2005-02-15
Flame Test Loop
A pretty trivial exercise really, you need a piece of wire and a glass tube for the handle. Ideally the wire should the Platinum, but Nichrome is also suitable. I used Platium/Iridium alloy wire and a Pasteur pipette to supply the glass tube.
A small loop, about 3 mm ID it turned into the end of the wire. A pair of pliers are used to hold the loop end and the wire and narrowed end of the glass tube are heated together in a hot torch flame until red heat. The glass will flow around and bond to Platinum or Nichrome quite well, the trick is keeping everything centred and symmetric as the glass cools in the edge of the flame.
It is wise to anneal the glass in the cooler parts of the flame for a few moments, followed by slow cooling, in glass wool is ideal. The expansion coefficients of the metal don't match soda glass very well, so the annealing will make a much more robust join in the long run, especially as it undergoes significant thermal cycling in use.
To use the loop dip it in moderately strong HCl solution and heat strongly in the hottest part of the oxidising flame (just above the pale inner code). If it colours the flame at all repeat until it no longer colours the flame. Once clean touch the HCl dampened end to the sample and heat in the oxidising part of the torch flame again. Repeat with the reducing flame (inside the inner cone), often there is a difference which will help narrow down the possibilities.
- Red
- Lithium is a distinctive purplish-red. Strontium is a deep scarlet. Calcium varies between brick red to a kind of orange.
- Yellow
- Sodium drowns out just about everything. Ammonia can be similar, but is somewhat more green, add a few percent NaCl to sample to disambiguate.
- Green
- Boron is a wonderful mint green. Zinc a whitish lime green. Copper is a crisp emerald green with tinges of blue.
- Blue
- Copper especially tinges at the edges of a green flame. Heavy metals often give a light sky blue.
- Purple
- Potassium, easily masked by just about everything.
Some cobalt-blue glass is useful as a filter, it cuts down on the largely red-orange black body light from the loop and also attenuates Sodium D-lines from minor contamination which can swamp delicate colours like that of Potassium. Blue cellophane works in a pinch, but the filter is unnecessary if all you want is to qualitatively say there is no significant sodium in something.
A little PVC can be used to provide Chlorine to the flame if you have no HCl handy. Just touch the hot loop to a piece of PVC and enough will stick to donate plenty of Cl. Other chlorinated polymers will probably work too. Burning the residue off the loop is annoying however.
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