WHAT IS THAT METAL?
Q. We have to strip deposits from steel-based substrates and are having trouble determining the metal that we are supposed to be stripping away. Our company does not want to invest in any “fancy” laboratory instruments, so a simple procedure for metal identification would be appreciated.
Thanks for your help. P.Q.S.
A. I am asked this question every so often. I believe the last time I answered it was more than five years ago, so a review of the procedure is in order.
The following sequence, taken from SOLUTION 4.1 Software by Lawrence J. Durney, Kushner Electroplating School, 408-749-8652, is a straightforward approach for determining the make-up of plated layer.
B. Reaction delayed or does not occur—gold
[Note: A chromate coating can sometimes mimic brass. However, with the addition of the nitric acid, the coating will usually bleach out. The drop will then usually turn yellow or green, and the surface will become white.]
2. Plating is not attacked—add concentrated hydrochloric acid.
- a. Green solution—chromium
b. No action—rhodium, platinum
2. Take up on filter paper, make ammoniacal, and add dimethyl glyoxime. [This solution should be reasonably fresh (less than 6 to 8 weeks old) andbe stored in a dark bottle to retard deterioration.]
- a. Red coloration formed—nickel
b. No red coloration—chromium
2. Reaction not positive—place nitric acid on a new spot, take up on filter paper, make ammoniacal, add ammonium sulfide.
- a. Spot becomes colorless or white—zinc
b. Spot becomes bright yellow—cadmium.
c. Brown-black spot—nitric acid on new spot, take up on filter paper, add NaOH.
- i. Brown-black spot—silver
ii. Colorless or white—lead