Iron, Cobalt and Nickel

I bought a couple of 10mm cubes, one of Nickel, one of Cobalt, because I read that cobalt and nickel are ferromagnetic, strongly attracted to magnets. I could not distinguish the two cubes based on density. I’m documenting my effort to distinguish them based on magnetic attraction.

My son, Everett, helped me with experiment design and execution.

Both cubes are attracted to magnets, but they do look quite similar. I was not able to distinguish the materials based on density, I measured both cubes at 8.76 g/cm³, a little less than the nominal densities of both nickel (8.907 g/cm³) and cobalt (8.834 g/cm³).

I paid $12.97 for the cobalt cube, and $29.99 for a set of 5 cubes that included the nickel cube, so no more than about $6 for the nickel cube. I can’t tell if the cubes are different materials based on density. Did I get burned on the cobalt purchase? Did some unethical element vendor sell me nickel instead of cobalt?

I also bought an iron cube ($9.97), so that I’d have “something else” that’s ferromagnetic to experiment with.

Surface Finish

Co and Ni cubes are almost identical. They have an almost identical dull gray color. I see the Ni cube as very slightly “yellower” than the other two. The Fe cube is slightly brighter to my eye. The Co cube has the smoothest finish, which I find odd. Cobalt is supposed to be “harder” and more resistant to grinding and wear. Without the decals labeling the cubes, I don’t believe I could identify the cubes, although I could probably distinguish differences reliably.

Magnetic Attraction

Authorities say that all 3 elements are ferromagnetic. All 3 of my cubes are indeed attracted to magnets, and it feels like some are more strongly attracted than others.

Can I distinguish them based on attraction to a magnet?

As near as I can tell, the “attractiveness” should be, from most to least attracted:

1. Iron
2. Cobalt
3. Nickel

Everett agreed based on what he knows about these elements’ orbitals.

Experiment Design

The goal is to measure attraction of a 1 cm³ cube of one of the three elements to a magnet. The idea is to put a cube of an element on the tray of a sensitive scale and place the magnet above the cube. Before and after weights of the cube reveal how much the magnet attracts the cube of the element.

There are experimental difficulties:

• Does the magnet effect the scale’s inner workings somehow? The scale is a “black box” to me.
• Element cubes should have the same position on the scale’s tray.
• The magnet should have the same position above and horizontally relative to each cube.

Experimental Instruments

AWS scale AWS-100 scale, “capacity 100g, graduation 0.01g”. This scale has a flip-top lid that helped consistently position the magnet left-right. We used the body of the scale to position the magnet forward-backward.

Magnet scavenged out of a greeting card that played a song. The magnet is in a little metal cup, which distorts the magnet field to be mostly in one direction. I needed a magnet strong enough to attract the element cubes, but not so strong that it lifts a cube off the scale’s tray.

I read that as: 8Ω 0.4W. Amazon lists 8Ω 0.4W speaker magnets, so I think my recollection is correct. This magnet did not lift any cube off the scale’s tray.

Weighing the magnet and a plastic box on the scale gave different weights if the magnet was directly on the scale’s tray, with the plastic box on top, versus plastic box on scale’s tray, magnet on top. We took this to mean that the magnet was distorting the scale’s measurement somehow, so we suspended the magnet above the scale via a clear plastic display stand.

Acrylic plastic mineral display stand If this item were made out of aluminum or steel, you’d call it a channel section. It’s a C-section, 3 inches wide. The top and bottom of the “C” are 1.31 inches long, the backbone of the “C” (bottom of the channel) is 3.44 inches long. The whole thing is about 0.09 inches thick.

This piece of acrylic places bottom of the magnet is 0.78 inches above the tray of the scale. Writing this post made me look at my images of the experiment more critically. One edge of the C-section was on the scale’s lid, and one edge was not. The backbone of the “C” was not parallel to the scale’s tray.

That’s the acrylic display stand with the magnet taped on top. My iron cube is next to it for scale.

Setup

I put tape on the scale’s tray. I located the center of the tray by drawing diagonal lines from its corners. I put a cube at the center of the tray and drew lines on the tape showing where the cube should rest.

There’s the iron cube in place. I tapped the cubes so that their bottom edges aligned with the interior of the right angle marks. I piously believe I was never off by more than half a millimeter.

I believe that documents how I positioned the cube consistently on the scale’s tray.

Here’s the whole experimental apparatus assembled.

You can see the upper left edge of the acrylic C-section locates the stand, and the attached magnet, left-right. The inside lower face of the acrylic C-section locates the attached magnet up-and-down by touching the edge of the AWS-100 scale.

The magnet is not positioned exactly vertically above the cube under test, but it should be consistently positioned.

Results

Material	Mass, grams	Mols	Attraction, grams	Attraction per gram	Attraction per Mol
Cobalt	8.85	0.150	0.87	0.098	5.80
Nickel	9.00	0.153	0.89	0.099	5.88
Iron	8.02	0.143	0.99	0.123	6.92

I’m not sure if I should be measuring attraction per Mol or attraction per gram. Fortunately, the absolute results and the per-unit results rank the 3 cubes the same way.

Conclusion

The experimental apparatus worked: the iron cube exhibited greater attraction to the magnet than either cobalt or nickel labeled cubes. There’s only a tiny difference in absolute attraction for nickel or cobalt cubes. I can’t distinguish them based on magnetic attraction.