# Is it possible to have KH greater than GH?



## ray-the-pilot (May 14, 2008)

This question was suggested somewhere in another thread and it is a pretty interesting one.

What do you think?


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## HeyPK (Jan 23, 2004)

Yes, Quite possible. In fact, my tapwater has a KH of 9 and a GH of 0. KH is a measure of alkalinity---how much acid it takes to lower the pH to around 4.5. GH is a measure of Calcium and Magnesium content. When you have hard water due to calcium and magnesium bicarbonate, you have both a KH and a GH value, because these compounds raise the pH as well as contain Ca and Mg. In my tap water the high KH is due entirely to sodium bicarbonate. It comes out of the faucet with a pH of around 8.5. There is no measurable calcium or magnesium---at least not measurable with hobbyist test kits.


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## ray-the-pilot (May 14, 2008)

*Good answer! but?*



HeyPK said:


> Yes, Quite possible. In fact, my tapwater has a KH of 9 and a GH of 0. KH is a measure of alkalinity---how much acid it takes to lower the pH to around 4.5. GH is a measure of Calcium and Magnesium content. When you have hard water due to calcium and magnesium bicarbonate, you have both a KH and a GH value, because these compounds raise the pH as well as contain Ca and Mg. In my tap water the high KH is due entirely to sodium bicarbonate. It comes out of the faucet with a pH of around 8.5. There is no measurable calcium or magnesium---at least not measurable with hobbyist test kits.


That is interesting tap water you have down there in Mississippi.

You are right about KH and GH but this question is still a little deeper.

OK it is obvious if you only have sodium bicarbonate in your water, you can have KH greater than GH. But what if you have Calcium in your tap water as well? Can you still have KH > GH?


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## HeyPK (Jan 23, 2004)

The answer is, "yes".


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## ray-the-pilot (May 14, 2008)

Yes is a good answer.

But I was thinking that increasing the KH might cause CaCO3 to precipitate from solution.

Ca(HCO3)2 --> CaCO3 + CO2 + H2O

What do you think? Could there be some upper limit to the level of KH in the presence of Ca?


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## HeyPK (Jan 23, 2004)

Calcium bicarbonate dissociates into calcium ions and bicarbonate ions:

Ca(HCO3)2 → Ca++ + 2(HCO3)-

Bicarbonate ions are involved in the following equilibrium reactions: 

CO2 + H2O ⇄ H2CO3- + H2O ⇄ H3O+ +HCO3- + H2O ⇄ H3O+ + CO3--

At a low pH, (high concentration of hydronium ions, H3O+) the equilibrium is displaced to the left. At a low concentration of hydronium ions (high pH), the equilibrium is displaced to the right, increasing the concentration of carbonate ions, which react with calcium to form calcium carbonate which has a low solubility, and precipitates out, removing calcium and lowering the concentration of calcium ions, thus reducing the GH. 

Is this a test? Is it over?


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## ray-the-pilot (May 14, 2008)

I believe that I was asking a thought provoking question that would result in some interesting discussions about the relationship between KH and GH. 

Maybe I was wrong? Is there only one master thought that I should be following?

This is not an exam just a discussion board.

BTW. I'm really sorry if my stupid questions offend you. I'm only interested in the advancement of this hobby.


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## kid creole (Mar 5, 2009)

ray-the-pilot said:


> I believe that I was asking a thought provoking question that would result in some interesting discussions about the relationship between KH and GH.
> 
> Maybe I was wrong? Is there only one master thought that I should be following?
> 
> ...


I think he was just kidding about the test part. Overall, I would think you would be satisfied with the answer, but I can't verify it it's way beyond my knowledge or memory of chemistry.


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## kimbm04r (Apr 22, 2005)

Yes, I would say it is also possible.

These are my various stats.

Straight from the well:

Kh - 22
Gh - 11

From Tap: (after water softener)

Kh - 22
Gh - 9

From RO: (after water softener)

Kh - 4
Gh - 2

29 g Tank using tap water:

Kh - 17
Gh - 4

29 g Tank w/ 5 gallon replacement of RO:

Kh - 15
Gh - 4


We are having problems with the water softener not working right and I believe I need to stop using the tap for my water changes. My number never used to be this high.


