Reef Chemistry Question of the Day #228 Ions and TDS

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #228

Which of the following would contribute the most to the apparent TDS measured by a standard TDS/conductivity meter?

A. 1 billion H+ ions
B. 1 billion Na+ ions
C. 1 billion Cl- ions
D. 1 billion NH4+ ions

Good luck!































.

 

[Beardo]

I will say A since H+ has the highest relative conductivity

 

[rayjay]

C: Cl

 

[Martin Kuhn]

B

 

[tgionet]

D. One of us has to be right

 

[dragon99]

A

 

[rayjay]

D. One of us has to be right

Unless the real answer happens to be "ALL" or "NONE".

 

[TbyZ]

D

 

[MnFish1]

A

 

[karimwassef]

B

 

[Randy Holmes-Farley]

And the answer is...

Which of the following would contribute the most to the apparent TDS measured by a standard TDS/conductivity meter?

A. 1 billion H+ ions

The H+ in solution is actually far more conductive than other ions, largely because it doesn't really have to "move" through the water at all (like a chloride ion would). I give some data and describe this mechanism in the article linked below:

AquariumFish.com :: Aquarium Fish International :: The monthly magazine for both novice and expert aquarium hobbyists.
http://web.archive.org/web/20090217...iumfish.com/aquariumfish/detail.aspx?aid=1804

from it:

Another interesting factor is the unusually large conductivity displayed by hydrogen ions (H+) and hydroxide ions (OH¯). They have very high apparent conductivities and high mobilities, but not because they are especially small. Instead, they conduct without actually moving very far. In this mechanism, a hydrogen ion, for example, makes a new bond with a nearby water molecule. The water molecule then releases a new hydrogen ion off its other side, resulting in the apparent motion of a hydrogen ion without any single ion actually moving. This process continuesthrough the solution (Figure 1) [edit: the article no longer has the figure associated with it, and I cannot find it]

resulting in high conductivities for H+ and OH¯. The most important fact for measuring seawater salinity is that anything that is not present atrelatively high concentrations just doesn’t contribute significantly to the total. Even H+ and OH¯ , with their high inherent conductivities, do not contribute much because they are present at very low concentrations. In fact, Na+ and Cl¯ really dominate in seawater, comprising >90 percent of the total ions present. Magnesium adds another 5 percent and sulfate adds another 2.5 percent. Thus, as long as these four ions are roughly right (>97 percent of the total ions), imbalances in the other ions will have relatively small contributions to the conductivity. So, for this measure of salinity, reasonable variations in phosphate, calcium and other ions that are so important for other aspects of marine aquaria are of no consequence.