How is drift due to static electricity eliminated from an analytical balance?

Question

I've recently encountered a problem when using an analytical (0.1 mg accuracy) scale to determine the mass of a sample of ferrocene powder. The reading in the balance seems to drift either upwards or downwards when I drop some of the powder in an erlenmeyer flask. The reading stabilizes when I add water to form a solution.

I've heard that static electricity might cause this kinds of problems when measuring powders so I suspect this is the case here, because the problem only appears when I add the ferrocene powder. I've try reaserching some ways to get rid of the problem, a static gun seems to work in some cases (no idea if it would work with a powder though) and I've heard that air ionisers also work, but don't have access to either of these.

Although this is the first time I've encountered this situation, I've heard its pretty common, so what do people usually do to get rid of the problem in a cheap manner? Is there an easy solution?

I'm using gloves to operate the scale, which I've been told to do in order to keep the flasks clean of oil from my fingers. Can the gloves be worsening the problem? What can I do about that?

Answer 1

Static charge can affect the measurements made by an analytical balance, as can magnetic materials because the mechanism in modern analytical balances involves a feedback loop between the load that is measured and an electromagnet. Insulating weighing vessels (plastic or glass) tend to accumulate static charge, especially when handled with another insulator (gloves) by the triboelectric effect.

You should first make sure that the body of the balance is grounded properly according to the manual for your model.

As a short-term solution to static charged glass, you might try arranging the sample so that it is father away from the balance. One way might be to place the sample on top of an empty sample bottle on the balance. As you mentioned, a more robust solution for discharging static build-up is to have a commercially available air ionizer nearby. The device will either produce an equal amount of positive and negative ions in the air at high voltage AC or DC electrodes, or with an ionizing radioactive material such as is found in anti-static brushes used for cleaning vinyl audio records.

If you are frequently measuring magnetic materials, many analytical balances can be configured in a bottom-loading arrangement. In this setup a platform is hooked beneath the balance, and the sample is supported sufficiently far away from the force-restoring motor so that it does not influence the measurement.

Answer 2

Wearing gloves may contribute to the static charges you observe, especially if you have to work with gloves not only to protect yourself, but per se you can't do otherwise (glove box) and a special, palm size ionizer is indeed a remedy.

While I do not pretend "it works every time", these are my suggestions, that sometimes work alone, sometimes in combination:

• Electric discharge of yourself. Sometimes over the day I become "charged", too, and a simple tip of my hand to the (air dry) water pipeline or heating pipe / radiator removes some of the charges that can't reach earth while walking in firm (but nonconducting) lab shoes over polymer-coated surfaces.

• Electric Discharge the clean spatula. Prior to transferring material to the balance, tip the metallic spatula against something that is both conductive to the earth and clean. Clean and air dry water pipeline, heating pipe (vide supra) are handy.

• Attempt the mass transfer with a spatula made of a different material rather than metal. Beside glass, different plastics may be represent a surprising alternative. Krischenbaum et al. reported recently how to deploy drinking straws as disposable spatulas in Journal of Chemical Education.

Answer 3

I have been using gloveboxes for several years in different labs across the world and I find that Ar gloveboxes completely eliminate the problem of static electricity commonly experienced with N2 gloveboxes. I have no explanation for this explanation but I am pretty sure N2 is the culprit because we once switched an N2 box to Ar and the problem disappeared. Before that we spent our time fiddling around the balance with static guns, air ionisers and such, without much success (the only exception being the Po ionisers which are pretty efficient). On the other hand, when purging an Ar box with N2, static electricity will usually appear for a couple of days. In my opinion the price difference between N2 and Ar is simply not worth the trouble.