Boffis
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Microchemical Tests and Reagents
Is there anyone out there interested in microchemistry as applied to semi-quantitative and purely qualitative analysis? This branch of chemistry has
been virtually eclipsed by modern instrumental techniques but it still fascinates me and there are limitless opportunities to pursue new avenues.
I got interested in this branch of chemistry after reading a US Bureau of Mines publication (link below) which looked at the use of fairly simple
microchemical tests to help identify opaque minerals under the microscope while trying to find ways to help me identify minerals I had collected.
Because this publication deals only with opaque minerals it deals mainly with a limited range of transition metals, semi-metals and their oxides,
sulphide, selenides and telluride plus titanium and vanadium oxides. This work led me to more general works on both inorganic and organic
microchemistry and spot tests such as the works of Feigl and Angers, Feigl alone, Chamot and Mason, Vogel etc. and many individual papers, an IUPAC
report and even an individual's web site.
For anyone interested in the field some worthwhile reading:
Qualitative Inorganic Analysis; A.I. Vogel and G. Svehla
Spot Tests in Inorganic Analysis; F. Feigl, V. Angers & R.E. Oesper
Spot Tests in Organic Analysis; F. Feigl
Handbook of Chemical Microscopy; E.M. Chamot & C.W. Mason (2 vols)
USBM publication No825 by N. M. Short (Can be download free, now from the USGS web site since the demise of the USBM) https://pubs.er.usgs.gov/publication/b825
Website; Microchemical Tests by Jesse Crawford accessed via Mindat.org https://www.mindat.org/article.php/545/Microchemical+Tests
Reagents and Reaction for Qualitative Inorganic Analysis; IUPAC report 5th edition; Ed R Belcher (downloadable from the internet) https://www.iupac.org/publications/pac/8/1/0001/index.html
The purpose of this post is an attempt to persuade closet or would-be microchemists out into the open to share their experiences and knowledge about
the preparation of reagents, their application and limitations of microchemical (ie under the microscope) reactions, spot tests, paper chromatography,
TLC and related techniques; both inorganic and organic. Interestingly there are numerous scattered threads related to specific tests for say cyanide
and I have posted a few for specific compounds while investigating organic reaction. Solo once posted a series of links to spot tests used for the
identification of street drugs and this, not too surprisingly, is probably one of the few points of contact most people will have had with
microchemistry and there is a considerable amount of data on these tests online.
Thee have been some important recent developments in microchemistry with the development of hormone and pregnancy "dipstick" tests and similar tests
for specific ailments but these are generally beyond the amateur as they use such things as protein specific enzymes prepared from GM microbes etc
As an example of the work still to be done; I read an obscure article about a substance called violuric acid many years ago that described this creamy
white crystalline compound and its brilliantly coloured crystalline salts with alkali and alkaline Earth metals. Attempts to identify a synthesis were
frustrated by the age of the references and the difficulty in obtaining them (most are now easily available on line), however, one reference gave a
series of synonyms including 5-nitrosobarbituric acid and this suggested ways to prepare it. Subsequent experiments with barbituric acid and either
aqueous sodium nitrite and hydrochloric acid or butyl nitrite and gaseous HCl both resulted in the same product but the former was much simpler.
Initial results with metal cations produced spectacular result under the microscope with strontium, barium, calcium, zinc etc. Though some of the
reactions were strange, sometime two different products formed with a single cation, one much less characteristic than the other. Set against this
disadvantage was the fact that under the microscope it is often possible to discern up to 4 different element in a mixture as many of the salts did
not form solid solutions (mixed crystals). The compounds have just about the right solubility to precipitate slowly as large crystals. A few elements
(Fe and Co) form intensely coloured but highly soluble complex that do not precipitate but are easily discernible in mixture. This reagent has proved
very useful in characterising natural manganese oxides which can often exceed 5% Co, Ni, Cu, Ba, K, Na, Li, Fe, Tl, Sr etc and while this does not
give absolute identifications to minerals of this group it certainly shortens the list of possibilities. Secondary uranium minerals are another case
in point violuric acid giving reaction with uranyl ions and those of K, Na, Ca etc simultaneously, this test and a few others can rapidly reduce the
list of possible minerals from 100+ to just a few. Work continues with this reagent and several related compounds described by Hantzsh et al.,
(Berichte., 1909 p986.). This reagent has a problem though, the test fails in the presence of strong mineral acid. The normal response is to add a
buffer such as sodium acetate, unfortunately all of the salts, including organic types, interfere with the test as they form brightly coloured
crystals too. My current avenue of research with this reagent is the use of OH form anion exchange resins to mop up the H+ ions.
