Sciencemadness Discussion Board - Efflorescence: testing the salt on the walls

Efflorescence: testing the salt on the walls

CHRIS25 - 20-11-2014 at 13:09

An aquaintance of mine showed me around his restored 19th century tower today and pointed out the problem with white stuff on the walls all over the place, it is efflorescence. I said I would look into it and took a sample back home for testing to see if I could identify which of many salts it could be.

I saw it as a brand new challenge for me to take an unknown chemical and go to work on something I have no experience with. I would like to run my methods and results by you in case I have missed something or done something wrong, or can be corrected on perhaps testing things in a different order:

Sample of white stuff placed in dilute HCl - bubbled vigorously
confirms the anion to be carbonate.
Sample placed in NaOH - Dissolved, but went milky white, possibly Calcium or Sodium
Potassium Ferricyanide - no change in colour, no Iron present
Placed in Sodium carbonate solution - white precipitate, but not on surface of solution, spread around inside it, not clumped together. Confirms this to be the Sodium cation and not Calcium.

Problem with above is that the tower is perched on a cliff, and after some research I read that Alkali chlorides and sulphates are most likely in the efflorescence due to the salt sea and air? Furthermore I could not do a flame test, each time I tried, the substance melted into an extremely tough solid within seconds.

At the moment I can not think of other tests for the cation.

aga - 20-11-2014 at 13:22

See if you can sprinkle some of the powder into the flame rather than a spoonful.

CHRIS25 - 20-11-2014 at 13:35

Quote: Originally posted by aga

See if you can sprinkle some of the powder into the flame rather than a spoonful.

Yep, that did'nt work, burnt my finger and the flame went out, tried a few times. Maybe I should taste it!

Ok, I sprinkled some into the wax and lit a second candle for comparison. The flame with the substance is a persistent bright orange around the central flame of the wick. This confirms Na and not Ca.

[Edited on 20-11-2014 by CHRIS25]

aga - 20-11-2014 at 13:44

Lol !
Sprinkle Less, and use a spatula maybe, or asbestos fingers.

Given it's on the coast, with plenty of NaCl knocking about, would Na not be the most likely candidate for cation ?

Does your stuff dissolve in water ?
If so, is it acidic or basic, strong or weak ?

Never ever taste any unknown chemical yourself.

The accepted best practice is to recruit 10 street urchins and get them to taste it in case it is poisonous.

10 is a convenient number for the LD50 calculations.

CHRIS25 - 20-11-2014 at 13:47

Sodium carbonate seems a good contender here judging by the tests. Just the above though, what I wrote in my first post: Problem with above is that the tower is perched on a cliff, and after some research I read that Alkali chlorides and sulphates are most likely in the efflorescence due to the salt sea and air? I'm against cruelty to urchins. (see edited previous post though).

aga - 20-11-2014 at 13:50

Don't All carbonates give orange flame test spectra ?

CHRIS25 - 20-11-2014 at 14:03

Quote: Originally posted by aga

Don't All carbonates give orange flame test spectra ?

Yes, but being that this was scraped off the wall and its consistency is more like snow-flakes, it is not the same as placing granules or crystals of the stuff into a flame. since I know this is a carbonate, it only remained for me to identify the cation. And calcium is a distinct orange red, this was a yellow orange, but clearly orange.

unionised - 20-11-2014 at 14:06

This
"Placed in Sodium carbonate solution - white precipitate, but not on surface of solution, spread around inside it, not clumped together. Confirms this to be the Sodium cation and not Calcium." does not make sense.
What would the precipitate be?

The last time I analysed a sample of efflorescence like this it was magnesium sulphate.
That would give a ppt with Na2CO3.
However it wouldn't bubble with dilute acid.
of course, yours could be something else (and is probably impure anyway)

aga - 20-11-2014 at 14:10

Someone correct me if i got this wrong, but would sodium carbonate AND calcium carbonate both give an orange flame test result ?

phlogiston - 20-11-2014 at 14:40

A 19th century tower in Ireland sounds like the perfect place for a mad scientist lab.

Was this found at ground level by any chance? then it may well be saltpeter (potassium nitrate), which is known to accumulate on the walls of old buildings when nitrate-rich ground water permeates the wall and evaporates on the inside.

Calcium chloride has been used as an additive to mortar to accelerate its setting, especially in cold weather.

Sodium emission is very dominant. Even very small amounts of it will yield a yellow flame, masking the color of many non-sodium salts that could be present (and may make up the larger part of your substance).

