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Flame Salts

In 1860 Kirchhoff and Bunsen drew the attention of chemists to the practical usefulness of analyzing the color of the flame of burning elements. Many of these colors had been observed before but their systematic analysis, coupled with the development by Bunsen of the clean gas burner (i.e. nearly invisible flame), actually enabled them to identify cesium, a previously unknown alakali metal. At a time when scientists were actively trying to complete the list of elements, that got them a lot of attention.

They were using spectral lines to identify the elements in Durkheim mineral water. Spectral lines for sodium, potassium, lithium, calcium, and strontium were observed. When these metals were chemically removed, the absence of their lines from the spectrum was observed. As the lines began to disappear, two blue lines were unexpectedly observed. The discovery of new spectral lines suggested a new element was to be discovered. Its name was taken from the Latin word coesius meaning "sky blue" [2].

Flame salts are simple compounds particularly easy to identify by flame color analysis. They include alkali metal salts and alkali earth salts. The alkali metals form the first column of the periodic table: Hydrogen (H), lithium (Li), sodium (Na), potassium (K). Rubidium (Rb) was discovered in 1861 and cesium (Cs) the year before, both using the flame method. Alkali earths form the second column of the table: Be, Mg, Ca, Sr, Ba, ... The salts formed by any one of these elements and chloride (Cl) appear as tasty white crystalline powders, one of them we know very well: NaCl or table salt. KCl is also marketed as sodium-free salt, as indeed it is, but it is also far from being as tastefully tasty as NaCl.

It is interesting to repeat the Kirchhoff and Bunsen tests, to see the lines they saw and correlate them to what we now know about the structure of the atoms of the elements they identified. It is a wonderful opportunity to mix some chemistry and some physics and peer with our own eyes into the world of the quantum.

Equipment

Salts: Used flame salts, home schooling kit, from eBay; about 10$.

Burner: Propane "benzomatic" welding torch, any hardware store; 20-40$.

Spectroscope: Hand held direct view double prism spectroscope, designed for gem identification. New from eBay, for about $80.

Lithium -- Deep Red

Li from LiCl.

Lithium is a hydrogen like atom. It has a single electron in a 2s state outside a Helium core (He)2s. The 2s-2p transition doublet is a beautiful red. A much fainter yellow line can also be seen, on the red side of the sodium line. This matches the description from Kirchhoff and Bunsen ([1], 1860). The association with modern atomic states can safely be made by selecting the simplest transitions in the NIST line spectra tables.


                   2s->2p        Red    -- bright
                       2p->1s    1s is full!
                       2p->2d    there is no 2d!
                       2p->3s    Yellow -- dim
                       2p->3d    IR     -- invisible

       -----------------------------------------------------------------------------+
        Elem. | Wavelength | Rel. |   Ei           Ek   | Electron      | Ji  -  Jk |
              | Air (nm)   | Int. | (cm-1)       (cm-1) | Config.       |           |
       -----------------------------------------------------------------------------|
  Yellow   Na |   588.995  |      |     0.0  - 16973.36 | (Ne)3s-(Ne)3p | 1/2 - 3/2 |
              |            |      |                     |               |           |
 Yellow    Li |   610.353  |  320 | 14903.66 - 31283.08 | (He)2p-(He)3d | 1/2 - 3/2 |
 Yellow    Li |   610.364  |      | 14904.00 - 31283.12 | (He)2p-(He)3d | 3/2 - 5/2 |
 Yellow    Li |   610.366  |  320 | 14904.00 - 31283.08 | (He)2p-(He)3d | 3/2 - 3/2 |
              |            |      |                     |               |           |
    Red    Li |   670.776  | 3600 |     0.0  - 14904.00 | (He)2s-(He)2p | 1/2 - 3/2 |
    Red    Li |   670.791  | 3600 |     0.0  - 14903.66 | (He)2s-(He)2p | 1/2 - 1/2 |
              |            |      |                     |               |           |
     IR    Li |   812.622  |   48 | 14903.66 - 27206.12 | (He)2p-(He)3s | 1/2 - 1/2 |
     IR    Li |   812.645  |   48 | 14904.00 - 27206.12 | (He)2p-(He)3s | 3/2 - 1/2 |
              +---------------------------------------------------------------------+

Detailed Li spectrum, from NIST.

