Reef Chemistry Question of the Day #165 Salinity of Skimmate

[JimWelsh]

So, back to my question of how do you define salinity in a skimmate matrix? Also, it is interesting that there is no discussion of centrifuging the skimmate first, and then testing the supernatant (I know this is a hobbyist forum, and most hobbyists won't have access to a centrifuge, but still....).

 

[ermartin]

I know these are just fun questions but my question to this one would be WHY WOULD I CARE? Is there any reason i would really care about this

 

[JimWelsh]

One reason I might care is to know how much salinity is being exported, so I can replace it with the appropriate amount of ASW. As the water (and salts) in the skimmate is taken out, my ATO is going to replace it with RO/DI water, which will tend to lower salinity over time if not compensated for elsewhere.

 

[Randy Holmes-Farley]

Also to just understand what is happening in a skimmer.

When someone first posed the question to me many years ago (before I saw any data), I really didn't know what it might be.

Here's a discussion we had more than 10 years ago where some of us measured it. Some got lower than tank salinity, and some (me) higher. Habib is in this discussion too.

http://archive.reefcentral.com/forums/archive/index.php/t-473700.html

 

[SROPE]

Excellent point Jim. My major issue running a AIO.

 

[Habib(Salifert)]

Also to just understand what is happening in a skimmer.

When someone first posed the question to me many years ago (before I saw any data), I really didn't know what it might be.

Here's a discussion we had more than 10 years ago where some of us measured it. Some got lower than tank salinity, and some (me) higher. Habib is in this discussion too.

http://archive.reefcentral.com/forums/archive/index.php/t-473700.html

Funny... Around the time you posted I was searching exactly for that post of mine; calcium and alkalinity of skimmate. Couldn't find it but found:
http://www.reefcentral.com/forums/printthread.php?t=1943610



FWIW, IMO is interpretation of conductivity results on skimmate tricky. I'll post in a moment my motivation for this.

 

[Habib(Salifert)]

And the answer is...2. Conductivity meter (IMO, I've not tried anything except this answer)
...

A hydrometer will be impacted by the effect of the organics (both dissolved and particulate) and particulate inorganics on liquid density. How big the effect is will, obviously depend on the concentration and what exactly is in the liquid
...
Happy Reefing!

I don't think that particles will contribute to buoyancy. Sure bulk density changes but not the fluid pressure.

Regarding conductivity, I think that one of the arguments against hydrometers, namely dissolved organics, also applies in various ways.
The volume is increased, lowering the molarity of the salts. That would equate to a lower conductivity.

Some of the organics very likely alter the viscosity of the skimmed water, resulting in a lower ionic mobility.

Certain organics might form a (mono) layer on the conductivity cell's surface impacting measurement.

Evaporation, which is very likely, would increase salinity and conductivity, ignoring other effects.

The various effects might or might not cancel each other to a certain degree.

IMHO, too many uncertainties requiring many assumptions to made.


Perhaps a hydrometer is the least worst?

 

[Cory]

Do you think if a foam head dries it would concentrate its salt content, but if the foam head is made more wet, than a dry head it would contain more water?

 

[Habib(Salifert)]

Drier means less water means more salt per volume.

 

[Randy Holmes-Farley]

I don't think that particles will contribute to buoyancy. Sure bulk density changes but not the fluid pressure.

Regarding conductivity, I think that one of the arguments against hydrometers, namely dissolved organics, also applies in various ways.
The volume is increased, lowering the molarity of the salts. That would equate to a lower conductivity.

Some of the organics very likely alter the viscosity of the skimmed water, resulting in a lower ionic mobility.

Certain organics might form a (mono) layer on the conductivity cell's surface impacting measurement.

Evaporation, which is very likely, would increase salinity and conductivity, ignoring other effects.

The various effects might or might not cancel each other to a certain degree.

IMHO, too many uncertainties requiring many assumptions to made.


Perhaps a hydrometer is the least worst?

I don't buy it.

