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solubility HCl vs water

Posted by cascaillou  
cascaillou December 27, 2013 11:58AM
I'd like some chemistry explanation:

I used to think that if a mineral is soluble in water then it is necessarly also soluble in strong acid solutions (as these are water based), and reciprocally that a mineral that isn't soluble in strong acid solution cannot be soluble in water...

However I came across this documentation about solubility of minerals:

This table shows that many minerals that are soluble in water still aren't soluble in HCl.
How is that possible, considering that concentrated HCl solutions (such as 37%) are largely composed of water?

Edited 1 time(s). Last edit at 12/27/2013 12:13PM by el cascaillou.
Alfredo Petrov December 27, 2013 01:27PM
It is silly to divide minerals into "soluble" and "insoluble", when there is a continuous scale of solubility. If only 1 gram dissolves in 1 liter of water, is that a soluble or insoluble mineral? If it is very soluble in warm water but very little in cold, do we list it as soluble or insoluble? I suspect a lot of the discrepancies in the table are due to this lack of a defined boundary. For example, I would list gypsum as "soluble", because it is slowly dissolved by running water - you can notice it after a few hours under a running tap - but lots of people would say "insoluble" because the crystal doesn't instantly disappear like sugar when you drop it in a glass of water. There is also the distinction between ease of solubility of the powdered mineral (as measured in the laboratory) and ease of solubility of a solid crystal while washing the mud off, which is more what concerns collectors.

Imagine if weather forecasters were not permitted to give us the temperature and were only allowed to use the words "hot" and "cold" :-S

Then there are the minerals that react with HCl to give an insoluble precipitate...

In conclusion, such a table is of only very limited use without numerical data.

Edited 1 time(s). Last edit at 12/27/2013 01:32PM by Alfredo Petrov.
cascaillou December 27, 2013 03:27PM
well, when a compound is stated to be soluble in water, or in an acid, with no indication of temperature, you have to assume that it means at ambient room temperature (meaning nearby 20°C), otherwise it should be mentionned that the solvent is hot (heating to 100°C at most)

Of course one shouldn't expect all minerals to solubilize within minutes, but keeping things at human size time wise, I always wait for 24h of soaking in the solvent before concluding that a compound is insoluble.

For a solubility test, I only use a tiny grain of the mineral (I mean 1 or 2mm sized grain), soaked in an excess of concentrated acid (meaning filling a good quarter of the test tube with solvent). Indeed a single tiny grain makes it easy to observe even slight dissolution over a 24h period.

Concerning acid concentration, I just stick to the highest concentration available to the general public (i.e. at the hardware store)

In the final, you don't really need to obtain numerical values (i.e. mg per liter) as long as your empirical solubility test is based on a standardized procedure.

Anyway, I still don't understand how a mineral compound that is soluble in water could be insoluble in a solution of hydrochloric acid???
Assuming that the acid solution is used in excess and that both experiments were conducted at ambient room temperature, and considering that an HCl solution is largely composed of water, I would expect the mineral to show at least the same solubility in the acid than in water, if not better.
So is that a mistake in the table, or is there some chemistry rule I'm not aware of that can explain such results?

Edited 4 time(s). Last edit at 12/27/2013 11:12PM by el cascaillou.
Doug Daniels December 27, 2013 05:14PM
To further muddy things, basically, everything is soluble in water.....just a matter of how much of copmound X goes into a given amount of water at a given temperature and pressure.

As far as the table, I perused thru the"k's", and have these observations:
Borax and kernite - borates - would react with acid to form boric acid, which isn't soluble in cold solution, so may be part of the explanation

Carnallite, halite, kainite - chlorides (I think kainite is a chloride) - there's some rule in chemistry regarding same anions - HCl wouldn't necessarily dissolve a chloride, because of the common ion effect (or at least has something to do with it - the chemists can chime in here)

Chalcanthite, copiapite, epsomite - sulfates - may have to do with common ion effect also. Maybe they were just ornery at the time.

