Alkalinity vs. KH
Contents:
- CO2/KH/PH Ratio
by cb77-at-namaste.cc.columbia.edu (Craig Bingman) (30 Jan 1994)
- newbie chemistry 2nd try
by Wilson Angerson <gqva06-at-udcf.gla.ac.uk> (Fri, 24 Feb 1995)
- CO2 Tables
by booth-at-lvld.hp.com (George Booth) (14 Mar 1995)
- Alkalinity
by psears-at-emr1.NRCan.gc.ca (Paul Sears) (Thu, 16 Jan 1997)
- total alkalinity and total hardness
by "Roger S. Miller" <rgrmill/rt66.com> (Tue, 14 Oct 1997)
- C02 Measurements
by busko/stsci.edu (Ivo Busko) (Mon, 29 Nov 1999)
- Carbonate Hardness (was APD V4 #551
by George Slusarczuk <yurko/warwick.net> (Mon, 18 Sep 2000)
- Carbonate Hardness
by "Roger S. Miller" <rgrmill/rt66.com> (Tue, 19 Sep 2000)
- Carbonate Hardness
by George Slusarczuk <yurko/warwick.net> (Wed, 20 Sep 2000)
- Carbonate hardness
by "Roger S. Miller" <rgrmill/rt66.com> (Wed, 20 Sep 2000)
- Aquatic Plants Digest V4 #556
by George Slusarczuk <yurko/warwick.net> (Thu, 21 Sep 2000)
by cb77-at-namaste.cc.columbia.edu (Craig Bingman)
Date: 30 Jan 1994
In article <2800565-at-hplvec.LVLD.HP.COM>,
George Booth <booth-at-hplvec.LVLD.HP.COM> wrote:
>In rec.aquaria, cineaste-at-uclink.berkeley.edu (Eric Yun-Sung Fang) writes:
>
> But, as Craig Bingman pointed out to me, KH is not CO3- but total
> alkalinity, so buffers could change the KH reading without affecting
> the others, and, as you said, all bets are off.
>
>My little Tetra "Water Chemistry" handbook states:
>
> "Measuring the general hardness involves calculating the concentration
> of Ca++ and Mg++ ions that are present, whilst measurement of the
> carbonate hardness involves the ions HCO3- and CO3--."
>
>Of course, this academic debate all depends on what your specific test
>kit is actually measuring, i.e., carbonates or carbonate equivalents.
>
>In my specific tanks, HCO3- is the predominant ion and the KH/pH/CO2
>tables hold true. Your results may vary.
>
>------
>George
>
OK, George, a little game of "telephone" seems to have gone on with my
original e-mail reply to E. Fang. ;-) Here is the original email I sent
him a while back.
-------------------------------------------------------------------
Again, the term "KH" screws up a moderately knowledgable person.
"KH" is an obsolete term, and should be eradicated from the aquarium
literature. Tests for KH measure alkalinity. Other substances than
carbonate/bicarbonate can contribute to the alkalinity of the tank,
for example, the "random buffer du jour" as George put it. They will
show up in your test for "KH" and the relationships expressed in that
table will no longer hold.
I would strongly prefer that everyone start using the word "alkalinity"
for the quantity measured by these tests. It is technically correct,
"KH" isn't. Others object to KH based on the use of the word "hardness"
to refer to the concentration of various anions.
(Feel free to post this to the net. I also suggest that you bang your
head into the wall 10 times while chanting "KH bad, alkalinity good...")
<grin>
Craig
_______________________________________________________________________
OK, so that is what I orignally wrote, before it was chainsawed.
There is now this issue of the little tetra book on water chemistry.
I don't care WHAT the little tetra book says, all the tests for "KH"
on the market are ALKALINITY tests. Alkalinity is measured by acid
titration. There is nothing about acid titration that is selective
for HCO3- or CO3--, the acid titrates any sufficiently strong base
present in solution. IF there was a test that specifically identified
HCO3- and/or CO3--, then it would be a genuine test for "carbonate"
hardness.
It may very well be true that in your tank, the vast majority of the
alkalinity is in the form of HCO3- and CO3--. That isn't always the
case, though. In fact, you were the one who originally pointed out that
other buffers would show up in "KH" readings and play havoc with the
Dupla CO2 tables. Does this really mean that you are justified in
calling the parameter you read "KH?" I guess so, but WHY, why, George?
