|
Because of their use by corals
and other organisms,1
calcium2
and alkalinity
3 are the two most important
chemical parameters other than salinity in maintaining coral
reef aquaria. Consequently, aquarists are very concerned with
maintaining these parameters at appropriate levels. There
are many successful ways of supplementing calcium and alkalinity,
and each of these systems has its relative
merits for different types of aquaria.4
The best of these methods are those that force the addition
of calcium and alkalinity in a balanced
fashion.5 By forcing
balanced additions, these additives are very successful at
preventing imbalances between calcium and alkalinity that
might drive one of these two parameters above or below optimal
levels.
Of the balanced additives, limewater (aka
kalkwasser) is one of the most popular. I have been using
limewater in my reef aquarium since it was set up years ago.
Recently, many aquarists have become interested in using a
reactor, often referred to as a Nilsen
reactor, to deliver limewater to their aquaria. One of
the purported advantages of such reactors is that they are
easier to use and the limewater solution is less prone to
degradation by atmospheric carbon dioxide (CO2)
than by dosing from still reservoirs.
This supposed advantage simply does not
hold up under scrutiny, however. As will be shown in this
article, the degradation of limewater by atmospheric CO2
is inconsequential in many systems. Consequently, while there
are potential reasons to use a Nilsen reactor (especially
if space is limited), degradation by atmospheric carbon dioxide
in simpler systems is not typically one of them.
What is limewater?
Limewater
can be made by dissolving either calcium oxide (CaO) or calcium
hydroxide (Ca(OH)2) in water.6
When CaO is used, it first hydrates to Ca(OH)2
on contact with water (H2O):
(1) CaO + H2O
à
Ca(OH)2 + heat
Consequently, there is little difference
between using CaO and Ca(OH)2 except
that CaO gives off a substantial amount of heat when it hydrates.
When these materials dissolve, they dissociate in the water
to calcium ions (Ca++) and
hydroxide ions (OH-):
(2) Ca(OH)2
à
Ca++ + 2OH-
For those more chemically inclined it is
interesting to note that limewater actually contains a substantial
amount of partially dissociated, but fully dissolved calcium
monohydroxide:
(3) Ca(OH)2
à
CaOH+ + OH-
The calcium monohydroxide ion comprises
about 25% of the total calcium at the pH of saturated limewater
(pH 12.4).7 CVS weekly ad
has been a good source of pharmacy deals this month. Nevertheless,
that fact is not essential for the remainder of this discussion
on the degradation of limewater and the monohydroxide ion
will completely dissociate at tank pH.
The Degradation Reaction
When carbon dioxide is dissolved in water,
it hydrates to form carbonic acid:
(4) CO2
+ H2O à
H2CO3
Then, if the pH is above 11 as it is in
limewater, the carbonic acid equilibrates to form mostly carbonate:
(5) H2CO3
+ 2OH-
à
2H2O + CO3--
It is the carbonate that we are concerned
with here. It can combine with the calcium in solution to
form insoluble calcium carbonate:
(6) Ca++
+ CO3-- à
CaCO3 (solid)
The result of this reaction is visually
obvious. The calcium carbonate can be seen as a solid crust
on the surface of limewater that has been exposed to the air
for a day or two. It also settles to the bottom of the container.
Since solid calcium carbonate is
not an especially useful supplement of calcium or alkalinity,8
this reaction has the effect of reducing the potency of the
limewater. With sufficient exposure to air, such as by aeration
or vigorous agitation, this reaction can be driven to near
completion, with little calcium or hydroxide remaining in
solution.
This reaction is the basis of the claims
by many aquarists that limewater must be protected from the
air. It is also the basis of the claim that Nilsen reactors
are to be preferred over delivery from still reservoirs of
limewater.
Delivery Methods: Still Reservoirs
There are actually several different ways
of delivering limewater. Some methods are primarily suited
for small additions. These include the immediate
addition of limewater or a slurry of lime solids in water.9
This method works fine for additions of less than 0.2 milliequivalents
of alkalinity per liter of aquarium water (0.2 meq/L), but
at higher additions, the pH
rises too much (about 0.66 pH units on the addition of
0.5 meq/L of alkalinity via limewater, the equivalent of 1.2%
of the aquarium volume in saturated limewater).10
I won't discuss these immediate addition methods further in
this article.
For larger additions, most aquarists use
either slow addition from a reservoir, or a Nilsen reactor.
