Reef Chemistry Question of the Day #46 General Hardness

Randy Holmes-Farley

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Reef Chemistry Question of the Day #46

What is general hardness (GH)?


A. A measure of alkalinity
B. A measure of calcium only
C. A measure of magnesium only
D. A measure of calcium plus magnesium
E. A measure of alkalinity plus calcium
F. None of the above
 

H2O

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F None of the above it measures dissolved minerals witch is mostly calcium and others depend where the water tested is from
 

Heatherrom

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F. None of these
General hardness is a measure of all +2 cations. Which is mostly calcium and magnesium, but includes many others such as strontium.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is... I'll accept either D or F.

General hardness is a measure of divalent ions, primarily calcium and magnesium, and also strontium.

Strontium is low enough in most tanks that it will hardly show on any GH test. All hobby GH tests measure these divalent ions by number of ions present (not weight present) and since strontium is so much heavier than the other two (more than twice that of calcium and 3.5 times that of magnesium), the 8 ppm of strontium in seawater will "count" as the same as 2.3 ppm of magnesium. Against a background of 1300 ppm of magnesium and several hundred more for calcium, that strontium won't be noticed.

It is possible that a GH test kit might chemically detect other divalent ions in seawater (such as ferrous iron), but I wouldn't be certain of it, and these other are at much lower concentration in seawater anyway.

One weird thing about GH is the units. It blurs these divalent ions together as if they were all calcium. That goes back to the counting by numbers of ions. A kit simply counts them one by one. So it is hardly a useful measurement for reefers to get the addition product of magnesium and calcium (and maybe strontium) in units of calcium equivalents. That is why marine aquarists (who care critically about the difference between magnesium and calcium) hardly ever use such a kit, while some freshwater people (to whom calcium and magnesium are fairly similar) use them. :)

Here's the section of one of my articles:

The Units of Measure of Reefkeeping by Randy Holmes-Farley - Reefkeeping.com

GH (general hardness)

Hardness is a characteristic of water due to the presence of dissolved calcium and magnesium. Water hardness is responsible for most scale formation in pipes and water heaters (calcium and magnesium carbonates, typically), and forms insoluble solids when it reacts with soaps. Hardness is often expressed in grains per gallon, parts per million or milligrams per liter, all as calcium carbonate equivalents. It is a measure often used in freshwater aquarium systems, but not often in marine systems, where its values are very large. Seawater has a total (general) hardness of about 6.3 g/L (6,300 ppm) of calcium carbonate equivalents.
 
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H2O

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This is good news I picked F :) what's the better answer D or F out of those 2
 
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Randy Holmes-Farley

Randy Holmes-Farley

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This is good news I picked F :) what's the better answer D or F out of those 2

Well, I suppose it depends on why you pickled F. If you thought GH was a measure of potassium, then F wasnt a better answer than D. :D
 

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