You write articles for the aquarium trade magazines. You are an individual
that people come to for advice and wisdom. You are in a position to lead,
and yet you are following, and reporting this dKH crap that was outmoded
a hundred years ago. Alkalinity should be called alkalinity, and the
proper unit for expressing it is meq/L (please don't use ppm CaCO3,
because there is a trap hidden there as well. Most of the alkalinity
in the aquarium is in the form of HCO3-, not CO3--, and there is actually
a factor of two conversion required to go between the two forms.)
When people stop using "dark ages" concentration scales, such as dKH,
ppm, degrees of Clark hardness, grains CaCO3 per King's washbasin and
god only knows what else, then maybe they can get on with the business
of really understanding what is happening in their tanks.
Sorry for the tirade.
Craig
by Wilson Angerson <gqva06-at-udcf.gla.ac.uk>
Date: Fri, 24 Feb 1995
> >> How do I raise or lower pH?
>
> >Maybe say that it's synonymous with changing KH in many
> >cases (so above section is relevant). Also mention (in passing) CO2
> >injection as a pH lowering technique.
>
> You know, I'm still not entirely comfortable with my understanding of
> buffering. The above statement is sort of true and sort of false (I
> think).
Changing pH *necessarily* involves changing KH, CO2 concentration
or both. The 3 parameters are related via the Henderson-Hasselbalch
equation:
pH = pKa + log([HCO3-]/[H2CO3]) (Eq.1)
where pKa is a constant (6.35 in FW), [HCO3-] is bicarbonate ion
concentration (which is essentially equal to KH in FW), and [H2CO3] is
effectively the dissolved CO2 concentration. This equation holds
irrespective of whether other buffers are present or not.
Some methods of changing pH directly target KH (e.g. adding baking soda)
or CO2 (injecting). Other methods also change KH (and CO2 in the short
term) but do so slightly less directly and hence wouldn't necessarily be
thought of in terms of manipulating these variables. For example, adding
NaOH to increase pH wouldn't normally be described as increasing KH or
reducing dissolved CO2, but that is nonetheless what happens. The
removal of H+ ions by combination with OH- ions causes more H2CO3 to
ionise, reducing dissolved CO2 (temporarily) and increasing HCO3-. This
increases the numerator and reduces the denominator of the log term in
eq.1 and allows the equation to balance at a higher pH. The reduction in
CO2 (and most of the increase in pH) is temporary because the long-term
equilibrium level of CO2 in an aquarium is mainly determined by physical
and biological processes (exchange with the atmosphere, respiration and
photosynthesis), and the end result when all the dust has settled is a
(very) small increase in both pH and KH.
So it *is* true that changing pH is synonymous with changing KH (or
CO2 concentration), but perhaps sometimes a bit misleading to put it
that way, especially if another buffer is introduced.
Erik wrote:
> What about the issue of two buffers competing? If you are trying to
> lower pH, it's probably because you have too much carbonate buffer
> already. Now you introduce phosphate buffer. Doesn't the carbonate
> buffer dominate anyway? Wilson, oh expert resident chemist, canst thou
> shed some light?
Depends on how much phosphate you add. As a general rule, if you add
enough to drop pH by a significant amount then phosphate will be the
main buffer. I'm being deliberately vague in using terms like "general
rule", "significant" and "main" because what happens is dependent on the
initial and final conditions (although easy to calculate in any given
case). Do you really want the ghastly details?
If KH is high to start with you need to add "a lot" of phosphoric
acid to have an impact on pH. I'd guess (on the basis of no practical
experience whatsoever) that it's not a good way to go in those
circumstances.
Thomas N wrote:
> With phosphoric acid, isn't it the case that you are actually
> replacing the KH with the phosphate ion as the buffer? At this point,
> we've messed with the chemistry of water enough to make the "KH ==
> buffering in aquariums" statement false.