Also, may have to do with the concentration of acid used; don't know if that is given in the table. I haven't physically tried to dissolve chalcanthite or epsomite with HCl (either dilute or straight from the hardware-store bottle) to see if they dissolve....maybe that's how I'll spend New Years eve. Anyway, my 2 cents worth.
David Von Bargen December 27, 2013 05:21PM
But the HCl solution has a lot of chloride ions in solution. The amount of halite that would be dissolved in a pure water solution is going to be larger than that in a concentrated acid solution (since you are part way to the maximum number of chloride ions in solution).
Rock Currier December 27, 2013 07:18PM
No wonder the chemists left us behind. We should be listing solubility constants and the temperature and solvent listed. We should be using something better for a hardness scale and for heaven sake if we do use that one make sure we which hardness scale we are referring to.

Rock Currier
Crystals not pistols.
Trevor Dart December 27, 2013 10:17PM
I'll try to use my knowledge of chemistry and acid-base equilibrium here.

It comes back to solute vs solvent. In acidic conditions there can be a negative effect on the solubility of some substances (as previously pointed out with borax) if the concentration of Hydrogen ions is already high. This would then show a tendency not to dissolve in acid when the same substance had limited solubility in water.

Again the key factor in solubility is temperature. If the water is hotter, then it has a greater ability to act as a solvent. (gases on the other hand have greater solubility in cold water). The dissolving of substances in acidic solutions is usually a chemical reaction involving the hydrogen ions and this may be the data that is being relayed in this table. A substance may dissolve in water, but have no apparent reaction with the HCl.

Just remember that water will dissolve quartz. But the temperature needs to be around 300+ oC and at a high pressure. Which is why it commonly forms in hydrothermal veins and at hot springs.


Cheers, from my little piece of the Sunny Australian Outback...
cascaillou December 27, 2013 10:52PM
I did my homework and opened my copy of the Merck Index (must have book).
It does indeed confirm that while NaCl (=halite) is soluble in water (1g dissolves in 2.8mL water at 25°C), HCl decreases its solubility in water, to the point that NaCl is almost insoluble in concentrated HCl (that suggest that NaCl is soly in diluted HCl but to a lesser extent than in water)

Please note that a solubility test might help to identify a mineral by elimination of other candidates, it's largely empirical, and for such purpose, the fact that almost everything is soluble in water isn't an argument. We are talking of a tiny mineral grain in an excess of solvant, at ambiant pressure and ambiant temperature, or heating around 100°C, duration of experiment not exceeding 24h. Upon such specifications, neither quartz nor calcite are going to noticeably dissolve in water, and therefore mineralogy books state that these are insoluble in water (even so they would indeed dissolve with time/pressure/heat).

Edited 3 time(s). Last edit at 12/27/2013 11:16PM by el cascaillou.
Ilkka Mikkola December 28, 2013 03:02PM
If you will use HCl for identifying minerals, use 10% HCl. See: marulla dot com, mineral solubility. For instance
most phosphates are soluble in 37% HCl but many Al- and Be-phosphates are not soluble in 10% HCl.
Of course they are theoretically soluble but after 1-3 hours they look intact. We have used 10% HCl for identifying
viitaniemiite. Fluorapatite at Viitaniemi could be very much like viitaniemiite but it is soluble in 10% HCl.
Steve Hardinger December 28, 2013 03:19PM
Being a chemist and mineral collector, let me add another layer of fuzziness: Person unfamiliar with the chemical idea of solubility (i.e., solubility at equilibrium with time not a factor) often confuse solubility with rate of dissolution. Some things which are highly soluble in water may dissolve slowly because the crystals are large (smaller surface area = less contact with solvent = slower dissolution), agitation of solvent, etc.

I'd like to see an ASTM-style procedural description for how to run a solubility test. Once this is done, and tried and tested, we can have a even play field to discuss solubility characteristics. Only then does solubility become a meaningful test for the masses.

