Magnesium test kit inconsistencies

[ckhorne]

I've got a bit of a dilemma with my magnesium testing.

I've been using a Seachem Reefstatus Magnesium test kit for about a year. It's the one that comes in the white plastic box, and involves adding the cotton ball to separate the interference ions, etc.

The other day I picked up a Salifert Mg test kit in an attempt to make testing magnesium easier. However, my test results are WAY different.

Same water, tests taken minutes apart. I did two tests with each kit (it was a fun evening...). I was very careful to follow the test procedure to the letter:

Salifert test #1: 1770ppm
Salifert test #2: 1710ppm

Seachem test #1: 1450ppm
Seachem test #2: 1425ppm

At least I'm consistent, but which one is right? 300ppm is a pretty serious difference...

Well, my Seachem kit comes with a 1100ppm reference sample that I've never opened. I broke the seal and ran both test kits against it:

Salifert reference test: 1500
Seachem reference test: 1150

Woah. I still tend to believe that the Seachem is a better kit, but which one can I trust? If I assume that the scales are linear and the Salifert is off by +400 (read 1500 from a 1100 reference) and the Seachem is off by +50 (1150-1100), then we get the adjusted test results:

Salifert test #1 adjusted: 1370ppm
Salifert test #2 adjusted: 1310ppm

Seachem test #1 adjusted: 1400ppm
Seachem test #2 adjusted: 1375ppm

But the big question is - can I normalize like that? More importantly, which one should I use? You can argue that stability is more critical with temperature, pH, salinity and some others, but I need to know what my Mg is - at least within 100ppm.

Any ideas? Can I add a certain amount of MgCl to RO/DI water and create a known reference sample?

[Ralph ATL]

Hello everyone! I'm just going to tag along for the ride!

[kretzkiller]

+ 1 same here. I was about to order a Salifert Mg test kit, but not am weary of doing so.

[David Grigor]

<a href=showthread.php?s=&postid=14168502#post14168502 target=_blank>Originally posted</a> by ckhorne
Can I add a certain amount of MgCl to RO/DI water and create a known reference sample?

Depending on how it was stored the water content in the MgCl is going to varry enough that I don't think you can make a reference sample that would be any better.

What salt do you normally use ? I would test on a fresh batch of salt and see what the reading is.

My guess is your not consistantly determining when the color change is occuring.

[ckhorne]

What salt do you normally use ? I would test on a fresh batch of salt and see what the reading is.

Kent and IO, but I don't know why that would make the reference samples show 300 ppm difference. I repeated both tests kits, so that a pattern was established. Even if I tested on a new batch, there's still the question of which test kit is the accurate one.

My guess is your not consistantly determining when the color change is occuring.

Hmm- I have a hard time buying that. The difference between 1400ppm and 1700ppm is 300ppm. That's about 0.3mL of the titrant. If it were one or two drops, then sure, I'm happy to mark it up to color differences. But being off by 0.3mL - 10 to 15 drops (and consistently, at that) - is unlikely.

Besides, while my eyes aren't a calibrated colorimeter, I've been testing long enough (15 years now?) to at least be consistent in when I call it. It'd be pretty unlikely that after 6 test kits, I'm off consistently to show a clear pattern...

[HighlandReefer]

Mix up a small batch of Instant Ocean salt mix to a salinity level of 1.0264 using a properly calibrated refractometer. Your test results should be close to:

Magnesium: 1070
dKH: 12
Cal: 350

See this thread: http://www.reefcentral.com/forums/showthread.php?s=&threadid=1287118&perpage=25&pagenumber=1

Everytime I check my IO salt mix I get about the same results. :)

[ssamick]

Wow I just posted something similar to this. Wierd

[GeorgeW]

Reliance on the salt mix isn't going to establish much since batch-to-batch, actual values can varry (wildly for some manufacturers):

http://reefsaltanalysis.googlepages.com/AWT_Salt_Analysis_0208.pdf

[Randy Holmes-Farley]

Well, we've found that it is a pretty good way to determine when folks have test kit errors that are large, especially when you make up a substantial portion of the salt mix container at once. For small errors I agree.

[ckhorne]

Thanks - I'll try finding some IO and mixing up 60g.

So there's no way to mix up my own solution of MgCl to come up with a known concentration that I could test against?

[Randy Holmes-Farley]

You may be able to do that, but you must recognize that a kit that works in perfectly a pure MgCl2 solution does not necessarily work perfectly in seawater.

Do you have a balance and a good source of pure MgCl2?

[rexdenton]

Test kits for Mg++ often use ammonium-hydroxide in the precipitation step. I suppose the ammonia can slowly evaporate down (out). Maybe it has lost its oomph? Just a guess.

[ckhorne]

<a href=showthread.php?s=&postid=14183176#post14183176 target=_blank>Originally posted</a> by Randy Holmes-Farley
You may be able to do that, but you must recognize that a kit that works in perfectly a pure MgCl2 solution does not necessarily work perfectly in seawater.

Ah - that makes perfect sense, and I certainly didn't take that into account.

Do you have a balance and a good source of pure MgCl2?

I have both a 10th and 100th gram scale. I get my MgCl2 from bulk reef supply, although I can get my hands on some "lab grade," through a friend in my local club.

I realize that it may not be possible to create a perfect reference solution with what I've got, but 300ppm difference is 20% at 1400ppm - I'm just looking for something a bit more accurate...


Test kits for Mg++ often use ammonium-hydroxide in the precipitation step. I suppose the ammonia can slowly evaporate down (out). Maybe it has lost its oomph? Just a guess.

That would make sense - is that a possibility?

[Billybeau1]

ck, magnesium test kits have the most test noise of any kit I've tested.

It is not unusual to get different readings from two successive tests for this reason.

From all of my testing, I have found the Salifert and Elos mag kits pretty accurate for our testing. I do not care for the Seachem as there are way too many steps to get to the final result, hence, more possibility for noise. :)

[diverdick]

Funny how these boards bring out some great discussion! Awesome..... Anyway, I have the Salifert test kit for mg. too and found the the powder materials has picked up some moisture and is clumping..... Do you think this contributes to flaky results?

Regards,

--Rich

[Billybeau1]

I do not think so. The powder is used for the color you see. :)