amazingchemistry
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Qualitative (and Possibly quantitative) tests for Na?
A friend of mine left me with a head-scratcher yesterday. He said he had to analyze a water sample to tell what sort of metal ions were in there, and
said that without some sort of spectroscopic method (he has one), its REALLY hard to test for sodium ion. Initially I thought of
precipitation, but sodium happily forms water-soluble salts with darn near everything so anything that makes the sodium crash out would likely make
other stuff in the multi-metal ion solution crash out too, and then you'd be left with a mess of precipitate and no idea of what you have. Then I
thought flame test, but realized that if you have a multi-metal ion solution, the test would probably give you all sorts of colors mixed together and
you'd again have no idea whether there was sodium in there or not. Is there an "old-school" (no spectrometer) qualitative test that's selective for
sodium? or do you just have to precipitate everything else, do a flame test and pray? In addition, can we perform some sort of quantitative test to
determine how much sodium is in the solution?
[Edited on 8-5-2013 by amazingchemistry]
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elementcollector1
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1) Flame test. Sodium overrides most, if not all other colors at concentrations of something like 1% or greater. If you see orange, you have some
amount of sodium.
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amazingchemistry
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Why does sodium override all other colors? Is the transition higher in energy or something of the sort?
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elementcollector1
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I think the wavelength of the light emitted is stronger than the others, or something. (Can you tell I'm not a physicist? I can.)
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Magpie
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There is a wet chemistry qualitative test for sodium. It requires a "sodium reagent," which is a saturated solution of zinc acetate and uranyl
acetate in acetic acid. You can probably find the procedure via Google Books by searching for "Qualitative Analysis," or such. If you can't find it
I can type it out for you here.
The single most important condition for a successful synthesis is good mixing - Nicodem
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amazingchemistry
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This looks like it merits some research. Please correct me if my (admittedly sketchy) reasoning is wrong but now that I think about it, the energy of
the electron transition only dictates the color of the light, not it's intensity. Although it could be that a shorter electron transition means that
more electrons can make the jump, resulting in more intense light. Could this be related to ionization energy trends? But then, wouldn't larger atoms
(with smaller ionization energies) produce more transition-capable electrons (and hence more intense light) than smaller atoms? Does it have something
to do with how we as humans see color? I seem to recall that we perceive yellow-green more clearly than other colors. Time to break out the books!
Edit: Thank you Magpie! A simple google search did tell me that uranyl zinc acetate reacts with sodium to produce an insoluble
(gasp!) sodium compound. It did say that some interfering metal ions have to be removed first, but those are small qualms. You managed to answer the
second question too, as the formation of precipitate allows for gravimetric analysis. I had heard that uranium forms a solid peroxide (which if I'm
not mistaken is also very rare) so I guess that makes sense.
[Edited on 8-5-2013 by amazingchemistry]
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woelen
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Are you working for a company or do you do these tests as a private person. Obtaining uranyl salts is not an easy thing in these times if you have no
connection with some research lab.
There are other compounds which give a precipitate with sodium and which allow you to do quantitative analysis on sodium ions. I do not have a
particular chemical in my mind, but I expect Google to be helpful.
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blogfast25
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The relative intensity (relative to other spectral lines) of the sodium D doublet is extremely high, the highest of all spectral lines, IIRW. The
intensity reflects the probability of that particular transition being extremely high and nothing else. Tabled values of the relative intensity of the
spectral lines of most elements are online, worth searching for. The high relative intensity also makes sodium lamps possible.
The intensity of the line makes it fairly unsuitable for flame tests: you tend to find sodium in EVERYTHING you test. I did some experiments by
recystallising (several times) technical KCl a while back, using a simple homemade spectroscope to check for sodium: no matter what I did, I
couldn’t eliminate the sodium line(s doublet) altogether (but I did weaken it substantially). So if you’re going to use spectroscopy to check for
sodium, use at a very minimum an actual spectroscope and compare the line to that of a nominally ‘sodium free’ sample. A flame test will be
deceptive, unless you have some means of quantifying the colour intensity, other than your retina.
[Edited on 8-5-2013 by blogfast25]
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confused
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well, sodium bismuthate is insoluble in cold water
the following reaction should precipitae out sodium bimuthate (i picked Ca(BiO3)2 and NaCl on a whim)
molecular equation= Ca(BiO3)2 + 2 NaCl = CaCl2 + 2 NaBiO3
net ionic equation= 2Na+ + 2BiO3- = 2NaBiO3
pretty sure there should be enough sodium ions for a qualitative test
(This is all speculation on my part)
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Boffis
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There are actually quite a few reagent for sodium but most lack specifity. Zinc and Magnesium uranyl acetates have been referred to above but others
are potassium fluorosilicate, nitrobarbituric acid, potassium hexahydroxyantimonate, alloxan-5-oxime, 6,8 dichloro 2,4 quinazolinedione (Sheibley's
reagent) and a couple of anthraquinone derivatives. The first two and fourth form characteristic shaped crystals, the third and fifth precipitates and
the last a coloured precipitate.
