drying seawater for salinity content

Cory · Nov 1, 2015

Im thinking the most accurate way to get salinity would be by drying a sample of water then weighing the remaining salt.

I was thinking of buying a small beaker and burner or perhaps a micro pipette and let the sample evaporate then weigh the salt residue to get salinity.

How would i do this accurately?

Cory · Nov 1, 2015

I should also add that temperature is not a problem as it would be with other forms of measurment like a refractometer, or hydrometer. Its just evaporation and whats left is whats left. So thats my purpose for doing it this way- there shouldnt be much error provided the scale is accurate to weigh it.

Dtackett · Nov 1, 2015

just seems like a lot more work to me lol but it is cool and am interested in reading randys response. afterall who doesn't like fire?

oldsaint · Nov 1, 2015

Look forward to Randy's response. But, I'm not sure that would be as accurate as you might think. My understanding during the drying out some salts change, loose weight and would cause a low measurement.

I'll stick with Randy's advice here
"When salts are dissolved in water, a variety of the water's attributes change, including its density, its refractive index and its conductivity. All of these changes can be used by aquarists as a way to measure the total amount of salt in solution. Consequently, aquarists need not dry out water to find the salinity by the amount of salts that remain behind, but can simply rely on tools that measure these other attributes"
http://reefkeeping.com/issues/2007-03/rhf/

Cory said:
I should also add that temperature is not a problem as it would be with other forms of measurment like a refractometer, or hydrometer.

I think you would be hard pressed to find a non ATC, (auto temp compensation) refractometer in a LFS. So temp would not be an issue.

Paul B · Nov 1, 2015

I went to Sicily where they have huge expanses of flat land to do just that to extract the salt. It takes about a year for it to dry but I don't remember how deep they start out with. Of course, they don't weigh it, they use it on Bacala.

d2mini · Nov 1, 2015

Cory said:
Im thinking the most accurate way to get salinity would be by drying a sample of water then weighing the remaining salt.

I was thinking of buying a small beaker and burner or perhaps a micro pipette and let the sample evaporate then weigh the salt residue to get salinity.

How would i do this accurately?

But why?
Salinity varies around the globe. Other than pure curiosity, what would be the benefit, assuming that it would even be more accurate than other methods?

Jimmyb819 · Nov 1, 2015

Unless this is mainly for curiosity's sake, I doubt any improved accuracy would be worth the trouble over using a good refractometer.

JimWelsh · Nov 1, 2015

http://www.sciencedirect.com/science/article/pii/0011747164903390

Randy Holmes-Farley · Nov 1, 2015

This is trickier than it might seem, and not worth doing at home, but it can work and was the standard back in the 1800's to early 1900's. There's a detailed historical description in Chemical Oceanography by Millero.

It requires pretty high heat (480 degrees C) since you need to dehydrate the salts that precipitate as crystalline hydrates (magnesium chloride, magnesium sulfate, etc.). Unfortunately, that also will convert carbonates and bicarbonates into oxides and hydroxides, causing weight loss. You also lose some chloride, bromide and iodide (as HCl, HBr, and HI and some Cl2 and Br2). Boric acid also vaporizes. All of these require corrections (or prevention) if high accuracy is a goal.

Cory · Nov 2, 2015

It seems complicated but possibly worth doing. I wonder we cant just let a small sample evaporate? Say a 10ml sample. Heat seems to mess with it.

What other methods are there besides conductivity, and a refractometer? Whats this gravimetric method jim welsh posted?

Randy Holmes-Farley · Nov 2, 2015

Cory said:
It seems complicated but possibly worth doing. I wonder we cant just let a small sample evaporate? Say a 10ml sample. Heat seems to mess with it.

What other methods are there besides conductivity, and a refractometer? Whats this gravimetric method jim welsh posted?

No, you can't just let it evaporate at room temp as you won't remove all the water that way.

Drying under heat is the gravimetric method. It is what the others need to be tied to to understand what they mean, but it is not as simple as heating and weighing.

Conductivity is the standard that oceanographers use now (since 1978 or so), and I'm not sure why you want to look for something else. They have done all the hard work already to show how it relates tot he solids in seawater. 35 PSU (Practical Salinity Units) is now DEFINED as seawater with a conductivity equal to the conductivity of a potassium chloride solution containing exactly 32.4356 grams in a mass of 1 kg of solution (53 mS/cm).

For a long time (before 1978) people used chlorinity by titrating with silver nitrate, but I doubt you want to do that either.