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## BryceM (Nov 6, 2005)

A few people have water with high bicarbonate concentrations with relatively low GH. That situation is pretty rare though.

Most of the time people who report very low GH and high KH are testing water that has gone through a home ion-exchange softener.

Chemically speaking, GH and KH are almost completely independent of each other. It's rather unfortunate that these properties have been given such confusing names.


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## JamesC (Jul 26, 2005)

What also is confusing is that KH test kits measure alkalinity and not carbonate hardness.

If we take KH to mean carbonate hardness then by definition KH has to be equal or less than GH. This is because GH is a measurement of total Ca and Mg whereas KH is a measurement of Ca and Mg carbonates. Having all Ca and Mg carbonates would therefore yield equal KH and GH. Add some calcium sulphate and you get higher GH compared to KH. Adding sodium bicarbonate doesn't affect anything as it contains no Ca or Mg.

But if we take KH to mean alkalinity then KH can equal, be lower or be higher than GH as alkalinity has nothing to do with hardness. ie Ca and Mg.

James


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## BryceM (Nov 6, 2005)

JamesC said:


> If we take KH to mean carbonate hardness then by definition KH has to be equal or less than GH.


James,

That's not true. CaCO3 and MgCO3 are not the only source of carbonates in nature. They're certainly the most prevalent, but natural water sources can have a higher concentrations of carbonate than divalent cations.


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## JamesC (Jul 26, 2005)

Yes, there are many forms of carbonates but when we talk about carbonate hardness then it's only the calcium and magnesium ones that it applies to.

When we discuss water hardness it only ever refers to calcium and magnesium. Water hardness comprises of Total Hardness (GH), Temporary Hardness (KH) aka Carbonate Hardness and Permanent Hardness.

Total Hardness = Carbonate Hardness + Permanent Hardness

Even Wikipedia has got the entry for carbonate hardness wrong as do many other websites. For more info - http://www.theplantedtank.co.uk/water.htm

James


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## BryceM (Nov 6, 2005)

JamesC said:


> Yes, there are many forms of carbonates but when we talk about carbonate hardness then it's only the calcium and magnesium ones that it applies to.


I don't mean to sound argumentative. I'm actually trying to come to a clear understanding on this. Do you have a source that supports your statement above that isn't from your own website? My understanding of this is somewhat different.


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## JamesC (Jul 26, 2005)

No problem. I just used my old chemistry books for my knowledge. There's loads of stuff online about water hardness which I have looked at but some sites contradict other sites so it all becomes a bit confusing.

Here's a fairly easy article to follow that I've just found - http://fins.actwin.com/aquatic-plants/month.9806/msg00254.html

Also here's a proper KH test kit - http://www.macherey-nagel.com/StartpageWaterAnalysisTesting/VISOCOLOR/VisocolorECO/VISOCOLORECOcarbonatehardness/tabid/4786/language/en-US/Default.aspx
Notice what it says at the bottom about Carbonate hardness being that portion of calcium and magnesium ions which are present as carbonate or hydrogen carbonate.

This is the definition I've got for carbonate hardness "Carbonate hardness is defined as that portion of all calcium ions (Ca2+) and magnesium ions (Mg2+) ("hardness ions," "hardening constituents") present in 1 liter of water for which there exists an equivalent amount of hydrogen carbonate ions (HCO3-) and carbonate ions (CO32-) originating from dissolved carbonic acid."

The problem with all this is that there are many websites that state that carbonate hardness is all the carbonates and not just the Ca and Mg ones. I was always taught that water hardness only dealt with calcium and magnesium ions, but perhaps I'm wrong or maybe it's different in different parts of the world.

James


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## kid creole (Mar 5, 2009)

JamesC said:


> No problem. I just used my old chemistry books for my knowledge. There's loads of stuff online about water hardness which I have looked at but some sites contradict other sites so it all becomes a bit confusing.
> 
> Here's a fairly easy article to follow that I've just found - http://fins.actwin.com/aquatic-plants/month.9806/msg00254.html
> 
> ...


