A salty tale ....

Way back in 2007, I wrote a piece on a simple chemical reaction which is now proving to be highly useful in chemical analysis for process and quality control. To save you having to scroll down to the bottom of the blog, here it is again:

You can use this reaction to determine by thermometric endpoint titration (TET) sodium, aluminium and fluoride (but not potassium), because the reaction is exothermic. I'm going to stick my neck out here, and say that this reaction offers the first viable titration method for analyzing sodium. Sure, there's been others, but none have proven to be successful. A titration based on the insolubility of zinc uranyl acetate has been around since 1931, and a complexometric procedure was reported in 1970, but my argument is that if they were of any real practical use, you would in fact see them being used. You don't. And yet titration is a great technique for routine process and quality control. Automated titration instruments don't take up much bench space, infrastructure is minimal, and lower-skilled operators can be used, providing they are well trained and well supervised.

In the case of TET, frequently it is unnecessary to filter sample solutions prior to titration. Further, the sensor is really just a fast-responding electronic thermometer. The probe doesn't need calibrating and requires little maintenance, the titrations take place in regular vessels (no insulation required), you don't need a reference electrode, the probe is always 24/7 ready - what could be easier? 

Thermoprobe thermometric titration sensor

"But wait a minute", I hear you say, "we're already titrating for sodium in food - we use silver nitrate as the titrant. That's all we need to do, isn't it?"

Well, no. It isn't good enough any more. The silver nitrate titration assumes that all sodium present is in the form of sodium chloride, common salt. You actually titrate chloride (and other halides) with the silver nitrate, and assume that all chloride is present as sodium chloride. The trouble is, that not all sodium in food comes from added sodium chloride. Lots of other sodium salts are added to processed foods; some are preservatives, some are flavour enhancers, and then there's emulsifiers and also stabilizers; all sorts of different sodium salts are used in foods. There's something else to consider: some manufacturers are seeking to reduce the sodium content of the food, while minimizing the impact of the "saltiness" of the food. They're doing this by substituting potassium chloride for some of the sodium chloride that's normally added. Look at this label from a container of margarine:
Potassium chloride is also being used as a sodium chloride replacement in snack foods as well. Check for the food code E508 on the label, if potassium chloride isn't mentioned by name.

The titration itself is quite straightforward. However, as in every analysis involving food, you have to pay attention to the sample preparation. The main aims are to liberate all the sodium from the food matrix, and to obtain a solution that is suitable for titration - not too viscous, and no lumps. Fortunately, you don't have to worry about turbidity or fine particulates, because there's no sensitive sensor membrane to foul or reference junction to clog. Even fatty foods can be handled with the appropriate sample preparattion. When you're considering the determination of sodium by TET in a food for the first time, there is a need to think carefully about the sample preparation options you need to consider.

Recently, I wrote a paper which gives some examples of the determination of sodium in various foods. You can download a PDF copy of the paper "Novel method for determination of sodium in foods by thermometric endpoint titrimetry (TET)" by clicking on the link. It was published in the January 2014 edition (Volume 3, Number 1B) of the open-access Journal of Agricultural Chemistry and Environment. If you don't have much time, there's a condensed version as a presentation-style PDF which can be downloaded from here. 

Goodbye to Gove (Part 2)

"Part 1" of my "Goodbye to Gove" dealt with one of the more significant technical challenges we faced in the laboratory during the early years of the Gove alumina refinery, which Rio Tinto Alcan is now in the process of closing down. This second part deals with the most critical challenge to the still young refinery, in which the lab. played an important part.

I've mentioned the word "fun" in Part One of these reminiscences of my technical life at Gove. Fortunately, there was more fun to come after we got the Bayer liquor analysis problem sorted out, which fortuitously put us in good shape for the next challenge. A few years after start-up, we were put in the position of changing the type of alumina that the company produced. If we didn't change the alumina product, the Gove refinery would go out of business.

A short explanation. Aluminium metal is smelted from alumina, Al2O3, That alumina is produced by decomposing aluminium hydroxide, Al(OH)3 at high temperatures.

In the illustration above, the red stuff is the bauxite, from which the aluminium hydroxide is refined (the white stuff). The white stuff is smelted to produce aluminium metal.

To produce aluminium, the alumina is put into a large carbon-lined electrolysis cell (a "pot"). The alumina is dissolved in molten cryolite, Na3AlF6, and lots of electricity is used to reduce the alumina to aluminium metal. You can read a description of the process here. The type of alumina the Gove refinery was designed to make was called "floury" alumina, which was a good descriptor. If you pinched a bit of this alumina between your fingers, it made little peaks, just like finely milled baking flour. It comprised mainly alpha-alumina (corundum), and was made by calcining Al(OH)3 along with a trace of AlF3 as a catalyst at very high temperatures, about 1200 deg. C.