CHRIS25 - 20-11-2014 at 15:37

Ah well, not enough chemistry studying or experience really then to work this one out. By the way it is on a cliff very exposed, and it a renovated living space with heating. I was pretty certain it was carbonate though. But solubility is difficult to determine in water since it weighs like a feather and just putting in a sample in water and seeing it dissolve does not mean that it is soluble, since 1g per 100g water would be considered an insoluble substance.

IrC - 20-11-2014 at 15:39

"efflorescence due to the salt sea and air"

Gets my vote unless for many years bats have been crapping all over the walls, which would be the previous nitrate suggestion on steroids as opposed to the ground water theory. If the walls have a hollow space going up for all you know many decades of guano could be filling them in from just below the roof area. Drawn out towards the drier inner surface of the walls through repeatedly moist mortar with a combination of chemical reactions taking place over time. A combination of sea salt laden air, mortar, and bat induced nitrates making a mix of chemical reactions possible.

I also wonder if any good ghost stories go along with the tower.

jock88 - 20-11-2014 at 16:25

I seen something similar in an old cattle house. Fluffy stuff coming out of a cement wall. I was sure it was KNO3 but when mixed with some charcoal it did not burn.

Perhaps it and old Napoleonic tower. The British built them on the coast of Ireland to keep an eye on Napoleon. (At least he wasn't firing rockets at the moon).

And yes a round tower is the IDEAL mad science lab. Just don't try to piss in the corner as it can be very confusing.........

[Edited on 21-11-2014 by jock88]

blogfast25 - 21-11-2014 at 06:09

For a flame test you'll need to dissolve the stuff in the smallest amount of HCl possible. Add drops of the solution to the flame, from a Pt wire, preferably. You need a hot, clean Bunsen flame and darkness (preferably). If there is sodium you'll see it very clearly but it will outshine everything else and you'd need a decent spectroscope to see any other lines.

Most likely the sample will contain Na, Ca, Mg and traces of others.

Check for sulphates with barium nitrate or chloride.

[Edited on 21-11-2014 by blogfast25]

Artemus Gordon - 21-11-2014 at 15:31

Quote: Originally posted by blogfast25

For a flame test you'll need to dissolve the stuff in the smallest amount of HCl possible. Add drops of the solution to the flame, from a Pt wire, preferably. You need a hot, clean Bunsen flame and darkness (preferably). If there is sodium you'll see it very clearly but it will outshine everything else and you'd need a decent spectroscope to see any other lines.

Most likely the sample will contain Na, Ca, Mg and traces of others.

Check for sulphates with barium nitrate or chloride.

[Edited on 21-11-2014 by blogfast25]

The method I've read in a number of books is to have a small loop (a few millimeters in diameter) of Pt or nichrome wire, and to dip the loop in clean HCl to both clean the loop and to wet it. Then touch the loop to your (dry) sample to pick up a few particles, and hold that in the flame. A piece of cobalt blue glass can be used to filter out the almost unavoidable yellow color from sodium.
I have done this, and it works pretty well, although of course it is sometimes hard to tell what the specific color is. I'd say it works best if you can narrow down your cation list of possibilities to only 2 or 3 choices.

CHRIS25 - 21-11-2014 at 16:07

Thankyou for the tips on how to do this properly.

unionised - 22-11-2014 at 03:55

Quote: Originally posted by CHRIS25

Placed in Sodium carbonate solution - white precipitate, but not on surface of solution, spread around inside it, not clumped together. Confirms this to be the Sodium cation and not Calcium.

I'm still waiting for someone to explain why sodium carbonate would give a precipitate with sodium carbonate.

blogfast25 - 22-11-2014 at 04:29

Quote: Originally posted by Artemus Gordon

A piece of cobalt blue glass can be used to filter out the almost unavoidable yellow color from sodium.

In principle that works but the intensity of any other element has to be very high to actually see it that way. As a 'filter,' a simple CD-R based hand held spectrometer works better but even with that tool it isn't easy, unless your sample is highly concentrated in whatever specific element. On top of that, atom excitation by Bunsen really only works on a very limited number of elements.

CHRIS25 - 22-11-2014 at 06:31

Quote: Originally posted by unionised

Quote: Originally posted by CHRIS25

Placed in Sodium carbonate solution - white precipitate, but not on surface of solution, spread around inside it, not clumped together. Confirms this to be the Sodium cation and not Calcium.

I'm still waiting for someone to explain why sodium carbonate would give a precipitate with sodium carbonate.

Some reactions work differently in Ireland