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I found an old bottle labeled "soda asbestos" in the lab. It is a granular material with a grain size of about 0.5 mm. A newer label put on top says "sodium hydroxide".

It is brown, and when dissolved in water makes a brown solution with some residual brown grains at the bottom. The solution has a very high pH as one would expect from sodium hydroxide.

I saw an MSDS online that says it does not contain asbestos, despite being called "soda asbestos".

So what is it? Impure sodium hydroxide?


EDIT

Ok, so let's go on an adventure.

Here's the bottle:

enter image description here

This is what it looks like inside. Has a plug that fell inside I guess? You can clearly see the beige material.

enter image description here

Is the red stuff soluble? Put in two bottles, one stirred, the other isn't. If after a few hours the left settles or the right dissolves, then we have an answer.

enter image description here

Final result - it is definitely insoluble.

enter image description here

Very high pH as one would expect from a "sodium hydroxide".

enter image description here

To identify the red stuff, I poured the solution out after it settled, and then waited for it to dry. I scraped some of the red residue to carbon tape and took it to our SEM. This is how it looks:

enter image description here

Overall chemical composition is dominated by sodium and iron, with traces of other things like silicon, calcium, and aluminium (I assume all as oxides):

enter image description here

You can find an occasional needle in there - I wanted to make sure it is not asbestos. A quick EDS map shows that it is not a magnesium silicate (proper asbestos) but rather some kind of aluminium-rich silicate.

enter image description hereenter image description hereenter image description here

I guess this mostly answers my initial question of what is "soda asbestos": it is sodium hydroxide with iron oxide impurities (that is, "rust") and some other minor non-hazardous solids.

Then a revised question would be why is it called "asbestos" if there is no asbestos in it, and why does it have iron impurities? Do they serve a purpose, or is it just a cheap version of sodium hydroxide for applications where the impurities don't matter?

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    $\begingroup$ Asbestos membranes were (are still?) used in the electrolysis process to make sodium hydroxide. Perhaps that was considered purer than other methods so acquired the name to distinguish it from other methods. $\endgroup$
    – Jon Custer
    Commented May 23 at 0:44
  • $\begingroup$ @JonCuster this is clearly not pure, being brown and with an insoluble brown residue. Looks a bit like rust. I might take it to the SEM to see what it is... $\endgroup$
    – Gimelist
    Commented May 23 at 0:56
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    $\begingroup$ Purer than the mercury process for NaOH would produce perhaps? At least no mercury in it… $\endgroup$
    – Jon Custer
    Commented May 23 at 1:02
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    $\begingroup$ Approximately, what mass percentage the insoluble stuff makes? Photos of the bottle, avalilable info and of the dry and dissolved stuff could help. But possibly all the think is not worthy the trouble, I would dispose. $\endgroup$
    – Poutnik
    Commented May 23 at 7:12
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    $\begingroup$ @Poutnik not worth the trouble, but absolutely fun to do. See update to OP. $\endgroup$
    – Gimelist
    Commented May 24 at 7:25

2 Answers 2

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Soda asbestos was a product of Merck and used in old-school analytical chemistry for organic analysis. The German name was Natronasbest and it can be found older books by German author (Fritz Pregl, Nobel Prize winner). It served as carbon dioxide adsorbent. In combustion analysis, one would burn an organic compound, determine the amount of carbon dioxide and water to eventually determine the molecular formula of a compound. One needed traps for water and carbon dioxide.

The only clear description I found is in Pechanec, V., & Horáček, J. (1965). Bestimmung von Kohlenstoff und Wasserstoff in organischen Verbindungen IV. Beitrag zur Untersuchung der die Kohlendioxidabsorption beeinflussenden Faktoren, Vergleich des Wirkungsgrades fester Absorptionsmittel und Vorschlag für ein neues Absorbens. Collection of Czechoslovak Chemical Communications, 30(4), 1082-1091.

"Natronasbest ist im wesentlichen eine granulierte Schmelze von Natriumhydroxid, Asbest... "

"Soda-asbestos is essentially a granular molten (mixture) of sodium hydroxide and asbestos."

No surprises as the name indicates. The brown color could from iron impurities or used up material stored in a bottle. Modern Ascarite as mentioned by the user Buck Thorn must be modern asbestos free material.

Updated response to comments and in the original post:

See update to OP. No asbestos in there, it's mostly iron oxide.

No, one cannot conclude that just on the basis of EDS alone. There is Al, there is Si and there is iron but this is not rust. As the bottle states, there is a synthetic silicate with NaOH.

