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Professor Robin Stokes was interviewed in 2009 for the Interviews with Australian scientists series. By viewing the interviews in this series, or reading the transcripts and extracts, your students can begin to appreciate Australia's contribution to the growth of scientific knowledge.
The following summary of Stokes’ career sets the context for the extract chosen for these teachers notes. The extract highlights Stokes’ PhD work on diffusion in liquids and how he overcame technical challenges to study the movement of electrolytes. Use the focus questions that accompany the extract to promote discussion among your students.
Robert (Robin) Harold Stokes was born in England in 1918 and moved to New Zealand at age four or five. Stokes earned a BSc (1938) and MSc (1940) from the Auckland University College and supported himself by working as an assistant chemist at the local coal gasworks. During the war (1941-45) Stokes worked as chemist and chief chemist at the Colonial Ammunition Company, New Zealand, where he was first intrigued by machining and instrumentation.
After the war Stokes moved to Australia to take up a position as lecturer in Chemistry at the University of Western Australia. In 1948 Stokes resumed his formal study and went to the University of Cambridge as an Imperial Chemical Industries fellow, where he graduated with a PhD in 1950. Stokes’ doctoral work focused on studying diffusion in liquids for which he developed the stirred diaphragm cell method (see interview extract below).
From 1950 to 1955, Stokes was senior lecturer and reader in chemistry at the University of Western Australia. In 1955 his definitive book Electrolyte Solutions, which he co-authored with Professor Robert Robinson, was first published. Also in 1955, Stokes moved to the University of New England, as the foundation professor of chemistry, a position which he held until his retirement in 1979. Stokes was made emeritus professor from 1980.
Professor Stokes has received numerous awards throughout his long career, including; the Rennie (1946) and H.G. Smith (1953) medals from the Australian Chemical Institute; the Meldola medal from the Institute of Chemistry (1946); the Queen’s Jubilee medal (1977); the inaugural R.H. Stokes medal from the Electrochemistry division of the Royal Australian Chemical Institute (1980) and the inaugural R.A. Robinson Memorial medal from the Faraday division of the Royal Society of Chemistry (1981). Stokes was elected a Fellow of the Australian Academy of Science in 1957.
A PhD at Cambridge studying diffusion in liquids
There you developed the stirred diaphragm cell method.
Well, after this couple of months, I decided that an interesting field to work on would be diffusion in liquids. Diffusion in liquids is very closely connected with two of my other interests, which were the thermodynamic properties of solutions and electrical conductivity. The difference between diffusion and conduction in an electrolyte solution is that in diffusion the ions are all moving one way from a concentrated solution to a dilute solution, whereas in conduction, of course, positive ions move one way and negative ions move the other. But there is clearly a very strong relationship between these two processes and more data was urgently needed on diffusion.
There were very few reliable data. At that time people were realising the importance of diffusion, in connection with the theory as well as practice, and lots of reviews were being written. In the chemical literature there were several reviews on diffusion, and they all reviewed the same things and there was hardly any experimental data to review. Except a few very recent measurements in very dilute solutions, which were being done at Yale, they all went over the same old ground. It was clear that there was a very strong need for the actual measurements, so I decided to do this.
One of the methods that had been tried was this diaphragm cell, in which there was a sintered-glass diaphragm in the middle of a cylindrical cell and solutions diffused through the diaphragm from one side to the other. The diaphragm is a device to stop the liquid from mixing mechanically and just let the ions go through so that the liquids don’t actually mix into each other mechanically at all. This had been used and had worked moderately well, but it had some difficulties. One of these was that there was a layer formed near the surface of this sintered-glass diaphragm, which was stagnant, and its immediate thickness was not calculable, so you didn’t really know the distance the ions were diffusing over. I was trying out various things about the effect of the angle at which the cell was tilted and whether, if the denser solution was on top, would it still work? and, if it was underneath, would it not work because of the formation of these layers? If you had the stronger solution underneath, you would get a bit of liquid going through but then staying on the diaphragm because it was heavier than the lighter liquid on top; whereas, if you put the cell the other way up and it was difficult to be sure that you weren’t getting actual liquid flowing through the diaphragm instead of just diffusing.
I was playing around with this and John Agar was interested in what I was doing at the time. I had just started to work on this for a PhD, and John (who ended up being my supervisor) suggested that it might be possible to stir it magnetically. At that time this was a pretty revolutionary suggestion because magnetic stirrers were quite scarce. When you wanted to stir something, you stirred it with a stirring rod. These little magnetic stirrers that are used everywhere in labs now were pretty rare and we didn’t have one anywhere in the Cambridge labs. But this idea was clearly what we wanted. So I found a way of making the stirrers using little bits of tubing, with a bit of iron wire inside, and made these rotate by having a big horseshoe magnet rotating around the outside of the cell. This made all the difference to the whole principle. You could still have the denser solution underneath, so that it didn’t have this liquid falling through by gravity, and the less dense solution on top, so it was gravitationally stable, and you could then get rid of this effect of the layers of liquid accumulating near the glass diaphragm by having these magnetic stirrers going around stirring the whole thing up so that each compartment was kept uniform in composition. This meant that the thing was a complete success in terms of measuring diffusion coefficients.
I did a lot of measurements on simple electrolytes with this stirred cell and got them into publication before the end of my second year. All the simple 1:1 electrolytes that have ions of a noble gas structure, I measured over the next year or so. It was a very slow process because these diaphragm cells have to run for three or four days to get enough diffusion occurring to make a useful difference to the concentration. What you do is know the concentrations to start with, then you set the diffusion process up and, after a few days, you measure the concentrations again. You find that the concentrated solutions become more dilute and the dilute ones become more concentrated. But it needed some pretty accurate measurement of the concentration to be able to calculate this with sufficient accuracy.
Focus questions
Select activities that are most appropriate for your lesson plan or add your own. You can also encourage students to identify key issues in the preceding extract and devise their own questions or topics for discussion.
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