Microchemical Testing

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CHEMICAL TESTS FOR SMALL SPECIMENS By Jesse Crawford

Introduction

This is a work in progress. The objective is to collect together the chemical tests that are useful for identifying minerals and that are also within the reach of the typical hobbyist with the typical hobbyist's budget. If anyone would like to suggest a test for the collection, please email me ([email protected]) with the details. Put "Mineral test" in the title so the message can get through my spam filter. Tests should be easy to perform, and should use materials that are reasonably easy to obtain.

The tests described here are intended for small samples. For most tests, a piece that's 1 or 2 millimeters across is adequate for at least 3 or 4 tests. That's about 5 to 20 milligrams.

Scientists have directed a lot of effort toward developing ways to make chemical tests on tiny samples using a microscope to interpret the results. Most of these tests have become obsolete in recent years, but they still offer useful and fairly low cost methods that amateur scientists can use to test minerals.

What follows is a description of some chemical tests that work reasonably well when scaled down to a size appropriate for testing tiny samples. The author has tried most, but not all, of the tests included. Not much detail is included about what positive or negative test results look like because it is assumed that the tests will be performed on both the unknown sample, and a sample that is known to contain the element being tested for. It's also a good idea to run the test on a blank sample known to not contain the element being tested for.

No test is perfect. Most of these tests offer a level of confidence probably no more than the 80 to 90 percent level, which is pretty good in a world as uncertain as this one. As I see it, it's the uncertainty that keeps things interesting. Remember, if you're not having fun, then you're not doing it right.

Chemists (at least old chemists) form the habit early in their careers of treating all chemicals as if they're dangerous.

BE SAFE! Respect the fact that chemicals can be hazardous. Scientists don't know all the ways that chemicals can injure people. You don't want to be the first to discover a new one. Don't let chemicals stay on your skin, and don't breathe them. If you can smell them, then you probably need to improve the ventilation.

Sample Preparation

After we have a sample to test, the next thing to do is dissolve it, or at least dissolve enough of it to test. Ideally, the objective is to dissolve as much as possible of the sample so that the result is a drop of clear liquid about 10 to 50 microliters in volume containing all of the ions of interest. (in other words, a small drop). Most of the time, it's not necessary to go for complete dissolution. Often there will be a part of the sample that remains as pulverized fragments or sometimes a gelatinous mass of silica. If the grinding of the specimen is done with a mortar and pestle, the acid can be added while the grinding is being done. Then touching the pestle to a slide makes a drop that's sufficient for a test.

Almost all samples are prepared by dissolving them in some kind of acid. The following is a list in the suggested order to use in trying to dissolve the sample. If nothing in the list attacks the sample, then that's a lot of information already. The list of minerals that are impervious to all acids is a comparatively short one. A table of solubilities of some minerals is included at the end of this paper. Acids should be full strength. When one is found that attacks the sample, the solution can be diluted with a drop of water before beginning the tests. Some of the tests need to be carried out in a neutral or basic environment. Ammonia is handy for neutralizing acids.

THESE ACIDS ARE DANGEROUS! Handle them carefully in a well ventilated environment. Don't breathe the fumes. Be especially careful with fluoride minerals. Hydrofluoric acid and sometimes elemental fluorine is evolved when fluorides are treated with some acids. It's very nasty stuff.

  • Water
  • Hydrochloric Acid
  • Nitric Acid
  • Sulfuric Acid
  • Aqua Regia (3 parts Hydrochloric 1 part Nitric) CAUTION! Chlorine is evolved from aqua regia.


Hydrofluoric acid, if it were less dangerous, would certainly belong on this list. It neatly solves the problem of dissolving silicate minerals by converting silicon to a gas, silicon tetrafluoride. With that goal in mind, there is an alternative to using a strong solution of hydrofluoric acid. Small samples of silicate minerals can be digested in platinum or teflon dishes with a mixture of sulfuric acid and calcium fluoride. Hydrofluoric acid is thereby generated ìin situî and immediately reacts with the silica in the mineral. The technique is not without dangers, but with proper precautions can be used when necessary. The hydrogen fluoride generated is still dangerous, and must be respected, but the risk is more manageable.

