Reef Chemistry Question of the Day #270 Settling of Salt Mixes

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #270

A number of factors can influence whether the ingredients in a salt mix settle during shipping to be nonhomogeneous. These include the density, size, and shape of the particles present. Since the only one of these we can generalize about is density (and it is likely a big factor), let's explore how salt mixes might settle based on density.

Salt mixes can be formulated in a number of ways, but some of the main ingredients are anhydrous sodium chloride, calcium chloride (anhydrous or dihydrate), magnesium chloride (anhydrous or hexahydrate) and magnesium sulfate (anhydrous or heptahydrate).

Assuming you used all of these ingredients, and based on intrinsic (crystal) density alone, which of these ingredients would be most likely to be enriched on the top of a salt bucket (that had remained upright since manufacture) and which would be most enriched on the bottom?

A. Anhydrous sodium chloride on the bottom and magnesium chloride hexahydrate on the top
B. Anhydrous calcium chloride on the bottom and anhydrous sodium chloride on the top
C. Magnesium chloride hexahydrate on the bottom and calcium chloride dihydrate on the top
D. Anhydrous magnesium sulfate on the bottom and magnesium chloride hexahydrate on the top

Good luck!




























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[CNDReef]

I pick C , final answer lol

 

[rock_lobster]

C!!

 

[JimWelsh]

The densities of all these salts are readily available online with a quick Google search. Answer is D.

 

[Chuk]

I’m going with d since magnesium is a heavy little bugger and sulfate is heavier than chloride.

 

[SKYHIGGh]

The densities of all these salts are readily available online with a quick Google search. Answer is D.

Don’t ruin it lol

I say b final answer

 

[Hans-Werner]

Just to sow some doubts: My old Merck catalogue says for magnesium sulfate anhydrous: Density 2,66 g/cm3, but for bulk density just ca. 600 kg/m3. Does bulk density play any role?

 

[Randy Holmes-Farley]

Just to sow some doubts: My old Merck catalogue says for magnesium sulfate anhydrous: Density 2,66 g/cm3, but for bulk density just ca. 600 kg/m3. Does bulk density play any role?

I'm sure you've thought through these issues, but I'll discuss them for others...

Bulk density can be very important, but how it does is a very complex function of size and shape factors (which I excluded from this question), as well as if other dry powers are in the same mix with different sizes and shapes (as a salt mix may).

Let me take an extreme example to illustrate the issue.

Suppose you have a huge pile of randomly arranged wood tree logs. The density of the wood is maybe 0.7 g/cm3. The whole pile might have a bulk density of 0.2 g/cm3 due to all the air in the pile.

If I take some small packing peanuts with an actual density of 0.3 g/cm3 and a bulk density of 0.1 g/cm3 and place them on top of the pile, they will fall to the bottom through the logs despite having a lower density, because they fit into the spaces between the logs without pushing up the logs at all.

If you glue those same packing peanuts into bigger loose structures that do not fit between the logs, and that still have an intrinsic density of 0.3 g/cm3 and a bulk density of 0.1 g/cm3, they will stay on top of the logs (if that's where you put them), even if you shake the mixture up. If they started on the bottom, they would eventually rise to the top.

Finally, if you glue those same packing peanuts into bigger, but very tight structures that do not fit between the logs, and that still have an intrinsic density of 0.3 g/cm3 and a bulk density of 0.27 g/cm3, they will move downward in the log pile if you shake the mixture up because they are effectively more dense than the wood pile. If they started on the bottom, they would stay on the bottom.

The purpose of this discussion is to just show that how a mix really settles will be a complicated amalgam of features relating to density of the solid, the bulk density, as well as the relative sizes and shapes of the components.

 

[Hans-Werner]

Randy, thank you for this very illustrative example. I already had something similar in mind: dried magnesium sulfate is also available as granules. The density of the single granules may be much lower that the density of the dried magnesium sulfate because the granules seem porous and quite lightweight. It is a bit similar to a feather and a hoof or a horn: All three are made of keratin but the one is so lightweight it will easily be blown away by a light breeze while the other two are massive and heavy.

 

[MnFish1]

Is the assumption that no humidity has entered the bucket?

 

[Randy Holmes-Farley]

Is the assumption that no humidity has entered the bucket?

Yes. And no trading moisture between the materials.

 

[Randy Holmes-Farley]

And the answer is...

Assuming you used all of these ingredients, and based on intrinsic (crystal) density alone, which of these ingredients would be most likely to be enriched on the top of a salt bucket (that had remained upright since manufacture) and which would be most enriched on the bottom?