Yes, you are mostly replacing it (again, assuming you've dropped pH
significantly). One reason is that the KH buffering system is "open":
i.e. one of its components - carbonic acid (dissolved CO2) - can enter
or leave the system relatively freely. You therefore can't have a high
KH and a low pH (as always, excepting the case of artificial CO2
injection) because that would entail an unsustainably high concentration
of H2CO3 (eq. 1). Dropping pH means that most of the original HCO3-
ends up literally out the window in the form of CO2. The phosphate
buffering system is "closed" (ignoring whatever enters and leaves via
feeding and metabolism). Whatever phosphate is added in the form of
phosphoric acid will stay there as one or other species of phosphate
ion. The amount of phosphate in molar terms will be 0.5-1 times as much
as there was bicarbonate destroyed, and this will normally be much more
than the amount of bicarbonate left. The relative contributions to
buffering will be pH-dependent. Shit! Sorry! A few ghastly details just
leaked out :-).
For practical purposes it doesn't very much matter that you can't equate
KH with buffering capacity. "KH" test kits will still give some kind of
indication of buffering capacity even if it's due to phosphate.
>
> Wilson? Help? :-)
Hope you don't regret asking :-)
Wilson (back at the keyboard after some distractions)
PS Don't some of these pH-down type products contain sodium dihydrogen
phosphate as opposed to phosphoric acid? That would be safer but would
introduce more phosphate for a given effect on pH.
by booth-at-lvld.hp.com (George Booth)
Date: 14 Mar 1995
Newsgroup: alt.aquaria,rec.aquaria,sci.aquaria
After a discussion with Wilson Angerson, I've realized the error in my
caveat I include with the pH/KH/CO2 tables. The intent of the tables
is to allow you to determine CO2 concentration by measuring pH and KH.
The table is accurate whether or not bicarbonate is the only buffering
agent in the water. The relation defined by the Henderson-Hasselbalch
equation for bicarbonate is still valid with other buffers in the water,
BUT you will have a difficult time determining KH with other buffers
present.
Most KH or alkalinity test kits measure total alkalinity, of which KH
is only a part. In typical soft freshwater (no phosphates or other
salts), bicarbonate is the predominate buffer and KH = alkalinity and
the table can be used to determine CO2 from pH and KH. If you have high
phosphates or other buffers present, the "KH" test kit will read high and
the table will give you incorrect values for CO2 based on pH and KH.
However, if you have a CO2 test kit, you CAN determine the true carbonate
hardness by measuring pH and CO2 and getting KH from the table.
-----------------------------------------------------------------------------
George Booth "Nothing in the world is more dangerous
booth-at-hplvec.lvld.hp.com than sincere ignorance and conscientious
Freshwater Plant Tank Technology stupidity" - Martin Luther King, Jr.
-----------------------------------------------------------------------------
by psears-at-emr1.NRCan.gc.ca (Paul Sears)
Date: Thu, 16 Jan 1997
> From: "James Purchase" <jpp-at-inforamp.net>
> Subject: Re: Water Chemistry Reference
>
>
> What is phenolphthalein alkalinity and how does it differ from total
> alkalinity (I measured my tank's water with both test kits and while the
> LaMotte and HACH test kits give identical readings for Total Alkalinity,
> the HACH kit indicates that my water has zero phenolphthalein alkalinity.
>
Phenolphthalein alkalinity is given by the amount of acid that
is required to titrate the sample _down_ to pH 8.3, the endpoint for
phenolphthalein. In effect, you are measuring the total of OH- and
CO3-- ions in your solution. If your pH _starts_ below 8.3, both
these are very low and you get a zero result.
The total alkalinity is given by a titration down to pH 4.3;
the methyl orange endpoint. This effectively measures HCO3-, CO3--
and OH-. If the phenolphthalein result was zero (as it would be for
most aquaria with plants) it just means that your "alkalinity" was
just about all bicarbonate.
>
> Can anyone recommend a comprehensive book on Water Chemistry as it applies
> to freshwater aquaculture? George Booth has made several references to one
> published by HACH but they advised me that it is no longer available. Are
> there any others which go from A-Z, in detail?
In general, aquarium books are at best weak on water chemistry.
Some screw things up really badly. All I can suggest is that you get
some general chemistry textbooks and read up first on equilibria in
general, and then look at the carbonate/bicarbonate system. The
only equilibria you really have to consider are:
H2O <-> H+ + OH- ([H+][OH-] = 10^-14)
H2CO3 <-> H+ + HCO3- ([H+][HCO3-]/[H2CO3] = 4.16 x 10^-7)
HCO3- <-> H+ + CO3-- ([H+][CO3--]/[HCO3-] = 4.84 x 10^-11)
[z] means the concentration of [z] in moles per litre.