There is a whole world of interesting reaction that can be carried under the microscope, these are extensively discussed in the USBM report and the
book of Chamot and Mason listed above. Many of them use simple reagents such KI, CsCl, alkali thiocyanates and oxalic acid. Apart from being useful
tests for specific ions the explanation of the chemistry is often highly insightful and instructive; Fiegl's works are often the best for these
explanations.
Carrying out reactions under the microscope often allows observation that cannot seen in bulk such as the homogeneity of precipitates. I routinely
examine the products of reactions under the microscope. Then there are techniques like paper chromatography and TLC, either with traditional media or
with ionic exchange resin or inorganic zeolitic, surface modified media etc for the separation and identification of both organic and inorganic
compounds.
Then there is the acquisition of the specialist reagent. As this branch of chemistry has fallen into a backwater many of the special microchemical
reagents have become difficult to obtain or disappeared altogether. On the other hand many are fairly simple to prepare in a basic laboratory but
often finding the published reference can be difficult particularly as the preparation of the compound may have occurred many years before the
description of its microchemical use. Microchemistry in the period from 1930 to 1970 was often hived off into very obscure journals that often do not
appear to have been digitized and finding a reference to a preparation which is often pre 1900 and in German can be frustrating, so once one member
has found such a reference this forum would be a place to share it.
A search on the internet today reveals that most of the current interest in microchemistry and spot tests is aimed at testing of or for illicit drugs:
A sad reflection of modern society or a good reason to keep alive an almost forgotten branch of chemistry?
I would like the aim of this thread to be to disseminate knowledge of the preparation and use of microchemical reagents, source papers and books and
methods.
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AJKOER
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Here are some definitions of this field:
From Merriam-Webster Dictionary:
"chemistry dealing with the manipulation of very small quantities for purposes of preparation, characterization, or analysis.
microchemistry"
From Collins English Dictionary:
"Microchemistry definition: chemical experimentation with minute quantities of material"
From The American Heritage® Stedman's Medical Dictionary:
"microchemistry mi·cro·chem·is·try (mī'krō-kěm'ĭ-strē) n. Chemistry that deals with minute quantities of materials, frequently less than one
milligram in mass or one milliliter in volume. "
From Webster Dictionary:
"Micro-chemistry(noun). the application of chemical tests to minute objects or portions of matter, magnified by the use of the microscopy; --
distinguished from macro-chemistry."
So, I have examined copper/aqueous ammonia reactions in the presence of O2 or H2O2. Such reactions can quickly produce colored solutions containing
very small amounts of copper-ammonium complexes. So working with such dilute solutions (of under one milliliter) may be considered to be
microchemistry per the first three definitions, but not the last which is restrictive to tests and may employ a microscope. But, as one can use/test
micro quantities of copper ions to kill microbes, which may be visible employing microscopy, this could meet the conditions of the last definition.
Opinions? Have I been engaged in microchemistry?
[Edited on 22-2-2018 by AJKOER]
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woelen
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The idea of qualitative (micro)chemistry may probably be considered part of a wider domain of chemistry, which I would call descriptive chemistry.
I have some pre world war II books, which describe a lot of chemistry of many many compounds. Some of this is used for qualitative analytical
chemistry, some of this is useful for synthetic purposes, but most of it is chemical curiosa, sometimes with really obscure and highly unknown
reactions.
Some examples:
Who knows of the yellow [I(SO2)n](-) complexes, with n being 2, 3, or 4? Just dissolve some sulfite in excess dilute HCl or dilute H2SO4 and add some
KI or NaI to it.
Who knows about the mysterious reaction between thiocyanate ion and nitrite ion at low pH. Just try it and be surprised.
