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What is solvent extraction ?

Dear Reader

You might ask the questions of what is the liquid-liquid extraction of metals and why is it important ?

I would say that the liquid-liquid extraction of metals is the more correct term for what is normally understood as “solvent extraction”. Solvent extraction is not the extraction of a solvent from a thing, but instead it is the extraction of a substance from one solvent into another. Commonly one of the liquids is water but there is no reason why the ideas of solvent extraction should not be applied to non aqueous systems such as silver being extracted from molten lead into molten zinc or the extraction of an organic species from methanol into hexane.

A typical metal ion in an aqueous phase is bonded to water molecules and/or chloride anions, this forms a water soluble metal complex. The metal complex normally has a very low solubility in an organic layer, but like many rules some exceptions do exist.

For example osmium(VIII) forms an organic soluble oxide (OsO4) which can be thought of as having formed from a Os8+ cation and four water molecules. The waters would have bonded onto the osmium before undergoing hydrolysis to hydroxyl ligands (OH-) which then react further to form oxide ligands. I suspect however that [Os(H2O)n]8+ is an impossible complex for several reasons so I do not think it will be possible to make it.

But I know that compounds such as [OsO3(OH)2] are well known, and this can be thought of as a hydrated form of OsO4, or alternatively OsO4 can be viewed as the acid anhydride of [OsO3(OH)2]. Yet another alternative is to stop thinking for a moment about osmium and then move onto some other things.

Well back to the solvent extraction of metals, if we take a hooks and eyes view of chemical bonding the electron poor metal ions can bond to the electron rich parts of solvent molecules such as the oxygens in water. These bonds to water can be broken and replaced with bonds to molecules with big long fatty groups, for example charged cobalt atoms in water are normally found with six waters around them. For example using 3D x-ray crystalovision (known to chemists as X-ray crystallography) we can glimpse a charged cobalt bearing six waters in the way it is thought to be in aqueous solution.

Cobalt with six waters

Cobalt with six waters

This cobalt ion was seen in a paper by some gifted chemists who made coordination complexes with tetrahydrofuran-1,2,3,4-tetracarboxylic acid.[i] I have to admire their compound, in some ways I wish I had done that chemistry myself.

Now it is well known that cobalt(II) salts in dilute water solutions are pale pink in colour, this is a sign that the cobalt is in a octahedral complex with six waters attached. However under some conditions deep blue complexes of cobalt are formed, this is normally a sign that a tetrahedral complex of cobalt as formed. It is well known that the solution of cobalt in bis-2-ethylhexyl hydrogen phosphate in a hydrocarbon solvent is deep blue. This made me want to look to see if X-ray crystallography has ever been used to characterize the idea mononuclear (complex with a single metal atom) of cobalt with two dialkyl phosphate ligands.

When I did the search I was in for a shock, I found a coordination polymer of cobalt and bis tert-butyl phosphate in which tetrahedral cobalts were linked by the dialkyl phosphates into a 1D chain. I also found bis-(((4,7,7-Trimethyl-3-oxobicyclo[2.2.1]hept-2-yl)phosphinato)-((4,7,7-trimethyl-3-oxobicyclo[2.2.1]hept-2-yl) hydrogen phosphinato))-cobalt(II) which is made from one cobalt(II) ion, two molecules of bis-(4,7,7-trimethyl-3-oxobicyclo[2.2.1]heptan-2-yl) phosphinic acid and two molecules of the conjugate base of this rather long named phosphinic acid.

I looked in the Cambridge database for metal complexes in which two dialkyl phosphate ligands chelate to a single metal atom to form a complex. None were found, I soon concluded that the oxygen oxygen distance is too large as the phosphorus atom is tetrahedral, this is different to a carboxylate where the central carbon is trigonal planar.

