Is carbonate hardness Total Alkalinity?

bfazio1030 · Aug 16, 2020

Little confused here. Is Carbonate hardness different then Alkalinity?

redfishbluefish · Aug 16, 2020

Carbonate hardness is the same as total alkalinity.

JimWelsh · Aug 16, 2020

redfishbluefish said:
Carbonate hardness is the same as total alkalinity.

No. Total alkalinity = carbonate hardness + other non-carbonate forms of alkalinity present, most notably, alkalinity from borate. Carbonate hardness is generally a little less than total alkalinity in a reef system. The difference isn't that great, but is worth mentioning, especially for those running higher borate levels.

Randy Holmes-Farley · Aug 16, 2020

To expand the discussion a bit...

In seawater, total alkalinity is:

TA = [HCO3–] + 2[CO3—] + [B(OH)4–] + [OH–] + [Si(OH)3O–] + [MgOH+] + [HPO4—] + 2[PO4—] – [H+]

Carbonate alkalinty is:

CA = [HCO3–] + 2[CO3—] – [H+]

Thus carbonate alk is not counting contributions to alkalinity from borate, hydroxide, silicate, magnesium monohydroxide, and some of the forms of phosphate.

In practice, all reef hobby tests that use a single titration actually measure total alkalinity (or try to).

Indirect measuring devices, like the Mindstream, give carbonate alkalinity.

As Jim noted, carbonate alk is a bit lower than total alk.

JimWelsh · Aug 16, 2020

Randy Holmes-Farley said:
In seawater, total alkalinity is:

TA = [HCO3–] + 2[CO3—] + [B(OH)4–] + [OH–] + [Si(OH)3O–] + [MgOH+] + [HPO4—] + 2[PO4—] – [H+]

Well, if you're going to go there, Dickson in 1981 published it as:

TA = [HCO3-]+2[CO3--]+[B(OH)4]+[OH-]+[HPO4--]+2[PO4---]+[SiO(OH)3-]+[HS-]+[NH3]-[H+]-[HSO4-]-[HF]-[H3PO4].

But, I was trying to keep the readers' eyes from glazing over! LOL