Another example is the brown silver(III) complex in nitric acid which can be formed by adding some silver nitrate and some sodium peroxodisulfate to
50...60% nitric acid.
Then we have the nowadays considered obscure trichromates and tetrachromates. Something better known in the good old days before world war II.
This kind of practical chemistry unfortunately is much less performed nowadays. It is this kind of (sometimes really obscure) reactions which raised
my interest in the subject.
The qualitative analysis techniques and schemes, developed in the first half of the 20th century also are little gems. They contain a lot of
information on specific precipitates, complexes and acid-base reactions. It is really remarkable how sensitive some tests are and what can be achieved
with simple aqueous chemistry.
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byko3y
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Houben-Weyl methods of organic chemistry. Volume II (1953) "Analytical methods".
| Quote: | | Who knows of the yellow [I(SO2)n](-) complexes, with n being 2, 3, or 4? Just dissolve some sulfite in excess dilute HCl or dilute H2SO4 and add some
KI or NaI to it. |
Sounds like elemental iodine to me.
| Quote: | | Who knows about the mysterious reaction between thiocyanate ion and nitrite ion at low pH |
Thiocyanate is
used to detect oxidizing agents, forming yellow product on positive reaction. Dunno whether it's the case.
upd: the reaction is actually well-studied, product is a red nitrosyl thiocyanate ONSCN: Kinetics and equilibria in the nitric acid–nitrous acid–sodium thiocyanate system.
| Quote: | | Then we have the nowadays considered obscure trichromates and tetrachromates |
Most likely a mixture.
[Edited on 22-2-2018 by byko3y]
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Boffis
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Actually I was thinking more along the lines of small scale chemistry as a means of investigating compounds, minerals etc. and the techniques of
manipulation at this scale.
Take the simple dimethylglyoxime test for nickel for example. Most chemists know you can do this reaction in a test-tube and get a red precipitate, if
Fe2+ is present you get a temporary red solution while if you have Fe3+ present you get a brown precipitate; both these reaction can obscure small
amounts of nickel. However, if you carry this reaction out on a drop on a microscope slide and render it alkaline with ammonia, all the iron
precipitates. If a drop of clean water is merged with the original drop to one side and a few grains of solid DMG or better its sodium salt added to
the water drop the concentration gradient set up causes beautiful crimson needles of the NiDMG complex to grow that are easily visible under the
microscope even within the FeOOH ppt. Interestingly this test can be used under the microscope to detect copper too in the presence of nickel.
But has anyone ever investigated any other related compound like 1,2-cyclohexadione dioxime, 1,2-cycloheptadione dioxime, furildioxime,
2,2'-pyridildioxime, camphodione dioxime and dimethylglyoxalmonoxime-semicarbazone etc. and how are these compounds prepared?
[Edited on 23-2-2018 by Boffis]
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woelen
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| Quote: | | Sounds like elemental iodine to me. |
Initially I also thought that to be the case, but further investigation made absolutely clear it is no iodine. Iodine in fact is reduced by sulfite or
SO2 quantitatively in aqueous solution and none of the other typical tests for iodine are positive (e.g. starch test, extraction in organic solvent).
With the help of someone else from a university with equipment who did spectral analysis and the finding of an old paper from the 1960's this yellow
compound could be identified. It is mentioned in an old book of Holleman and it can be isolated when KI is dissolved in liquid SO2. I, however, could
not do that, I only have access to aqueous solutions.
Yep, indeed it is this compound. But it is absolutely not well-known. No common textbook mentions this and you have to dig deep to find a paper like
the one you mention. And still, there are some riddle-like things around this reaction. White fumes appear from concentrated solutions, what are
these?
| Quote: | | Quote: | | Then we have the nowadays considered obscure trichromates and tetrachromates |
Most likely a mixture.
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Definitely not a mixture. These are well-determined compounds. I myself isolated (NH4)2Cr3O10 and K2Cr3O10. The tetrachromates are more difficult to
isolate, but it should be possible for the well-equipped home chemist.
All of these things you can find on mu web site with different experiments and pictures.