Here are the XYZ coordinates of a typical model phosphorus compound

O(1) -1.368 1.517 -0.000
P(2) -0.171 0.647 -0.000
O(3) 1.160 1.562 -0.000
C(4) -0.185 -0.423 1.516
C(5) -0.185 -0.424 -1.516
H(6) 1.876 0.950 -0.000
H(7) 0.715 -1.078 1.517
H(8) -1.102 -1.054 1.516
H(9) -0.177 0.220 2.424
H(10) -1.091 -0.199 -2.122
H(11) 0.726 -0.221 -2.122
H(12) -0.199 -1.495 -1.213

 

The two oxygen atoms are 2.5 Å apart,  now we can compare the distance with that in acetic acid. Again here is the xyz table, and in acetic acid the oxygens are 2.2 Å apart which makes it much more easy to form a chelate ring.

C(1) -7.031 -0.130 -0.000
C(2) -5.522 -0.130 -0.000
O(3) -4.918 -1.176 -0.000
O(4) -4.853 1.029 -0.000
H(5) -7.403 -1.179 -0.000
H(6) -7.402 0.394 0.909
H(7) -7.402 0.395 -0.909
H(8) -3.910 0.796 -0.000

 

Now some of my readers may ask “what is a chelate ring”, the idea of chelation is where a molecule bonds to a metal with more than one atom. I would like you to do a thought experiment, we have gone to the beach and a fish has bitten my foot, now how do I remove it ?

I have to open its mouth and then I can pull it off and throw it back in the sea.

Now imagine that the crab has grabbed by foot, now to remove the crab I must release both of its pincers at the same time. If I remove only one at a time while I am dealing with the second one the first one will grab me again. The crab is able to chelate to me.

Things get even worse with a scorpion which has two front grabbers and a tail to grab with, and frankly if the octopus gets me with all eight tentacles then it is going to be very hard to escape from its grasp.

If a ligand is able to chelate to a metal then it normally makes it very hard to remove the metal from the ligand’s grasp.

Thinking about the extraction of cobalt we might suspect that it should be given by the following equation

DCo = k[(RO)2PO2H]2[(RO)2PO2-]2

I suspect that the (RO)2PO2H will form hydrogen bonded dimers in the organic phase which could alter the equation to

DCo = k[{(RO)2PO2H}2]1[(RO)2PO2-]2

This might be true when the concentration of (RO)2PO2H is very low, the problem will be that when the concentration of (RO)2PO2H is high then it increases the dielectric constant of the organic phase, this will disfavor the extraction of the lipophilic (fat loving) cobalt complex [Co((RO)2PO2)2 ((RO)2PO2H)2]

This will make the equation which relates the distribution ratio closer to

DCo = k[{(RO)2PO2H}2]0[(RO)2PO2-]2 = k[(RO)2PO2-]2

As the concentration of (RO)2PO2- depends on the proton concentration we can make the equation slightly more complex and more useful.

As

Ka = [(RO)2PO2-][H+]/[(RO)2PO2H]

Then

Ka / [H+] = [(RO)2PO2-]/[(RO)2PO2H]

So

Ka [(RO)2PO2H] / [H+] = [(RO)2PO2-]

So next

DCo = k{[(RO)2PO2H]}2 ka2 / [H+]2

So

DCo = k{[(RO)2PO2H]}2 ka2 [H+]-2

This equation does explain how to change the distribution ratio by altering the concentration of acid, by making a change of pH we can adjust our D value. Now as my physics teacher told me we should use dimensional analysis to decide if the equation is good or bad.

Now DCo = [Co]organic / [Co]aqueous

So the D value has no units

So the k value has the units of mol-2 dm6 as [(RO)2PO2-] has the unit of mol dm-3

OK what next.

Ka has the units of mol dm-3

So we have

mol-2 dm6 . mol2 dm-6 . mol2 dm-6 . mol-2 dm6 = mol-4 dm12 . mol4 dm-12 = mol0 dm0

Which finally confirms that the equation

DCo = k{[(RO)2PO2H]}2 ka2 [H+]-2

Is a viable equation which is at least mathematically correct.

At this point I think we can call it a day and have another solvent extraction lesson another day.


 

[i] Liang-Fang Huang, Chang-Chun Ji, Zhen-Zhong Lu, Xiao-Qiang Yao, Jin-Song Hu, He-Gen Zheng, Dalton Trans, 2011, 40, 3183.

