Using Electrographic Techniques in Microchemical Metal Tests

Figure 1: Electrographic apparatus showing copper coin on an Al cathode before analysis James J. Benko 1

Sometimes it is better to use an electric current to get metals into solution rather than acid(s), especially when it is necessary to preserve the integrity of the sample surface.

Useful for such items as coins or metallic art objects, electrographic techniques are, for all intents and purposes, virtually non-destructive since only traces of metal are removed. Sufficient metal dissolves for microchemical tests when a 10-50 milliamp current, supplied by one or two flashlight batteries, passes through a specimen for a few seconds. The specimen rests on a small piece of filter paper moistened with an aqueous electrolyte solution of sodium chloride or sodium nitrate.

A clever apparatus for doing this is described by Nordmann (1) and also by Weisz (2), both authors attributing it to Fritz (3). I have constructed this apparatus from ordinary materials readily available today. It is simple to make and simple to use.

MATERIALS

  1. Aluminum cathode made from a disposable weighing dish or soda can bottom (use sand paper to remove any can liner/coating material)
  2. Plastic battery holder for two "D" cells (available from Radio Shack)
  3. Screw driver with plastic handle
  4. Small lengths of copper wire
  5. Small alligator clip (available from Radio Shack)
  6. Ashless filter paper, 5.5 cm diameter.
  7. Electrolyte solution, 2% NaCI in deionized water
  8. Two "D" cell batteries

DIRECTIONS

  1. Cut off the tip of the screwdriver with a hacksaw leaving a flat-edged rod. File this surface smooth. The screwdriver could be used "as is" but the flat edge tends to minimize scratching and provides somewhat better contact with the sample.
  2. Solder one end of a copper wire to the screwdriver. This is the anode. The other end of the wire is connected to the (+) terminal of the battery holder, the anode.
  3. Solder an alligator clip to another piece of copper wire. This clip is attached to the aluminum cathode. The other end of the wire is attached to the (-) terminal of the battery holder.
  4. Put batteries in the battery holder when ready to use.

The finished apparatus is shown in Figure 1. Obviously, numerous variations can be made such as adding an ammeter, using different sized batteries/holders (Nordmann' uses a 1.5-volt dry cell), using different metals for anodes/cathodes, using different electrolytes, etc.

Figure 2: After 4 seconds contact with the electrodes, copper transferred to the filter paper tested positive with 0.1% cuproine in ethanolMETHOD

  1. Place a circle of filter paper moistened with electrolyte on the aluminum cathode.
  2. Place the sample to be tested which can be a piece of metal, coin, conductive mineral, or conductive powder on top of the paper.
  3. Connect the alligator clip to the cathode.
  4. To complete the connection firmly press the screwdriver post onto the sample for 3-4 seconds.

This should allow enough metal to be transferred to the filter paper for analysis (Figure 2). Microchemical tests for suspected metals can then be made using reagents commonly employed such as dithizone, dimethylglyoxime, thioacetamide, etc. The filter paper discs can also be used in ring-oven tests for further separation and identification as described by Weisz (2). Changing the physical size of the anode/cathode may be necessary to test large objects such as lamps, statues, etc., but the basic principle remains the same.

REFERENCES

  1. Nordmann, J. Qualitative Testing and Inorganic Chemistry, J. Wiley, N.Y., 1957, pp. 446-7.
  2. Weisz, H., Microanalysis by the Ring Oven Technique, Pergamon Press, 1961, pp. 17-119.
  3. Fritz, H., Z. Anal. Chem, 78, 418 (1929).

FOOTNOTES
1 Microspec Analytical, 3352 128th Ave., Holland, MI, 49442