Reef Chemistry Question of the Day #55 Salinity Rise from Calcium

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #55

Assume your reef aquarium has a salinity of 35.0000 ppt (specific gravity = 1.0264000).

You then determine that your calcium level is unusually low, and you need to raise it.

You use solid calcium chloride to boost the calcium level by 100 ppm (adding a solution is usually preferable, but I don't want to introduce the additive liquid volume into this question).

The final salinity will be closest to:

A. 35.0010 ppt (1.0264008 sg)
B. 35.0100 ppt (1.0264075 sg)
C. 35.1000 ppt (1.0264750 sg)
D. 35.0028 ppt (1.0264021 sg)
E. 35.0280 ppt (1.0264210 sg)
F. 35.2800 ppt (1.0266100 sg)
G. 35.0000 ppt (1.0264000 sg)

Good luck!




























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[TSwizzy]

This is pure guess but C

 

[MrDJeep123]

F. 35.2800 ppt (1.0266100 sg)

 

[s.h.moradi]

I guess B is correct

 

[beaslbob]

too much math

i guess G so I don't have to do the math.

 

[Cory]

Salinity measures all ions? I'd guess c. I thought it was just a measure of salt! Haha

 

[Randy Holmes-Farley]

Salinity measures all ions? I'd guess c. I thought it was just a measure of salt! Haha

I will say that salinity does count everything in the seawater.

 

[Randy Holmes-Farley]

Anyone else want to try before I give the answer?

 

[DFW]

B because it moves the decimal 2 places?:second:

 

[Randy Holmes-Farley]

And the answer is...F. 35.2800 ppt (1.0266100 sg) The winner today is MrDJeep123. Good job!

As mentioned above, salinity measures all ions in the water.

If you boost calcium by 100 ppm (100 mg/kg), then the calcium part of that addition boosts salinity also by 100 ppm (100 mg/kg).

Salinity is measured in ppt (parts per thousand or g/kg) or PSU. PSU stands for Practical Salinity Units. For most of our purposes, it differs little from ppt.

So that 100 ppm (100 mg/kg) boost in salinity is equivalent to 0.1 ppt (g/kg) by simple math since 100 mg = 0.1 g. Thus the calcium boosted the salinity by 0.1 ppt.

But we cannot neglect the chloride that also comes in with calcium chloride, and chloride actually weighs more in calcium chloride than does the calcium.

Calcium has a molecular weight of 40. Chloride has a molecular weight of 35.45, and there are two chlorides in calcium chloride.

So the addition of the chloride is actually larger than the calcium, by a factor of (2 x 35.45)/40 = 1.77.

Consequently, the chloride boosts the salinity by 1.77 x 0.1 ppt = 0.18 ppt.

So the total addition tot he salinity is 0.1 + 0.18 = 0.28 ppt, giving the final answer of 35 ppt + 0.28 ppt = 35.28 ppt.

The effect of normal calcium additions is usually small, as we can see here, but folks often boost magnesium by 100 or 200 or more ppm, and that can have a substantial impact on salinity, especially since magnesium is a smaller percentage of the weight of magnesium chloride or magnesium sulfate than calcium is of calcium chloride.


Happy Reefing.

 

[g6gang]

I came in late - and was going to say G because I never would have thought that raising Calc would also impact salinity. As always you learn something new. Love these Chemistry questions of the day.

 

[Cory]

Wow I just did that to my tank, 100ppm magnesium sulphate and 40ppm calcium chloride increased. So I guess my salinity would be around 1.027 now instead of 1.026?

What is the proper way to combat these increases? Add more water?

 

[Randy Holmes-Farley]

I came in late - and was going to say G because I never would have thought that raising Calc would also impact salinity. As always you learn something new. Love these Chemistry questions of the day.

Great! Glad it was useful.

 

[Randy Holmes-Farley]

Wow I just did that to my tank, 100ppm magnesium sulphate and 40ppm calcium chloride increased. So I guess my salinity would be around 1.027 now instead of 1.026?

What is the proper way to combat these increases? Add more water?

If it was just magnesium sulfate, that 100 ppm boost to magnesium boosts salinity by 0.6 ppt, or from 1.0260 to 1.0264.

 

[Cory]

Thanks for the help. I was trying to understand your post here about how much cqlcium chloride would raise salinity. You said:

"Salinity is measured in ppt (parts per thousand or g/kg) or PSU."

Is this the same as saying full strength seawater has got 35% dissolved salts, and 65% water? That's quite a lot of salt! That doesn't seem right to me though based on how much salt I add. I suppose because different ions weigh different amounts?

 

[Cory]

You said:

"Calcium has a molecular weight of 40. Chloride has a molecular weight of 35.45, and there are two chlorides in calcium chloride."

Where can I find how much molecular weight a certain element has? I'd like to know where to get this for future info.

You said:

"So the addition of the chloride is actually larger than the calcium, by a factor of (2 x 35.45)/40 = 1.77."

I'm sorry I suck at algebra but 2x35.45 equals 70.9 , so how do we get this 1.77? I understand that chloride is here counted as two molecular weights, but the 1.77? I also see the 40 is calciums molecular weight but don't understand why it's at te end.

Thanks Randy!

 

[Randy Holmes-Farley]

The molecular weights of the elements are readily available in tables online and in books.

For example:

Chemical elements of the periodic table sorted by Atomic Mass


Explaining the calculation...

(2 x 35.45)/40 = 1.77

We could have done that many ways. What I calculated above was the chloride relative to calcium in calcium chloride.

Perhaps it is easier to see it this way.

The molecular weight of calcium is 40 g/mole/

The molecular weight of calcium chloride (CaCl2) is 40 + 35.45 +35.45 = 110.9 g/mole.

So the total weight of calcium chloride is 110.9 g/mole, which is 2.77 times the molecular weight of the calcium alone (2.77 x 40 =110.9).

If the calcium alone went up by 100 ppm, then the total of calcium and chloride must go up by 2.77 times that value, or 277 ppm.

Make sense?

 

[Cory]

Much more sense!

I hope I'm not frustrating you! However a mole, this is the unit of weight per one atom of an element like calcium is 40?

 

[Randy Holmes-Farley]

Thanks for the help. I was trying to understand your post here about how much cqlcium chloride would raise salinity. You said:

"Salinity is measured in ppt (parts per thousand or g/kg) or PSU."

Is this the same as saying full strength seawater has got 35% dissolved salts, and 65% water? That's quite a lot of salt! That doesn't seem right to me though based on how much salt I add. I suppose because different ions weigh different amounts?

Sorry, I missed this.

ppt is parts per thousand, so 35 parts per thousand = 3.5 parts per hundred, or 3.5% salt and 96.5% water.

 

[Randy Holmes-Farley]

However a mole, this is the unit of weight per one atom of an element like calcium is 40?

Close. It is the number of atoms in exactly 12 grams of carbon twelve (the isotope of carbon with a molecular weight of 12).

Other atoms are close but not exactly the same for reasons I'll give in one of the current questions of the day. So 1 mole of calcium weighs 40 grams per mole.