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byko3y
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You are correct, halosulfites are actually existing and are know for a long time. Fluorosulfite was the first one discovered. There are also
chlorosulfite and bromosulfite:
Ab initio studies on halosulfite ions
Structure of the SO2F- Anion, a Problem Case
Particulary about iodosulfite:
(a) M. R. Snow and J. A. Ibers, Inorg. Chem., 1973, 12, 224–229; (b) P. G. Eller, G. J. Kubas and R. R. Ryan, Inorg. Chem., 1977, 16, 2454–2462;
(c) P. G. Eller and G. J. Kubas, Inorg. Chem., 1978, 17, 894–897.
In strongly acidic conditions reverse reaction between hydrogen iodide and sulfur dioxide happens: SO2 + HI -> S + I2 + H2O. In those conditions
sulfur is further reduced to hydrogen sulfide.
| Quote: | | White fumes appear from concentrated solutions, what are these? |
Thiocyanic acid? Boiling point 146-147 C,
vapor pressure 4.7 mmHg at 25 C.
| Quote: | | These are well-determined compounds. I myself isolated (NH4)2Cr3O10 and K2Cr3O10. The tetrachromates are more difficult to isolate, but it should be
possible for the well-equipped home chemist. |
Okay, you are right, indeed it is known: The orthorhombic polymorph of diammonium trichromate(VI) decaoxide, α–(NH4)2Cr3O10
The reason why those reactions are scarcely know is because they have no real application. As you already mentioned, you cannot easily isolate the
iodosulfite, but there also exists some really obscure mad shit, which exists only in complex, like A new type of iodosulfite ion formulated as I2SO22−.
You might know about NaI-acetone adduct, but fewer know about NaI2-acetone adduct and even NaI3-acetone adduct, which are all crystalline compounds.
If you crystallize salts in form of hydrate, you might get different crystals depending on the proportions. For this reason sulfuric acid has a crazy
melting point-concentration graph.
But all this information can be found only in specialized sources built around particular compounds, there's not much reason to compile them into a
general book.
I agree that a lot of knowledge has been lost and is becoming forgotten, but that's a general trend caused by the mainstream organization of society.
What seems more important to me is that some usefull knowledge is lost as well. For example, my first messages on this board were about reviving an
old procedure of making anthranillic acid, which became forgotten about hundred years ago (last mentioning of the correct procedure in the literature
at 1937) https://www.sciencemadness.org/whisper/viewthread.php?tid=10...
I can provide you an example of reaction which I failed to revive, although I have some insights into it: nitroprusside from ferrocyanide and nitric
acid:
Inorganic Laboratory Preparations - Schlessiner - p. 105
Preparative Inorganic Chemistry (Brauer) - p. 1768 - Sodium Nitrosyl Cyanoferrate
If you try to repeat the procedure it's most likely you will fail because some details were lost by time. I abandoned the research because I bought
the nitroprusside.
[Edited on 23-2-2018 by byko3y]
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woelen
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This all is fascinating stuff. I agree with you that most of this chemistry has seen no practical application, and that certainly can be the reason of
loss of knowledge. I am fascinated by this kind of things, purely for academic reasons and for curiosity. So many strange things are possible with
just a few hands full of different elements. That is what fascinates me. What I really find a pity is that I have no real access to such old
documentation and papers. I once had access to a science library at a university, but now I don't have that access and Google is not really helpful
when it comes to finding really interesting info about this kind of unknown/forgotten chemistry. I really wish I had more of those old books. . .
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Boffis
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Sigh....
Gentlemen, while the points you have made above are interesting and some are relevant to microchemical analysis you have completely failed to explain
why they are relevant and in doing so you have totally derailed this thread.
Take byko3y's brief reference to nitroprussides; it could have contained information about the use of this reagent for detecting various anions or a
discussion of his attempt to prepare this very important microchemical reagent. It could have been a discussion of the derivatives prepared by
replacing the "nitro" ligand with ammonia or other ligands and the uses of these compounds in microchemistry/organic analysis, ... but it didn't. By
the way I have prepared sodium nitroprusside on numerous occasions using both the main techniques without problem, you just have to careful
particularly with the sodium nitrite route where hydrogen cyanide is evolved.
