Telling elements apart

19 February 2019

Interview with

Leesa Ferguson, Anglia Ruskin University

BUNSEN-BURNER-CARTOON

Graphic of a bunsen burner.

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150 years ago the average scientists’ laboratory looked very different to what we have today.  So how exactly did scientists test what each element was, beyond just looking at them? Katie Haylor went to Anglia Ruskin University to find out, with the help of forensic scientist Leesa Ferguson...

Leesa - Well they typically use flame tests. So what they would do is put individual elements into a flame and the flames would give characteristic colours and that's how they identified different elements.

Katie - Remember those Bunsen burners from school science class. You've got a gas inlet and a valve to control the airflow and these burned together to produce a clean flame. Crucial for doing a flame test. And this was something that chemist Robert Bunsen back in the 1950s or so was particularly interested in.

Leesa - He originally started work on flame tests but he found that the flames that he was using sometimes produced soot and weren't necessarily clean flames. And sometimes interfered with the actual colours of the elements he was trying to get. And so what he developed was the Bunsen burner because that gave you a flame that was non luminescent and therefore did not interfere with the colours that the actual elements produced when they were put in the flames.

Katie - So what's actually going on in a flame test? How does burning an element tell you what it is?

Leesa - So typically what is happening is that as the actual element is heated up, the electrons in the element get excited and they go from what we term a ground state to a higher energy state. So when they're in that higher energy state they then drop back down to the ground state and as they do that they emit wavelengths of light, coloured light, that is characteristic of the element.

Katie - Almost like jumps, jumping up and down?

Leesa -It is indeed, a little bit like jumping up and down a ladder. On a ladder you can step on one rung to the next rung, but you can't step in between the rungs. So that's a little bit like the energy levels.

Katie - And the distance between the rungs is separate for each element?

Leesa - It is indeed. And so when they actually emit light and it drops back down to the ground state, that would be characteristic of the element.

Katie - Well that's the theory down now let's put it into practice. Once me and Lisa were kitted out with lab coats safety specs and gloves, we got stuck in. We've got a Bunsen burner some matches, just putting the gas on now, let's get this Bunsen burner lit. Now what are you holding, and what are you about to do?

Leesa - It's just an implement that's going to enable us to actually put the element that we're looking at, in this case copper. into the flame.

Katie - But you're going to clean it to make sure we don't get any contamination?

Leesa - Yes I'm initially going to clean it with hydrochloric acid and then put the implement into the flame. So it just makes sure that any contaminants that may be present are no longer there.

Katie - So you're not doing a flame test for the wrong thing?

Leesa - Absolutely yeah.

Katie - So you've got some copper pieces. I guess the idea is to attach it to this implement and then see what happens when you put it in the flame. Oh that does look a bit green or a different kind of blue.

Leesa -You can see a characteristic sort of green flame that you do actually get with copper.

Katie - Is this how fireworks work? Because you get lots of different colours in fireworks and there's lots of different metals involved.

Leesa -Yes, fireworks typically contain lots of different metals, when they're heated up they actually emit energy of different wavelengths which is the different colours that you actually see.

Katie - Flame test in action, very expensive flame test. So what colours would you typically see with other elements?

Leesa -So with sodium you typically get quite an intense yellow colour, with potassium a lilac colour with lithium it tends to be a red colour. So there's some of your basic elements. Bunsen and Kirchhoff by using flame test coupled with a spectroscope actually identified two new elements, caesium and rubidium.

Katie - The spectroscope that Leesa mentioned contained a prism inside Think Pink Floyd's Dark Side Of The Moon album cover which splits light into all its coloured components. They found that the lines that appeared within the spectrum corresponded to the light being emitted by the elements. The wavelengths of light that each element emitted was unique, and this is why a spectrum is often thought of as an elements fingerprint. So are scientists using these same techniques to identify elements now?

Leesa - We don't typically use flame tests, we've got more advanced techniques that we tend to use to try and identify elements present in samples. Typical technique that we would use is something called inductively coupled plasma optical emission spectroscopy or ICP-OES.

Katie - I can see why it has an acronym carry on.

Leesa - And so basically it works on the same sort of premise really. You've got, let's say a sample of pure of element, the actual instrument would suck up the sample into something called a plasma. It has a really high temperature. So when the sample is in the plasma the actual solvent its in evaporates off, and the actual elements there are atomised. And what happens is that the electrons in the elements again get excited from a ground state up to a higher energy state and it's different for each element. And as they fall back down to the ground state the elements they emit a light of a specific wavelength.

Katie - As it turns out it's not just flames or even plasma that can be used to excite electrons in spectroscopy. Other wavelengths or parts of the electromagnetic spectrum can get involved.

Leesa - So different parts of the electromagnetic spectrum have different energies associated with them. So something like X-rays is very energetic, so it's so energetic that actually when you irradiate a sample with X-rays, in a technique like X-ray fluorescence, what that causes is actually electrons from the core, the inner electrons, one of those to be ejected actually out of the sample you're looking at. So because this electron is ejected from the core, one of the outer electrons drops down and fills that unoccupied space and because the electron has gone from a higher energy outer orbital to a lower energy inner orbital, it gives off an X-ray that's characteristic of the actual element you're looking at.

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