DC Arc
Definition and meaning of DC Arc in chemistry.
A DC arc is a continuous, high-temperature electrical discharge that forms between two electrodes connected to a direct-current power supply when the electrodes are separated after contact, ionizing the intervening gas into a conducting plasma.
In more detail
The arc is sustained by thermionic emission of electrons from a hot cathode, which collide with and ionize gas atoms to maintain a conductive plasma channel typically reaching 4,000-7,000 K. Because DC arcs vaporize and excite even refractory materials efficiently, they historically served as excitation sources in atomic emission spectroscopy, particularly for qualitative and semiquantitative analysis of powdered solid samples such as minerals and metals. Compared to AC arcs or spark discharges, the DC arc is more stable but tends to show greater selective volatilization, causing signal drift as easily vaporized elements are depleted from the sample before more refractory ones.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Typical arc temperature | 4,000-7,000 K |
| Power source | Direct current (DC) |
| Common use | Excitation source in arc emission spectroscopy |
In DC arc emission spectroscopy, a powdered ore sample is packed into a graphite electrode cup, and a DC arc struck between it and a counter-electrode vaporizes and excites the sample, producing characteristic emission lines used to identify trace metals.
Frequently asked questions
How does a DC arc differ from an AC arc in spectroscopy?
A DC arc uses a unidirectional current and a fixed cathode/anode pair, giving a steadier, hotter discharge, while an AC arc reverses polarity each half-cycle, which reduces electrode erosion asymmetry but can be less thermally stable.
Why can a DC arc cause inaccurate quantitative results?
Selective volatilization means volatile elements evaporate from the sample early in the arcing period while refractory elements evaporate later, so emission intensity ratios can drift over the course of the burn, complicating quantitative calibration.