So, if I understand this correctly, simple test kits measure Alkalinity, not just carbonate hardness. I can see how that can cause a lot of confusion. And, it sounds like measuring the true carbonate hardness wouldn't be very useful anyway, even if it were named accurately.


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## BryceM (Nov 6, 2005)

Again, not to be argumentative, but I'm looking for a technical paper or scientific article, not a hobbyist-written piece.

The terms themselves are severely lacking in clarity. Like so many other concepts, our current description is hampered by archaic terminology.

Once an ionic compound dissolves in water it's meaningless from a scientific point of view to discuss how the carbonate got there. If it came from CaCO3, MgCO3, K2CO2, or even XYZCO3 it doesn't matter. Free carbonate ions are floating around, waiting to interact with fish, plants, or test kits.

It is true that our usual test kits don't measure carbonate. They measure the buffering capacity of water using a simple titration. We express the result as KH, knowing all the while that we're only making an approximation. For our purposes, we're right 99% of the time. Just keep in mind that CaCO3 and MgCO3 aren't responsible for the buffering capacity in 100% of people's tapwater. 

I wrote a paper recently (again hobbyist-written, not scientific) about GH/KH/pH, and the related concepts. I'll try to find it and post it here.


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## JamesC (Jul 26, 2005)

I do totally see where you are coming from. The problem I would guess comes from that natural fresh waters contain nearly all calcium and magnesium carbonates so this is what is talked about when discussing carbonate hardness. All I know is that through my school and college days I was taught that carbonate hardness or as it's more commonly known, temporary hardness, is defined as the amount of carbonate or hydrogen carbonate that comes from calcium and magnesium. If it was all carbonates then the nice equation Total Hardness = Temporary Hardness + Permanent Hardness would no longer be valid as Total Hardness, or GH, would now no longer be a measurement of just Ca and Mg.

As it goes Carbonate Hardness is of very little use really and tells us very little. This is because, as you said, a carbonate or hydrogen carbonate is the same regardless of where it comes from. What is important is the GH and the Alkalinity which is what we measure now. What would be great is if the test kit maufacturers could label their test kits correctly.

If I find anything more scientific I'll post up here.

James


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## kid creole (Mar 5, 2009)

JamesC said:


> All I know is that through my school and college days I was taught that carbonate hardness or as it's more commonly known, temporary hardness, is defined as the amount of carbonate or hydrogen carbonate that comes from calcium and magnesium.


Now that I'm reading this all again, some stuff is coming back to me. I seem to remember that the measurement is of CO3 is independant of where it came from, and expressed as though it came solely from Calcium. If you get your hardness from baking soda, NaHCO3, you still express the hardness in weight of CaCO3 it would have taken to get that same alkalinity.


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## JamesC (Jul 26, 2005)

I've done quite a bit of Googling to try and find out more about water hardness. It appears that I may have been slightly wrong in my assumption that water hardness only includes calcium and magnesium salts. Some web sites seem to suggest that it is all divalent cations that make up water hardness, with a couple saying that it is all multivalent cations. There are also plenty of websites that state it is only calcium and magnesium. There does seem to be some confusion with all of this and I'm still not 100% sure which one is accurate, but I'm happy to accept that multivalent is probably the correct one and the one I'm going to use from now on. This website explains it quite well - http://www.ca.uky.edu/wkrec/Hardness.htm

Going back to the initial question the answer I gave earlier I believe still stands. You can't have a carbonate hardness greater than the total hardness (GH) because the measurement of water hardness only involves multivalent cations. As sodium isn't a multivalent cation adding sodium bicarbonate will not affect the carbonate hardness, only the total alkalinity. It mentions this in the link I gave above.

James


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## BryceM (Nov 6, 2005)

OK, again, it's a semantics issue. In the hobby we use the terms carbonate hardness, alkalinity, and KH interchangeably. For the most part, that's perfectly acceptable for our purposes.

It is clear that it's possible to have a buffering capacity, that, when expressed as an equivalent carbonate hardness (or KH), will exceed the GH of the water. _In that sense_, it is possible for the KH to be higher than the GH. Our test kits measure entirely unrelated water properties when we test for GH and KH. There is no instrinstic property of total hardness that limits the ability to establish a high buffering capacity.


