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Re: Hardness, Calcium, Magnesium
Charles Kuehnl wrote:
> This however does not explain what is going on with the test which was the
> reason for my question. Perhaps expressed more clearly, what is going on in
> the test? From what I think I understand, Ca and Mg are the primary
> constituents of general hardness and CaCO3 and Total hardness are the most
> easily measured. That permits one to extrapolate the concentration of Mg
> from measurements of total hardness and calcium hardness. Deriving the
> percentage of Ca in CaCO3 is pretty straightforward from atomic mass but I
> do not understand the how the numbers used in the calculation for Mg
> concentration are derived?
I tried last time to explain briefly how the hardness test works.
Obviously, too briefly.
Hardness is a measure of the amount of alkaline earth elements dissolved
in the water. Formally, that means magnesium, calcium, strontium and
barium. Even radium might be included. In almost all cases of natural
or natural-like water the hardness is all magnesium and calcium because
the remaining alkaline earth elements are relatively rare and/or absent
from solution. Calcium and magnesium are dissolved as charged particles
(ions), with each ion carrying a charge of +2.
As I understand it, kits that measure total hardness by titration (drop
counting) contain two main ingredients. One is a dye and one is a
chelating agent. The dye binds to calcium or magnesium and when bound
that way it is one color; the dye is a different color when it isn't
bound to calcium or magnesium. The dye used in my test kit is red when
bound to calcium or magnesium and green when it is not. The chelating
agent (usually EDTA) binds either calcium or magnesium and it binds them
strongly enough to not only take up all the dissolved calcium and
magnesium ions but also to take the calcium and magnesium away from the
At the beginning of a test (say, after the first drop of the test
solution is added) there is usually enough calcium and magnesium in the
water to exceed the amount of chelating agent added, so that the
chelating agent is saturated with calcium and magnesium and there's
enough left over in the water to react with the dye and probably some
unreacted ions still in the water.
With each added drop the chelating agent binds with more and more of the
calcium and magnesium. After you have added a number of drops to your
water sample (the number depending on the hardness in your water and the
reagent concentrations in the test kit) the sample will contain enough
of the chelating agent to bind with all of the free calcium and
magnesium ions in the water. All additional drops you add start taking
calcium and magnesium away from the dye, and the dye starts to change
color. When finally you have added enough of the chelating agent to
bind all of the calcium and magnesium then the dye changes to it's
final, calcium- and magnesium-free color and the test is done.
The number of drops it takes to bind all of the calcium and magnesium
measures the amount of calcium and magnesium initially in the water.
Exactly how the concentration is calculated from the drop count depends
on the design of the kit; the concentration of the chelating agent in
the test solution, the amount of calcium or magnesium that can be bound
per weight of the chelating agent, the volume of the water sample and
the size of the drops. Fortunately you don't have to worry about those
details, the manufacturer works all that out, tells you how much sample
to use and how much hardness is present per each added drop of test
solution. Pretty convenient.
But wait, there's a complicating factor. How does the manufacturer
express the amount of hardness?
The chelating agent binds with a certain amount of electrical charge,
with specific preferences for different ions depending on the size of
the ion and the amount of charge on each ion. With usual abundances
that means the chelating agent will bind with a certain amount of charge
on calcium and magnesium ions. Both calcium and magnesium carry two
units of charge per ion, so the amount of charge converts directly to a
number of ions.
The chelating agent will also bind with a variety of metal ions that are
similar to calcium and magnesium in size and ionic charge. The metal
ions are found at relatively low concentrations, so they don't have a
significant effect on the test.
So, what the test actually measured was the *number* of calcium and
magnesium ions in the water, not the weight of those ions. Moreover,
the kit doesn't distinguish between calcium ions and magnesium ions. It
responds equally to either ion. Without knowing which ion we're
measuring there's no way to convert from the concentration of ions to
the weight of the ion and no way to express the results in normal
concentration (e.g. mg/l) units.
Hardness is a fairly ancient measure of water quality and the
traditional measures are independent of which ion is being measured.
George Slusarczuk wrote a discussion of those units that you can read at
www.thekrib.com. If you haven't found thekrib yet then you *really*
should look into it.
Briefly, what the various measures do is assume that all of the reacting
ions are calcium, determine the weight of the calcium that would be
required to provide the amount of electrical charge that was measured by
the test and then express the result in terms of the concentration of
some calcium compound. The kit you're using expresses the concentration
in terms of calcium carbonate.
