Reef Chemistry Question of the Day #164 TDS and Ions

[Randy Holmes-Farley]

Which of the following in the effluent from an RO/DI will contribute most to the measured TDS (total dissolved solids) as measured by a typical conductivity meter?

1. 50 H+ ions
2. 100 Na+ ions
3. 75 K+ ions
4. 75 Mg++ ions

Bonus:

Rank order them from highest to lowest.

Good luck!


























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[JimWelsh]

I'm going with ionic strength (concentration times charge number) on this one, so from highest to lowest:

75 Mg++
100 Na+
75 K+
50 H+

 

[Taservices]

I have to agree with Jim Welsh.

I took too long to be proud of my thought process as I have had a bit much to drink and didn't read the options properly.

 

[JimWelsh]

Now that I've cheated and done my Googling, I see that I'm wrong.

 

[Cory]

How would you know? Would it be best to know the mass on yhe periodic table? The more dense, the less chance of electricty flowing through it, sort of like a resistor or resistance.

 

[Habib(Salifert)]

How would you know? Would it be best to know the mass on yhe periodic table? The more dense, the less chance of electricty flowing through it, sort of like a resistor or resistance.

The more charge per unit time flows/moves the lower the resistance and the higher the conductivity (more or less). Charge can move even faster if no movement is required but the charge is passed on to the "neighbour".

 

[Cory]

The more charge of the atom? I cant visualize what's happening too good.

 

[Randy Holmes-Farley]

The more charge of the atom? I cant visualize what's happening too good.

Conductivity is the measure of the current flowing through the water in an applied electric field.

An ion with a single charge on it carries the current of 1 unit as it moves in response to that field. An ion with two charges on it carries twice as much current, but may itself move faster or slower, so doesn't necessarily contribute twice as much conductivity.

As Habib hinted, a very small number of ions have ways of allowing charge to move through the water without actually moving the whole ion. That's the tricky part of this question.

 

[Cory]

Randy and his sneaky trick questions lol!

Im just goimg to say sodium at the top because we all know saltwater (which is mostly Na+) carriers electricty pretty good.

 

[Randy Holmes-Farley]

And the answer is...1. 50 H+ ions

I discuss this in detail in next months ReefEdition article on using conductivity to measure salinity, but here's a preview:

Variations in Ion Concentration and Conductivity

In order to understand whether changes in the levels of ions such as calcium or magnesium are a significant issue when using conductivity to measure salinity, we need to understand a few facts about conductivity. The most important fact is that most ions contribute to conductivity to about the same extent. Table I shows the relative conductivity of several ions. Ions with higher charges tend to have higher conductivity because they not only carry more charge but they respond more strongly to an electrical field. Good examples are sulfate (SO4--) and calcium (Ca++), which have higher conductivities than sodium (Na+) or chloride (Cl¯).

Another effect is that larger ions tend to have more “drag” as they move through the water, and thus have lower conductivity. In such comparisons one needs to take into account the tightly bound water molecules that get dragged along as well, so one cannot simply look at molecular weights or ionic radii. This is, for example, why lithium (Li+) is so much less conductive than sodium, which in turn is less conductive than potassium (K+). Table II shows the ionic mobility — a measure of how readily the ion can move through water — for several ions. This value tracks well with the conductivity of the ions, except that ions with more charge and the same mobility, such as calcium or sulfate, conduct more.

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 continues through the solution, resulting in unusually high conductivities for H+ and OH-.