PH vs Alkalinity

[cybervic]

Hi, something for the chemical guru's.

I have it that pH and alkalinity are directly related to each other. So if your pH = 8 then your alkalinity = 8.

So what we are actually testing in our systems is carbonate hardness or kH.

Why are everyone calling it alkalinity?

[sunburst]

Closely related like brother and sister. Brother always looks after sister. Sister pH brother Alkalinity.
Alkalinity is a measure of a systems resistance to a change in pH. ie alkalinity stabilizers pH

[cybervic]

But according to some reading and a very good friend of mine, ph=alkalinity. So if you pH = 8 then your alkalinity = 8. So what are we actually testing for when we test for "alkalinity"? From what I've read, it seems that we are testing carbonate hardness which is not necessarily alkalinity.

And that is where my problem lies. Have a look at this http://en.wikipedia.org/wiki/PH "pH is a measure of the acidity or alkalinity of a solution"

[Alan]

Hell a really confusing issue, PH is a value for how acidic or alkaline a solution is, the alkalinity that we test is actually the KH of the water.

[Mekaeel]

im lost.the other day i had a low Kh of 5 and my Ph was 8.2 at the same time?

[irie ivan]

But according to some reading and a very good friend of mine, ph=alkalinity. So if

you pH = 8 then your alkalinity = 8.

Ummm NO!
Rather, if ur water is at a pH of 8, ur water is termed alakline.

"pH is a measure of the acidity or alkalinity of a solution"

What it means is based on the pH scale, which is based on how acidic or alakaline a solution is> the pH scale is measured form ) to 14. 7 is exactly in the middle, and is defined as a neutral solution. Any number above 7 is termed an alkaline solution or a base solution. Any value below 7 is termed as an acidic solution.

Alkalinity which we measure as dkh, alk (measured in caco3, mille equivalents, or degrees carbonate hardness) is done by adding a powder (reagent 1) or substance which appears a certain colour at a certain ph to a sample of water. Then an acid is dripped into the solution until a certain pH is reached (usually an acidic value) when the lower pH is reached, the colour of the sample changes, as it shows a different colour at the lowered pH.
Exactly what Sunburst said.

[sihaya]

Sorry, I don't have time to read all the responses here, but pH and alkalinity are NOT the same thing...

pH is "power of Hydrogen." It's literally the concentration of Hydrogen atoms in solution.

Alkalinity is, as stated, a measure of "hardness" or a measure of carbonate/bicarbonate. Carbonate/bicarbonate in solution act as a *buffer*... a weak acid and it's conjugate base. Buffers temper pH changes within a certain pH range.

For more on all this...
http://www.asira.org/anotherlookatchemistry

[palmerc]

In reefkeeping alkalinity does not mean the same as in chemistry.

In Chemistry alkalinity refers to an alkaline solution (i.e. having a pH greater than 7).

In reefkeeping the same term is used as a measure of carbonate hardness.

It is confusing but that is the way it is.

An extreme example - It is possible that a low pH solution (eg. Ph 6.0) has a carbonate hardness when tested with a marine test kit. The reason is that the end point for these tests lies at about pH 4.5 so it can measure any "alkalinity" in the solution between 4.5 and 6.0. So in this case you have an acidic solution that still has alkalinity.

Another example - A saturated solution of kalkwasser has a very high pH (12.0) but relatively low alkalinity when compared to a saturated solution of sodium bicarbonate.

So don't try to compare the two. Reefkeeping has it's own terminology. I suppose it would be better if, instead of alkalinity, that reefkeepers rather refer to buffering capacity.

[cybervic]

Thanks Clinton. Your answer clears up my question. My neighbour having a degree in chemical engineering argued with me when I told him my tank have a pH of 8.4 but the alkalinity is 5dkH. That is where the whole "issue" started.

Now I can go back to him and explain why it is like that, thanks to your clear explanation.

Thanks

Wikus

[Galibore]

Originally Posted by palmerc

I suppose it would be better if, instead of alkalinity, that reefkeepers rather refer to buffering capacity.

I agree. Reefkeeping is hard enough as it is without confusing terms. I have always wondered about this and then I decided that my chemistry teacher was screwed up. Now I know he isn't.

[Reef Maniac]

Originally Posted by palmerc

In reefkeeping alkalinity does not mean the same as in chemistry.

In Chemistry alkalinity refers to an alkaline solution (i.e. having a pH greater than 7).

In reefkeeping the same term is used as a measure of carbonate hardness...

... So don't try to compare the two. Reefkeeping has it's own terminology. I suppose it would be better if, instead of alkalinity, that reefkeepers rather refer to buffering capacity.

Very well explained

To summarise... pH is a measure of how acid or alkaline the water is, whilst alkalinity is a measure of the buffering capacity (resistance to change in pH) of the water.

Just to confuse the issue even more, though, there are different types of alkalinity. Borate is a very good buffer (increaser of alkalinity...), and will resist the change in pH very well, but it does not add any carbonate/bicarbonate to the water. Thus, if you use borate, you could have a high alkalinity, and still not have water which is good for coral growth, as the corals need carbonate, (and calcium of course) to grow their calcium carbonate skeletons...

So, as Clinton mentioned, what we reef aquarists really mean when we talk about alkalinity is actually the carbonate/bicarbonate content of the water.

Hennie

[Kanga]

Originally Posted by Reef Maniac

Very well explained

To summarise... pH is a measure of how acid or alkaline the water is, whilst alkalinity is a measure of the buffering capacity (resistance to change in pH) of the water.

Just to confuse the issue even more, though, there are different types of alkalinity. Borate is a very good buffer (increaser of alkalinity...), and will resist the change in pH very well, but it does not add any carbonate/bicarbonate to the water. Thus, if you use borate, you could have a high alkalinity, and still not have water which is good for coral growth, as the corals need carbonate, (and calcium of course) to grow their calcium carbonate skeletons...

So, as Clinton mentioned, what we reef aquarists really mean when we talk about alkalinity is actually the carbonate/bicarbonate content of the water.

Hennie

whoa, i couldnt follow a sentence in this thread and suddenly everything makes sense

Thanks Hennie, you are DA MAN, no doubt

[Tom]

Umm.. can i use a pool test kit to measure salt water pH & TA?

[Alan]

I wouldn't, not accurate enough.

[mandarinman]

man alive, i am now enlightened, thanks for understandable explanation

[herkie]

Look at this link. Anthony Calfo wrote it and its very easy to understand.
www.wetwebmedia.com/calcalkmar.htm

[cybervic]

Thanks, that is a nice link, it also clears up my confusion on ph and alk and alk and buffering.

Wikus