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Tin oxide based ion exchange solids

Dear Reader,

I was reading the other day about ion exchange solids which are based on metal oxides such as tin and titanium dioxide. The surface of these metal oxides can absorb cations, this is a bit different to the organic resins such as DOWEX 50 which is a polystyrene with sulfonic acid groups.

One likely way in which the metal oxides such as tin oxide can bind to the metals is at the surface, when the crystal is cut then the surface will have some dangling bonds. I suspect that the oxygens which do not coordinate to the same number of metal atoms as those oxygens which are fully within the solid will bind to protons thus forming hydroxide groups on the surface.

If we look at tin dioxide we will see that a oxygen is shared between three tins while each tin binds to six oxygen atoms. If we were to cut the crystal in the right way we could expose the oxygen atoms, these oxygen atoms can then bind to protons to thus form hydroxyl groups like those on some silicas.

In common with silica it is possible to add organic groups onto a tin oxide by simple treatment with a trialkoxy silane such as 3-aminopropyl triethoxysilane[1] or 3-methacryloxypropyl trimethoxysilane.[2] Now lets get back to the structure of tin dioxide.

It has a tetragonal cell which is a 4.7402 by 4.7402 by 3.1856 Å box, the fractional coordinates of the atoms are as follows.

Element x y z
Sn 0 0 0
Sn 0.5 0.5 0.5
O 0.3073 0.3073 0
O 0.6927 0.6927 0
O 0.8073 0.1927 0.5
O 0.1927 0.8073 0.5

We will discuss tin oxide in further detail soon

 

[1] C.J. Liu, K. Oshima, M. Shimomura and S. Miyauchi, Journal of Applied Polymer Science, 2006, 100, 1881-1888.

[2] W. Posthumus, J. Laven, G. de With, R. van der Linde, Journal of Colloid and Interface Science, 2006, 304, 394-401.

 

 

Zirconium phosphates

Dear Reader,

A while ago I wrote a lot about Prussian blue the wonder solid which captures cesium, the fact that people want to read what I write about Prussian blue made me think about the other inorganic ion exchange solids. One important one is the 2D network of zirconium hydrogen phosphate.

This can be quite simple to make, one synthesis is simply boiling together a zirconyl salt with phosphoric acid. This forms a layered solid which contains hydrogen phosphate groups, the hydrogen phosphate groups can be deprotonated and then cations can bind to the solid. One important person in this field is Abraham Clearfield who has written many papers on the subject of this class of solid. (Brian M. Mosby, Agustín Díaz and Abraham Clearfield, Dalton Trans., 2014, 43, 10328-10339).

Here we can see one of the layers in the solid it is a 2D network of zirconium atoms and hydrogen phosphate groups.

A layer of zirconium hydrogen phosphate

A layer of zirconium hydrogen phosphate

The 2D sheets then make a layered solid with many many layers. Here are some pictures of the layers of the solid.

Three layers of the layered zirconium hydrogen phosphate

Three layers of the layered zirconium hydrogen phosphate

When the zirconium phosphate is immersed in a solution of a metal some of the protons in the hydrogen phosphate groups can be replaced with metal ions, for example in the following diagram the layers in a potassium exchanged zirconium hydrogen phosphate can be seen.

One layer of the zirconium phsophate after exchanging with potassium ions

One layer of the zirconium phosphate after exchanging with potassium ions

Again the layers can be seen

Three layers of the potassium exchanged zirconium phosphate

Three layers of the potassium exchanged zirconium phosphate

We will have a look at some other interesting solids soon,

All vs Some

Dear Reader,

Sadly one of the classic errors in thinking has shown its ugly face again, this is the issue of “all vs some”. This can be understood as a person thinking that because one example of a person, thing or idea from a class has one set of properties then all things from that class, set or group must share this property.

For example I know a Romanian woman who is a chemist who likes motorcycles and is training for her motorcycle license. Now I sincerely hope that none of readers would be so silly that they then think that all women from Romania are trained in chemistry and have a liking for motorcycles. But sadly when it comes to chemical substances a pandemic of this stupidity exists.

