Seachem Reef Status strontium test

[WillpoleReefers]

I am working on developing my seawater analysis skills and getting good results titrating for calcium using EDTA, will work on the.magnesium side next. However as expected strontium proves a problem using basic lab techniques. Separation of strontium on a quantitative basis from Ca and Mg in seawater doesn’t seem to be a very practical process. I would like to know about strontium to at the least correct my calcium results for the element rather than guessing.

Best aquarist test based on claims that I can see is the Seachem Reef Status kit, giving a claimed 0.5 ppm resolution. Has anyone used this kit, any use? It uses some sort of proprietary strontium absorbtion and then elution I think. Anybody know what the technology is and whether it’s anything I could replicate? Ion exchange?

I know about the Salifert kit which seems to be pretty basic in terms of quantification. From what I can see it maybe involves first complexing all the Ca and Mg using EDTA excess , then reverse titrating to get the calcium. There then seems to be an addition of the SR4 reagent which allows strontium to be measured as a second step? Not sure what chemistry is going on here and again wonder if it is something I could replicate and refine.

Any info on the chemistry of strontium testing in seawater is welcome

Steve

 

[taricha]

i know @Rick Mathew tried to make a Sr kit work and was not pleased with results.

 

[Randy Holmes-Farley Verified Community Expert]

I've not looked into the current strontium kits available. Long ago I examined the Salifert and Seachem kits, but I do not think those are currently used. I vaguely recall a sulfate precipitation step (strontium sulfate is less soluble than calcium or magnesium sulfate), but that may be incorrect.

I would note that at the natural seawater concentration of 8 ppm, strontium only gets counted in a titration kit as 4 ppm of calcium, so with a 50% variability range in strontium (8 +/- 4 ppm), that only adds +/-2 ppm error to a calcium titration.

There are strontium selective chelating agents and Seachem may be using one of those.

For example:

https://www.sciencedirect.com/science/article/abs/pii/S1293255819301402?via%3Dihub

Crown ether is an ion-selective reagent which is usually used to extract isotopes of strontium by liquid-liquid extraction. The high efficiency is given by the good match between the ionic radius of the metal ion and the cavity size of the crown ether [4]. The ionic size of strontium ions is able to fit well in the size of 18-crown-6 ether cavity due to ionic size fitting effect. As a consequence, strontium can be selectively extracted by using 18-crown-6 ether as a chelating agent.

 

[Rick Mathew]

i know @Rick Mathew tried to make a Sr kit work and was not pleased with results.

That is correct. I was working with the Salifert Kit and was unable to get accurate measurements using known standards.

 

[WillpoleReefers]

I've not looked into the current strontium kits available. Long ago I examined the Salifert and Seachem kits, but I do not think those are currently used. I vaguely recall a sulfate precipitation step (strontium sulfate is less soluble than calcium or magnesium sulfate), but that may be incorrect.

I would note that at the natural seawater concentration of 8 ppm, strontium only gets counted in a titration kit as 4 ppm of calcium, so with a 50% variability range in strontium (8 +/- 4 ppm), that only adds +/-2 ppm error to a calcium titration.

There are strontium selective chelating agents and Seachem may be using one of those.

For example:

https://www.sciencedirect.com/science/article/abs/pii/S1293255819301402?via%3Dihub

Crown ether is an ion-selective reagent which is usually used to extract isotopes of strontium by liquid-liquid extraction. The high efficiency is given by the good match between the ionic radius of the metal ion and the cavity size of the crown ether [4]. The ionic size of strontium ions is able to fit well in the size of 18-crown-6 ether cavity due to ionic size fitting effect. As a consequence, strontium can be selectively extracted by using 18-crown-6 ether as a chelating agent.

Thanks both!

I had forgotten about crown ethers, read something about those some time ago and will pursue. I think expensive reagents but if a procedure could work then of intetest. I guess some sort of bind then displace process is involved.

I read an old paper about precipitation. I think combined sulphate and oxalate salts were involved, with heat decomposition of strontium oxalate as a step. That method while possible seemed too involved to be practical.

I am titrating seawater or a seawater standard prep with either murexide or eriochrome using disodium EDTA. I was under the impression that Sr would be scored equally with Ca as a divalent cation, giving an 8ppm average error. Hoping to get Ca error down to minimum and track dosing accurately

Steve

 

[WillpoleReefers]

That is correct. I was working with the Salifert Kit and was unable to get accurate measurements using known standards.

Thanks, I read that thread. I wondered what their reagents were and whether the method could be improved upon.

 

[Randy Holmes-Farley Verified Community Expert]

I am titrating seawater or a seawater standard prep with either murexide or eriochrome using disodium EDTA. I was under the impression that Sr would be scored equally with Ca as a divalent cation, giving an 8ppm average error. Hoping to get Ca error down to minimum and track dosing accurately

Steve

Equal moles, yes, but not equal ppm. 8 ppm strontium is the same moles as 4 ppm calcium.

 

[WillpoleReefers]

Equal moles, yes, but not equal ppm. 8 ppm strontium is the same moles as 4 ppm calcium.

Ah yes! I was asleep there sorry!