All the numbers are temperature-dependent to some extent.
For almost all aquarium conditions, the only one you really have
to worry about is the second, but for a complete understanding, look
at all three, remembering that _all_ must be satisfied at the same time,
and that there must be a charge balance as well (all the ions in solution).
For a start, at any given pH, [OH-] is fixed, and the
ratios of [H2CO3] to [HCO3-] and [HCO3-] to [CO3--] are also fixed.
- --
Paul Sears Ottawa, Canada
Finger ap626-at-freenet.carleton.ca for PGP public key.
by "Roger S. Miller" <rgrmill/rt66.com>
Date: Tue, 14 Oct 1997
>
> I think I understand that KH measures carbonate ion concentration and GH
> measures total dissolved solids. Knowing the total alkalinity and total
> hardness, and the concentrations of Cl-, NO3-, SO4--, Ca++ and Mg++ can
> we deduce CO3--, Na+ and K+ concentrations? (assuming typical ground
> water from a well)
>
KH normally measures bicarbonate rather than carbonate, and GH measures
the concentration of alkaline earth elements (Ca, Mg, Sr, Ba), not total
dissolved solids.
You can easily calculate CO3 from alkalinity and pH; CO3 is negligible
until the pH gets well over 8. The concentration of Na+K can be
estimated by a charge-balance calculation, but it isn't possible (without
additional information) to say how much of the total is Na and how much is
K.
I've added some values to your table:
mg/l mg/meq meq/l
Chloride 45 -35.45 -1.27
Nitrate 22.2 -62.00 -0.36
Sulfate 24 -48.03 -0.50
Barium <0.02
Calcium 92 20.04 4.59
Magnesium 12.2 12.15 1.00
Ferrous iron <0.02
Manganese <0.02
Copper <0.02
Zinc 0.36
Chromium <0.02
Phosphate 0.041
pH 7.1
Total Hardness 280 50.04
Total Alkalinity 330 50.04 -6.59
Na+K 72 22.99 3.13
HCO3 402 61.02
CO2 51
CO3 0.28
The values in the "mg/meq" column lists the weight in milligrams needed
for each major constituent to make one millimole of charge - that is, one
milliequivalent. The values in the "meq/l" column show the concentration
of electrical charge due to each major constituent; the sign of the value
shows the the polarity of the ionic charge.
It's usually assumed that the water must be electrically neutral. That
is, the sum of the milliquivalents for all major species must be near 0.
The values listed above (excepting that shown for Na+K) total -3.13 meq/l.
In order for the solution to have a net charge of 0 the Na+K in solution
must provide a balancing charge of +3.13 meq/l. Sodium is usually more
common than potassium in natural freshwater but without additional
information it isn't possible to tell how much of the charge comes from
sodium and how much is from potassium. The normal procedure is to
represent all that balancing charge as sodium, and call it Na+K as Na.
72 mg/l of Na are required to provide 3.13 meq/l.
I assumed that the nitrate concentration is listed as nitrate. If it is
shown as nitrogen, then this is polluted water. Also, these calculations
need to be adjusted.
The table also shows the calculated concentrations of the dissolved
inorganic carbon species. The numbers are based on the alkalinity and pH
and the (usually good) assumption that the alkalinity is provided entirely
by bicarbonate (HCO3). Bicarbonate is calculated by dividing alkalinity
in mg/l by 50.04 mg/meq, then multiplying that by 61.02 mg/meq for
bicarbonate. CO2 is calculated from bicarbonate and pH with
CO2 = HCO3*1.6*10^(6-pH)
and carbonate is calculated with
CO3 = HCO3*0.553*10^(ph-10)
which uses pK = 10.25. The calculated CO3 concentration (0.28 mg/l) is
functionally zero because of the limited precision of the alkalinity
value. If the CO3 concentration were significant then the assumption that
alkalinity is all provided by bicarbonate would be wrong and a more
complicated method would be needed.
Calcium and magnesium together total 5.59 meq/l. The general hardness
due to calcium and magnesium is then 5.59*50.04 = 280 mg/l, exactly the
value shown in your table for the general hardness.