There is a method that can be employed to dissolve even the minerals that resist all the above acids. Heating the sample with a flux to a high temperature until it is thoroughly fused alters the composition of most minerals so that they can be dissolved in water or one of the above acids. The usual flux used is sodium or potassium carbonate, or for some minerals sodium or potassium bisulfate.

Fusing the mineral sample at red heat with a flux can induce almost any mineral to dissolve either in water or in one of the acids. These are extreme measures, and because they involve a lot more handling of the sample than simply treating it with acid it's usually good to start with a larger piece. 50 to 100 milligrams is good. Carbonate fusions can be carried out in a platinum crucible or piece of platinum foil, but bisulfate fusions should not be made on platinum, as the platinum will be attacked. Fusions with carbonate can be done in a small ceramic crucible, or on a block of charcoal, or a loop of platinum or nichrome wire using pretty much any small torch.

To do a carbonate fusion, start by grinding the sample as fine as possible. Add about twice the volume of dry sodium carbonate, and mix them. If you have a platinum crucible, then put in the sample mixed with flux, cover with a little more pure flux and support the crucible for heating. Begin heating the side of the crucible and as the mass begins to fuse, regulate the heat so as to avoid any loss of sample. The melt will evolve carbon dioxide and water vapor and possibly other gasses, and it will probably do a lot of bubbling. After the bubbles stop, raise the heat to redness and continue heating for 10 or 15 minutes, until it's thoroughly melted. Let everything cool down and add a few drops of nitric acid and a little water, and let it sit for a while. The melt will loosen and dissolve. Put the contents into a beaker, rinse the crucible with water, and add the washings to the beaker. Then set the beaker on a low source of heat so that the water and nitric acid can evaporate. It should not boil at any time. A double boiler arrangement is desirable for this phase of the operation. When the contents of the beaker are dry, add the minimum amount of water necessary to dissolve the soluble part. There may be an insoluble residue of silica. If the dried sample doesn't dissolve in water, one of the acids may be necessary. The fusion can also be done using a wire loop. Start with a hot loop, pick up as much sample and flux mixture as will stick to it, and fuse it. Then, touch the fused bead to the sample mixture to pick up a little more, and continue. Repeat the process until enough of the sample is fused.

The procedure for a bisulfate fusion is similar, but should be carried out in a porcelain crucible. It's messier, and the fumes are more toxic, so BE CAREFUL. During the bisulfate fusion, there's a lot of bubbling at first. After the bubbling stops, there comes a point where the melt solidifies, and a higher heat is needed to get it to fuse again. This is the point at which the generation of sulfur trioxide and other corrosive sulfur oxides begins, which is the objective of the procedure. If the melt is allowed to cool at this point, the process will not be complete. The heat should continue until the mass fuses again, and no further changes are in evidence. At this point, cool the melt, add a drop of concentrated sulfuric acid (carefully) and resume heating. This is repeated two times. Then the melt is cooled and removed from the crucible as above using a little sulfuric acid and water. Sulfuric acid gives off dense clouds of white fumes when it is heated to dryness. DON'T BREATHE ANY OF IT. This procedure is not for the faint hearted. It's noisy and hot and frightening and suitable only for a well ventilated garage or lab. Have a fire extinguisher close by and an escape route cleared in case of emergency. Other than that, it's kind of fun.

Vycor labware works well for fusions.

Sodium peroxide also makes a good flux. One author asserts that any mineral can be brought into solution by sodium peroxide fusion. Peroxide fusions are ordinarily carried out in a zirconium crucible.

Deciding What Tests to Perform

In deciding what tests to make, it's sometimes handy to remember that it can be just as valuable to know what isn't present in a sample as what is.

Once we know what will dissolve the sample, tables of the solubility of minerals can be consulted to help in selecting which further tests to undertake. Try to find a test that will split the list of possibilities in half. This has been called the half-split technique.