Then a revised question would be why is it called "asbestos" if there is no asbestos in it, and why does it have iron impurities? Do they serve a purpose, or is it just a cheap version of sodium hydroxide for applications where the impurities don't matter?

There is most likely due to historical reasons, well the composition is similar to asbestos although the bottle clearly says synthetic silicates. The main purpose of soda asbestos was carbon dioxide absorbent.

Soda asbestos is not the only term we would find in historical chemistry literature, platinum asbestos, copper oxide asbestos etc. are common too. There is nothing mysterious about this bottle. It is NaOH with synthetic silicates, which mimic asbestos or perhaps an equivalent of synthetic version of granular asbestos so that gas can pass through this material. It is not supposed to be dissolved in water. This material would be filled in a U-tube or something equivalent, and the weight gain would correspond to the weight of carbon dioxide obtained from combusting an organic material.

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    $\begingroup$ So, what is it, chemically? Is it supposed to be brown, or it should be (and normally was) white, like neat sodium hydroxide should be? I think the gist of the question is where does the brown color come from. $\endgroup$
    – andselisk
    Commented May 23 at 16:31
  • $\begingroup$ Brown color could be anything, but most likely iron impurities or gunked up used material. The OP does not provide the history of this antique material. $\endgroup$
    – AChem
    Commented May 23 at 19:08
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    $\begingroup$ @AChem see update to OP. No asbestos in there, it's mostly iron oxide. $\endgroup$
    – Gimelist
    Commented May 24 at 7:24
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    $\begingroup$ How would carbon dioxide absorption work? By reaction of NaOH to bicard soda? $\endgroup$
    – Gimelist
    Commented May 25 at 3:19
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    $\begingroup$ @Gimelist, Yes, imagine gas pass a glass tube (say a U- shaped tube) packed with soda-asbestos...NaOH will react with CO2 forming bicarb first. The entering gas was dried by P2O5 or by other powerful dessicants. $\endgroup$
    – AChem
    Commented May 25 at 3:28
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A search for "soda asbestos" reveals that similar compounds are available from major chemical suppliers under various names (CAS # 81133-20-2, 1310-73-2, 7631-86-9).

ThermoFisher sells this as Ascarite™ II, Sigma-Aldrich as Ascarite (both are trademarks). Ascarite is a synonym for $\ce{NaOH}$ coated silica (as explained in the Sigma-Aldrich website). It is indeed composed of beige to light brown granules (20-30 mesh), and as noted in another answer serves as a $\ce{CO2}$ absorbent (the ThermoFisher product has a capacity of 40-50%).


Update

The label suggests that the substance is an impure form (>=60%) of $\ce{NaOH}$ containing synthetic silicates.

enter image description here

Synthetic silicates (clay) include amorphous magnesium silicate, used as an adsorbent material, the clay serving as a support.

The analysis by the OP indicates however that the support is an aluminosilicate with some magnesium and plenty of iron. That composition would not be inconsistent with asbestos (chrysotile):

The idealized chemical formula of chrysotile is $\ce{Mg3(Si2O5)(OH)4}$, although some of the magnesium ions may be replaced by iron or other cations.

Why retain the name "soda asbestos" if it does not contain asbestos? Presumably for historical reasons but it still remains the biggest mystery.

Why was asbestos used at all? Chrysotile (serpentine) is resistant to strong bases but reacts with acids and can presumably react with $\ce{CO2}$.

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  • $\begingroup$ It is very strange that if we search Ascarite in CAS, it comes up as sodium hydroxide only. $\endgroup$
    – AChem
    Commented May 23 at 18:45
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    $\begingroup$ Modern ascarite must be asbestos free. Old school name is true to its name :-) Molten NaOH and asbestos granules. $\endgroup$
    – AChem
    Commented May 23 at 19:09
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    $\begingroup$ See edit to OP - this is not NaOH coated silica. $\endgroup$
    – Gimelist
    Commented May 24 at 7:22
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    $\begingroup$ Note this answer is based on information scraped from the Wikipedia but might provide a Bayesian prior for further thought. There is plenty suspicious about much of this. At least it's nice to have some powerful instruments to check the material! $\endgroup$
    – Buck Thorn
    Commented May 24 at 8:48
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    $\begingroup$ @BuckThorn very easy when I have a free-to-access SEM at my disposal. :) I guessing XRDing it would solve the question of what it is, but now that I discovered it's essentially rust, I'm not sure it is worth the effort. $\endgroup$
    – Gimelist
    Commented May 24 at 13:28

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