General hardness does not measure total dissolved solids (TDS). You can
calculated the TDS concentration if you like: sum all of the major
constituents other than bicarbonate, then add in 49% of the bicarbonate
concentration. The total dissolved solids estimate is then about 393 mg/l
- - substantially higher than the general hardness. If your analysis
included silica then the calculated TDS would be more accurate and probably
10 or 20 mg/l higher.
The 49% correction on the bicarb concentration is to keep the estimate
consistent with wet-chemistry results. The normal lab procedure calls for
drying the water sample at 105 degrees C, then weighing the residue to
determine the solids content. Before the sample is completely dry the
bicarbonate breaks down by the reaction:
2(HCO3-) -> H20 + CO2 + CO3--
The weight of CO3 that remains to be measured is 49% of the weight of the
original bicarb.
Gee, I hope that's all worth it.
Roger Miller
by busko/stsci.edu (Ivo Busko)
Date: Mon, 29 Nov 1999
There has been a number of posts lately about peat wreaking havoc with
the pH/kH/CO2 relationship. I saw a *huge* effect when adding a small bag
with about 200 ml of fine ground gardening sphagnum peat to my aquarium.
With everything else kept the same, including kH and the CO2 bubbling rate,
in less than two days the pH dropped from 6.4 - 6.6 to below 6.0 (the lower
limit of my test kit). The pH had been stable for several months before, only
occasionaly raising to the 6.8 - 7.0 range when the CO2 yeast is near
exhaustion (tap water has pH = 7.4). So in this case in particular the
introduction of peat in the water *did* break up the relationship.
- -Ivo Busko
Baltimore, MD
by George Slusarczuk <yurko/warwick.net>
Date: Mon, 18 Sep 2000
Hello George,
You are, of course, right that measurement of alkalinity (the so-called
KH) in many cases does NOT measure "carbonate hardness" -- if other
alkalinity components are present in the sample and thus introduce an
error.
There IS a relatively simple way to measure "true" Carbonate Hardness.
It is based on the fact, that carbonate hardness equals to "temporary
hardness" (that's where the term "carbonate hardness" came from).
Measuring Total Hardness and then Permanent Hardness, and then
subtracting the second from the first gives one temporary (carbonate)
hardness.
To get a measure of Permanent Hardness" -- measure water hardness on the
original sample. Then boil another water sample gently for about 15
minutes, cool it without exposure to the atmosphere (so that CO2 does
not get reabsorbed), filter out (or let settle out) any precipitated
calcium carbonate and measure hardness again. Your second measurement
will be of Permanent Hardness.
The difference between the initial reading and that done on the boiled
sample IS "temporary" or "carbonate" hardness.
Of course, many waters might not have any Permanent Hardness or any
Carbonate Hardness, but most will probably have a mixture of both.
Waters in the South-Western US will probably have a relatively large
"other alkalies" component that will add to the _alkalinity_
measurement, but NOT to the water hardness value.
Best,
George
> Date: Mon, 18 Sep 2000 08:52:50 -0600 (MDT)
> From: George Booth <booth@lvld.agilent.com>
> Subject: Re: Minor technical corrections
>
> >Date: Sat, 16 Sep 2000 13:47:49 -0700
> >From: Wright Huntley <huntley1@home.com>
> >Subject: pH hogwash?
> >
> >4 ) pH is a useful thing to measure, along with alkalinity (unfortunately
> >confused with hardness by calling it "carbonate hardness") primarily to use
> >the CO2 concentration equations or charts to set the CO2 level for proper
> >plant nutrition.
>
> Just to be perfectly clear: The pH/CO2 charts are based on the measure of the
> carbonate ions present in the water, often called carbonate hardness or KH. KH
> can NOT be measured directly, so an alkalinity test is usually substituted. If
> carbonates are the main form of alkalinity, this will produce useful results. If
> other forms of alkalinity are also present (such as phosphates), the results
> will be garbage.
>
> >Date: Sun, 17 Sep 2000 11:44:03 -0700
> >From: "Dixon, Steven T. (BEn)" <stdixon@ben.bechtel.com>
> >Subject: CO2
> >
> >Julius Odian wrote: "[lowering pH with CO2 is bad]"
> >
> >Let's look at the implications of Julius' first sentence. The equilibrium
> >level of dissolved CO2 in a water column exposed to air is fairly low,
> >around 2 - 3 ppm CO2.
