Ca/Alk balance equation

[MikeS]

Man...if I had a dollar for every Chem class I either skipped or slept through....molar solutions make my head hurt...

Here's one for you molar solution gurus....

I was looking at an online Ca/Alk calculator...

http://www.andy-hipkiss.co.uk/index.htm?http://www.andy-hipkiss.co.uk/caalkcalc.htm

the website owner had this to say about the equation used to generate these balances...

If you look at the periodic table then you can see calcium as a atomic weight of around 40. This is important because its a lot easier to figure out chemical maths when the unit of measure is on moles. To get from the usual parts per million (ppm) to micromoles, just divide the ppm figure by the atomic weight.

Therefore: 400 / 40 = 10mM. (NOTE: I'll round down from 40.08)

The other side of the issue is the alkalinity. When in balance the alkalinity as meq/l is

meq/l = (mM of Ca - 9) * 2. NB: The Red Sea has a difference balance, so rather than 9 we need to use 11

Note, once you go below NSW levels then the calculations go screwy.

By way of an example, let's say I have an alkalinity level of 3.5meq/l, what is the balanced Ca level.

(x - 9) * 2 = 3.5
(x - 9) = 1.75
x = 10.75 mM which in ppm
10.75 * 40 = 430 ppm

Turning it the other way round, let's say I have a Ca level of 450, what is the "balanced" alkalinity level.

450 / 40 = 11.25 (turn into millimoles)
(11.25 - 9) * 2 = 4.5 meq/l.

Now I understand the math just fine...I'm actually very good at math because my job requires that I be so....

My question is this...what happens to the whole Ca/Alk thing once you drop below 360ppm Ca? You start generating negative numbers in the balance...obviously you can't have a negative alkalinity level...would Ca levels below 360ppm automatically be at an imbalance? How does this all work?

MikeS

 

[Katchupoy]

Hi MikeS,

This will not answer your question about what alk if you have a below 360 CA... But this will give you a quick chart to look at...

http://66.213.194.54/myreef/misc/calciumcalc.htm

 

[MikeS]

Thanks for the chart Cesar....Nice quick reference instead of a calculator....

I crunched the numbers and the molecular "balance" between the two hits 0 alk at 360ppm Ca, just like the chart says....but I'm puzzled what happens below 360ppm Ca....

Mike