What is the difference between PH and Hardness?

[FINWIN]

Ever since I grew up with fishkeeping these two terms have been interchanged and confusing...because of this I always thought ph = hardness. (alkaline hard, acidic soft)

Acidic, Neutral and Alkaline my understanding this has to do with ion charge?

Hardness is what, dissolved solids? I would wonder what neutral water would be like then. More recently I 've read the term grains.

 

[Midwater]

They can be related. Alkaline water is usually hard.

Pure soft water would be neutral, not acid. But has nothing to buffer it against PH changes with any impurities, which is why it goes acid easily.

 

[duanes]

Water Hardness is determined by calcium and magnesium, which resists change (the waters buffering capacity).
Water can be Mineral rich and still be considered soft and have a low pHif other minerals are present that don't resist change.
Certain dissolved gases can also temporarily lower pH.
Water hardness for aquariums, and water hardness for producing suds in the laundry, or other household concerns, are related, but slightly different concepts.
Water can be softened for household use, by exchanging the calcium ions (in hard water) with sodium ions (this is considered soft for humans).
But for fish it's different, it's not about replacing certain ions with others. (Why water from a water softener can be just as bad for certain species, as hard water)
Naturally soft water (like Amazonian water) is basically devoid of any ions at all.

Note the difference in Total Hardness, Carbonate Hardness, and Conductivity of a Mexican water source,
with the South American Water source below

and another couple South American water sources below

 

[Leonardo_CH]

Water Hardness is determined by calcium and magnesium, which resists change (the waters buffering capacity).
Water can be Mineral rich and still be considered soft and have a low pHif other minerals are present that don't resist change.
Certain dissolved gases can also temporarily lower pH.

Its technically not the hardness minerals (GH) that resist the change but rather the carbonate ions (KH), in the hobby these 2 values are often confused as they are almost always directly proportional (as long as extra CO2 isnt added by the water supply) but in reality a very hard solution of Calcium chloride wont resist pH changes any more than distilled water.

Its also worth noting that carbonate ions arent the only buffers present in aquarium (and natural) water, tannins for example are usually made up of weak organic acids and therefore also produce a buffer that contrary to carbonates tends to keep the water more acidic, which is why the soft waters of the amazon lacking in both GH and KH dont suffer major pH swings as long as they have at least SOME low concentration base for the acids to make a buffer with like phosphate or sulfate ions.

 

[FINWIN]

Here's a report of the water in D.C. They list it as 'moderately hard' but almost seems more neutral to me. There is a seasonal shift with higher ph (up to 8.2) in the summer with an average of 7.6. Not sure if this is standard for all water locales or not. Not clear on what 'total' hardness means.

 

[Leonardo_CH]

Not clear on what 'total' hardness means.

I feel like everybody has a different definition of what 'total' means, even different governament entities cant seem to be able to decide.
Some say its just calcium and magnesium, others say its the sum of all divalent cations (ions with a 2+ charge).
The fact they used ppm makes it more confusing as it doesnt immediatly tell you what that concentration actually rapresents, with hardness degrees its always clear and comparable.

 

[duanes]

Its

Here's a report of the water in D.C. They list it as 'moderately hard' but almost seems more neutral to me. There is a seasonal shift with higher ph (up to 8.2) in the summer with an average of 7.6. Not sure if this is standard for all water locales or not. Not clear on what 'total' hardness means.

its interesting the DC chart uses both "grains" and ppm in order to to depict the hardness units that different people understand.
Grains, is usually the unit the home water softener crowd understand.
ppm is normally used by the U.S. scientific community.
When I lived in Milwaukee if our water when described in grains, was the same at 7 grains per gallon, and considered neutral, or not really hard, but not soft. But. in the lab, we used ppm...
Milwaukee water in ppm usually averaged 250 ppm (AKA mg/L).
The pH of raw lake Michigan water averaged 8.2 to 8.4, but the treatment process takes pH down to about 7.6 by the time it's coming from the tap.
Lake Michigan sits on an ancient coral reef of dolomite rock, a highly calcified aquifer, so it naturally resists pH change.
DC uses the Potomac as its drinking water source, so much more valuable.