding of chemistry so I feel reef safe.
 
Did not see this before but total hardness as CaCO<sub>3</sub> should come in at around 6100-6200 mg/l. Subtract the Ca as CaCO<sub>3</sub> from that and that gives you Mg as CaCO<sub>3</sub>. Dividing that by 4.12 gives you Mg as Mg. Just a little simplification of jdieck's formula he gave above.

The digital titrator is great but you will probably need to do a sample dilution for SW as it is mainly for FW hardness testing.
 
<a href=showthread.php?s=&postid=9584142#post9584142 target=_blank>Originally posted</a> by Billybeau1
Thats because he is old as dirt :lol:

Yes, Boomer knows his stuff. :thumbsup: :blown:

I still miss Randy :(
Click to expand...

Yes.. Yes.. and me too :D
 
Most people dont even test the parameters and have thriving reefs.

You are very, very wrong here, most do by far ;) And sorry you are way off in your assessment. I have talked to Chemetrics (Hardness) about this kit and they themselves say this kit is not suitable for our needs by far. It is a wild guess.

In my book, between 700 and 1000 is about 850

You will not know where you are that is the issue, it can be anywhere 700 and 1,000. That is a crap shoot.

IMO if Im around 350-450 Ca++

Then you are one of the .1 % in this hobby that feels that way :D

The inherent error in all these test, especially hobby grade

There are inherent error in any kit not just hobby kits. Many labs kits are not set to or give bad readings in seawater. I have been at this with HACH for many years. A lab kit does NOT mean BETTER. Many lab grade companies make kits that say they work in seawater, when in fact they can't. CO2 is just one example


its just a friendly stab if u will

Sorry, I'm not a chemistry but have spent 40 years playing with test kits ;)

not distinguish between 400 and 450 anyway

That is you, you are color blind :lol:

The inherent error in all these test, especially hobby grade test like salifert can not distinguish between 400 and 450 anyway

Based on what tests have you done to prove that. What do you mean hobby grade ? Many kits in this hobby are lab kits, by Chemetrics, HACH and others relabled and re-boxed and have hobby names stuck on them. All of the Kordon, SeaTest/FasTest, Instant Oceans and Aquatronics are HACH kits. Precession Aquarium Testing kits are Chemetrics.
 
not distinguish between 400 and 450 anyway
That is you, you are color blind

This gave me a good laugh!! Thanks!

I havent done any test to prove the inherent error in test kits but like you said, there is always going to be some error.

Can u recommend off hand any test kits that would utlize a spectrophotmeter to read the results?
 
No, but I wish I could. How easy it would be to just read a number off a meter. They do not work right in seawater :( accept a very few parameters like PO4, Silica, Cooper. A no deal period for Ca++, Mg+ and Alk. There is no such thing as a meter for Alk, other than automated Alk meter that do their own titrations and work off of a pH probe usually and at the tune of $4,000 or so.

Jd is playing with one now for PO4. NO3- can be done but you need to make up a seawater std. So, for most of us, the only thing they are good for is PO4. The issue with seawater is almost always the choride ion which is like 19,000 ppm and in some cases the Ca++ and Mg++ or the pKa being not set to seawater.
 
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