Elk River spill III

Dear Reader,

What is safe ?

This is a good question for us to ask. Recently in the US a coal washing plant accidentally spilled a large volume of 4-methyl-1-hydroxymethyl cyclohexane into the Elk river, now while I do not like water which is tainted with something I have to ask how safe / dangerous is it.

I was interested to see that a Midwife (Lesley Rathbun) in the US has been quoted as saying “Nobody knows what this chemical is or what it can do, or how much is not good. Pregnant women have all sorts of things they can be worried about when pregnant, and they have to have some sort of trust in the state and federal agencies.”

I have emailed her to ask why she is concerned about this chemical to see if there is something about it which causes her to be concerned about it, and I am awaiting her reply.

Lets look at this short quote and take it apart and examine it.

  • The statement that “Nobody knows what this chemical is”

This is a statement which can be understood in several ways, if we use super formal English then this is a statement which suggests that no person on this earth (above it or below it) knows what the chemical is that was involved in the accident. I think that this would be an unreasonable statement to make as it is already known what the identity is of the chemical which has been spilled is.

An alternative reading of the text is that “none of the general public have heard of the chemical involved in the spill”, I can quite believe this statement. Before the Elk river spill I have to confess that I had never heard of or considered 4-methyl-1-hydroxymethyl cyclohexane. I have less of an excuse than the typical member of the public as I have two degrees in chemistry and I am a docent in organic chemistry, but it is impossible even for docents to be able to have considered every possible molecule in the big wide world.

Well we can have a go at educating the public as to what this molecule is, so maybe we can do something about this first issue.

  • “or what it can do, or how much is not good.”

This is a great question which is caused by a lack of data, I have looked quickly at the molecule in question and the aldehyde / carboxylic acid which can be formed by oxidation of the alcohol group in the body. None of these compounds have a structural feature which screams out at me “toxic”, and a quick search of the literature suggests a general lack of knowledge about the substance. I would say that for an industrial product to have been used for some years and not attract the attention of the biochemists it is likely to be a substance with little if any biological activity.

If it was an acute poison then I would expect it to have been noticed as a result of an accident, the chances are that people have been splashed with the chemical and it has been fed to rodents, in rodents the substance does not seem to be very toxic and as no reports exist of human poisonings then I think we can conclude that it is not a quick acting poison.

I would also say that as no large scale fish kills have occurred in the river it does suggest that the chemical is not toxic.

Longer term exposure is harder to deal with, also the problem exists that in todays society routine toxicology testing on animals is not socially acceptable. I also feel unhappy about the idea of doing animal experiments with every chemical in existence just to obtain the data. To my mind the work cannot be justified, the use of animals in research must be restricted to experiments where it can be argued that a clear need exists for the data and that no non animal alternative exists.

Now as I can not find any sign of the CDC work on the substance which suggests that it is “safe” then I can not make a judgement of how good the work is, but right now I can see a case for testing the chemical with cell lines or with bacteria / yeasts in things like the Ames test. Looking at the compound I think that it will not be a mutagen according to the Ames test, even if you include the mashed up rats liver which will give you metabolic activation of some carcinogens. Some carcinogens are in reality precarcinogens, they need to be activated by the body to form the thing which induces cancer.

While the Ames test is not perfect (some false positives and false negatives do occur) it is a good test for carcinogens which act by damaging the DNA. The majority of carcinogens act by harming DNA thus making it a good screening test.

I would also say that maybe some testing with pregnant fish could be justified to find out if the substance which humans have now been exposed to at trace levels is able to pose a reproductive threat. I would imagine that the Zebra fish would be used for such an experiment. The fact that the substance does not appear to kill adult fish will make it slightly more easy to test it for reproductive effects in fish.

  • “Pregnant women have all sorts of things they can be worried about when pregnant”

This is a rather grey statement, a woman can worry about all manner of things while pregnant. Some of these things are perfectly valid things to worry about, I hold the view that a woman who is pregnant who needs radiotherapy for cancer of the cervix should consider the real threat to her baby much more than the woman who is concerned that laughing at a joke is going to cause her baby to change into an alien. Both are things which a woman could worry about but I have chosen two examples from opposite ends of the spectrum of how serious a threat they are to a fetus.