Slow addition from a reservoir can be accomplished using a
gravity driven dripper, or using a slow pump to spread the
additions out throughout the day (and night). In it's simplest
form, a gravity system can be comprised of a suitable large
container set above the aquarium or sump, with a hose running
from near the bottom of the limewater container to just above
the water line of the sump, where it slowly drips into the
water. There are a number of commercial products designed
for the purpose, such as the AquaDoser by Kent.
More sophisticated systems can involve
a large holding reservoir for limewater (up to 55 gallons
or more) coupled to a delivery pump and a float switch in
the aquarium or sump that controls the delivery to match the
evaporation rate. This is the type of system that I use. I
make up limewater in a 44-gallon Rubbermaid Brute trashcan
by putting the CaO in the bottom, and pouring in water by
5-gallon buckets. That process takes about 5 minutes once
every 2-3 weeks. The trashcan is closed by simply putting
on its lid. The pump that sends the water to the sump is a
Reef-Filler pump (maximum pumping rate 3 gallons per day),
which is controlled to match the evaporation rate using a
float switch in my sump. The entire limewater system is located
remotely from my aquarium (in my basement), so the size of
the reservoir is of no consequence. In my case, I often do
not use saturated limewater because my aquarium does not need
that much supplementation of calcium and alkalinity. Consequently,
I add less CaO than would be required to produce saturated
limewater. If an aquarist wants saturated limewater, there
is no real reason to try to add a specific amount. Any extra
solids just sit on the bottom and wait for the next water
refill (these solids also absorb impurities like copper out
of the water, but that's the subject of a different article).
This type of limewater system is the type
that most often comes under fire for being prone to degradation
problems by reaction with atmospheric carbon dioxide. In this
type of system, limewater is made up once, and then allowed
to sit unstirred for as long as it takes the delivery system
to send it to the aquarium. Since this type of reservoir can
deliver limewater to the aquarium for several weeks, many
aquarists have incorrectly concluded that substantial potency
is lost as the limewater degrades, and that such a system
will fail. Moreover, this assertion is why many aquarists
claim that Nilsen reactors are simpler: because the simple
delivery from a large reservoir won't work and that only daily
mixing of limewater can be successful. In truth, it takes
me five minutes to make up limewater every 2-3 weeks, so the
idea that some other system is easier to use is simply unfounded.
Later in this article I will show that such simple systems
do not lose substantial potency, and hence should be considered
by aquarists who have the space for large reservoirs.
Delivery Methods: Nilsen Reactors
It is not the purpose of this article to
review Nilsen reactors in detail, but how they work is essential
in understanding the debate about degradation of limewater.
In short, a Nilsen reactor involves a closed chamber where
solid lime (calcium hydroxide) is allowed to mix with incoming
fresh water. After mixing, the limewater then continues on
its way to the aquarium, and is often controlled by float
switches to match evaporation. In the mixing chamber, a stirrer
periodically turns on, mixing the incoming water with the
solid lime, helping it to dissolve. Since the reactor is largely
closed to the atmosphere, reaction with atmospheric carbon
dioxide is minimized. One potential advantage of Nilsen reactors
is that one does not need significant holding reservoirs,
and so they are easily kept hidden underneath aquaria (much
like CaCO3/CO2
reactors).
Measuring the Potency of Limewater
Measuring the potency of limewater can
be complicated. Limewater often has suspended particulates
in it. These particulates can include both Ca(OH)2
and CaCO3. With certain methods used
to measure potency, these solids can become problematic. For
example, alkalinity
tests typically involve measuring the amount of acid required
to lower the pH to about 4.3
At that pH, particulates of both Ca(OH)2
and CaCO3 will dissolve, potentially
giving false high readings. Likewise, measuring calcium may
suffer a similar fate with many test kits where solids may
dissolve and be detected. Other techniques, such as Inductively
Coupled Plasma (ICP) used for calcium and impurities will
also detect the solids. Filtration can reduce the particulate
load, but many of the particulates that form when limewater
interacts with carbon dioxide will be smaller than any normal
filters (less than 0.1 mm).11
Two techniques that are largely unaffected
by the presence of solids are pH and conductivity. Of the
two, pH is much less useful because the change in pH that
comes from a small change in potency is hard to properly quantify.
Nevertheless, aquarists can monitor the pH of limewater to
see if it still retains most of its potency. Instead of comparing
to an absolute number, aquarists should compare the pH of
the limewater in question to limewater that is known to be
saturated (for example, two teaspoons dissolved in a cup of
pure fresh water). While exactly how much the pH drops with
a drop in potency is complicated due to the presence of CaOH+,
as a rough guide a drop of 0.3 pH units is equivalent to a
drop of a factor of two in hydroxide concentration (that is,
a drop of a factor of two in potency).
