Humidity of a saturated NaCl solution with anticaking additive

[JT Smith]

TL;DR Summary

Will the presence of a small amount of an anticaking agent (calcium silicate or sodium aluminosilicate) present in commercially available table salt significantly affect the equilibrium humidity of a saturated salt/water solution?

I want to test a humidity sensor with one or more saturated salt solutions. The table salt that I have on hand contains one of two anticaking agents, calcium silicate or sodium aluminosilicate. Will the presence of either of these additives (or iodine for that matter) significantly affect the equilibrium humidity?

I searched and all the how-to-do-it guides did not address this question. One research paper I found reported that at 1.5% w/w calcium silicate increased the deliquescent point by 1.5% RH but I'm not entirely sure what to do with that. The search AI assist responses warned me that these agents could be an issue but could not be specific. ChatGPT provide a response that was tainted with some obvious nonsense.

So I came here. :-)

 

[Borek]

Unless my English fails me big there is no such thing as "humidity of a solution", I suppose you mean "humidity of the air saturated with a vapor in equilibrium with the saturated salt solution"?

Such highly concentrated solutions are about as far from being ideal as possible, so their behavior is very difficult to predict quantitatively, but qualitative prediction here is rather obvious and follows the Raoult's law: if the anticaking agent is soluble and dissolves in the presence of the saturated salt it will lower molar fraction of water in the mixture, lowering water vapor pressure above the mixture (to make things easier salts are not volatile, so it is safe to assume water vapor is the only gas in equilibrium with the solution).

 

[JT Smith]

Thank you, your English seems fine.

I believe humidity implies water vapor in air. And I did say saturated salt solutions and equilibrium so maybe you didn't read it well. Or perhaps I didn't word it well. Either way you got the gist of it so it's good.

From what I read calcium silicate has a very low solubility in water (0.01% w/w) so from a Raoult's Law perspective it wouldn't make much of a difference. But since it is added as an anticaking agent I wonder if its effect on water vapor might be larger somehow. Or maybe not. I just want a reliable calibration.

I could always buy reagent grade NaCl but would rather not if it's not going to make a difference. I'd like something that's accurate to about 1% relative humidity given that my sensor isn't supposed to be any better.

 

[Dullard]

Evaluation of the boiling point of a target solution (compared to a theoretical 'pure' salt solution) may give some insight into the magnitude of the effect of the anti-caking agents. FWIW: My experience with humidity sensors suggests that the accuracy claims can be misleading. Most have extremely long equilibrium times - humidity changes too quickly (in most situations) for the indication to reach the degree of accuracy inferred by an optimistic user.

 

[JT Smith]

Thank you, that's a good idea and something I can try. Of course it then brings up the question of the accuracy of my temperature sensor(s), a thorny problem all its own.

I'm expecting to be disappointed with my humidity sensor with respect to its specifications. I'd just like to know far off it is and correct it as much as possible.

The datasheet claims a response time (63%) of 4 seconds. Perhaps there are caveats to that which are not stated. I have another sensor with a 1 second response time provided there is a 1 m/s airflow in a specified direction, which isn't especially practical. What would it be in still air? The ambient relative humidity might change on the order of a few percent an hour. Following that doesn't seem like a particularly tall order but maybe I'm wrong.

 

[Mayhem]

Can't you use general purpose salt (non food grade) containing fewer additives.

What does the salt used for dish washers contain aside from NaCl?

 

[JT Smith]

I looked at some overpriced bags of sea salt in a fancy grocery store but most of those appeared to trumpet natural impurities as health benefits.

I had never heard of dishwasher salt. But I quickly found an inexpensive offering that claims it has no additives, is 99.9% pure, and is food grade, vegan, non-GMO, and kosher! Ha ha.

Thank you for that suggestion!

 

[Mayhem]

I mean salt for technical use, like for water softening (dish washers) or deicing, should be cheaper than food grade table salt.

 

[JT Smith]

The product I found is explicitly for use in dishwashers, is 99.9% pure, and contains no anti-caking agents. It costs more than table salt. Table salt is very inexpensive.

The product that Bosch recommends for their dishwashers is 99% pure (which is the same purity as the table salt that I have in my cupboard). They don't say that there are no anti-caking agents. They don't say anything about the impurities. It costs less on a per weight basis but would cost me more to buy the smallest amount as compared to the other product.

 

[Mayhem]

Purity of x% usually just means "we made sure it is x% pure, we literally have no conclusive data on the remaining percents". Also 0.1% anything should be inconsequential.

 

[JT Smith]

That's what I'm counting on but I'm not sure I completely believe the label. The product says 99.9% pure salt. But they have another product that is not labeled as being for dishwashers ("unrefined to preserve essential minerals") that is also 99.9% pure. Does "pure salt" = "pure NaCl"? I don't know the regulations. I can't even be certain that the product is legit.

The table salt I have specifies 99.7% purity but with a footnote that explains that this is "before additives". After including those it's closer to 99.0%. And of course one of those additives is an anti-caking agent at 0.2-0.7%. That's the one I am mainly concerned with, perhaps unnecessarily so.

I can also buy USP grade, 99.5% NaCl. It's "pharmaceutical" grade and has no additives. It's a little less expensive. The price is more about what I have to pay for the smallest available package than the the $/kg.


What if I took my table salt, dissolved it, and then recrystallized the salt. Would that form pure NaCl crystals or would impurities still come along for the ride?

 

[Borek]

What if I took my table salt, dissolved it, and then recrystallized the salt. Would that form pure NaCl crystals or would impurities still come along for the ride?

In theory recrystallization is one of purification methods, In practice you will never really know without precise analysis.

Plus, NaCl is one of these salts that are difficult to recrystallize, as its solubility doesn't change much with the temperature.

 

[chemisttree]

I believe Kosher salt does not contain anticaking agents. Not Morton but Diamond Crystal brand.

 

[JT Smith]

Plus, NaCl is one of these salts that are difficult to recrystallize, as its solubility doesn't change much with the temperature.

Now that you mention it I remember reading that. I suppose it's related to the fact that the humidity of a saturated solution is also nearly constant across a wide temperature range.

 

[JT Smith]

I believe Kosher salt does not contain anticaking agents. Not Morton but Diamond Crystal brand.

Thank you. I guess what you're saying is that some kosher salts don't have anti-caking agents.

A container of Diamond Crystal is about the same price as the other options I've found. There's another brand that has no additives that I believe I can obtain locally for about half the price. They don't make any claims about purity.