Flame Test Colors — Metal Ion Identification
| Element | Ion | Flame Color | Dominant Wavelength (nm) | Intensity | Common Test Compound |
|---|---|---|---|---|---|
| Lithium | Li⁺ | Crimson red | 671 | Strong | LiCl |
| Sodium | Na⁺ | Intense yellow | 589 | Very strong | NaCl |
| Potassium | K⁺ | Lilac / violet | 767 | Moderate | KCl |
| Rubidium | Rb⁺ | Red-violet | 780 | Moderate | RbCl |
| Cesium | Cs⁺ | Blue-violet | 456 | Moderate | CsCl |
| Calcium | Ca²⁺ | Orange-red | 622 | Strong | CaCl₂ |
| Strontium | Sr²⁺ | Bright red | 641 | Strong | SrCl₂ |
| Barium | Ba²⁺ | Yellow-green | 554 | Strong | BaCl₂ |
| Copper | Cu²⁺ | Blue-green | 510 | Strong | CuCl₂ |
| Copper (with halide) | Cu²⁺ | Bright green | 520 | Very strong | CuCl₂ (excess HCl) |
| Boron | B³⁺ | Bright green | 518 | Strong | H₃BO₃ in methanol |
| Lead | Pb²⁺ | Pale blue-white | — | Weak | Pb(NO₃)₂ |
| Iron | Fe³⁺ | Gold / orange-brown | — | Weak | FeCl₃ |
| Zinc | Zn²⁺ | Blue-white / pale green | — | Weak | ZnCl₂ |
| Manganese | Mn²⁺ | Yellow-green | — | Weak | MnCl₂ |
| Indium | In³⁺ | Indigo blue | 451 | Strong | InCl₃ |
| Thallium | Tl⁺ | Bright green | 535 | Strong | TlCl |
| Arsenic | As³⁺ | Pale blue | — | Weak | As₂O₃ |
Dominant emission wavelengths are taken from NIST Atomic Spectra Database principal lines (Na D-line 589 nm, K 766.5/769.9 nm, Li 670.8 nm, etc.); intensity ratings reflect typical Bunsen-flame visibility, not absolute photon counts. Sodium contamination from skin oils or glassware will mask the K, Rb, and Cs lines — viewing through cobalt-blue glass filters out the yellow D-line. Chloride salts are used because HCl volatilizes the metal cleanly. Several entries (Pb, Fe, Zn, Mn, As) give weak or non-distinctive flames and should be confirmed by AAS, ICP-OES, or wet chemistry. Source: Vogel's Qualitative Inorganic Analysis, NIST ASD.
Frequently Asked Questions
Why does sodium produce such a strong yellow flame?
The 589 nm sodium D-line corresponds to the 3p → 3s transition, which has an unusually high oscillator strength — roughly 0.65, near the theoretical maximum. Combined with sodium's low ionization energy (496 kJ/mol), this makes excitation efficient at Bunsen-flame temperatures (~1500 K). Trace contamination from sweat or glassware (Na is everywhere) gives a visible yellow flame from microgram quantities, which is why sodium overwhelms K, Rb, and Cs unless filtered through cobalt-blue glass.
How do you perform a flame test?
Clean a nichrome or platinum loop in concentrated HCl and burn it in the flame until no color persists. Dip the clean loop into the sample (solid or dissolved in HCl) and hold it in the hottest part of the flame, just above the inner blue cone. Observe immediately — some colors fade in seconds as the salt vaporizes. For K, Rb, or Cs, view through cobalt-blue glass to suppress sodium D-line interference. Repeat with a fresh loop if the color is ambiguous.
Why can't flame tests identify all metal ions?
Bunsen flames (~1500 K) cannot excite the high-energy d-d transitions of most transition metals, and many of those transitions emit in the UV or IR rather than the visible. Iron and zinc give pale, washed-out flames that are easy to confuse with sodium contamination. For Fe, Zn, Mn, Ni, Co, and most heavy metals, run AAS, ICP-OES, or qualitative wet tests (e.g., thiocyanate for Fe³⁺, dithizone for Zn²⁺) instead of relying on flame color.