  • “and they have to have some sort of trust in the state and federal agencies.”

I hold the view that both the state and federal agencies should exist to serve the needs of the citizen, but serving the needs of the citizen does not mean that the agency should attempt to ban everything in an attempt to make the world a safer place. While some substances and activities should be restricted or even banned, a ban should not be imposed without good evidence or at least solid reasoning.

The idea of banning a chemical because it has a funny smell is not reasonable, I would say that no relationship exists between how offensive the smell is and how harmful a substance is. I can think of plenty of nasty things which either have nice pleasant smells (or no smell) and plenty of chemical sheep in wolves clothing which stink to high heaven but are quite harmless.

I also think that if the state starts to suffer from banamania (mania for banning things) then the citizen will suffer as medical products will be impossible to obtain and also employment is impossible to obtain as every job requires some chemicals. For example a school teacher needs a stick of calcium sulphate (blackboard chalk) which they use to write on a slate board (old fashioned blackboard). While you can replace the blackboard with some modern electronic white board smart board gadget this will require a far wider and more complex array of chemicals to make it and to operate it. So if we ban all “chemicals” then we will not even be able to write on the blackboard.

Furthermore the citizen will suffer under banamania when many household products are outlawed, for example in Sweden we make great use of glass fibre to keep our homes warm. Without the glass fibre the house would be much harder to heat and also if we take the glass from our windows things are going to get rather disagreeable in winter. Part of me would like to challenge anyone who has banamania to sleep naked in my garden during the worst part of a Swedish winter without a sleeping bag, or maybe I should not as I do not want to have to deal with the paperwork with the local police which is required when frozen dead bodies are found on the lawn.

Trust me unless you have a very thick sleeping bag (made from “chemicals” you will die if you try sleeping outside in -20 oC winter.

So I hope that my reader can understand that banamania is not good for society, on the other hand a refusal or failure to ban some substances is equally bad for society. When I was a boy, I once saw a toy which would make most chemical safety inspectors very upset. It was a mercury maze, a plastic maze with a drop of mercury inside it.

Now regardless of what you might think about mercury, I strongly hold the view that toys containing mercury in this form should be illegal. Another household item I think deserves a ban is the very old home science kit which contained 106Ru and other radioactive sources.

Also it is important that the regulator does not serve the wants and desires of industry at the expense of the needs of the citizen.

The Elk river spill II

Dear Reader,

When I looked at the case of the Elk river spill I considered the question of where does the 4-methyl 1-hydroxymethyl cyclohexane come from, it is not as if the aliens land in the car park and hand over drums of the chemical to the humans (who are glad to get it). Instead it is made by the chemical industry, one route would be from the reduction of dimethyl terephthalate using hydrogen, while in the lab chemists like to use expensive reagents such as lithium aluminium hydride in cheap glass flasks in the bulk chemical industry they like to use cheap reagents such as hydrogen in expensive metal reactors.

S. Richard Turner, Journal of Polymer Science Part A-Polymer Chemistry, 2004, volume 42, page 5847 explains how dimethyl terephthalate is hydrogenated using a palladium cataylst to make dimethyl 1,4-cyclohexanedicarboxylate which is then reduced using a copper / chromium catalyst to make 1,4-cyclohexanedimethanol. This type of catalyst is reported to have some ability to do a hydrodeoxygenation reaction on an alcohol. But the Cu/Cr system is more well known for the hydrogenation of carbonyls.

I suspect that if some acid is present then the 1,4-cyclohexanedimethanol will dehydrate to form an unsaturated monoalcohol which can be hydrogenated to form 4-methyl 1-hydroxymethyl cyclohexane.

The Elk river spill

Dear Reader,

It has come to my attention that a coal treatment plant in the USA has accidentally spilled a large volume of 4-methyl-1-(hydroxymethyl)cyclohexane into the Elk river. This chemical (also known as 4-methylcyclohexane methanol) is used in a froth floatation process to remove unwanted minerals from coal with is intended for use in the production of coke for use in the metal extraction industry.

