Alkalinity vs Carboante Hardness

[rtparty]

Randy,

Do you have an article about the difference between the two and how it relates to our systems? Is there a difference?

I ask because I am in the middle of some pretty extensive testing and have sent off multiple ICP tests and the carbonate hardness always comes back ~1dkh lower than my 3 alkalinity tests (Salifert, Hanna, and Red Sea.)

I am not so worried about "which one is right" but more about understanding what is happening and why.

 

[Nano sapiens]

This was asked in a older thread from a different reef site:

Tobman
09/19/2006, 01:04 PM
I went to my LFS and asked if they had an alkalinity test kit. They said no, but do have a hardness test kit which "is effectively the same". The hardness is measured in dKH.

Now as I understand it, hardness is made up mostly (>90%) by Mg and Ca. And alkalinity is made up of buffering compounds, primarily bicarbonate and carbonate. So how are hardness and alkalinity "equivalent" in terms of testing?

Tobman
09/19/2006, 01:34 PM
In an atempt to answer my own question, dKH for hardness is the hardness in CaCO3 equivalent. So if the results indicate hardness = 7 dKH, I can say that it is equivalent to 7 dKH alkalinity, yes?

Randy Holmes-Farley
09/19/2006, 02:54 PM
1 meq/L alkalinity = 2.8 dKH carbonate hardness = 50 ppm calcium carbonate equivalents.

This confusion results from a lot of crappy units being used for water parameters.

Hardness usually relates to calcium and magnesium (total hardness is the sum of those two).

Carbonate hardness is a poor name for a chemical principle, but in short, most marine aquarists treat it as being exactly the same as total alkalinity.

These articles may be helpful:

Reef Aquarium Water Parameters

http://www.reefkeeping.com/issues/2004-05/rhf/index.htm

What is Alkalinity

http://www.advancedaquarist.com/issues/feb2002/chemistry.htm

The Units of Measure of Reefkeeping

http://reefkeeping.com/issues/2005-08/rhf/index.php

What is Alkalinity

http://www.advancedaquarist.com/issues/feb2002/chemistry.htm

The Units of Measure of Reefkeeping

http://reefkeeping.com/issues/2005-08/rhf/index.php

 

[Randy Holmes-Farley Verified Community Expert]

Randy,

Do you have an article about the difference between the two and how it relates to our systems? Is there a difference?

I ask because I am in the middle of some pretty extensive testing and have sent off multiple ICP tests and the carbonate hardness always comes back ~1dkh lower than my 3 alkalinity tests (Salifert, Hanna, and Red Sea.)

I am not so worried about "which one is right" but more about understanding what is happening and why.

That might be about right.

Carbonate hardness is 2 x carbonate + bicarbonate.

total alk is

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

Chemistry and the Aquarium: What is Alkalinity?

Randy provides an overview of alkalinity as to why it's important, how it's measured, and how can it be tested.

reefs.com

from it:

Other Definitions of Alkalinity​

Any definition of alkalinity other than the total alkalinity seems to lead to confusion. For example, Millero defines the carbonate alkalinity (AC) as the alkalinity coming from just bicarbonate and carbonate (equation 8). Some test kits use this definition as well.

(8) AC = [HCO3–] + 2[CO3—]

Unfortunately, another leading author, Pankow, defines carbonate alkalinity (CO3— – Alk) as the total alkalinity down to the pH where all carbonate is converted into bicarbonate (the bicarbonate equivalence point or endpoint; about pH 8.3 in fresh water; about pH 7.3 in seawater). Consequently, it doesn’t count bicarbonate at all, and does count borate and other ions that take up acid above the carbonate endpoint. For freshwater, this type of alkalinity is represented by the phenolphthalein endpoint used in the Hach and other kits.