drying seawater for salinity content

[Cory]

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.

If i do this at home on the oven what steps would i do? Will try it just to veryifiy that my refractometer is working correctly or in range.

 

[JimWelsh]

Anybody wanting to do this will have to be able to measure two things accurately in order to be successful: 1) The volume of the saltwater sample, and 2) The weight of the dried sample. Actually, the temperature of the sample would need to be known, too, at the time the volume of it was measured.

In order to determine the salinity to a resolution of 1 PPT, you would need to know both the volume and the dried weight to at least three significant figures. Then there is the challenge of determining the weight of *all* of the dried sample; you can't just scrape it out of the beaker it was dried in, for example, since some would likely stay stuck on the surface, or get lost, etc., so your best bet is to be able to weight the beaker when clean and dry before the experiment, and then weigh the beaker plus the dried salt, and determine the salt weight by the difference between the two weights.

Anybody with the tools (volumetric glassware and a good scale) to do the above is also in a position to do something much easier: Simply weigh accurately the original sample of a known volume at a known temperature. With that information, the density could be determined, and then the salinity from that. Much easier, and less error-prone.

 

[JimWelsh]

So, I went ahead and attempted to do what I just suggested in the previous post. I have a Class A 100 mL volumetric flask, which has an uncertainty of +/- 0.08 mL, or 0.08% when used at 20 degrees C. I also have a scale that can measure up to 300 grams with a precision of 0.01 g. I also have a NIST-traceable thermometer with an uncertainty of +/- 0.5 degrees C. I also have a refractometer that I've calibrated with a DIY standard made according to Randy's article.

I took about 250 mL of my tank water in a sampling flask, and let it sit on my lab bench together with the clean, dry volumetric flask, the scale, and the refractometer at 20 degrees C for a couple of hours for the temperature to stabilize. I then measured the salinity using the refractometer as being 34.9. I then carefully weighed the empty flask. Next, I added 100 mL of 20C tank water to the volumetric flask, using a black/white card to correctly read the meniscus. I then weighed the full flask, and determined the weight of the 100 mL of tank water, which was 102.41 g, meaning a density of 1.0241.

There are various seawater density calculators, most of which take temperature and salinity as inputs, and give density as an output, such as the one found here: http://www.csgnetwork.com/h2odenscalc.html. I'm not aware of any that take temperature and density as inputs, and give salinity as an output, but you can play around with the salinity input until you get the right density output. For the purposes of this discussion, though, the chart found here is also very helpful: http://www.msc.ucla.edu/oceanglobe/pdf/temp_densal_chart.pdf.

Now, according to either the calculator or the chart, a density of 1.0241 at 20C indicates a salinity of right at 34.1. But, how certain am I of that number? What is the measurement uncertainty of what I just did? Well, fortunately, I can readily calculate that from the uncertainties of the various measurements taken to arrive at that 34.1 number, and determine the uncertainty of the salinity result.

The uncertainty in the density value is the sum of the uncertainties of the volumetric flask and the scale, which are 0.08% and 2 * 0.01% (because we used the scale twice), or a total of 0.10%. That means that the actual density could be within plus/minus 0.10%, or 0.001, meaning the real density value could easily be anywhere between 1.0231 and 1.0251. Plugging these values into either the calculator or the chart, that shows us that the actual salinity could be anywhere from 32.8 up to 35.3. And that isn't even taking into account the uncertainty in temperature, which widens the uncertainty even greater, from 32.6 to 35.6!

My point is that even with quite reasonably accurate equipment, used correctly to perform a very straightforward analysis, the ability to measure salinity gravimetrically is fraught with challenges. Using this method I can say with good confidence that my salinity is somewhere between 32.6 and 35.6.

Randy is right. Why not just use conductivity?

 

[Randy Holmes-Farley]

Thanks for doing that Jim!

Cory, if you just want to check the refractometer, I'd do it by making (or buying) a 35 ppt standard. The uncertainty from weighing out sodium chloride to make a standard will not be any more error prone than weighing the mass left after drying seawater, and avoids nearly all of the issues relating to moisture remaining or driving off CO2.

