Salifert Magnesium consistently low?

[saltyhog]

I've done a couple of ICP testing (Triton) recently and my Salifert Calcium and Hanna ULR Phosphorous tests have been spot on. My Salifert Magnesium test has been 90-100 points lower than the actual value on ICP testing.

Has anyone else noticed that? What test are you using for Magnesium?

 

[Hitman]

I use Salifert and mine was off by 3 ppm for mg per my ATI ICP test.

 

[Larry L]

reefhacks.com did a test last year and found Salifert test kits to compare very closely to ATI ICP-OES: https://reefhacks.com/how-accurate-are-salifert-test-kits/

Maybe you're getting consistently low readings because of "jumping the gun" and stopping at the start of the color change but not when it's complete? (FWIW, I never feel like I really know the exact point to stop...)

A reading of 90 or 100 higher would only take about an extra 0.05 ml of reagent. Maybe next time you do the Salifert test, record it the way you have been doing, but then continue to add reagent and see if the color continues to change, or stops changing at some point within the next 0.05 ml or so.

 

[saltyhog]

Wow, I have always thought the color change to be pretty sudden and definitive but I'll give that a try. I'll order a new test kit too (since they are not expensive) and compare to my old one. Although, I believe the ICP comparison was done with two different Salifert kits already.

 

[Randy Holmes-Farley Verified Community Expert]

I've not seen convincing evidence that the Salifert kit reads low.

Also note that mg/L (usual ICP test result, at least by Triton) and ppm are not exactly the same.

1 ppm = 1 mg/kg and does not equal 1 mg/L. Since 1 L of seawater weighs about 1.025 kg, mg/l cannot exactly equal mg/kg.

1 ppm = 1 mg/kg = 1 mg/kg * 1.025 kg/L = 1.025 mg/L

Thus an ICP reading of 1300 mg/L is equal to a reading of 1268 ppm.