quick alk question

[wasp9166]

the paper that came with my salifert alk kit says that seawater has an alk of 2.9, but the back of the bionic bottle says alk should be maintained between 3.00 and 3.75, why is this?

id say im ready for rock, the turbo worked well,ph is a bit low still with the house closed up even tho i ran the skimmer hose outside, im hoping it will jump up when some macro goes in the sump

ph-7.93

cal-420

alk-3.54

[Randy Holmes-Farley]

Many if not most aquarists maintain higher than NSW alkalinity because corals grow faster and pH is more stable with higher alkalinity.

These articles may help:

Reef Aquarium Water Parameters (a summary general article)
http://www.reefkeeping.com/issues/2004-05/rhf/index.htm

and

The Chemical & Biochemical Mechanisms of Calcification in Corals
http://www.advancedaquarist.com/issues/apr2002/chem.htm


from the second one:

"Implications for Reefkeeping: Alkalinity
Unlike the calcium concentration, it is widely believed that certain organisms calcify faster at higher alkalinity than in normal seawater. This result has also been demonstrated in the literature, where it has been shown that adding bicarbonate to seawater increases the rate of calcification in Porites porites17 Figure 6). In that case, a doubling of the bicarbonate concentration resulted in a doubling of the calcification rate.

Much, though not all, of the carbon source for calcification comes from external bicarbonate. As the alkalinity is reduced (at a given pH) the bicarbonate concentration (which comprises the bulk of the alkalinity in seawater) will also be reduced. Diffusion of bicarbonate or diffusion of CO2 from the coelenteron can apparently become rate limiting in many corals. In part this may be due to the fact that both photosynthesis and calcification are competing for bicarbonate, and the fact that the external bicarbonate concentration is not that large to begin with (when compared, for example, to the calcium concentration)."


and from the first one:

"Alkalinity

Like calcium, many corals also use "alkalinity" to form their skeletons, which are composed primarily of calcium carbonate. It is generally believed that corals take up bicarbonate, convert it into carbonate, and then use that carbonate to form calcium carbonate skeletons. That conversion process is shown as:

HCO3- ---> CO3-- + H+

Bicarbonate Ã* Carbonate + acid

To ensure that corals have an adequate supply of bicarbonate for calcification, aquarists could very well just measure bicarbonate directly. Designing a test kit for bicarbonate, however, is somewhat more complicated than for alkalinity. Consequently, the use of alkalinity as a surrogate measure for bicarbonate is deeply entrenched in the reef aquarium hobby.

So, what is alkalinity? Alkalinity in a marine aquarium is simply a measure of the amount of acid (H+) required to reduce the pH to about 4.5, where all bicarbonate is converted into carbonic acid as follows:

HCO3- + H+ ---> H2CO3

In normal seawater or marine aquarium water, the bicarbonate greatly dominates all other ions that contribute to alkalinity, so knowing the amount of H+ needed to reduce the pH to 4.5 is akin to knowing how much bicarbonate is present. Aquarists have therefore found it convenient to use alkalinity as a surrogate measure for bicarbonate.

One important caveat to this surrogate measure is that some artificial seawater mixes, such as Seachem salt, contain elevated concentrations of borate. While borate is natural at low levels, and does contribute to pH stability, too much interferes with the normal relationship between bicarbonate and alkalinity, and aquaria using those mixes must take this difference into account when determining the appropriate alkalinity level.

Unlike the calcium concentration, it is widely believed that certain organisms calcify more quickly at alkalinity levels higher than those in normal seawater. This result has also been demonstrated in the scientific literature, which has shown that adding bicarbonate to seawater increases the rate of calcification in Porites porites.4 In this case, doubling the bicarbonate concentration resulted in a doubling of the calcification rate. Uptake of bicarbonate can apparently become rate limiting in many corals.5 This may be partly due to the fact that both photosynthesis and calcification are competing for bicarbonate, and that the external bicarbonate concentration is not large to begin with (relative to, for example, the calcium concentration).

For these reasons, alkalinity maintenance is a critical aspect of coral reef aquarium husbandry. In the absence of supplementation, alkalinity will rapidly drop as corals use up much of what is present in seawater. Most reef aquarists try to maintain alkalinity at levels at or slightly above those of normal seawater, although exactly what levels different aquarists target depend a bit on the goals of their aquaria. Those wanting the most rapid skeletal growth, for example, often push alkalinity to higher levels. I suggest that aquarists maintain alkalinity between about 2.5 and 4 meq/L (7-11 dKH, 125-200 ppm CaCO3 equivalents), although higher levels are acceptable as long as they do not depress the calcium level.

Alkalinity levels above those in natural seawater increase the abiotic (nonbiological) precipitation of calcium carbonate on objects such as heaters and pump impellers. This precipitation not only wastes calcium and alkalinity that aquarists are carefully adding, but it also increases equipment maintenance requirements. When elevated alkalinity is driving this precipitation, it can also depress the calcium level. A raised alkalinity level can therefore create undesirable consequences.

I suggest that aquarists use a balanced calcium and alkalinity additive system of some sort for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part additive systems.

For rapid alkalinity corrections, aquarists can simply use baking soda or washing soda to good effect."