Not sure I want to lead this project, but I would if there was enough interest.
cascaillou December 28, 2013 03:34PM
well I think solubility test is interesting considering that it is accessible to everyone: it requires nothin but a pyrex test tube and a few acids (and bases by the way) that are largely available at the hardware store (HCl, H2SO4, HNO3, NaOH, KOH, NH4OH)

Also, I don't understand why mineralogy handbooks often mention solubility in acids, but never solubility in bases (which would be just as useful information).

Another concern is that concentration of solution is never mentionned.
This being said, it would make more sense to stick to the highest concentration available to the general public, meaning at the hardware store, not through chemical suppliers (and anyway I do not like to deal with 90%+ HNO3 or H2SO4 which are nasties)

Edited 9 time(s). Last edit at 12/28/2013 03:47PM by el cascaillou.
Steve Hardinger December 28, 2013 07:39PM
El, no, that doesn't make sense because the concentrations vary from source to source and country to country, and even between two containers of the same material from the same manufacturer at the same supply source. For example the concentration (and purity) or muriatic acid varies widely. Since both of the variables are critical components to a definitive solubility test, a standardized solubility test procedure would require running some known samples first before trying the unknown. This is why a standardized procedure would be needed before we can begin to use solubility as a reproduceable characterization tool.

And even with a standardized procedure, the solubility results won't necessarily be perfect. What if you're testing something which is a mixture, and one component dissolves in (for example) hydrochloric acid but another does not?
Ilkka Mikkola December 28, 2013 11:21PM
Small fluorapatite grain dissolves in dilute HCl in minutes, if mineral grain looks intact after an hour, it is not apatite.
That is the point. If the HCl is strong (undiluted), I think that all phosphates dissolve.
Does anyone use diluted (about 10%) HCl with calcite identification?
Rock Currier December 28, 2013 11:27PM
Perhaps sometime time in the not too distant future Mindat will have the ability to host video clips. I think that this would be the time to start pushing for show and tell clips on hardness, use of acids, solubility etc. There are tons of written references that could be sited, or copied and be included here, but what I would like to see is at least a series of YouTube like videos about basic simple ID techniques. Some are probably out there that we could get permission to host if we had a suitable platform to show them.

Rock Currier
Crystals not pistols.
Reiner Mielke December 29, 2013 01:02AM
The best way to conduct a solubility test is to use a standard, that way temperature and acid strength are not a factor. By this I mean placing a grain of approximately the same size and shape of a known mineral of known solubility next to an unknown and comparing the reactions.

Edited 1 time(s). Last edit at 12/29/2013 02:08PM by Reiner Mielke.
cascaillou December 29, 2013 01:53AM
steve: I understand, however my comment was essentially pragmatical: the problem I was pointing to is that many chemical suppliers won't sell to private individuals and not even to societies which are not directly involved in some kind of chemical processing. So a standard procedure based on concentrations not available to the general public (meaning at the hardware store) wouldn't be practical.

For instance a standard of 90% for H2SO4 would be very impractical (only chemical suppliers have it for sale), on the other hand I guess that any hardware store selling H2SO4 for batteries will be offering it at 30% at least (or most likely higher concentration), so 30% might be a reasonable standard for H2SO4, in terms of supply I mean.

Btw, if the label is announcing 35% concentration but the product is actually 32% only, are those 3% error actually going to make a big difference in a solubility test?

ps: concerning Mohs hardness, beside that it is empirical and approximative, it would be nice to advice people not to work from rough samples of reference material, I've seen a lot of hardness test kits composed of rough pieces of each mineral of the mohs scale. Quartz is 7 and orthoclase is 6, but I still have to see one producing a visible scratch on a rough cleaved feldspar (messy step like cleavage) with a quartz point. I think one should only work with flat piece of POLISHED reference material.

Edited 10 time(s). Last edit at 12/29/2013 02:16AM by el cascaillou.
Steve Hardinger December 29, 2013 03:10AM
Reiner stated my idea in words simpler than I used.
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