It's important to realize that the kit isn't measuring *actual* calcium
carbonate. In fact there may be no calcium and no carbonate in the
sample (for instance, the solution contains only magnesium chloride) and
the kit will still report calcium carbonate.
I'm not sure how the calcium-specific test works. I think I read into
it once, but whatever I knew then went the usual way of unnecesary
knowledge. I think they work on the same principle as the total
hardness kit, but somehow cancel-out the effect of the magnesium.
Most commonly hardness is used as a direct measure of water quality and
you don't need any additional information to interpret the results. If
you do want to take it further then you will need additional information
-- usually alkalinity and/or the calcium/magnesium breakdown.
Calculating Ca++ and Mg++ concentrations from hardness is one example.
> For something more practical, my test kits finally came and I was able to
> take some measurements last night and today. All of these are LaMotte test
> pH - <7.2 using octet comparator I would guess ~7.15
> Alkalinity - 200 mg/l
> Total Hardness - 108 mg/l
> Ca Hardness - 60 mg/l => Ca is 24 mg/l, Mg is 11.5 mg/l
> CO2 - Not sure which value to use. 14-15 was first and very faint hint of
> change to pink but did not turn clearly faint pink until 17 (19 by pH/KH
Probably the 19 is your best number. It looks like you can use the high
value from the CO2 test kit if you find it convenient.
> Nitrate - 6 ppm
> Test says NO3 - N and to multiply by 4.4 for ppm NO3. Does this mean 26.4
> mg/l NO3?
> When I increase CO2 to the where the pH goes to 7.0, some of the fish look a
> little funny - particularly the Otos and 2 Corys while the Tetras seem to be
> happier (perhaps it's the pH). Otos' breathing looks a little stressed or
> they look a little lethargic and they have lost some color.
At a pH of 7 you should have about 20 mg/l of CO2. That *shouldn't* be
enough to hurt your fish, but you really need to trust the response of
your fish more than you trust the test kits. Those sound like symptoms
of CO2 excess.
> With pH around
> 7.1-<7.2, plants are producing sufficient O2 to show bubbles on leaves and
> some of the Gloss produces nice bubbles almost proportional to the bubble
> rate of the CO2.
If you go to higher CO2 levels (lower pH values) then you will be in the
realm of diminishing returns. Tom Barr might (with reason) encourage
you to higher values, but I would stop at the point were the plants give
a visible response.
> My inclination is to keep the pH around 6.9-7.0 and drop the KH closer to
> 4-6 so CO2 won't be too high. When I last had fish the theory was that fish
> health was improved and easier to maintain if the pH were neutral to
> slightly acidic. Is this still the accepted reasoning? If I keep KH in the
> 11-12 range where it is now, a pH near neutral or slightly below would
> require CO2 around 34-42 mg/l. Will the fish get used to this or is this
> asking for trouble?
It's asking for trouble.
> I notice that in the Dupla and Dennerle books they recommend running CO2
> around 35-40 mg/l but this seems much higher than what I perceive is the
> general consensus among everyone here (around 15-25). Dupla also runs NO3
> around 25 mg/l, about 15-20 mg/l higher than I recall seeing most of those I
> recognize here suggesting.
If you have a pH-regulated tank and you find it very dependable then
maybe you can safely push the CO2 up to those levels. At 35 -40 mg/l
you don't have much margin of safety; if it drifts up for some reason
you will kill fish.
> If the lower KH route is best, I would guess I need to add DI (LFS sells it)
> or distilled water to get KH down to 5-6 range, dosing it with trace and
> macro nutrients to keep the other things in the proper range.
You could use a 50/50 mix of distilled or DI water with your regular
water supply. You'll probably need to dose with some sort of
fertilizer. Without it the plants will use up what's available now,
growth will slow down (not necessarily a bad thing) and deficiency
symptoms will set in. The alternative would be to back off the light
and CO2 and let things go nature's way, but that doesn't seem to be
where you're headed.
> Any suggestions would be greatly appreciated.
As you're fairly new at this I suggest a slow approach. I think it
would be best for you keep you water as is, leave the CO2 at the point
were you get visible response from the plants (pH of 7.1-7.2) and take
some time to make sure that you have the rest of your fertilizer regime
adjusted to where it needs to be. In the mean time, watch your plants
and fish carefully and try to understand their responses.
Work on details like mixing DI water to lower alkalinity only after you
have stabilized things at this point. Also, a larger tank would be a
good idea. A 10 gallon tank requires a lot of work for a relatively