Dosing problem

[ssavader]

I have been using the RC chemistry calculator for some time but have been trying to be more exact as to how much of each additive I need to dose to make corrections.

The problem is this: in my 125 gallon tank with sump, I started with a Mg++ of 1200 ppm and added 1000 ml of ESV Mg, which raised the Mg++ level to 1275 ppm (+75 ppm). I then added another 1000 ml of ESV Mg, which raised the Mg++ level to 1425 ppm (+150 ppm). So the first dose raised the Mg++ 75 ppm, and the second (same) dose raised the Mg++ 150 ppm! A similar incongruity happened in my 25 gallon tank with sump.

What am I missing? Is the dose/change equation not "linear"?

Randy or any other chemistry Wiz?

[Dan_P]

I have been using the RC chemistry calculator for some time but have been trying to be more exact as to how much of each additive I need to dose to make corrections.

The problem is this: in my 125 gallon tank with sump, I started with a Mg++ of 1200 ppm and added 1000 ml of ESV Mg, which raised the Mg++ level to 1275 ppm (+75 ppm). I then added another 1000 ml of ESV Mg, which raised the Mg++ level to 1425 ppm (+150 ppm). So the first dose raised the Mg++ 75 ppm, and the second (same) dose raised the Mg++ 150 ppm! A similar incongruity happened in my 25 gallon tank with sump.

What am I missing? Is the dose/change equation not "linear"?

Randy or any other chemistry Wiz?

You are correct in expecting the Mg level to increase in an arithmetic fashion. It is interesting that you were able to duplicate the error.

If the Mg additive does not require vigorous shaking before adding, we can rule out inconsistent amount of material being added. Your test procedure and the error in the test method might be a contributing to the odd results. The first Mg test might have been 25 ppm low and the second 25 ppm high, making the real difference 100 vs 125 ppm a difference no one would worry about.

When I obtain unexpected test results, I perform the test two more times. In principle, I should be doing triplicate measurements all the time :-)

[bertoni]

I agree that the numbers are odd. My first guess would be a testing problem, but the ESV requires some mixing before dosing, if my memory is correct. Which magnesium kit are you using?