>
> The theoretical equilibrium value is roughly 0.5 mg/l, +/- some based on
> temperature and altitude. Typical values seen in a non-CO2-injected aquariums is
> 2-3 mg/l due to CO2 generated by the bioload present.
>
> George Booth
> Keeping those myths from getting out of control
by "Roger S. Miller" <rgrmill/rt66.com>
Date: Tue, 19 Sep 2000
On Tue, 19 Sep 2000, George Slusarczuk wrote:
> There IS a relatively simple way to measure "true" Carbonate Hardness.
> It is based on the fact, that carbonate hardness equals to "temporary
> hardness" (that's where the term "carbonate hardness" came from).
> Measuring Total Hardness and then Permanent Hardness, and then
> subtracting the second from the first gives one temporary (carbonate)
> hardness.
"Carbonate hardness" as you describe it here measures that part of the
total hardness that is balanced by carbonates. If the water contains more
carbonate than general hardness then this method does not measure the
amount of bicarbonate in solution (which is, after all, what we want to
measure).
My tap water for instance, contains about 1 degree of general hardness
and 7 degrees of alkalinity--all apparently from bicarbonate. Going
through the procedure you describe, the water would have 1 dGH before
boiling (Total Hardness) and 0 dGH after boiling (Permanent Hardness) and
the method would determine 1 dGH of "carbonate hardness".
Uh... That's not a very useful result.
The boiling method works in some water, but not in others. The tip off
comes from the measure of Permanent Hardness. If that is 0 then the
method can't be used. As it happens this value often is 0.
As an aside, the USGS (and EPA, I believe) use the term "non-carbonate
hardness", but I have found no use in modern English-language
water-quality literature for the term "carbonate hardness".
> Of course, many waters might not have any Permanent Hardness or any
> Carbonate Hardness, but most will probably have a mixture of both.
> Waters in the South-Western US will probably have a relatively large
> "other alkalies" component that will add to the _alkalinity_
> measurement, but NOT to the water hardness value.
For what it's worth, its very rare for unpolluted natural water from the
southwest to contain a significant amount of anything that will contribute
to alkalinity except bicarbonate. I have seen analyses that contained
significant amount of HS- and I understand that acetate may be more common
than is widely recognized. Either of these could appear as part of the
measured alkalinity but these are rare in natural water; HS- might appear
in some oil field brines, but typically both of these flag heavily
polluted water such as septic tank or landfill leachate and will never
show up in a public water supply. These pollutants are probably more
common outside the southwest US.
Roger Miller
by George Slusarczuk <yurko/warwick.net>
Date: Wed, 20 Sep 2000
Hello Roger,
It seems that we are talking here a bit past each other, probably
because the term "Carbonate Hardness" is often misapplied to mean
"Alkalinity".
If one wants to measure ALKALINITY, then titration is both an easy and
accurate method. Of course it does not tell one where the alkalinity is
coming from, but that is another story.
In nature, alkalinity _might_ be equal to carbonate hardness, but there
is no guarantee. If baking soda is added to the water, as many aquarists
do, then the two values DEFINITELY will NOT be equal! So that is the
best reason why one should not use the term "Carbonate Hardness" for
"Alkalinity". It is simply wrong.
CARBONATE HARDNESS is just that -- that component of dissolved calcium
and magnesium in water that, as you say it, is counterbalanced by
bicarbonate ion. It is a _component_ of alkalinity. It is also called,
properly, "Temporary Hardness", because it can be removed by boiling.
If, as you say, the EPA talks about "non-carbonate hardness", then, by
implication, there IS a "carbonate hardness" (perhaps of no interest to
them), and, I am certain, that they do not mean "Alkalinity" by it!
The example you cite with your water is right: Your water has 1 "degree"
of Carbonate Hardness and 7 "degrees" of Alkalinity. Nothing more,
nothing less. I place "degree" in quotes, because *a priori* one would
not know WHICH "degree" you have in mind -- American, English, French,
German, or ... As you know, the EPA uses milligrams-per-liter (or ppm)
as units for Water Hardness. Wouldn't it be a good idea for aquarists
also to use it, exclusively?