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## Left C (Jun 14, 2005)

Larry Frank wrote a summary article in 1998 called "Hardness alkalinity" where he summarized some of the posts about the somewhat archaic and confusing way that GH and KH is described and measured. It may be helpful.
http://fins.actwin.com/aquatic-plants/month.9801/msg00174.html

*Hardness alkalinity*

* * To: Aquatic-Plants at actwin_com
* Subject: Hardness alkalinity
* From: larry at creative_net (Larry Frank)*
* Date: Tue, 13 Jan 1998 01:11:43 -0800 (PST)

Here is a post I sent in December on hardness and alkalinity.

After reading all the posts for hardness and alkalinity, I tried to go back
through the aquarium literature that I have, and make some sense of all the
different definitions, also tried to understand how all this affects CO2
addition. The following is what I came up with. My thanks to Dave
(eworobe at cc_UManitoba.CA) for his help with understanding alkalinity. Any
errors are mine not his.

Water Hardness

All fresh water sources contain calcium and magnesium in varying
quantities. These are cations with a +2 charge. They form salts with
anions which have a negative charge. The most important of these are
bicarbonate (HCO3(-1) ; carbonate CO3(-2) ; and sulfate
SO4(-2).

General Hardness (GH) measures the cations (+ charge); for calcium and
magnesium.

Carbonate Hardness (KH) Refers to only the bicarbonate, and carbonate
anions (-charge); it does not measure the sulfates and other anions.
Carbonate Hardness is a confusing term because it does refer to
hardness, but rather to the alkalinity (the ability of a solution to resist
a Ph change with an addition of an acid.) from the carbonates and
bicarbonates. Other anions (such as hydroxide, borates, silicates, and
phosphates) can contribute to the alkalinity. To be absolutely correct, you
should NEVER use the term 'kh'; however, this is often refered to in
aquarium literature. It should be noted that it is the
bicarbonate/carbonate buffering system which provides the majority of the
alkalinity in aquariums plant aquariums.

Kh and Gh are usually close two each other, but the GH can be the same,
higher or lower to the KH depending on the Cations and Anions in the
sample. For example, a large amount of NaHCO3 would raise the (KH) and not
effect the (GH). A large amount of MgSO4 would raise the (GH) and not the
(KH).

Usually, in fresh water most of the cations are calcium and magnesium (In a
3:1 ratio) and most of the anions are carbonates. The levels for (GH) and
(KH) will often be similar.

Units

It would make sense to measure the general hardness in # of ions/liter or
molarity, but this is not used. The common units found in the literature
are degrees of general hardness dgh (GH) from the German system or ppm Ca
from CaCO3 . Carbonate hardness (KH) is a term which has nothing to do with
hardness, rather it is the amount of carbonate or bicarbonate equivalents
which effect the alkalinity or acid buffering capacity. (KH) is equated to
ppm CO3 from CaCO3

Converting from dgh and dkh to ppm CaCO3 can be accomplished by multiplying
by 17.86

How the conversion factors were derived:
(GH)
By definition 1dgh = 10mg/liter CaO
Atomic Weight Ca= 40 O=16 CaO= 56
So 10 mg/liter CaO contains 40/56 *10 = 7.143 mg/liter of Ca

By definition ppm Ca is not for elemental calcium but for ppm CaCO3 .
Atomic weight CaCO3=100
So 7.143 mg/liter of elemental Ca would be expressed as
100/40 * 7.143 = 17.8575 mg/liter(ppm)CaCO3.

1dgh = 17.86 ppm CaCO3 and 7.143 ppm Ca+2
(KH)

1 dgh is defined as 10mg/lit CaO this can be related to ppm of CaCO3 as in
above. Now the definition for dKh must have come from the amount of
carbonate in 17.86 ppm CaCO3 which has nothing to do with GH wich is
defined by CaO!
Historically GH must have been defined first in terms of CaO; hardness in
terms of ppm CaCO3 second, then KH third?

