B. T. Sturman and K. L. Kosanke
Abstract: A simple lilac flame composition consisting of 80% potassium nitrate and 20% shellac was investigated by emission spectroscopy and thermodynamic modeling. The flame from the burning composition had a reddish-pink core with a brighter pale lilac outer envelope. The core of the flame is presumably close to the equilibrium conditions predicted by thermodynamic modeling. The calculated equilibrium flame temperature is 1526 K; seven gases (CO, H2O, N2, CO2, H2, KOH, K) and one liquid (K2CO3) account for 99.7% of the molecules in the core of the flame. Of these, liquid potassium carbonate (mole fraction 9.6%) is expected to emit a continuous spectrum while atomic potassium (mole fraction 2.5%) imparts a red colour from the resonance doublet (766.491 and 769.897 nm), which is considerably broadened by self-absorption. The outer fl ame envelope is presumably a diffusion fl ame in which flammable gases from the core burn in entrained air. The maximum adiabatic temperature of such a flame was calculated as 1723 K; seven gases (N2, CO2, H2O, KOH, K, Ar, K2O2H2) account for 99.8% of the molecules in the outer flame envelope. The emission spectrum of atomic potassium superimposed on a continuous spectrum arising from the combining of atomic potassium with OH radicals to form gaseous KOH is responsible for the lilac colour of laboratory flames containing potassium and is the likely cause of the lilac colour of the outer regions of this pyrotechnic flame. The article includes a brief tutorial outline of some relevant aspects of the atomic spectroscopy of potassium.
Keywords: lilac flame colour, potassium nitrate, shellac, thermodynamic modeling, visible spectrum
Ref: JPyro, Issue 22, 2005,pp28-43
(J22_28)
© Journal of Pyrotechnics and CarnDu Ltd