Unless I misunderstood your other example, the boiling method CAN be
used on waters without any Permanent Hardness: The second value
(Permanent Hardness) just will be "zero". Subtracted from Total Hardness
it will give you the correct answer -- all water hardness is due to
Temporary or Carbonate Hardness.
What I had in mind, mentioning "other alkalies" in the water in the SW
US, was borates and silicates. Perhaps my geographical designation was
not accurate enough.
Best,
George
> Date: Tue, 19 Sep 2000 08:02:43 -0600 (MDT)
> From: "Roger S. Miller" <rgrmill@rt66.com>
> Subject: Re: Carbonate Hardness
>
> On Tue, 19 Sep 2000, George Slusarczuk wrote:
>
> > There IS a relatively simple way to measure "true" Carbonate Hardness.
> > It is based on the fact, that carbonate hardness equals to "temporary
> > hardness" (that's where the term "carbonate hardness" came from).
> > Measuring Total Hardness and then Permanent Hardness, and then
> > subtracting the second from the first gives one temporary (carbonate)
> > hardness.
>
> "Carbonate hardness" as you describe it here measures that part of the
> total hardness that is balanced by carbonates. If the water contains more
> carbonate than general hardness then this method does not measure the
> amount of bicarbonate in solution (which is, after all, what we want to
> measure).
>
> My tap water for instance, contains about 1 degree of general hardness
> and 7 degrees of alkalinity--all apparently from bicarbonate. Going
> through the procedure you describe, the water would have 1 dGH before
> boiling (Total Hardness) and 0 dGH after boiling (Permanent Hardness) and
> the method would determine 1 dGH of "carbonate hardness".
>
> Uh... That's not a very useful result.
>
> The boiling method works in some water, but not in others. The tip off
> comes from the measure of Permanent Hardness. If that is 0 then the
> method can't be used. As it happens this value often is 0.
>
> As an aside, the USGS (and EPA, I believe) use the term "non-carbonate
> hardness", but I have found no use in modern English-language
> water-quality literature for the term "carbonate hardness".
>
> > Of course, many waters might not have any Permanent Hardness or any
> > Carbonate Hardness, but most will probably have a mixture of both.
> > Waters in the South-Western US will probably have a relatively large
> > "other alkalies" component that will add to the _alkalinity_
> > measurement, but NOT to the water hardness value.
>
> For what it's worth, its very rare for unpolluted natural water from the
> southwest to contain a significant amount of anything that will contribute
> to alkalinity except bicarbonate. I have seen analyses that contained
> significant amount of HS- and I understand that acetate may be more common
> than is widely recognized. Either of these could appear as part of the
> measured alkalinity but these are rare in natural water; HS- might appear
> in some oil field brines, but typically both of these flag heavily
> polluted water such as septic tank or landfill leachate and will never
> show up in a public water supply. These pollutants are probably more
> common outside the southwest US.
>
> Roger Miller
by "Roger S. Miller" <rgrmill/rt66.com>
Date: Wed, 20 Sep 2000
On Wed, 20 Sep 2000, George S. wrote:
> It seems that we are talking here a bit past each other, probably
> because the term "Carbonate Hardness" is often misapplied to mean
> "Alkalinity".
>
> If one wants to measure ALKALINITY, then titration is both an easy and
> accurate method. Of course it does not tell one where the alkalinity is
> coming from, but that is another story.
I think that bicarbonate concentration is the quantity we actually want to
get at -- it's just that we normally use it in units of hardness, rather
than as the actual bicarbonate concentration. Temporary hardness (your
carbonate hardness) isn't something we want to analyse.
Incidentally, the USGS and EPA standard methods both analyse bicarbonate
with the alkalinity titration, with the results reported as ppm HCO3-
rather than as ppm CaCO3. Other acids can effect the alkalinity
titration, and these are regarded as interference in the analysis. Our
problem is to determine when we have interference and to try to work
around the problem.
> If, as you say, the EPA talks about "non-carbonate hardness", then, by
> implication, there IS a "carbonate hardness" (perhaps of no interest to
> them), and, I am certain, that they do not mean "Alkalinity" by it!
That's a pretty solid logic, but they use the term "temporary hardness"
instead. Perhaps they would just like to avoid the confusion inherent in
associating the word "carbonate" with something that actually measures
calcium and magnesium, not carbonate.