Sodium -- Bright yellow

Na from NaCl

Sodium is the next hydrogen like atom. It has a single electron in a 3s state outside a Neon core (Ne)3s. The 3s-3p transition doublet is a bright yellow and is the famous signature of the omnipresent sodium. The next transition, 3p-3d is in the infrared.

                   3s->3p        Yellow  -- bright
                       3p->3d    IR      -- invisible
                       3p->4s    ?
                       3p->4d    Green   -- too dim
                       3p->5s    Yellow  -- too dim

      --------------------------------------------------------------------------------+
       Elem. | Wavelength | Rel.  |     Ei          Ek    | Electron      | Ji  -  Jk |
             | Air (nm)   | Int.  |   (cm-1)      (cm-1)  | Config.       |           |
      --------------------------------------------------------------------------------|
   Green  Na |  568.2633  |   280 | 16956.172 - 34548.766 | (Ne)3p-(Ne)4d | 1/2 - 3/2 |
   Green  Na |  568.8193  |    70 | 16973.368 - 34548.766 | (Ne)3p-(Ne)4d | 3/2 - 3/2 |
   Green  Na |  568.8205  |   560 | 16973.368 - 34548.731 | (Ne)3p-(Ne)4d | 3/2 - 5/2 |
             |            |       |                       |               |           |
  Yellow  Na |  588.9950  | 80000 |     0.0   - 16973.368 | (Ne)3s-(Ne)3p | 1/2 - 3/2 |
  Yellow  Na |  589.5924  | 40000 |     0.0   - 16956.172 | (Ne)3s-(Ne)3p | 1/2 - 1/2 |
             |            |       |                       |               |           |
  Yellow  Na |  615.4225  |   120 | 16956.172 - 33200.675 | (Ne)3p-(Ne)5s | 1/2 - 1/2 |
  Yellow  Na |  616.0747  |   240 | 16973.368 - 33200.675 | (Ne)3p-(Ne)5s | 3/2 - 1/2 |
             |            |       |                       |               |           |
      IR  Na |  818.3255  |  4400 | 16956.172 - 29172.889 | (Ne)3p-(Ne)3d | 1/2 - 3/2 |
      IR  Na |  819.4790  |   800 | 16973.368 - 29172.889 | (Ne)3p-(Ne)3d | 3/2 - 3/2 |
      IR  Na |  819.4824  |  8800 | 16973.368 - 29172.839 | (Ne)3p-(Ne)3d | 3/2 - 5/2 |
             +------------------------------------------------------------------------+

Detailed Na spectrum, from NIST.

Potassium -- Colorless

K from KCl

Potassium is another hydrogen like atom, where one electron orbits an Argon core: (Ar)4s. The 4s-4p doublet is visible bordering the infra-red region. This matches the dark (absorption) Fraunhoffer A line in the solar spectrum. There should be a violet line in the deep violet region, but I was unable to see it. (The "Lavender" color reported in some places is probably due to a weak sodium contamination.)

                   4s->4p        Far Red
                       4p->3d    ?
                       4p->4d    ?
                       4p->5s    ?
                   4s->5p        Far Violet
                   4s->6p        UV
       ---------------------------------------------------------------------------
        Elem. | Wavelength | Rel. |   Ei        Ek   | Electron      | Ji  -  Jk |
              | Air (nm)   | Int. | (cm-1)    (cm-1) | Config.       |           |
       --------------------------------------------------------------------------|
      UV  K   |  344.637   |   11 | 0.000 - 29007.71 | (Ar)4s-(Ar)6p | 1/2 - 3/2 |
      UV  K   |  344.737   |   10 | 0.000 - 28999.27 | (Ar)4s-(Ar)6p | 1/2 - 1/2 |
              |            |      |                  |               |           |
  Violet  K   |  404.414   |   18 | 0.000 - 24720.13 | (Ar)4s-(Ar)5p | 1/2 - 3/2 |
  Violet  K   |  404.721   |   17 | 0.000 - 24701.38 | (Ar)4s-(Ar)5p | 1/2 - 1/2 |
              |            |      |                  |               |           |
  Yellow  Na  |  588.9950  |      |                  |               |           |
              |            |      |                  |               |           |
     IR   K   |  766.491   |   25 | 0.000 - 13042.87 | (Ar)4s-(Ar)4p | 1/2 - 3/2 |
     IR   K   |  769.897   |   24 | 0.000 - 12985.17 | (Ar)4s-(Ar)4p | 1/2 - 1/2 |
       ---------------------------------------------------------------------------

Detailed K spectrum, from NIST.