Rybren · Nov 2, 2015

Paul B said:
I went to Sicily where they have huge expanses of flat land to do just that to extract the salt. It takes about a year for it to dry but I don't remember how deep they start out with. Of course, they don't weigh it, they use it on Bacala.

Paul, can you still get Bacala anywhere? My grandmother used to make it all the time when I was a kid. Of course, I lived in little Italy and every corner had a little market that sold it. I'm off on a quest.

Now back to our regularly scheduled thread...

Cory · Nov 2, 2015

Randy Holmes-Farley said:
No, you can't just let it evaporate at room temp as you won't remove all the water that way.

Drying under heat is the gravimetric method. It is what the others need to be tied to to understand what they mean, but it is not as simple as heating and weighing.

Conductivity is the standard that oceanographers use now (since 1978 or so), and I'm not sure why you want to look for something else. They have done all the hard work already to show how it relates tot he solids in seawater. 35 PSU (Practical Salinity Units) is now DEFINED as seawater with a conductivity equal to the conductivity of a potassium chloride solution containing exactly 32.4356 grams in a mass of 1 kg of solution (53 mS/cm).

For a long time (before 1978) people used chlorinity by titrating with silver nitrate, but I doubt you want to do that either.

I see what you mean. Well i wanted to not use conductivity because im worried about poorly desighned electronics, a faulty probe, wrongly made calibration standards, unknown impurities in the standard possibly causing inaccuracies, and air bubbles of large and microscopic order. And what about temperature? What if its not calibrated properly either. Plus i wonder about radio wave interference and magnetism. Lol. Lots of uncertainties with conductivity but i may be just over exaggerating.

With a refractometer there is a little less uncertainty. Because it has a lot less points of failure unlike an electronic machine with its circuits and such. But the same comes to mind about air bubbles over the collection plate, fairly imprecise scale to read it, and calbibration uncertainties like impurities. Not only that but how can i verify that its ATC is functioning properly?

With a hydrometer i worry about air bubbles raising it, impregnation of the plastic swing arm of water, possible mineral deposites. With the glass floating version how can i know its been made correct? Then how do i know my thermometer is measuring accurately?

This is why i thought evaporating the water then weighing it would be the most accurate- no error just salt in the end. But you proved its not as easy either. So now im still on the lookout for the easiest most reliable way of getting salinity.

Im thinking boyancy (floating glass hydrometer) and a nist thermometer is probably the best way considering uncertainties of the above. The forces in play here dont change much, and youd know ita broken easily, without a degree in electronics, or science right?

Randy Holmes-Farley · Nov 2, 2015

We'll, floating glass hydrometers with appropriate temperature corrections are a fine way to go.

Waters · Nov 2, 2015

I think the easiest, most accurate way would just be to purchase a Milwaukee digital refractometer

Randy Holmes-Farley · Nov 2, 2015

Waters said:
I think the easiest, most accurate way would just be to purchase a Milwaukee digital refractometer

While, I agree that can be a good way to go, I don't agree it is either easier or more accurate than a good conductivity meter.

cu455 · Nov 2, 2015

Give it a try, even if it isn't going to be 100% accurate, it will still be interesting. Test the water with what ever test kits you have before dehydrating it, weight it after the water loss, weight it, then make it and test it again.

All you will need is a small beaker and heat it up on the stove. Next water change save a little of the fresh made water and use it. Get 2 beakers and heat one up above boiling and right below boiling and compare the results. It will take hours to do it. I had to make peroxide concentrate using this method and it took hours for it to evaporate.

Randy Holmes-Farley · Nov 2, 2015

cu455 said:
Give it a try, even if it isn't going to be 100% accurate, it will still be interesting. Test the water with what ever test kits you have before dehydrating it, weight it after the water loss, weight it, then make it and test it again.

All you will need is a small beaker and heat it up on the stove. Next water change save a little of the fresh made water and use it. Get 2 beakers and heat one up above boiling and right below boiling and compare the results. It will take hours to do it. I had to make peroxide concentrate using this method and it took hours for it to evaporate.

FWIW , just heated to a gentle dryness, 35 ppt seawater will probably read very roughly 4 ppt too high due to hydrated magnesium chloride and sulfate.

There won't be much solids, but I wouldn't reuse it as the alkalinity will mostly be gone into calcium and magnesium carbonate, which won't redissolve.

JimWelsh · Nov 2, 2015

Randy Holmes-Farley said:
For a long time (before 1978) people used chlorinity by titrating with silver nitrate, but I doubt you want to do that either.

Why not? It works great! Very sharp endpoint.