How to make a calcium standard for titration tests?

[Miami Reef]

As title states. I’m using Red Sea Calcium Pro for my calcium.

Is there a standard I can make?

 

[Randy Holmes-Farley Verified Community Expert]

A calcium kit needs to properly distinguish calcium from magnesium and maybe strontium. Thus, a true standard may need a real seawater matrix, which makes it much harder to do.

 

[JimWelsh]

I use a standard containing 50 mM of MgCl2 and 10 mM of CaCl2, with both the MgCl2 and CaCl2 stock solutions, as well as the mixed standard being standardized by a Mohr titration of the Cl. The MgCl2 and CaCl2 stock solutions were also standardized in parallel by densitometry using an Anton-Paar DMA5000 densitometer.

 

[Randy Holmes-Farley Verified Community Expert]

I use a standard containing 50 mM of MgCl2 and 10 mM of CaCl2, with both the MgCl2 and CaCl2 stock solutions, as well as the mixed standard being standardized by a Morh titration of the Cl. The MgCl2 and CaCl2 stock solutions were also standardized in parallel by densitometry using an Anton-Paar DMA5000 densitometer.

Thanks, Jim!

 

[JimWelsh]

Thus, a true standard may need a real seawater matrix, which makes it much harder to do.

Thinking more about this, I have, in the past, when working on certain projects, developed a controlled ASW mixture where I was able to take a base ASW mixture without any alkalinity, calcium, or magnesium (apart from whatever was present as contaminants in the source salts, most of which were pretty darned pure), and then add arbitrary known amounts of alkalinity, calcium, and magnesium, plus pure RODI water as needed to achieve specific concentrations of "the big three", while maintaining a target final salinity of 35 PPT. It was a lot of work, but I was able to successfully and repeatably produce such standards. So, it is doable, but, as @Randy Holmes-Farley said, it is quite difficult to do.

As far as strontium goes, it gets picked up by calcium test kits as though it were calcium, but on a molar basis, so since it's atomic mass is 87.62 vs. 40.078 for calcium, 10 PPM of strontium would inflate the calcium reading by 10 * 40.078 / 87.62 = 4.6 PPM.

For the purposes of the OP's question, my simpler recipe posted above works really well, IMHO.

 

[chemfun]

Gentlemen (@Randy Holmes-Farley and @JimWelsh),

My wife has developed a love for the home salt water reef, and therefore rekindled the joy in our life. I'fe got a 5' acrylic tank running with 3 vaiarspectra lights.

I've been using API test kits, but would rather create my own titration solutions and standards. In my search for procedures and concentrations I found this post. When I'm at school next, I'll make both the 50mM MgCl2 and 10 mM CaCl2 solutions.

I would love any assistance in what you're using for titrations. I can mix up a batch of AgNO3 to test the Cl concentration, but would love to test the Ca more accurately.

I do have a portable vernier spectrophotometer (GoDirect LED bluetooth model). I'd love to use that to expedite making a nice repeatable curve for my tests. I contacted them about their Ca specific

Would love the help. Thanks.

-P

 

[taricha]

I do have a portable vernier spectrophotometer (GoDirect LED bluetooth model). I'd love to use that to expedite making a nice repeatable curve for my tests. I contacted them about their Ca specific

spectrometer isn't really helpful for titrations (I use that spectrometer for all kinds of stuff - love it, btw). By which I mean the precision of endpoint color is not a significant source of error. eyeball comparison to a color card is plenty for establishing the endpoint of a titration.

 

[chemfun]

spectrometer isn't really helpful for titrations (I use that spectrometer for all kinds of stuff - love it, btw). By which I mean the precision of endpoint color is not a significant source of error. eyeball comparison to a color card is plenty for establishing the endpoint of a titration.

As I've been reading about the Ca titration, you're correct. I did use the spectrometer to get baselines for my phytoplankton cultures and copepod cultures. I believe I then killed the phytoplankton (ordering more stock). I was thinking more of using the spectrometer to read the test kits like phosphate and nitrate after making standards. I've always found the test solution cards difficult to read accurately.

 

[jda]

I mixed up 50g bag of IO to whatever salinity that they say on the box - like 1.022, or something - so 50 gallons worth. The whole bag to avoid settling. It tested out perfect on Salifert to whatever it said on their website, or maybe even on the box. It has been a lot time.

Check out the reliability and expense of Salifert before you make too many things. I looked at this once and they are just too cheap at $15 for calcium and $18 for alk and you can sometimes find them cheaper other places. Of course, this will not be much fun...