Reef Chemistry Question of the Day #77 Calcium

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #77

Being the brilliant reef scientist that you are, everything seems to be at appropriate levels in your reef tank, but some of the creatures are just not looking as good as they did at the same time last year.

Finally you discover that your refractometer must be broken and is way off. The salinity is much too low.

You decide to top off for evaporation with new saltwater (Instant Ocean made to 35 ppt) in order to slowly bring up the salinity.

Normally you top off with saturated limewater (kalkwasser). Since you won't be topping off with limewater during this period, both calcium and alkalinity in the tank might be impacted. But in what way?

Which of the following is the most accurate statement?

A. The limewater has more than 5 times as much calcium as the seawater you are now topping off with.
B. The limewater has about twice as much calcium as the seawater you are now topping off with.
C. The limewater has about the same calcium as the seawater you are now topping off with.
D. The limewater has about half as much calcium as the seawater you are now topping off with.
E. The limewater has less than 1/5 as much calcium as the seawater you are now topping off with.

Good luck!

















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[Zero Nitrates]

Going with C!

 

[beaslbob]

I have no idea but A sounds good.

But then the first answer (or last) is almost never the correct one.

 

[Coraloliseffect]

"A" please

 

[Rausch]

I like C as well! at least i hope!

 

[Kyuss414]

IIRC saturated limewater has about 800ppm of calcium, so I'll go with B.

 

[Ledo]

B
Instant Ocean = ~320 ppm
Limewater = ~600 ppm

 

[Randy Holmes-Farley]

Anyone else want to give a try before I give the answer?

 

[tonizzy22]

Answer is B Instant Ocean calcium is 400ppm at 35ppt
saturated kalk calcium is 800ppm

 

[Rybren]

My vote goes to B as well.

35ppt IO has about 400 ppm Ca (according to a recent Triton test that I saw on RC)
Saturated kalwasser has about 610 ppm of Ca (according to a table I found online - I think it came from one of Randy's articles)

Closest answer is B.

 

[Randy Holmes-Farley]

And the answer is...B. The limewater has about twice as much calcium as the seawater you are now topping off with.

Good job folks!

The concentration of calcium in saturated limewater depends a bit on the temperature, with more at lower temperature.

At 25 degrees C (77 degrees F), saturated limewater contains about 808 ppm calcium. It also has a ph of 12.54 and an alkalinity of about 41 meq/L (115 dKH).

Instant Ocean typically has a calcium level a bit below 400 ppm at 35 ppt, so the limewater has about twice as much calcium.

In terms of alkalinity, the limewater has about 10 times as much alkalinity as the new salt water.

So replacing the top off with seawater won't be nearly as noticeable in terms of calcium as it is in therms of alkalinity.