Difference between KH & GH?

[NathanB]

Hi all, just getting started with mixing RO water for my caridina shrimp and i'm trying to better understand water chemistry. What actually is the difference between KH and GH? I know that the higher the KH is, the more stable the ph will be? But I heard lots about needing to remineralize the RO water with only GH+ mixes, because the KH will buffer the ph up? I'm just a bit confused with this!

 

[dw1305]

Hi all,

What actually is the difference between KH and GH?

You need <"Some handy facts about water"> & <"Confusion concerning 0dKH and nitrification">. All the derivations and descriptions are in <"Water Hardness">. One thing to note is that @_Maq_ referred to dGH as "hardness" and dKH as "alkalinity" to remove any confusion.

I know that the higher the KH is, the more stable the ph will be? But I heard lots about needing to remineralize the RO water with only GH+ mixes, because the KH will buffer the ph up?

It is a bit of a strange one, limestone, (calcium carbonate (CaCO3)) is technically insoluble in water, but against that our hard tap water is saturated with calcium (Ca++) and bicarbonate (2HCO3-) ions. This occurs because the a small proportion of atmospheric CO2 goes into solution as "carbonic acid" (H2CO3) and calcium carbonate is soluble in acids. Rainwater dissolves a little bit of CaCO3 before the acid is neutralised and by this process caves are created and calcium and bicarbonate ions enter the water.

This dissolved CO2 leads to the CO2 ~ carbonate ~ pH equilibrium, where pH is dependent upon the relative proportions of the differing forms of Total (Dissolved) Inorganic Carbon (T(D)IC).

Have a look at the <"Bouncy Castle"> analogy, which explains the pH drop when you inject CO2 <"DIC and pH">.

cheers Darrel

 

[Andy Pierce]

I know that the higher the KH is, the more stable the ph will be?

No, the higher the KH is, the higher the pH will be - both a higher and lower pH/KH will be about equally stable for most normal purposes. KH doesn't really change pH stability unless you are at really low levels (approaching zero) of KH. One less thing to worry about. You can also lower pH by injecting CO2 gas - that won't change the KH or stability either.