Reef Chemistry Question of the Day #263 Conductivity of Limewater/Kalkwasser

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #263

I have shown previously that conductivity is a good way to monitor the potency of limewater (aka kalkwasser). Some products also claim to have magnesium in their solid product to make limewater.

At 25 degrees C, saturated limewater contains about 1.5 grams of calcium hydroxide per liter, and has a conductivity of about 10.3 mS/cm.

If I add 0.1 g of anhydrous magnesium chloride to this solution, what happens to the conductivity?

A. It rises
B. It remains the same
C. It drops

Good luck!













































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[johnzena]

Great question. Because it is conductive, I am guessing that it will rise.

 

[Sleepydoc]

Conductivity in water is due to ions. MgCl readily dissociates in water, so by itself, MgCl would increase the conductivity.

Limewater is a saturated solution of Ca(OH)2; Mg(OH)2 is also not very soluble in water, so the question is whether adding the Mg to the water will decrease the number of ions or not. Since you typically have an excess of Ca(OH)2, I'm going to guess that it increases the conductivity.

 

[SDReefer]

I'm going to say A because the more ions in solution, the greater the conductivity.

 

[SDchris]

C : Most will precipitate. 2 OH for 2 Cl. Even swap, the total amount of ions would be unchanged, or very slightly increased. But guessing Cl is less conductive than OH, so conductivity would decrease.

 

[Chuk]

I’m going with with raises A since I feel like it has to since you are adding more electricity conducive ions. I did look up the conductivity of a mgcl2 solution and I found that a .5% sln of mag chloride is only 8.6 mS/ cm and this is only 0.01 so based that it should drop but I can’t get my head around that so I’m sticking with A.

 

[Randy Holmes-Farley]

And the answer is...C

If I add 0.1 g of anhydrous magnesium chloride to this solution, what happens to the conductivity?

A. It rises
B. It remains the same
C. It drops

The answer is certainly counter intuitive.

If I were to have added sodium chloride instead of magnesium, it will dissolve into Na+ and Cl- ions. Those conduct, and the conductivity will rise.

If I add magnesium chloride, it initially dissolves into magnesium and chloride:

MgCl2 --> Mg++ + 2Cl-

Both of those would certainly conduct and raise conductivity if they stayed in solution, but magneisum will very rapidly precipitate as magneisum hydroxide:

Mg++ + 2OH- --> Mg(OH)2

Since it is so insoluble, nearly all of it will precipitate. Thus, the net overall effect of adding it is:

MgCl2 (solid) + 2OH- --> Mg(OH)2 (solid) + 2Cl-

In terms of what is in solution able to conduct electricity:

2OH- --> 2Cl-

Thus, 2 OH- are lost and 2 Cl- are gained in solution.

While many ions have roughly similar conductivity, two ions have far higher conductivity than any others: H+ and OH-. The reason they conduct so well is that they do not need to move through water the way other ions do. They can "move" by rearranging hydrogen bonds and some reorietentation of the molecules.

This page details these special mechanisms:

http://www1.lsbu.ac.uk/water/grotthuss.html

This page shows some comparative data:

http://www.aqion.de/site/70

Equivalent molar conductivity:
[S cm2 mol-1]
H+ 349.6
OH- 197.9
Na+ 50.0
Cl- 76.2
K+ 73.6
Br- 75.5

Consequently, the conductivity will decline as OH- (more conductive) is swapped for chloride (less conductive).

Happy Reefing!