A. Anhydrous sodium chloride on the bottom and magnesium chloride hexahydrate on the top
B. Anhydrous calcium chloride on the bottom and anhydrous sodium chloride on the top
C. Magnesium chloride hexahydrate on the bottom and calcium chloride dihydrate on the top
D. Anhydrous magnesium sulfate on the bottom and magnesium chloride hexahydrate on the top

I determined this answer based on published densities. From them, we might also speculate that magnesium is more likely to separate out (up or down) than is calcium, regardless of whether any of them are hydrated or not, since the magneisum forms deviate more from sodium chloride than do the calcium forms.

Anhydrous sodium chloride 2.17 g/cm3
Anhydrous calcium chloride 2.15 g/cm3
Calcium chloride dihydrate 1.85 g/cm3
Anhydrous magnesium chloride 2.32 g/cm3
Magnesium chloride hexahydrate 1.57 g/cm3
Anhydrous magnesium sulfate 2.66 g/cm3
Magnesium sulfate heptahydrate 1.68 g/cm3


https://en.wikipedia.org/wiki/Magnesium_sulfate
https://en.wikipedia.org/wiki/Magnesium_chloride
https://en.wikipedia.org/wiki/Sodium_chloride
https://en.wikipedia.org/wiki/Calcium_chloride

 

[Charles Silva]

In this case should we use the entire bucket or pack to prepaire the salt water? My doubt is if this known salt settle during shipment can play any significant changes in KH, Ca, Mg concentrations (of the new water) if I use for example 1/4 of the pack?

 

[Randy Holmes-Farley]

In this case should we use the entire bucket or pack to prepaire the salt water? My doubt is if this known salt settle during shipment can play any significant changes in KH, Ca, Mg concentrations (of the new water) if I use for example 1/4 of the pack?

Using the entire container is best, followed by mixing it up in the dry form, but mixing it up also risks actually causing separation depending on how you do it. I've not seen any studies of how to mix up a salt bucket, but many people roll them around, presumably just because it is easy.

 

[Charles Silva]

I've been watching several salt reviews & comparisons (that I assume bizarre) with the guy sampling 1 kg of each brand and preparing the water with the same salinity for some elements comparisons. I presume he adds a huge number of uncertantites like the refractometer error, the hobby test kits and this salt settle during transportation. I think even in a lab is hard to perform such type of comparison.

 

[Randy Holmes-Farley]

I've been watching several salt reviews & comparisons (that I assume bizarre) with the guy sampling 1 kg of each brand and preparing the water with the same salinity for some elements comparisons. I presume he adds a huge number of uncertantites like the refractometer error, the hobby test kits and this salt settle during transportation. I think even in a lab is hard to perform such type of comparison.

For sampling solid mixtures, industries use special devices to collect samples from the bottom, top and middle to allow looking for differences.

We used them extensively for pharmacueticals and raw materials that came in large drums.

Like this:

http://www.qclabequipment.com/SAMPLERPOWDERTHIEF.html

 

[*HOVA*]

Here I came thinking this topic would be about saltwater already mixed and the effects of leaving it unused for a period of time!

Thanks for the info as always though

 

[SteadyC]

@mrsaltwatertank i remember you telling us to shake our buckets, and people questioning you. You have been vindicated!

 

[Randy Holmes-Farley]

Here I came thinking this topic would be about saltwater already mixed and the effects of leaving it unused for a period of time!

Thanks for the info as always though

lol

You're welcome.

 

[chefjpaul]

And the answer is...

Assuming you used all of these ingredients, and based on intrinsic (crystal) density alone, which of these ingredients would be most likely to be enriched on the top of a salt bucket (that had remained upright since manufacture) and which would be most enriched on the bottom?

A. Anhydrous sodium chloride on the bottom and magnesium chloride hexahydrate on the top
B. Anhydrous calcium chloride on the bottom and anhydrous sodium chloride on the top
C. Magnesium chloride hexahydrate on the bottom and calcium chloride dihydrate on the top
D. Anhydrous magnesium sulfate on the bottom and magnesium chloride hexahydrate on the top

I determined this answer based on published densities. From them, we might also speculate that magnesium is more likely to separate out (up or down) than is calcium, regardless of whether any of them are hydrated or not, since the magneisum forms deviate more from sodium chloride than do the calcium forms.

Anhydrous sodium chloride 2.17 g/cm3
Anhydrous calcium chloride 2.15 g/cm3
Calcium chloride dihydrate 1.85 g/cm3
Anhydrous magnesium chloride 2.32 g/cm3
Magnesium chloride hexahydrate 1.57 g/cm3
Anhydrous magnesium sulfate 2.66 g/cm3
Magnesium sulfate heptahydrate 1.68 g/cm3


https://en.wikipedia.org/wiki/Magnesium_sulfate
https://en.wikipedia.org/wiki/Magnesium_chloride
https://en.wikipedia.org/wiki/Sodium_chloride
https://en.wikipedia.org/wiki/Calcium_chloride

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