Calcium -- Color?

Ca from CaCl2. The flame appears yellow but most of the light is from sodium contamination.

The spectrum shows 4 lines but only 2 are original, one red and 1 green, on either side of the yellow sodium doublet. A second red line coincides with the red Li line, and is most probably just that.

The calcium ground state is (Ne)4s2. With two electrons now equally easily excited the spectrum becomes significantly richer and thus more complicated. The red line can only be 4s2->4s4p. Identifying the green line is harder, I'm guessing 4s.4p-4s.5d. A better spectroscope would allow an estimate of the wavelength.

                   4s2->4s4p        Red
                        4s4p->4s3d  ?
                        4s4p->4s4d  ?
                        4s4p->4s5s  Orange
                        4s4p->4p2   Green
                        4s4p->4s5d  Green

        ------+------------+------+---------------------+---------------+-----------+
        Elem. | Wavelength | Rel. |    Ei         Ek    | Electron      | Ji  -  Jk |
              | Air (nm)   | Int. |  (cm-1)     (cm-1)  | Config.       |           |
        ------+------------+------+---------------------+---------------+-----------+
        Ca I  |  518.8844  |   25 | 23652.30 - 42919.05 | 4s.4p-4s.5d   |    1 - 2  |
  Green Ca I  |  551.29    |   23 | 23652.30 - 41786.27 | 4s.4p-4p2     |    1 - 0  |
        Ca I  |  558.19    |   25 | 20349.26 - 38259.12 | 3d.4s-3d.4p   |    2 - 3  |
        Ca I  |  558.87    |   27 | 20371.00 - 38259.12 | 3d.4s-3d.4p   |    3 - 3  |
        Ca I  |  585.7451  |   30 | 23652.30 - 40719.84 | 4s.4p-4p2     |    1 - 2  |
              |            |      |                     |               |           |
 Yellow Na I  |  588.9950  |      |                     |               |           |
              |            |      |                     |               |           |
 Orange Ca I  |  610.2723  |   27 | 15157.90 - 31539.49 | 4s.4p-4s.5s   |    0 - 1  |
 Orange Ca I  |  612.2217  |   29 | 15210.06 - 31539.49 | 4s.4p-4s.5s   |    1 - 1  |
  Red   Ca I  |  657.2779  |   23 |     0.0  - 15210.06 | 4s2-4s.4p     |    0 - 1  |
              |            |      |                     |               |           |
  Red   Li I  |  670.776   |      |                     |               |           |
        ------+------------+------+---------------------+---------------+-----------+

Detailed Ca spectrum, from NIST.

Strontium -- Orange

Sr from SrCl2

The flame color of strontium appears a very nice orange. Precisely as Kirchhoff and Bunsen report ([1], 1860), eight original lines are clearly visible: one blue, one orange and six red. The red lines fall on either side of the red lithium doublet. Dipping the wire in water before flaming helps brighten the lines at least for a moment.

The strontium ground state is (Kr)5s2. In this case the NIST data does not identify the electronic configuration of the prominent lines, so the identification of the orange line is just a guess.

                   5s2->5s5p       Orange

              ------+------------+-------+
              Elem. | Wavelength |  Rel. |
                    | Air (nm)   |  Int. |
              ------+------------+-------+
         Blue Sr I  |    460.733 | 65000 |
                    |            |       |
       Yellow Na I  |    588.995 |       |
                    |            |       |
       Orange Sr I  |    640.847 |  9000 | 
                    |            |       |
              Sr I  |    650.400 |  5500 |
              Sr I  |    655.026 |  1700 | 
              Sr I  |    661.726 |  3000 |
                    |            |       |
        Red   Li I  |    670.776 |       |
                    |            |       |
              Sr I  |    679.105 |  1800 |
              Sr I  |    687.838 |  4800 |
              Sr I  |    707.010 |  5500 |
              ------+------------+-------+
              

Detailed Sr spectrum, from NIST.

References

[1] Chemical Analysis by Observation of Spectra, Gustav Kirchhoff and Robert Bunsen, Heidelberg, 1860.
[2] Cesium, BCIT Chemistry Resource Center.