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byko3y
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Boffis, derailed? Okay, let's bring it back to topic: Iodide ion as a qualitative reagent. Detection of nitrite and sulfite.
| Quote: | | I have prepared sodium nitroprusside on numerous occasions using both the main techniques without problem, you just have to careful particularly with
the sodium nitrite route |
I was specifically mentioning another route, not the one using nitrite. I'm yet to
find a single person who can reproduce the nitric acid procedur, but I don't think somebody cares, despite the fact nitroprusside provides a wide
range of different qualitative reactions.
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Boffis
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There you go byko3y! I wasn't that hard after all was it? I am trying to keep his thread focus on the concept of mainly qualitative testing of tiny
mounts of material.
I have only tried the nitrite route to one but I have used the nitric acid-potassium ferrocyanide route at least three times and I do recall that the
key to success it to get just the right degree of concentration on the first evaporation so that the K nitrate crystallizes but leaves the Na
nitroprusside in solution. This is what I meant by care. The "nitro" group can be replaces by numerous other ligands to give complex cyanoferrate
derivatives that have all manner of microchemical uses. I had started collecting information on their preparation and use but this project is stalled
at present do to work pressures.
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Boffis
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Back in the late 70 and 80s a British chemical supplier called BDH supplied solutions of Diethylammonium diethylthiocarbamate as a substitute of
hydrogen sulphide under the name "Emdite". The solution is easily made by adding 1 Molar equ of carbon disulphide to 2 Molar Equ of diethylamine
solution.
The point is that the use of this solution required significant modifications to the classical scheme of qualitative analysis. I can't find anything
in the academic literature apart "....was also proposed as a substitute" without reference but I do recall that BDH published a pamphlet outlining its
use. Does anyone out there happen to have a copy of this pamphlet? I would be most grateful for a scan. Alternatively does anyone have a reference to
the original work? I have done a great deal of searching online but to no avail, I could only find one reference to Emdite and all of the papers on
analysis are about specific tests or measurements rather than a general qualitative scheme to replace the usual hydrogen sulphide method.
There are several general qualitative analytical schemes available that do not use hydrogen sulphide or alkali sulphide solutions but I would like to
investigate several of them and given the ease of preparation and the moderate stability of the solution in alkaline conditions this one attracted my
attention.
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byko3y
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Diethylditiocarbamate is not a substitute for sulfide, but instead a substitute for carbon disulfide or xanthate. Diethylthiocarbamate ( https://en.wikipedia.org/wiki/Thiocarbamate ) might be a substitute for sulfide, but it cannot be made from carbon disulfide, instead carbonyl
sulfide ( https://en.wikipedia.org/wiki/Carbonyl_sulfide ) shall be used:
KSCN + 2 H2SO4 + H2O → KHSO4 + NH4HSO4 + COS
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DJF90
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Thioacetamide is commonly touted as a safer and easily handled surrogate for sulfide precipitation. It undergoes hydrolysis under aqueous conditions
to generate hydrogen sulfide in situ.
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Boffis
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Sorry I accidently missed out the second "di" from diethyldithiocarbamate and it is prepared from carbon disulphide and diethylamine. PS byko3y you
missed the "h" out of Diethyldithiocarbamate; it seems like even the pernickety make mistakes.
If it were a SIMPLE substitute for sulphide ions in qualitative inorganic analysis it would not have been necessary to significantly change the
methodology. I does not function in this case as a sulphide substitute in the way that, say, thioacetamide does but as a complexing agent and
precipitant similar to thionalide but it doesn't precipitate quite the same group of metals as either hydrogen sulphide or thionalide and the
properties of the compounds that resulting compounds are interesting. Many are soluble in chloroform etc, some are acid stable others are not, some
are cyanide stable others are not. For this reason it would be nice to know BDH intended Emdite to be used. You can't work this out by looking at the
classical protocols for qualitative analysis. You either read up on it or try it out.
So byko3y, can I take it that you don't have anything constructive to add to this thread like a copy of the BDH pamphlet?
By the way the best route to carbonyl sulphide for me is via the hydrolysis of xanthates since the mine I work on uses them by the ton  .
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