They are fairly well covered in Fiegl and Anger's book on spot tests and Chamot & Mason's Chemical Microscopy vol 2. There is also Vogel's
Textbook of Qualitative Analysis. I have experience of some of these but not Sheibley's reagent or the anthraquinone derivatives.
If you're interesrted I'll dig out more info on these tests but I am current a long way from home so it may take some time. I had thought about
startinga thread specifically to discuss the use and preparation of this type of microchemical reagent; anyone interested?
By the way I would be interested if anyone has information on Sheibley's reagent in particular the original synthesis.
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Eddygp
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Well, as Na forms usually soluble in H2O, white and crystalline compounds with almost anything... no idea. Separating those won't be easy without
double salts or complex procedures, bearing in mind that most reactions that could bring out the sodium will also bring out the potassium.
there may be bugs in gfind
[ˌɛdidʒiˈpiː] IPA pronunciation for my Username |
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Hexavalent
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My book "Quantitative Inorganic Analysis" by Belcher and Nutten states that "Sodium is determined gravimetrically as sodium zinc uranyl acetate
(NaZn(UO2)3(CH3COO)9 . 6H2O."
"It is recommended that 10 volumes of reagent are used for each volume of the test solution, which should not contain more than 8 mg of sodium per
mililitre. In this way, precipitation of the triple acetate is complete within 1 hour."
It specifies that the reagent is prepared by dissolving "100 g of uranyl acetate and 27.8 g of zinc acetate in 27 ml of glacial acetic acid in 900 ml
of distilled water", warming to dissolve the solids. It then says to "add a few miligrammes of sodium chloride", allowing it to stand "for 24 hours"
and then filtering. "The reagent is then saturated with respect to sodium zinc uranyl acetate".
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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amazingchemistry
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| Quote: |
Are you working for a company or do you do these tests as a private person
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Both actually. My friend and I work for a chemical company, he does water analyses quite often and has a very nice spectrometer for that purpose. I
work in quality control, so my interest is mostly theoretical. I figured uranium compounds weren't easy to obtain. As soon as Magpie suggested uranyl
acetate I went to my copy of the Aldrich catalog. I wasn't too surprised when I didn't find it (or any other uranium compounds).
| Quote: |
If you're interesrted I'll dig out more info on these tests but I am current a long way from home so it may take some time. I had thought about
startinga thread specifically to discuss the use and preparation of this type of microchemical reagent; anyone interested?
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I'm definitely interested. Chemical problems of the "find the unknown" type have always tickled my curiosity. Besides, I think it'd be useful for
amateur chemists, who generally don't have access to spectroscopic equipment. I second the idea of a dedicated thread. Are there any books
specifically on this type of thing in the forum library? I know we have Vogels, but that has a bunch of other things and is mainly focused on organic
chem.
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The intensity reflects the probability of that particular transition being extremely high and nothing else.
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Ah, but why is it 'extremely high'? Extremely high relative to what? And why are the probabilities of transitions for the other elements lower? Do
decent pchem textbooks talk about this? My reading extends to general chem and ochem (I have been somewhat afraid to crack open a pchem text because
I'm not that confident in my math skills)
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Magpie
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for uranyl salts:
http://stores.intuitwebsites.com/beaglewebstore/Categories.b...
The single most important condition for a successful synthesis is good mixing - Nicodem
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blogfast25
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Quote: Originally posted by amazingchemistry  |
| Quote: |
The intensity reflects the probability of that particular transition being extremely high and nothing else.
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Ah, but why is it 'extremely high'? Extremely high relative to what? And why are the probabilities of transitions for the other elements lower? Do
decent pchem textbooks talk about this? My reading extends to general chem and ochem (I have been somewhat afraid to crack open a pchem text because
I'm not that confident in my math skills) |
The explanation lies squarely in quantum mechanics. My uni text book on that subject matter, in particular the treatise of the time-dependent
Schrodinger equation for atoms, explains which transitions are allowed, which are forbidden and how the probability of the allowed ones can be
established. Not light reading  . The resulting line intensities are relative to
the weaker ones, I'll see if I can dig up the tabled values of the sodium spectrum to illustrate the principle a little better [see below].
As regards, "(NaZn(UO2)3(CH3COO)9 . 6H2O", there seems to be a bracket missing. At first glance I think it should be
(NaZn(UO2)3)(CH3COO)9 . 6H2O or simpler: NaZn(UO2)3(CH3COO)9 . 6H2O. With a MM of over 1500, a gram of Na would yield about 67 g of precipitate!
And going back to the spectra, here’s a really nice page about the spectra of some important elements:
http://astro.u-strasbg.fr/~koppen/discharge/index.html
If you scroll down to below the beautiful pictures, you’ll find *.txt files that give the relative intensities for the individual lines. For the
sodium D doublet, the line strengths are 80,000 and 40,000 (eighty and fourty thousand)!
By contrast, for instance magnesium's strongest line is about 400. We all know that flame tests for magnesium are almost impossible.
[Edited on 9-5-2013 by blogfast25]
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