Salinity reference solution and water impurity of salt.

[Reefinmike]

Is there any way to have 0% or an exact known water weight in the salt? Can it be baked off or absorbed using silica desiccant?

I’ve grown to distrust my refractometer despite it being calibrated with two off the shelf solutions, I believe it to be 2-2.5ppt off. I spoke with Lou from Tropic Marin and he sent me a quality glass hydrometer. I created a batch of calibration solution using TM salt. They list 6.7% water impurity, the weight required for the hydrometer to read 1.0264 indicates it has 12% water weight assuming the hydrometer is accurate. To get my refractometer to read 35ppt I needed 173gr salt to 3785ml water indicating a 20% water impurity if it were accurate. I ran across your article on creating reference solutions using table salt but didnt see any mention of water impurity. I made 3785gr of solution using 140.6gr non iodized NaCl. My hydrometer read 1.0268 at 77*. This would indicate 1.15% water impurity if my math and hydrometer are correct. I mixed the solution for 3 hours and now 24 hours later it’s still pretty foggy. Is this an issue?

Thank you for any wisdom you send my way.

 

[JimWelsh]

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

 

[Reefinmike]

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

??? Yes that is the article i reference. My question is on accounting for water impurity in the salt.

 

[JimWelsh]

NaCl is not hygroscopic, and will have no "water impurity". There is no water to remove from it.

 

[Reefinmike]

NaCl is not hygroscopic, and will have no "water impurity". There is no water to remove from it.

I believe the nacl i was using had 1% or less water assuming my hydrometer was accurate. I figured randy didn’t address this because table salt is consistent but i have to wonder. Ever go to a beach side restaurant and see clumped up salt in the shakers or rice to prevent clumping?

 

[JimWelsh]

OK, to answer your original question directly: Yes, if you want to be certain that it has no water, then you can bake it for a while, and then let it cool inside a vacuum dessicator over calcium chloride or some other effective dessicant, such as silica gel or concentrated sulfuric acid, but that's really overkill and I doubt you will find a measurable difference. If your pure table salt with no additives, e.g., Diamond Crystal, isn't clumped and flows freely, then it's perfectly fine for this application.

 

[Reefinmike]

Thank you Jim! I will have to look up how to build a vacuum desiccator. It may not be worth the effort, like i said the table salt i used should only be 1.15% water if my hydrometer is accurate. Compared to the 12% i see in tropic marin salt this is rather minuscule.

 

[redfishbluefish]

I thought Randy's calibration solutions used IODIZED salt. You mentioned you used non-iodized.

I'm also amused that the water is considered an impurity. I have to believe that reef salts, containing a mix of salt, calcium, etc have a number of components that do "attract" to water and weakly associate with those compounds.

 

[Reefinmike]

I thought Randy's calibration solutions used IODIZED salt. You mentioned you used non-iodized.

I'm also amused that the water is considered an impurity. I have to believe that reef salts, containing a mix of salt, calcium, etc have a number of components that do "attract" to water and weakly associate with those compounds.

Good catch, i’ll have to pick up some iodized and see if there is any difference.

Reef salts are known to contain anywhere between 6.7-18.5% water by weight. I’d consider it an impurity when you are measuring salinity by creating a known reference solution. Calcium chloride dihydrate(the normal stuff most everybody uses) contains something like 23% water.

 

[Randy Holmes-Farley]

The iodide makes no difference. Jim was, I think, exaggerating for effect on the dessicator. It’s only needed if you want super precision. No reefer needs that. Use salt right from a new box and it will be fine. Bake it if you are super anal about precision. Don’t bother with a dessicator. [emoji23]

 

[Reefinmike]

The iodide makes no difference. Jim was, I think, exaggerating for effect on the dessicator. It’s only needed if you want super precision. No reefer needs that. Use salt right from a new box and it will be fine. Bake it if you are super anal about precision. Don’t bother with a dessicator. [emoji23]

Thanks for the reply Randy, I have been in the hobby 15 years and your articles have always been very helpful. I wasn’t sure how big of a factor water impurity could be in “pure” sodium chloride seeing that reef salts can have upward of 20% water by weight. If you aren’t worried about it, i’m not .

This hydrometer has been rather eye opening. My refractometer is off 2+ppt despite being calibrated with two solutions.

 

[Randy Holmes-Farley]

Thanks. [emoji3]

Reef salts are very different in moisture content than NaCl. Suppose it has 1% moisture in it. That means a solution that you think has a sg of 1.025 is really 1.02475. Not a concern for any Reef tank, and not discernible with any normal reef refractometer.

 

[Dkeller_nc]

I suspect Randy's correct, and Jim was simply explaining a laboratory procedure for effect.

But if you want to bake off moisture in materials for other reasons (such as baking sodium bicarbonate into sodium carbonate), you're likely to have all that you need in your kitchen. You simply need a way to seal the material from the atmosphere while it cools from being heated in the oven. A wide-mouth mason jar works well for this purpose - simply add the material as soon as it's removed from the oven and seal it. Allow it to cool all the way down while sealed before you open it to access the material.

One thing to note - there's a bit of difference between adsorbed water in a hygroscopic material like sodium chloride, and water that's actually incorporated into the chemical compound as waters-of-hydration. Sodium chloride that's left out for a while will have water in it, but it's still sodium chloride. Some amount of the water that's in a seawater mix, however, is actually incorporated as part of the chemical structure of the components - examples include calcium chloride dihydrate and sodium bicarbonate.

I note this because one could say that sodium chloride that's been left out is simply "wet". Calcium chloride dihydrate can actually be "dry", but it still has 2 moles of water per mole of calcium in the compound itself. So as Randy notes, it's a bit misleading to think of a seawater mix as having water in it as an "impurity".