One area within chemistry where this occurs is “ionic liquids”, my own view of ionic liquids is that they are a very wide class of substances I was reading the news at chemistry world in which a comment “Ionic liquids have previously generated much excitement, but also some fierce criticism owing to some being toxic” was made. I would like to point out that this is a good example of “all vs some”. I will admit that some ionic liquids are perfectly horrible but the horrible nature of one ionic liquid does not change how nice/horrible the next one which you encounter is. One of the best books about how to think (straight and crooked thinking) in chapter two considers this problem of “all vs some”.

Plenty of ionic liquids exist which are made of food grade chemicals which are close to harmless, for example choline salts are used as vital nutrients in some animal feed. From choline salts it is possible to form ionic liquids with anions like lactate (carboxylic acid found in milk) can be made. Such an ionic liquid will not be toxic, it is possible to use it as food for growing microbes.

I think that we need to be careful of the “all vs some” problem, you will also get a reverse problem where a person attracted by the good features of one example of a set then assumes that all other examples of that class are equally good. While if you get lucky and find something which makes you happy or satisfies some other need it is reasonable to continue to search within that class for another example of that class which is equally good or even better. It is impossible to give any warranty that it will be possible to find a better example within the class.

For example carboplatin and cisplatin are drugs which can cure cancer, they contain platinum but it is unreasonable to then assume that all platinum compounds can cure cancer. Also despite the fact that a lot of platinum compounds have been screened as anticancer drugs, very few were found to be suitable for the treatment of cancer. The fact that cisplatin / carboplatin were so good does not make it a certinity that a new effective cancer treatment system based on platinum chemistry will be possible.

Uranium glass again and how to make a radiometric measurement I

Dear Reader,

I have checked most of the uranium glass with a geiger counter, the geiger counter uses a tube which is known as a Geiger-Muller tube which is a gas filled high voltage discharge tube which uses an avalanche effect to increase the number of free electrons and ions formed in the tube after the absorption of radiation by the gas inside the tube. It is important to understand that GM tubes are not all born equal, it is possible through careful design to optimise a tube for an application. I borrowed a GM tube based device to check my uranium glass, this first device was a bit of a disappointment.

It has a tube with a thin mica end window and it has some beta sensitivity, but it is not very sensitive. It was intended as a gamma / beta detector which has a full scale reading of 1000 rem per hour. I think that such a device is the tool of choice when dealing with a high dose rate event. It would be very suitable for nuclear warfare use assuming that the fragile GM tube survives the bomb detonation, it could be very useful when dealing with a industrial radiography accident such as a lost source or a radiotherapy accident which involves a lost source. This detector has a rate meter for GM tube events which goes as low as 1 count per second.

With this high dose rate meter I could not get any reading from my uranium glasses, except for a very dark green one which gave 2 counts per second, the background in my house was 1 count per second. It is hard to work out if 2 cps is different to 1 cps as radiation from radioactive decay and cosmic rays is occurring randomly.

So I then tried a different GM tube based device, I choose one which is optimised for looking for low to moderate levels of beta emitting radioactive contamination. Note while from personal experience I know it works for carbon-14 it will never work for tritium. The device is a “Radiation Alert Inspector” made by S.E. International INC (Summertown Tennessee). This device can give the count rate or it can be set to record the number of counts over a given time.

As I was dealing with very weak sources I choose a counting time of five minutes, I measured the background in my house three times. The results for the background were 248, 224 and 263 counts. The total number of counts in these three determinations of the background count rate were 735 counts, which makes the count rate in my house to be 0.82 counts per second, the ESD on this count rate is 0.03 counts per second which is about 3.7 %. This ESD is based on the number of events observed. So based on this count number we should expect 245 events (give or take 9 events) in five minutes.

I measured a glass object which does not fluoresce when exposed to UV light, I got a total of 224 counts in five minutes. The difference between the count number for this object and the background is 21 counts, the sum of the ESDs is 24, so this difference is unlikely to be significant in a statistical sense.

I then went and measured a green glass milk jug which fluoresces nicely with UV light, this gave a count of 580 events. The difference between this count number and the background count number is a staggering 335, while the sum of the ESDs is 33, as the difference is ten times the sum of the ESDs it is very real which suggests that the milk jug does contain something which is radioactive.

The lowest count number I got in five minutes on uranium glass was 441 on a pale green thin blown glass vase, this value still suggests that it is radioactive. The difference between the background and the sample is 196 and the sum of the ESDs is 30. This is still very convincing.