Particles in the volume that is displaced by a hydrometer do change the effective density. Same for trapped bubbles. Whatever the weight in the displaced volume is what matters. That is why quicksand is such a problem:

"Quick sand can not support the weight of man or animal and it behaves like a liquid with a unit weight about twice that of water."
http://theconstructor.org/geotechnical/quick-sand-condition/3455/

I agree that organics in the water may reduce the conductivity a little bit, but not by displacement (IMO). In a sense this goes back to the definition used for salinity. If it is salts per unit volume (my definition), and organics displace some, then one expect the salinity and conductivity to be lower. That is not a false reading. IMO, the question is whether organics slow down ions travelling in the electric field, and they probably do to a small extent by creating more effective viscosity (as you note).

"Evaporation, which is very likely, would increase salinity and conductivity, ignoring other effects."

That is what one hopes for in a perfect tool for measuring salinity. It is not a tool problem, only a problem extrapolating from the skimmate as is to what came out at the instant the foam spilled into the cup.

Yes, organics might well coat the electrode and reduce readings a bit, but a very good probe takes this into account with 4 electrodes instead of 2. I wonder if, in the link I posted, that might be partly why I got higher than tank water readings when everyone else got lower than tank readings. I use a 4 electrode probe, but hobby ones are likely 2 electrodes.

"Two-electrode method :
This contacting conductivity sensor and is most suitable for low-conductivity applications for pure and ultra-pure water such as demineralizers and boiler water in Power Plants.
Four-electrode method :
Because this measuring method is not significantly affected by dirt or the electrodes, it is suitable for measuring dirty objects such as industrial wastewater and sewage.

"http://www.yokogawa.com/an/faq/sc_isc/sc_detector.htm

 

[Cory]

Drier means less water means more salt per volume.

So maybe thats why randys skimmates salinity was higher?

 

[Cory]

Also im still wondering if organics would absorb certain colors and caise a refractometer to read improperly.

 

[Randy Holmes-Farley]

So maybe thats why randys skimmates salinity was higher?

Mine might have evaporated making it drier and saltier.

But drier and unevaporated skimmate wouldn't have explained it, IMO, since I think fresh dry means more organics and less water, but not more salts. To get drier foam, the water drains out, taking the salts along with it.

 

[Randy Holmes-Farley]

Also im still wondering if organics would absorb certain colors and caise a refractometer to read improperly.

That can be a source of error, yes.

 

[JimWelsh]

Bumping the centrifuge question.

 

[Randy Holmes-Farley]

Bumping the centrifuge question.

Sorry, I missed it.

That would remove particles, and if high enough, some very large dissolved organics. That would remove many of the concerns of the different methods, but leave whatever effect the dissolved organics have.

I don't really know in skimmate how much organic is dissolved and how much is particulate to know how big of an improvement it would be, but it might make all of the methods usable to within some reasonable error.

Do you have one in your basement lab to try?

 

[JimWelsh]

Do you have one in your basement lab to try?

Not yet, but I can use the one at work that accepts 60 mL sample tubes.

The basement lab did recently get a scale upgrade to a 200g x 0.001g analytical balance.

 

[Cory]

I know these are just fun questions but my question to this one would be WHY WOULD I CARE? Is there any reason i would really care about this

Its important because as skimmate is disposed of so is the saltwater with it. With a skimmer overflow, even more. With an ato it will replace water and not salt, subsequently lowering salinity.

 

[Cory]

Not yet, but I can use the one at work that accepts 60 mL sample tubes.

The basement lab did recently get a scale upgrade to a 200g x 0.001g analytical balance.

I seen a cheap hand crank one i was thinking of buying.

 

[Myka]

The dissolved organics in the water will perturb the refractive index and potentially lead to an off reading by a refractometer. Particulates will scatter the light, making the "line" harder to see and less well defined. The color and light scattering may make it hard to see through it at all, depending on the concentration.

I wonder just how inaccurate it is. I never had trouble seeing the line.