I’ve been thinking about microchemical test recently. Part of this is due to the job related need to reconcile a positive microchemical test for sulfate and the X-ray diffraction which detected calcium sulfate as well as other sulfates. I and the outside lab (the scientists) are happy with the agreement of results, but management (MBAs) appear to be unhappy.
“Why couldn’t you detect calcium sulfate and same us the cost of sending the sample out for additional analysis?” the MBAs ask.
Let’s see, I worked with a water extract, the x-ray people with the solid. Calcium sulfate has solubility of about 0.2%. The x-ray people need a solid crystalline material. I look for a characteristic reaction or crystal formation based on the reaction of dissociated sulfate. The X-ray people look for characteristic crystal plane spacing. Even if I know all the ions present in water solution, I don’t know the original combinations. This seems to be beyond the MBAs.
So I’ve been charged with finding a chemical test specific to calcium sulfate. Moreover, while I know isolated crystals can be identified with dispersion staining and a pol scope, my samples are never isolated crystals, but lumps of combined gunk. I suspect an IR microscope might be able to distinguish between calcium sulfate and some other sulfate salt, I also suspect the difference in IR spectra might be too subtle for this light microscopist.
While I haven’t made too much progress in this area, it has caused me to think about microchemical testing. Microchemical testing is a balance between solubility and time. Most microchemical tests depend on a reaction between two or more material to product a unique product with reproducible morphology.
If the solubility is too low the reaction product simply precipitates out as tiny indistinguishable particle. While these crystal may have unusual morphology, if they require examination with the TEM to determine their shape the utility of the test is poor. These some of the test have poorly shaped precipitates that can be re-crystallized and retested by a second procedure. The Silver nitrate screening test for halogen is one. The whitish precipitate we’re familiar with can be re-crystallized in NH4OH to produce well-behaved crystals. In principle, the crystals can be isolated, dried and based on refractive index separated. According to Winchell (Optical Properties if Artificial Minerals):
Material Refractive Index
AgI 2.20
AgCl 2.07
AgBr 2.25
It’s simple, mount the recovered crystals in 2.20 refractive liquid (don’t poison your self, I believe they still contain selenium and arsenic) and you can separate out these three.
Other tests require too much time to form characteristic crystals. The growth of well-formed crystals the instant before the droplet of test material, reagent, conditioners (yes-microchemical test use conditioners) goes to complete dryness may be of limited usefulness.
Still I do have a current favorite test. I don’t have a need for it very often, but I do like it. Chamot discusses a test for thiosulfates with lead acetate.
While not the easiest test due to complications of solubility, what I like are the striking images that are formed. The test initially produces a dense white precipitate. These types of precipitates are so common as to be almost ubiquitous.
(Formation of second crystal material in test solution of thiosulfate and lead. Forget about magnification value...I photographed it with a 10X objective…)
(Second crystal formed during test for thiosulfate with lead. Crossed polars showing birefringency colors.)
Ah, but if the test is allowed to sit in a relatively short time better behaved crystal start to grow at the expense of the white precipitate.
In truth I don’t know the chemistry. Is it a change in oxidation state of lead? Development of a more thermodynamically stable phase? If you know, drop me a line, but the new crystals appear in a clear pocket surrounded by the initial precipitate. I find the images striking.
The best think about microchemical chemical testing is it leaves the MBAs puzzled.
“Why couldn’t you detect calcium sulfate and same us the cost of sending the sample out for additional analysis?” the MBAs ask.
Let’s see, I worked with a water extract, the x-ray people with the solid. Calcium sulfate has solubility of about 0.2%. The x-ray people need a solid crystalline material. I look for a characteristic reaction or crystal formation based on the reaction of dissociated sulfate. The X-ray people look for characteristic crystal plane spacing. Even if I know all the ions present in water solution, I don’t know the original combinations. This seems to be beyond the MBAs.
So I’ve been charged with finding a chemical test specific to calcium sulfate. Moreover, while I know isolated crystals can be identified with dispersion staining and a pol scope, my samples are never isolated crystals, but lumps of combined gunk. I suspect an IR microscope might be able to distinguish between calcium sulfate and some other sulfate salt, I also suspect the difference in IR spectra might be too subtle for this light microscopist.
While I haven’t made too much progress in this area, it has caused me to think about microchemical testing. Microchemical testing is a balance between solubility and time. Most microchemical tests depend on a reaction between two or more material to product a unique product with reproducible morphology.
If the solubility is too low the reaction product simply precipitates out as tiny indistinguishable particle. While these crystal may have unusual morphology, if they require examination with the TEM to determine their shape the utility of the test is poor. These some of the test have poorly shaped precipitates that can be re-crystallized and retested by a second procedure. The Silver nitrate screening test for halogen is one. The whitish precipitate we’re familiar with can be re-crystallized in NH4OH to produce well-behaved crystals. In principle, the crystals can be isolated, dried and based on refractive index separated. According to Winchell (Optical Properties if Artificial Minerals):
Material Refractive Index
AgI 2.20
AgCl 2.07
AgBr 2.25
It’s simple, mount the recovered crystals in 2.20 refractive liquid (don’t poison your self, I believe they still contain selenium and arsenic) and you can separate out these three.
Other tests require too much time to form characteristic crystals. The growth of well-formed crystals the instant before the droplet of test material, reagent, conditioners (yes-microchemical test use conditioners) goes to complete dryness may be of limited usefulness.
Still I do have a current favorite test. I don’t have a need for it very often, but I do like it. Chamot discusses a test for thiosulfates with lead acetate.
While not the easiest test due to complications of solubility, what I like are the striking images that are formed. The test initially produces a dense white precipitate. These types of precipitates are so common as to be almost ubiquitous.
(Formation of second crystal material in test solution of thiosulfate and lead. Forget about magnification value...I photographed it with a 10X objective…)
(Second crystal formed during test for thiosulfate with lead. Crossed polars showing birefringency colors.)
Ah, but if the test is allowed to sit in a relatively short time better behaved crystal start to grow at the expense of the white precipitate.
In truth I don’t know the chemistry. Is it a change in oxidation state of lead? Development of a more thermodynamically stable phase? If you know, drop me a line, but the new crystals appear in a clear pocket surrounded by the initial precipitate. I find the images striking.
The best think about microchemical chemical testing is it leaves the MBAs puzzled.
posted by Searcher12 at
6:15 AM
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