There were process advantages in making floury alumina. Plants making floury alumina could be highly productive, making a lot of alumina per cubic metre of the recirculating aluminate liquor in the plant. There were manufacturing advantages to the smelters as well. However, there was one overwhelming disadvantage - these smelters were highly polluting. When the carbon anodes in the smelting pots burnt, the off-gases which also contained fluoride-containing gases and particulates polluted the surrounding countryside. Many of the smelters would scrub these off-gases with water, but they weren't very efficient. The solution was to employ a property of alumina to contain the fluoride in the smelting pots, and not release it into the environment.

If you don't calcine the alumina all the way through to corundum, you have a product which possesses a high adsorption affinity for all sorts of gases and gas-entrained fine particulates. The smelter design could be changed first to put lids on the pots, and then to lead the off-gases over the incoming alumina feed to the pots. Thus the potentially polluting fluoride could be recycled back to the pots, instead of escaping into the atmosphere. Modern aluminium smelters are highly efficient and essentially non-polluting. Trouble was, we weren't making the right kind of alumina. It wasn't just a case of calcining our product less, we would also have to make the particles coarse and able to withstand the rigours of the new dry gas scrubbing process. 

The scanning electron micrographs at left illustrate a coarse particle of agglomerated aluminium hydroxide, and below it, a particle of the coarse "sandy" alumina which is produced by calcining the aluminium hydroxide.
We didn't know how to make this product. Our parent company Alusuisse approached companies who did know how, but they held the technology very tightly. The technology could be purchased at a crippling price, but it would also result in a considerable reduction in the production capacity. The company faced a stark choice: either pay the exorbitant asking price and suffer a loss of production as well, or be eventually obliged to close down due to market pressure. Alusuisse did neither, thanks to their brilliant chemist and engineer, Dr Otto Tschamper. Dr Tschamper invented a process which held the promise of maintaining plant productivity while manufacturing a grade suitable for use in modern smelters. It was a massive improvement over the technology being used by our competitors. Dr Tschamper told me later that when he first announced the new technology at a technical meeting in the USA, he was laughed at. It's nice when you eventually get the last laugh, isn't it?

The technology had been successfully trialled in Europe, now the confirmatory work had to be done at Gove. Fortunately, our lab. was up to the challenge. Although Alusuisse wasn't originally keen on the idea, we had built up some considerable expertise in modelling plant processes at the lab. scale in order to service the demands of our process engineering colleagues of the technical department. We also had the new thermometric titration liquor analysis, which played a critical role in accurately measuring the mass balances required in evaluating the new process.

I was also fortunate to have a staff of excellent graduate chemists, among them Hans-Peter Breu, who would take a leading role in solving the many problems involved in adapting the new process to the Gove plant through clever experimental design and execution. Cut off from the mainstream of the then highly secretive world of alumina R&D, we had to teach ourselves. Firstly, we had to learn how to make coarse, strong gibbsite particles, then we had to learn how to do this in Gove plant aluminate liquor which was heavily contaminated with organic carbon compounds whose source was the Gove bauxite itself. The process which had worked in Europe didn't work with the organic-laden Gove liquor. It was an exciting time as we gradually learned how to make it work in laboratory-scale reactors. We had no access to the American laboratory test technology, so we had to learn everything from scratch. It wasn't just exciting, it was fun. 

I left Gove around the time of the successful pilot plant tests, firstly to work for Dr Tschamper in Zurich, then later to lead a laboratory supporting Alusuisse' alumina operations around the world in their research institute in Neuhausen, Switzerland. Hans-Peter Breu kept on developing his expertise in the highly successful new process, seeing it implemented and adapted in alumina refineries around the world. 

I heard the other day that after the refinery is shut down, the lab. staff will be reduced to just three, as activities at Gove will comprise just the mining and export of bauxite. 

A sensor-ble solution

A confession: I like cool stuff. I like products which solve problems. Being an analytical chemist, I particularly like products which solve problems in analytical chemistry. Not so long ago, I was shown a new development from Metrohm which I regard as just straight out, simply cool. It's an ion selective probe for calcium determinations, which uses replaceable, thick-film technology sensor tips.


 Now, as you will note from previous posts, my niche speciality is thermometric endpoint titrimetry (TET). In comparison with TET, I don't know all that much about measurement with potentiometric sensors outside of pH electrodes, and previous experiences with ISE's haven't been all that positive. However, this new approach by Metrohm seems to be a most pragmatic approach to the problem of limited service lives of polymer membrane ISE probes, and that is, simply replace just the sensing tip rather than the entire probe.

I guess that following through on this pragmatic approach, they've chosen a Ca-selective probe, as this could have considerable market appeal in the titrimetric analysis of water hardness. It will be interesting to see how the market accepts it, and if they follow this with tips which are selective for other ions.