To make a DIY refractometer standard, dissolve 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater. This recipe can be scaled to any appropriate size if suitable instruments are available (36.5 grams in 963.5 grams (mL) of water, 0.365 grams in 9.635 g (mL) of water, etc.). This solution has exactly the refractive index of 35 ppt (sg = 1.0264) seawater.

 

[JimWelsh]

An update: I took the sample I was working with last night into the lab I work at today, and had them check it on the densitometer. The density of my sample is 1.023924 at 20.004 degrees C, which corresponds to a salinity of 33.9. I also had them check the refractive index, which came in at 1.33925, which corresponds well with a salinity of 34. As it turns out, my density calculation done at home (I got 1.0241) was in error by only 0.017%!

 

[Randy Holmes-Farley]

An update: I took the sample I was working with last night into the lab I work at today, and had them check it on the densitometer. The density of my sample is 1.023924 at 20.004 degrees C, !

How does the densitometer work? That's a lot of significant figures!

 

[JimWelsh]

The instrument used is an Anton Paar DMA5000. It uses the "oscillating U-tube" technique (https://en.wikipedia.org/wiki/Oscillating_U-tube). The laboratory usually reports one less sig fig to our clients than the raw number I quoted above.

 

[Cory]

What a cool thread this turned out to be

Im surprised the error in your test jim! I may try it, once i pay off this ultra hd tv i just bought...

Randy ill make that standard in the next few days.

 

[Cory]

An update: I took the sample I was working with last night into the lab I work at today, and had them check it on the densitometer. The density of my sample is 1.023924 at 20.004 degrees C, which corresponds to a salinity of 33.9. I also had them check the refractive index, which came in at 1.33925, which corresponds well with a salinity of 34. As it turns out, my density calculation done at home (I got 1.0241) was in error by only 0.017%!

What were you aiming for in your tank? 34ppt?

 

[JimWelsh]

What were you aiming for in your tank? 34ppt?

Actually, I'm targeting 35 PPT, but I made my calibration solution a long time ago, and probably did something wrong. I last calibrated my refractometer just before I sent off my Triton tests in August. At that time, my refractometer showed the salinity to be right at 35.0. Curiously, when I use my spreadsheet for calculating salinity from Triton results on my own test results, the calculated salinity at 20C is 34.00. (I also sent in a sample of my freshly-made Instant Ocean mix to Triton, and my refractometer showed 35.0 for that sample, too, but the Triton calculation returns a value of 34.02 for that sample.)

I now have five different data points of 34.00, 34.02, 34.1, 33.9, and 34 for my salinity, from the Triton calculations, my density calculation, the DMA5000 densitometer, and the professional laboratory refractometer, respectively, all of which agree well with each other, while my refractometer reads 34.9-35.0 for the same samples. Clearly, I need to revisit my refractometer calibration.

 

[Cory]

Actually, I'm targeting 35 PPT, but I made my calibration solution a long time ago, and probably did something wrong. I last calibrated my refractometer just before I sent off my Triton tests in August. At that time, my refractometer showed the salinity to be right at 35.0. Curiously, when I use my spreadsheet for calculating salinity from Triton results on my own test results, the calculated salinity at 20C is 34.00. (I also sent in a sample of my freshly-made Instant Ocean mix to Triton, and my refractometer showed 35.0 for that sample, too, but the Triton calculation returns a value of 34.02 for that sample.)

I now have five different data points of 34.00, 34.02, 34.1, 33.9, and 34 for my salinity, from the Triton calculations, my density calculation, the DMA5000 densitometer, and the professional laboratory refractometer, respectively, all of which agree well with each other, while my refractometer reads 34.9-35.0 for the same samples. Clearly, I need to revisit my refractometer calibration.

Thats the kind of certainty id like to have too. Where can you find all this equipment online? The volumetric flasks ive found dont state the precion.

 

[JimWelsh]

For the volumetric flask, you're looking for one classified as "Class A". If they don't say that, then they have looser tolerances, and this greater uncertainty will exacerbate the problem I described above. Here is an example: http://www.amazon.com/gp/product/B007FFXBIQ?*Version*=1&*entries*=0&redirect=true

I bought a cheap $40 scale, because I can use the ones at the lab when I need real precision. It serves me well for a lot of things. I got it here: http://www.hometrainingtools.com/digital-platform-scale-300-g-x-01g .