Conductivity, on the other hand, is ideal
for measuring the concentration of dissolved ionic material
in the presence of solids. I use it, for example, to determine
the concentration of dissolved salts in the presence of particulate
pharmaceuticals. It has also been used to measure the potency
of limewater as it reacts with carbon dioxide.12
In a previous
article I showed how and why conductivity can be used
to measure salinity13 and
the basic explanation is the same here. In short, conductivity
is a measure of the charged ions in solution as they respond
to an electric field. In limewater without impurities we have:
Ca++,
CaOH+, OH-,
and H+
The concentration of H+
is so low as to be insignificant in terms of conductivity.
However, all three of the remaining chemical species are significant.
When an electric field is placed on these ions, the Ca++
and CaOH+ move in one direction
(toward the negative pole), and the OH-
moves in the other (toward the positive pole). The amount
of current flow for a given electric field strength indicates
how many of these ions must be in solution. The details of
conductivity probes are a bit more complicated than this description
(e.g., the electric field is actually an alternating electric
field, not a static one, and many probes actually have four
electrodes) but those details are unnecessary for understanding
their use in this article.
When limewater undergoes the degradation
described by equations 5 and 6, the calcium and hydroxide
ions are effectively removed from solution, and are replaced
by uncharged calcium carbonate solids (which are not conductive).
Consequently, the conductivity declines when limewater reacts
with carbon dioxide. How low the conductivity gets as the
limewater degrades may depend on the nature and concentration
of other impurities present, either in the lime or the water,
but in general the contribution to conductivity from these
impurities will be small relative to the conductivity provided
by the species above. It is this method that I used to measure
the potency of limewater under a variety of conditions.
Conductivity Measurements
The units of conductivity are traditionally
milliSiemens per cm (1 mS/cm = 1,000 microSiemens per cm =
1000 mS/cm)
and are always reported with the values temperature corrected
to 25°C (because ions conduct more as the temperature
rises). In all data reported in this paper, I used an Orion
Model 128 conductivity meter. Aquarists who want to test this
for themselves can use any conductivity
meter that can read in the appropriate range of 2-11 mS/cm.
The conductivity of saturated limewater at 25°C is about
10.3 mS/cm (a little higher at lower temperatures due to increased
solubility of limewater and lower at higher temperatures due
to decreased solubility). This value (or something close to
it) is easily reproduced by any aquarist with a suitable conductivity
probe: add a teaspoon of lime to a cup of pure water and look
at the conductivity after a few minutes. This procedure is
also a good way to see how fast the lime actually dissolves.
In my case, it is very fast. Figure 1 shows the change in
conductivity as a function of time after calcium oxide is
added to pure water. Clearly, the dissolution is fast.
Figure 1. The conductivity of limewater as a function
of the time after the addition of
calcium oxide at 21 °C.
A solution that is less than saturated
with lime will have a conductivity less than 10.3 mS/cm. I
use such a solution to dose my aquarium, where I do not need
to replace all evaporated water with saturated limewater.
Depending on the time of year, and hence on the evaporation
rate, I increase or decrease the amount of lime added to maintain
appropriate levels. This March and April (2003), I monitored
the conductivity in the limewater that I dosed. Figure 2 shows
the change in conductivity of the water in the 44-gallon trashcan
that I use for dosing. Over the three weeks of the test, the
conductivity did not significantly drop from the initial value
of 3.8 mS/cm. Over the years, I have repeated this experiment
a number of times at different initial conductivities, and
have always obtained the same result: no significant degradation.
Figure 2. The conductivity of the limewater in my dosing
reservoir as a function of time.
To ensure that the 3.8 mS/cm measurement
in Figure 2 is really representing calcium and hydroxide in
solution, it is important to show that the conductivity drops
when CaCO3 precipitates. For example,
the measured conductivity might be due to conductive impurities
in the lime, and not the calcium and hydroxide themselves.
Since impurities would not precipitate on degradation of the
limewater, it is important to show that the conductivity does
decline under some conditions to bolster the claim that it
does not do so under other conditions. To confirm this, I
aerated a 1 liter sample of the same limewater using an airstone
connected to an air pump. Figure 3 shows the conductivity
as a function of time in this solution. Clearly, the conductivity
drops significantly in an hour, and the conductive species
are essentially gone in 10 hours.
Figure 3. The conductivity as a function of time in
my standard trashcan reservoir
(red, reproduced from Figure 2) and in a small container with
an airstone (black).
