Dkh or kh what's the difference?

[Steve Bullington]

So I've been in the hobby more than 10 years but always used the 2 or 3 part dosing chemicals. So basically I want to get more advanced in my chems... So I'm not sure why my Api master reef kit and API test strips read calcium the way they do. I here of levels that are supposed to be X ph, or are supposed to be X ppm. And as well as magnesium levels. I know mag is needed ( I know it's a salt ) and I believe it gets depleted. So When Mg gets depleted I assume calcium ions fill the void and change the pH to a higher base? So is Mg a buffer or what am I missing...please fill in the pieces or point me in the right direction to read....thanks

 

[rkpetersen]

For our purposes, calcium, magnesium, potassium and so on are measured on linear scales indicating how much is present by mass (weight) per a specific mass (weight) of water. So if 1 gram of calcium is present in 999,999 grams of water, it's 1 part per million (ppm). Another way of saying ppm is mg/l (milligrams per liter).
Depletion or addition of calcium or magnesium doesn't directly change pH.

pH is an entirely different thing. It's a measurement of the concentration of hydrogen ion in water. But because this has a much higher range of values than elements like calcium, it's convenient to measure it in the dimensionless unit pH, which is not linear but logarithmic, and also inverted. So every change of 1 pH is a change of 10X the hydrogen ion concentration. For example, water with a pH of 7 has 10X the hydrogen ion concentration of water with a pH of 8.

Alkalinity in saltwater tanks refers to how much pH buffering capacity is present, mostly carbonate/bicarbonate.
It's measured by testing how much acid is required to neutralize the carbonate down to a certain pH level.
It's a linear measurement, and can be measured in ppm, just like calcium, etc.
But historically, two other measurements have also been used to measure alkalinity, dKH and mEq/l.
All 3 are interchangeable; 1 dKH = 0.358 mEq/l = 17.86 ppm.

 

[Steve Bullington]

Thanks, I am a little rusty on the ole chemistry, been 20 years since I took a class. But it all comes back when you start hearing different parts and so on. Totally get it, and appreciate the refresher so to speak.

 

[JimWelsh]

1 dKH = 0.358 mEq/l = 17.86 ppm.

While you are correct, it's a little easier to comprehend and remember that 1.0 meq/L = 2.8 dKH = 50 PPM.

 

[rkpetersen]

While you are correct, it's a little easier to comprehend and remember that 1.0 meq/L = 2.8 dKH = 50 PPM.

I generally work in dKH and only occasionally want to convert to other units, so the way I remember it just requires multiplication.

 

[Randy Holmes-Farley Verified Community Expert]

So When Mg gets depleted I assume calcium ions fill the void and change the pH to a higher base? So is Mg a buffer or what am I missing...please fill in the pieces or point me in the right direction to read....thanks

I'm sorry, but that isn't making sense.

Always target calcium and magnesium in ppm or mg/L. All kits sold to hobbyists report this.

Target alkalinity in dKH or meq/l (just different units like inches and centimeters).

If you want to understand the multiple, complex relationships between calcium, magnesium, and alkalinity, these will explain in great detail:

A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH by Randy Holmes-Farley - Reefkeeping.com

http://reefkeeping.com/issues/2006-06/rhf/index.php

The Relationship Between Alkalinity And pH

Chemistry And The Aquarium: The Relationship Between Alkalinity And pH

Randy answers many questions in this complex relationship.

www.advancedaquarist.com

Magnesium In Reef Aquaria

Aquarium Chemistry: Magnesium In Reef Aquaria

This article details the nature of magnesium in seawater, how it is added, measured, and removed from marine aquaria, and how it impacts the maintenance of calcium and alkalinity.

www.advancedaquarist.com