> The example you cite with your water is right: Your water has 1 "degree"
> of Carbonate Hardness and 7 "degrees" of Alkalinity. Nothing more,
> nothing less. I place "degree" in quotes, because *a priori* one would
> not know WHICH "degree" you have in mind -- American, English, French,
> German, or ... As you know, the EPA uses milligrams-per-liter (or ppm)
> as units for Water Hardness. Wouldn't it be a good idea for aquarists
> also to use it, exclusively?
As long as I'm using consistent units, it makes no difference who's degree
I use, or whether I use ppm -- the conclusion is the same. My kits
measure in degrees, and I like using small round numbers, so I see no
value in converting things.
> Unless I misunderstood your other example, the boiling method CAN be
> used on waters without any Permanent Hardness: The second value
> (Permanent Hardness) just will be "zero". Subtracted from Total Hardness
> it will give you the correct answer -- all water hardness is due to
> Temporary or Carbonate Hardness.
True, but I think in the original context George B. was talking about
bicarbonate concentration (however you want to express it), not hardness.
> What I had in mind, mentioning "other alkalies" in the water in the SW
> US, was borates and silicates. Perhaps my geographical designation was
> not accurate enough.
Ah. Silicates are present in significant quantities in most groundwater
supplies, but borates are pretty rare. In either case, they won't effect
alkalinity unless the water your titrating starts out with a pH > 9.
That's fairly unusual in aquarium water. The lowest pKa for boric acid is
9.14, and the lowest pKa for silicic acid is 9.66.
Roger Miller
by George Slusarczuk <yurko/warwick.net>
Date: Thu, 21 Sep 2000
Hello Dave,
Not that I disagree with the definitions you presented, but I would like
to enlarge some concepts and add a few explanatory comments that might
make the entire subject of Water Hardness clearer (or more confusing).
> On the subject of hardness, I have uncovered and tried to define > the following terms:
>
> Hardness: Ca+Mg
Iron and aluminum ions also add to water hardness, but they are normally
present in much lower concentrations, so their influence on water
hardness is usually nil.
> General Hardness: Soap killing (tested by titration with > "standard" soap solution)
The term "General Hardness" is a translation artifact, similar to that
of "lead as plant nutrient" discussed recently on these pages.
The translator(s) being neither chemists nor aquarists, were faced with
translating the German word "Gesamthaerte" and its abbreviation "GH".
"Gesamthaerte" means "Total Hardness" in German. Not knowing the English
term "Total Hardness" and trying to find something that would fit the
abbreviation "GH", the translators created the term "General Hardness",
which is meaningless. It stuck.
It is interesting, that (as far as I know) the first English-language
book on "Aquarium Water Chemistry" was a translation from German of a
book by Rolf Geisler, published by TFH in 1963. It has the correct terms
"Total Hardness" and "Carbonate Hardness". (It has other serious
translation errors, which is another story.)
So, "General Hardness" is a misnomer for "Total Hardness". It would be
good if the term disappeared from general use, but I don't think that
this will happen: Too few people know the facts.
> Permanent hardness: General hardness after boiling the water
Correct. That is water hardness produced by salts of calcium and
magnesium OTHER than carbonates and bicarbonates.
> Temporary hardness: General hardness - Permanent hardness
Correct. That is water hardness produced by carbonates and bicarbonates
of calcium and magnesium.
> Carbonate content: Total CO3-- (and relatives) content
> Alkalinity: Resistance to titration with an acid
An interesting definition! What is actually measured is the amount of
cations counterbalanced by anions of weak acids (like carbonic acid)
> Carbonate hardness: Minimum of Hardness and Carbonate content (per Slusarczuk)
I would rephrase that definition :-).
Carbonate Hardness = Temporary Hardness = Water Hardness produced by
carbonates and bicarbonates of calcium and magnesium = Total Hardness -
Permanent Hardness.
> KH: German for carbonate hardness?
Yes -- Abbreviation for "Karbonathaerte"
> Non-carbonate hardness: General hardness - Carbonate hardness????
Right. It is the same as "Permanent Hardness" = water hardness produced
by salts of calcium and magnesium OTHER THAN carbonates and
bicarbonates, i.e. sulfates, chlorides, etc.
> Does anyone disagree with any of these definitions????
Nope.
Best,
George
The Krib
Plants
Plants