The DMA5000 runs about $5,000 used http://www.equipnet.com/densitometers-equipment-48372/.

EDIT: A key to success with trying to do what I did to measure density is being able to properly read the meniscus, and know exactly when the flask is full to the marking line. The use of a black/white card helps a LOT: http://www.nist.gov/pml/wmd/labmetrology/upload/GMP_3_20130424.pdf

 

[Cory]

Great! I got a scale thats .01 x 500g, which should do. I need to buy a flask and thermomemter then ill try the experiment.

Id like to know if my scale is accurate. I was thinking using a quarter, as their weights are listed online as a calibration check. Good idea?

 

[JimWelsh]

I got a set of weights when I got my scale: http://www.hometrainingtools.com/mass-set-advanced. I then took them to the lab and recorded their weight as measured on two different 0.0001 g analytical balances, and averaged the results. The weights were pleasantly accurate for the price (and the purpose intended), with a maximum 0.01 g error only one of the larger weights. Using the calibrated weight set, I was then able to calibrate my scale, which is very linear, but does have a slight slope error. To find the actual weight of something on my scale, I take the value the scale reads, and multiply by 1.0006.

 

[cmcoker]

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.

Could potassium chloride solution be used to calibrate a refractometer?

I know from your articles not all saltwater conductivity calibration solutions can be used on a refractometer, IIRC. SO I kinda was doubting it
Just wondering since we have 20mEq/10ml KCl solution at work, but I don't have a very accurate scale to measure salt to make your diy recipe.

Unless, my Chem prof will let me use one in the lab...

 

[Randy Holmes-Farley]

Potassium chloride can certainly be used to calibrate a refractometer, as long as you know what refractive index corresponds to what concentration.

The refractive index of 35 ppt seawater is 1.33940 at 20 deg C.

Potassium chloride at 0.65 moles/L should have the same refractive index according to my Handbook of Chemistry of Physics.

The solution you have is 20 meq/ 10 mL (pretty strange units, if you ask me, I'd double check) = 2 moles/L

So you need to mix 0.325 volumes of the solution with 0.675 volumes of RO/DI. You can do that on any scale you want (32.5 mL + 67.5 mL, for example).

 

[Paul B]

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...

Not everywhere but a good seafood market will have it. I saw it yesterday

 

[cmcoker]

Potassium chloride can certainly be used to calibrate a refractometer, as long as you know what refractive index corresponds to what concentration.

The refractive index of 35 ppt seawater is 1.33940 at 20 deg C.

Potassium chloride at 0.65 moles/L should have the same refractive index according to my Handbook of Chemistry of Physics.

The solution you have is 20 meq/ 10 mL (pretty strange units, if you ask me, I'd double check) = 2 moles/L

So you need to mix 0.325 volumes of the solution with 0.675 volumes of RO/DI. You can do that on any scale you want (32.5 mL + 67.5 mL, for example).

Thanks.

I can do this more accurately, I think.

The KCl we have is for IV use (veterinary). I am 99.9% certain it is 20mEq but I'll double check.

Another question regarding calibration. If you are running a tank at hypo salinity, (14ppt for monogenean parasites) would a refractometer be better calibrated with RODI or 35ppt solution?

 

[Randy Holmes-Farley]

Thanks.

I can do this more accurately, I think.

The KCl we have is for IV use (veterinary). I am 99.9% certain it is 20mEq but I'll double check.

Another question regarding calibration. If you are running a tank at hypo salinity, (14ppt for monogenean parasites) would a refractometer be better calibrated with RODI or 35ppt solution?

For hypo, I'd calibrate with RO/DI because 0 ppt is a little closer to the salinity in question (14 ppt) than is a 35 ppt standard, and there's no chance of it being made incorrectly.

 

[cmcoker]

For hypo, I'd calibrate with RO/DI because 0 ppt is a little closer to the salinity in question (14 ppt) than is a 35 ppt standard, and there's no chance of it being made incorrectly.

Thanks!