Is a duel titration magnesium test possible?

[KingNebTheGreat]

To my understanding one of the biggest issues with at home magnesium tests is that if the reagent for masking the calcium is in anyway degraded it can't mask the calcium, resulting in some calcium being shown as magnesium. Here's my question, is it possible to make a test that combines both magnesium and calcium into the same titration test to get a combined value of both the magnesium and calcium, then a second separate test for calcium alone, then subtracting the results from the alone calcium test from the combined magnesium/calcium test for the magnesium? In my eyes that would eliminate the need for the reagent that masks the calcium. I'm also aware that strontium causes inaccuracy because it is chemically similar to both calcium and magnesium and gets bundled into the final result of at-home tests. Could a third, strontium test be used at the same time and be used to subtract from the mixed test? I'm not sure how accurate at-home reef strontium titration tests are as it's not something I really see discussed at all.
I'm not the best at chemistry let alone reef chemistry and I'm not asking if I could do this with at home tests myself to be clear. It was just something I thought about while doing some tank maintenance right after chemistry at school lol
Thank you so much!

 

[Randy Holmes-Farley]

Yes, the old Hach magnesium kit worked like this:,measure total mag and calcium and measure calcium and get magnesium by the difference.

 

[KingNebTheGreat]

Yes, the old Hach magnesium kit worked like this:,measure total mag and calcium and measure calcium and get magnesium by the difference.

Does that method measure the magnesium accurately? Like by comparison to magnesium tests today I assume it is much better but would that be good enough to reliably test magnesium at home?

 

[Randy Holmes-Farley]

I am not sure how well it does in seawater. I used it for a long time, but never evaluated it. It’s obviously more tedious, and working by differences doubles the inherent potential random errors.

I don’t recommend testing it at all now.

 

[KingNebTheGreat]

I am not sure how well it does in seawater. I used it for a long time, but never evaluated it. It’s obviously more tedious, and working by differences doubles the inherent potential random errors.

I don’t recommend testing it at all now.

Yeah ik you don't recommend testing. But it's super cool to know that it has existed and that something I thought about on the spot actually had a product behind it!
Thank you so much! I'm glad to have gotten an answer I was hoping for!