Problems Making a 35 ppt Salinity Reference Solutions

[Mono]

So, I tried to make 35 ppt Salinity Reference Solution. When compared to my Two Little Fishies Accurasea Reference solution, there is a about 10 ppt difference.

My solution is about 10 ppt less than Two Little Fishies.

I got the same result checking it against AccuBrate.

So, I guess it's a fail.

I can't understand why my results should be so far off.

I checked my scale with reference weights and it's ok.

I used a 1 liter volumetric flask 3/4 filled with RODI water.

I added 35 g Instant Ocean salt from a newly opened bag and gave the flask a good swirl.

I topped of the flask with RODI water to the 1 liter line.

I added a magnetic stirrer and let it stir for about 45 minutes on high.

I did not notice any precipitate in the flask.

I removed the stirrer, put in a stopper and let it sit about 5 hours.

I checked the salinity and there is about a 9 or 10 ppt difference with my solution measuring about 10 ppt less than both commercial solutions.

What could cause such a wide discrepancy? Does the salt pick up moisture from the air?

I mean, if 10g of salt failed to make it into the flask, I would surely notice.

Any thoughts on how I can get this right would be greatly appreciated.

Thanks.

 

[Randy Holmes-Farley Verified Community Expert]

That method will always fail. Salt mixes contain lots of water.

 

[Randy Holmes-Farley Verified Community Expert]

I give suitable diy recipes here:

http://reefkeeping.com/issues/2004-06/rhf/index.htm

 

[Mono]

I give suitable diy recipes here:

http://reefkeeping.com/issues/2004-06/rhf/index.htm

Randy, Thanks a million again.

I re-read your article which I had read several years ago with a new appreciation.

I made a reference solution by first filling my volumetric flask 3/4 full with RODI, adding 36.5g of Morton Iodized Salt, then filling the flask to the 1 liter mark. The solution measured 35 ppt on my Milwaukee Digital Refractometer. So, I'll count that as a success. However, the solution does look a little cloudy. I assume that's because of the anti-caking agents.

So, just two quick follow-up questions.
1. I used Morton Iodized Salt because that's what mentioned in your article. Would it make any difference if one used un-iodized salt, salt without anti-caking agents, or a course salt like kosher salt?

2. What is it exactly (or inexactly) that makes synthetic sea salt more prone to absorbing moisture from the air as opposed to good old NaCl?

Thanks again.
Michael

 

[Randy Holmes-Farley Verified Community Expert]

Randy, Thanks a million again.

I re-read your article which I had read several years ago with a new appreciation.

I made a reference solution by first filling my volumetric flask 3/4 full with RODI, adding 36.5g of Morton Iodized Salt, then filling the flask to the 1 liter mark. The solution measured 35 ppt on my Milwaukee Digital Refractometer. So, I'll count that as a success. However, the solution does look a little cloudy. I assume that's because of the anti-caking agents.

So, just two quick follow-up questions.
1. I used Morton Iodized Salt because that's what mentioned in your article. Would it make any difference if one used un-iodized salt, salt without anti-caking agents, or a course salt like kosher salt?

2. What is it exactly (or inexactly) that makes synthetic sea salt more prone to absorbing moisture from the air as opposed to good old NaCl?

Thanks again.
Michael

If you are using a scale to weigh the salt, and it is not damp, it makes no difference what brand to use. Weighing is best and totally pure NaCl is optimal, although the amounts of other ingredients are down in the noise of what we can measure.

If you are measuring the salt more crudely by volume, the brand can matter because different brands may have different bulk density (the fluffiness factor) and I can only be sure the one I used has the same mass to volume ratio.

In sea salt, two of the main ingredients that are usually added with a large moisture content in the solid are the magnesium salts (magnesium sulfate heptahydrate and magnesium chloride hexahydrate) and calcium chloride dihydrate. All those hydrates add up to a lot of mass.

One could make it with a lot less (but not zero) moisture, but the dehydrated ingredients are more expensive and are no advantage for a salt mix.