Others have already commented on this event, but I would like to warn my reader to keep their wits about them when they read a newspaper, press release or even a blog. The blog which I linked to at the start of this paragraph is written by someone from Greenpeace, which is well known for its dislike of the use of coal. I imagine that a person writing a blog who works in the coal industry would have a different view of the same event and would write about it in a different way.

The Greenpeace blogger comments that 4-methylcyclohexane methanol is toxic, but looking at the compound it does not have any features which would make me view it as a strong poison. I would not want to drink it, but compared to a real nasty like sarin or vinyl chloride it does not look so bad. I do not think that either Acros or Aldrich sell it so it is a little harder to get hold of a MSDS for this chemical, I found one online and it does suggest that in mice the LD50 dose for an acute oral intake is more than 2 grams per kilo of bodyweight. This suggests that it is not a strong acute poison. I made a quick web of science search for “methylcyclohexane methanol” and the words “toxic” or “poison” and I got no hits suggesting that little if anything has been written about its biological effects.

A Reaxys search on the compound also suggests that little has been written about this substance in terms of biochemistry. I would like to suggest that the people in the USA where the tap water has been affected should pay attention to the advice given out by the public health authority in their state, I know in some areas that people have been advised to stop using tap water for some purposes. As I do not know the full facts of the case I can not offer advice regarding the safety of tap water.

I would advise people to be careful when reading a MSDS sheet, bear in mind that the MSDS is often written for a worst case exposure. If we consider the toxin in chilli peppers (Capsaicin), when you look this up at Sigma-Aldrich you are warned that this substance is Toxic if swallowed (R25), Irritating to the respiratory system (R37), Irritating to skin (R38), poses a threat of serious damage to eyes (R41), may cause sensitization by inhalation or skin contact (R42/R43). Sounds like a real terror of a chemical.

While the pure toxin in powder from from the chemical company is a real nasty, I would say that at the concentration it is found in the typical chilli pepper you can get eye and skin irritation but it will be unlikely that a normal person using chilli peppers in the kitchen will be able to experience the worst of the effects such as a serious lasting eye injury or a fatal poisoning. However I am aware of a fatal case of capsaicin poisoning where a infant was fed a infusion of a chilli powder, so do not either eat chilli powder or feed it to your children. For more details see T. Snyman et. al., FORENSIC SCIENCE INTERNATIONAL, 2001, Volume:124, Issue:1, Pages:43-46.

I am also aware that people who are occupationaly exposed to chilli powder can start to experience adverse health effects, the thing to keep in mind is that as Paracelsus commented “All things are poison, and nothing is without poison; only the dose permits something not to be poisonous” which is often summed up as “The dose makes the poison”. Bear in mind that many “harmless” household substances (such as drinking water, sugar and salt) are harmful when consumed in vast amounts. For example a teenaged girl in the USA recently died from drinking too much water while playing “water poker” while in my home town a man fatally poisoned himself with carrot juice. While a moderate amount of water or carrots is healthy these vast doses of “healthy” substances proved to be fatal.

Benchcoat

Dear Reader,

I keep meaning to get around to showing the world which way up benchcoat should go, for those of you who do not know it is a simple but important matter when working with open radioactive sources. All fumehoods and trays should have a water proof layer at the bottom which has an absorbent side upwards. The idea is that any droplets spilled will be absorbed into the absorbent layer and will not be able to pass downwards.

One way of doing this is to wrap a plastic tray in a plastic bag, then you place in the tray a pad of paper towel like matter. If you spill liquid then it will be rapidly absorbed and will not be mobile. If you leave out the plastic bag then you can contaminate your plastic tray, while if you leave out the paper towel then the contamination can spread when the droplet whizzes around like an ice skater.

Now when I get the chance I will take a postage stamp sized bit of bench coat and I will put 20 or 50 microlitre drops of a blue dye on it, I will repeat the process with a stamp of bench coat the wrong way up. I intend to photograph the event to let you see what happens.