Now while I have done quite a trivial experiment I would like to ask other people who are considering doing independent radiation measurements to up their game a bit. I sometimes see data shown on the internet where the people making the measurements do not explain their experimental method fully or state the number of counts which they use to estimate the dose rate or radioactivity level. For example Greenpeace have been using NaI spectrometers in Japan, they used a thing called a “Georadis RT-30″ which is a nice bit of kit. The only problem is that they did not give full details of how they obtained dose rates with these machines.

While I know that a NaI spectrometer will never give as good energy resolution as a high purity germanium detector this type of NaI detector can distinguish between different radionuclides (based on the gamma photon energy), what I would like Greenpeace to report are the gamma spectra and all the details such as the counting time, details of the dead time correction. This would allow the contribution of Cs-137 to the dose rate to be separated from the gamma rays from the uranium decay chains.

I would also like spectra obtained at different distances from a known cesium-137 source at different distances. This could be used to calibrate the spectrometers. I would also like to see the spectra obtained using natural uranium, natural thorium and uranium ore samples. This would allow me to see how well the machine is able to separate the signals from the different gamma photons.

When I write gamma spectrum I always mean a table or chart of counts per channel against channel number. The Greenpeace NaI spectrometer has 1024 channels so with some luck it should be able to separate the photopeak for Cs-137 (662 keV) from all the other gamma photons or at least allow a partial cleanup of the data.

Now I will not pretend that Greenpeace are neutral regarding the question of “should the world have nuclear power ?”. I know that they are opposed to nuclear power, the fact that they are opposed to it does not either disqualify them from commenting on nuclear issues or make them more trustworthy. I hold the view that if Greenpeace put in extra effort into their radiometric measurements then in the long run it will be good for them and the rest of society.

Firstly it would make their results more trustworthy, people would be more willing to accept their results as true.

Secondly it would avoid problems such as “I will not trust it until I have checked to see if another explanation exists for their observation”. For example if an antinuclear activist claimed that hot spot exists in Aberdeen (Scotland) as a result of a discharge from a Scottish nuclear reactor, then I would want to know that they had not been fooled by the high gamma background due to the rocks in Aberdeen (Granite). One way of proving to me that a high gamma level was not due to the granite is to show me the gamma spectrum.

In recent times the higher background radiation levels on some beaches in the western part of the USA have provoked great excitement. The radiation has been blamed by some on the Fukushima accident, however a close examination of the site indicates that the radiation is coming from daughters of uranium / radium rather than cesium 134 or cesium 137. As the Fukushima event released mainly cesium and iodine this radionuclide signature is not reasonable for the beach.

A person or group which has a track record of making hasty statements will carry much less weight than a group which takes its time and makes sure that its statements are correct.

The problem with making a statement which is quickly shown to be false is that the person or group which made the statement will lose credibility, so by taking additional care to improve the quality of the work which is behind a statement then in the long run you will be more persuasive. I will get onto another point about radiometric measurements soon.

More uranium glass

Dear Reader,

I have been in the second hand shops today of Lidköping, I have managed to find some more uranium glass. The glass does glow a nice green / yellow when you expose it to UV light. I will have to make a radiometric examination of the glass to determine if it is uranium glass or some other glass which has similar optical properties.

I think in future I will carry a UV torch in my pocket when I am out and about, here is a photo of the glass under normal light.

Mark's new uranium glass

Mark’s new uranium glass

Here is a photo when the only light is UV light from my UV torch.

My new uranium glass with UV light, notice it is glowing green

My new uranium glass with UV light, notice it is glowing green

I will examine the glass as soon as possible, then I will post the results of my tests.

Candlestick III

Dear Reader,

A lady from the USA who collects glass has been in communication with me regarding the uranium glass candlesticks, she has a pair of very similar candlesticks to my uranium glass one. It is possible that they all came from the same maker.

I did collect a gamma spectrum from my candlestick, as the activity level in a candlestick is low it took many hours to get a good spectrum. Here is the full range gamma spectrum from 10 keV photon energy to about 3 MeV photon energy. This covers almost all of the gamma spectrum. OK I can think of gamma events which are outside this range like the n + N-14 reaction to form C-14 which emits photons at about 10 MeV but such a high energy photon is very rare.