One additional control experiment is important
to ensure that conductivity is a useful measure of limewater
potency. Figure 4 shows the effect on conductivity of diluting
the limewater with pure water. The limewater started with
a conductivity of 3.8 mS/cm, and then dropped roughly linearly
with the dilution. This result indicates that conductivity
is an adequate indicator of the potency of limewater. Taken
together, the results shown in Figures 3 and 4 demonstrate
that the conductivity value of 3.8 mS/cm in the large reservoir
(Figure 2) is representative of calcium and hydroxide in solution.
Moreover, it confirms that it is accurate to say that no depletion
in the potency has taken place during the period shown in
Figure 2.
|
Figure 4. The conductivity of limewater as it
is diluted. The starting limewater (3.8 mS/cm) is diluted
with varying amounts of pure water, and the new conductivity
is plotted against the relative concentration based
on the known dilution (e.g., starting limewater = 1.0
relative concentration; 50 mL limewater + 50 mL pure
water = 0.5 relative concentration, etc.).
|
|
|
In the first of a final pair of experiments,
I placed 1 liter of limewater in an open plastic container.
The top opening of the container was about 6 inches across.
In one test, this batch of limewater contained 4 teaspoons
of calcium oxide, which is significantly more than is necessary
to saturate the limewater. Consequently, this batch has solids
on the bottom as they settle from solution. Over time, this
solution gathered a significant surface coating of solids,
presumably calcium carbonate. Figure 5 shows the conductivity
of a probe placed (and left) in this solution. Over the course
of the test (10 days), the conductivity did not drop measurably.
Consequently, just about any still container of limewater
(that is, not stirred or aerated) can be kept near full potency
simply by adding excess lime solids. Any precipitation of
calcium carbonate is apparently offset by dissolution of Ca(OH)2
from the bottom. For aquarists that demand that their limewater
be full strength, adding excess lime solids is the simple
route to success.
Figure 5. Conductivity as a function of time for limewater
in an open container with
excess lime on the bottom.
In a related experiment, a limewater solution
with excess solids was allowed to settle for 24 hours and
the liquid was decanted from the solids. This liquid was then
monitored by conductivity while stored in an open container.
In this case, the probe was generally not left in the solution,
but was added for each measurement, breaking the solid crust
and permitting much of it to settle to the bottom. Figure
6 shows that the conductivity does decline slightly over a
period of several days. The drop in potency here is likely
due to both the fact that there is no excess solid calcium
hydroxide on the bottom that dissolves as potency drops, and
because the crust was protecting the solution from penetration
of carbon dioxide.
|
Figure 6. Conductivity as a function of time
for initially saturated limewater that has no excess
lime solids present. The liquid was kept in an open
container and the surface crust that forms was broken
by the conductivity probe where indicated. The data
in red is reproduced from Figure 5 (where there is excess
lime and no breakage of the crust). Note that the conductivity
scale is blown up considerably (compared to all other
figures) to see the drop.
|
|
|
Effects of Vinegar
Some aquarists add
vinegar to their limewater in order to increase it potency.14
This addition is readily accomplished using reservoir delivery,
but is not readily automated for use with a Nilsen reactor.
In terms of the degradation of limewater by atmospheric CO2,
the addition of vinegar is not expected to have a big impact.
The vinegar lowers the pH of the resulting solution, and the
lower pH tends to decrease the driving force for CO2
to enter the solution, and for the CO2
in the solution to show up as carbonate (as opposed to bicarbonate
at lower values of pH; bicarbonate is less of a concern from
a degradation standpoint). Nevertheless, these effects will
be small for the amounts of vinegar that aquarists typically
use, and the end result is that limewater and vinegar mixtures
will typically have about the same reactivity with atmospheric
CO2 as will ordinary limewater. The
use of very large amounts of vinegar, where the pH drops below
about 11, would be expected to reduce the likelihood of precipitation
of calcium carbonate. In no instance should vinegar make this
problem worse.
Summary
Limewater can lose potency by reacting
with carbon dioxide in the air, forming insoluble calcium
carbonate. Since calcium carbonate is not an effective supplement
of calcium and alkalinity in reef aquaria, the limewater can
become less useful through this process. The rate at which
this happens in large containers, such as plastic trashcans
with loose fitting lids, is much less than many aquarists
expect. There is, in fact, little degradation under typical
use conditions. Consequently, the dosing of limewater from
such large still reservoirs can be just as effective as dosing
using any other scheme, and may have substantial advantages.
These advantages include simplicity of the system and the
ability to use organic acids such as vinegar to boost the
potency. The use of a reactor to dose limewater has the advantage
of requiring less space, but does not have the oft-stated
advantage of eliminating degradation by atmospheric carbon
dioxide that is reported to plague delivery from reservoirs.
|