I will warn you that while bench coat might be great for radioactivity work it is not universally good, I would like to point out that while it is a anti-contamination feature it is a pro-fire thing. I have fought petrol fires in chemistry labs, and I have always been lucky that I was fighting a fire which was a pool of fuel on a hard and non flammable surface (tiled floors of fume hoods). I hold the view that a petrol fire on top of a pile of paper or in long dry grass would be a real snake in the grass and a pain in the neck. If you thought I was going to swear in my blog then I have to disappoint you !

The tap and he moves forwards

Dear Reader,

Since seeing the chainlink fence in the defunct Ågesta nuclear power plant I have been experimenting with flash photography of water jets passing through mesh, here is a photo I took today of the kitchen tap showing the bubbles of air formed by the passage of the water through the mesh in the tap head.

kitchen tap

I hope to get the garden hose out soon and to take some good quailty photos with and without a sieve present.

By the way my son Jacob worked out today how to crawl forwards, he had been moving backwards for weeks but now he can go forwards.

Barium and DTPA

Dear Reader,

Within my group a student will start soon on a project on the recycling of fireworks, now we have to be careful with fun sounding projects. While it is possible to enjoy doing chemistry which relates to fun topics it is important to make sure that the academic quality remains high. I would like to warn people working in fashionable areas of science and technology that for their own good that they need to avoid dumbing down their science and also avoid trying to hide poor quality work behind a fashionable label. But I will save this for another day.

Barium is commonly used in fireworks, a range of different barium compounds (barium nitrate, barium carbonate, barium oxalate and barium sulphate) are all used for different purposes in fireworks.

Today because my mind is fixed on barium we should consider some of the barium chemistry which occurs in oil fields. In an oil field the formation of barium sulphate is a major pain in the neck. Often the water in an oil field is very rich in barium but has little if any sulphate in it, while sea water is very sulphate rich. So if sea water is injected into an oil field to displace and push out the oil / gas then it is possible to form insoluble barium sulphate in the pores and cracks in the rock. This then glues up the oil field and slows down the extraction of oil or even gums up the well to the point that no more production is possible.

The great problem is that barium sulphate has a very low solubility in water, so it is not possible to rinse out the barium sulphate using reasonable amounts of water. For a poorly soluble 1:1 salt such as barium sulphate in water at a given temperature the following equation holds true.

Ksp = [Ba2+][SO42-]

When the water contains no other sources of barium or sulphate then we can write.

Ksp = [Ba2+]2

Now that was fun, now we should consider a bug bear of the oil and gas industry which is barium’s heavier sister which is radium. Now radium and barium sulfates are thought to form solid solutions (solids where radium and barium atoms are randomly replaced by each other) and their solubility constants (Ksp) are similar.

Now if we treat radium as a radioactive version of barium and assume that the properties of the two elements are identical then we can write

Ksp = ([Ba2+]+[Ra2+])[SO42-]

Now this is a great simplification of the system and I recall that the real world is a little more complex. The reason why the radium is a bug bear is that the formation of the barium sulphate scale on pipes and other equipment tends to deposit radium onto the same surfaces thus contaminating the oil field equipment. This then makes maintenance work more difficult to do.

One method of making the maintenance work more easy and also cleaning out the oil well is to use a barium sulphate scale dissolving mixture. One common method is to use DTPA, this is Diethylene Triamine Penta Acetic acid. This is a larger version of EDTA which is able to form water soluble barium complexes from the insoluble sulphate.

While I have searched the crystallographic literature for barium complexes of DTPA and I found none, but I did find a dinuclear EDTA complex of barium. It is important to note that barium is larger than calcium which is the classic metal for binding to EDTA. While calcium fits nicely inside an EDTA the cavity is too small for a barium ion.

The barium tends to spill out of the EDTA ligand’s cavity and then an oxygen from the second EDTA binds to it through an oxygen which bridges between the barium atoms. The complex contains two bariums and two EDTAs.

See G.G.Sadikov, A.S.Antsyshkina, V.S.Sergienko and A.L.Poznyak, Zh.Neorg.Khim., 2002, 47, 54 for details.

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