1dKh = 17.86 ppm CaCO3

From above; 1dkh = 17.8575 mg/liter CaCO3. 7.143 mg/liter of this is Ca,
the rest ;(17.8575-7.143)= 10.7145mg/liter CO3

1dKh = 10.7145 ppm CO3

For bicarbonate:

CaCO3 forms Ca(HCO3)2 in water at ph less than 10.25 . (Two bicarbonates
are formed from each carbonate ion):

CaCO3 + H20 + CO2 ---> Ca(HCO3)2

CO3 mw = 60
HCO3 mw = 61

Therefore
10.7145mg/liter CO3 from CaCO3 (each CO3 carbonate anion forms two HCO3
bicarbonate anions; 61/60*2 *10.7=21.8 mg/liter HCO3

Another way to calculate this is using molarity:

1dkh = 17.86 mg/liter CaCO3
mw CaCO3 =100
17.86 mg/liter CaCO3 = .179 m Mole CaCO3

This will form 2* .179 m Mole = .358 m Mole
Multiplying moles *mw will give mg:
0.358*61(mwHCO3) = 21.8 mg/liter HCO3

1dkh= 21.8 ppm HCO3

How to use these conversion factors:

If you have alkalinity in ppm or hardness in ppm divide by 17.86 to get degrees.

If you want to raise the alkalinity by 1dkh using CaCO3:
use 17.86 mg CaCO3
If you want to raise the alkalinity by 1dkh using NaHCO3:
mw Na =23
mw HCO3= 61
mwNaHCO3= 84
1dkh= 21.8 ppm HCO3
21.8 *84/61=30 mg/liter of NaHCO3

using molarity:
0.358 mMoles * 84(mwNaHCO3) = 30 mg/liter of NaHCO3


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## ray-the-pilot (May 14, 2008)

BryceM said:


> OK, again, it's a semantics issue. In the hobby we use the terms carbonate hardness, alkalinity, and KH interchangeably. For the most part, that's perfectly acceptable for our purposes.
> 
> It is clear that it's possible to have a buffering capacity, that, when expressed as an equivalent carbonate hardness (or KH), will exceed the GH of the water. _In that sense_, it is possible for the KH to be higher than the GH. Our test kits measure entirely unrelated water properties when we test for GH and KH. There is no instrinstic property of total hardness that limits the ability to establish a high buffering capacity.


You are right on about the problems with GH and KH.

From my perspective GH is a holdover from the 19 th century, when people were interested in how much soap they needed to clean their clothes! To express the amount of Ca+2 and Mg+2 in terms of ppm of CaCO3 is primitive at best! There are plenty of kits on the market that can give ppm of Ca+2 and Mg+2. Why do we need deg. GH? As a force (ha ha!) in the hobby, we should press to get this kind of standardization.

KH is also a problem but for a different reason.

As you pointed out, there is no clear understanding of the difference between total alkalinity and KH and even what KH is? This is not semantics!

I read this someplace but cannot find it now. It is the rigorous definition of KH that I use:

KH = {[HCO3-] + 2[CO3(-2)] + [OH-] - [H3O+]} * 100.1*1000 
Where KH is the hardness in ppm of CaCO3 
[HCO3-], [CO3(-2)],[OH-] and [H3O+] are the concentration of the respective ions in moles per liter 
100.1 is the gram molecular weight of CaCO3 
1000 converts grams to milligrams.

The way I find the absolute KH is by first titrating a sample using a standardized HCl solution to a pH of 4.5 (this is the standard test for total alkalinity). 
I boil the resulting solution for 5 minutes to remove any residual CO2. 
Then I back titrate the sample with a standardized NaOH solution. 
The difference (on a molar basis ) between the amount of NaOH and HCl used is the amount of KH in the sample.

The amount of NaOH needed to back titrate (non specific alkalinity) is usually low. For example 6 ppm of PO4(-3) only adds about 10 ppm or 0.5 deg. of non specific alkalinity: so, most tests that measure total alkalinity are reasonably accurate for KH. Actually, if you are interested in controlling the pH of your water, it doesn't make any difference at all since the total alkalinity is what you need to know, not the KH.

Actually, KH is the single most important parameter of your aquarium water chemistry in determining how well your plants grow. But that is a different topic.


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