Full range gamma spectrum of the candlestick

Full range gamma spectrum of the candlestick

You should be able to see that almost all of the peaks are at low energies (below 500 keV), I have stretched the y axis by using a log scale so you can see some more of the minor peaks.

Candle stick full range log scale for counts

Candle stick full range log scale for counts

Now I have replotted the graph to show the lower energy end only, it should be clear that a series of peaks are present due to the gamma rays from the candlestick. Also on the left hand side is a big peak due to X-ray formation. What happens is that high speed electrons (both beta particles and photoelectrons) fly around inside the shielding and form X-rays by hitting the copper inner shielding on the lead castle in which the detector is placed. The copper inner shield reduces the fraction of photoelectrons which are ejected from the lead surfaces that are able to strike materials close to the detector crystal.

Also any X-rays formed in the surface layer of the lead nearest the radiation detector and the object being measured will tend to be stopped by the copper metal. By experience is that 1 mm of copper sheet is able to stop almost all copper K x-rays. And I imagine that it will stop many moderate energy photons like the moly K x-rays which are used for single crystal crystallography work and for mammograms. In the ideal world we would also have an aluminium layer inside the copper shield to further reduce the back ground at low energies due to secondary X-rays, and then a layer of a plastic like PMMA to finish the job. This is an example of graded Z shielding.

Even if the shielding is designed in such a way to prevent X-rays reaching the detector, it is possible to create X-rays inside the sample. The ideal sample for gamma counting would be a very small sample with a very high specific activity (radioactivity per mass or unit volume). If this was wrapped in plastic then any beta particles which come from the sample would lose their energy gently in the plastic without making horrible X-rays. But in our case the volume of the sample is large and the glass contains plenty of elements such as silicon which can form bremsstrahlung as the electrons pass close the atomic nuclei.

But we will not let a little bremsstrahlung spoil our fun. Here is the lower end of the gamma spectrum.

Candle stick spectrum lower end

Candle stick spectrum lower end

What we need to do next is to work out the relative efficiency for the detector for the different gamma lines, what we need to do is to measure the size of the peaks which we can identify and then use the fact that the yield of the different gamma lines are known. This will allow us to reconstruct the relative efficiency for the detector. In case anyone is thinking that we could do it with more easy by using a mixed photon energy radioactive source such as a radium-226 source. There is a problem, the odd shape of the sample makes it have a sum of many different counting geometries and a lot of self adsorption for the lower energy gamma photons.

I could make it more simple by crushing or melting my candle stick to make it into a more simple shape, but that would spoil my candle stick so I am not going to do that.

X-ray energy and getting the terms right

While reading the article entitled “the art detectives” in the RSC’s Chemistry World magazine I saw the statement that high energy X-rays are used for XRF of elements such as zinc. I strongly suspect that a misunderstanding has occurred, for example the zinc k lines will come at 8.6 keV which is hardly high when compared with the X-ray photons commonly for the industrial radiography of steel objects. To excite an atom in a X-ray fluorescence (XRF) experiment only moderate energy photons are needed (tube voltage of 40 kV is acceptable) while for industrial X-ray radiography it is common to use much higher accelerating voltages (100 kV and higher). For very thick metal objects photons in the MeV range are used.

What I think the article should have stated is that the object in XRF was illuminated with a high intensity of x-ray photons, to my mind intensity (photons cm-2 s-1) is very different to photon energy. But why would anyone use an expensive intense x-ray source rather than a weaker and cheaper one ?

If we assume that the increase measured above background is directly proportional to the concentration of an element and the intensity of the incoming exciting x-ray beam, then if the background is 10 cps, then with a weak x-ray source then we could get a reading of 20 cps on a spot on a painting. As for a random events the standard deviation on the count number is the square root of the count number after 1 second then the sum of the two SDs is 7.634 which is close to the difference between the two count numbers. If we were to use a source ten times brighter then the sum of the standard deviations (10.49 + 3.16 = 13.65) is small compared to the difference in counts after 1 second.

The great problem is that people writing about science sometimes tend to throw words about, almost randomly, without thinking about the fact that the word already has a meaning. To write clearly about science we must first avoid confusion.

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