Whenever we read about a test, it usually starts out with a list of needed equipment and reagents, then a description of the procedure, and somewhere near the end will be a list of ions that interfere with the test. That's always the catch. There are very few tests that respond only to one element. Usually there's a list of them.

A lot of the difficulty with interfering ions can be sidestepped by careful selection of the sample. Picking a well formed crystal of the mineral of interest improves the chances that there won't be a lot of interfering ions. Naturally, those are always the prettiest crystals.

Materials for Spot Tests

  • A small mortar and pestle for grinding samples.
  • A box of microscope slides.
  • A box of cover slips.
  • A glass or plastic ring about 15 or 20 millimeters in diameter ( it must be smaller than the cover slips) and about 2 to 3 millimeters thick
  • A glass rod 1 to 2 millimeters in diameter.
  • A small bulb type pipet (eyedropper).

Reagents for Spot Tests

  • Acetic Acid (Glacial)
  • Acetylsalicycilic Acid (Aspirin)
  • Ethyl Alcohol
  • Aluminon 0.1 percent solution
  • Ammonium Acetate Solution 3N
  • Ammonium Chloride
  • Ammonium Hydroxide
  • Ammonium Molybdate
  • Ammonium Oxalate
  • Ammonium Phosphate (Dibasic)
  • Aniline hydrochloride
  • Barium Chloride
  • Cesium Chloride
  • Chloroplatinic Acid
  • Citric Acid
  • Curcumin
  • Dimethylglyoxime
  • Hydrogen Peroxide
  • Hydroquinone
  • Lead Acetate
  • Oxalic Acid or sodium or potassium oxalate
  • m-phenylenediamine hydrochloride or sulfate
  • Potassium Dichromate
  • Potassium Iodide
  • Potassium Mercuric Thiocyanate (This reagent is made by combining mercuric nitrate with potassium thiocyanate in molar proportions of 1 part mercuric nitrate to 4 parts potassium thiocyanate. Tabular and needle-like crystals separate easily from acidic aqueous solution).
  • Potassium Nitrite
  • Potassium Phosphate (Dibasic)
  • Potassium or Sodium Sulfite
  • Rubidium Chloride
  • Silica sand
  • Sodium Acetate
  • Sodium Chloride
  • Sodium Fluoride
  • Sodium Phosphate (Dibasic)
  • Silver Nitrate
  • Starch
  • Tartaric Acid
  • Thiourea
  • Uranyl Acetate

The following are spot tests that are carried out on microscope slides and viewed through the microscope. Some authors recommend coating the microscope slides with wax or some other hydrophobic material to make it easier to control the drops. Some manufacturers make microscope slides with small wells that prevent solutions from running off the slide or to use with the "hanging drop" method (to be described below). They're all good ideas, yet just a plain microscope slide works fine for most tests. It's also a good idea to have a piece of black paper and a piece of white paper handy to put under the slide for contrast when viewing crystalline precipitates.

Techniques

The most general method for carrying out tests is to place a drop of the solution of the sample on a slide and put a drop of a reagent solution near it. Then a thin glass rod is used to bring the two drops together. The entire process is observed under the microscope.

Another important technique that's used is the hanging drop method. It's used to trap gaseous reaction products that are evolved from the sample as it reacts with a test reagent. For this technique a glass or plastic ring supports a cover glass with a drop of reagent or water hanging from the underside. The hanging drop is positioned over the sample, so it's close but not touching.

It is occasionally desirable to separate a drop of a solution from solid material, such as a precipitate or the fragments that remain after grinding the sample. It's often possible to precipitate an interfering ion and then move the clear sample solution to another slide for further tests. To remove the iron, for example, from a drop of solution, the pH of the sample solution can be raised by adding a drop of ammonia. At high values of pH, iron forms a dark gelatinous precipitate. To separate the sample from the iron precipitate a small piece of filter paper, an eighth of an inch or so in diameter, is placed on the slide near the sample. Then a dropper tube with an opening a little smaller than the diameter of the paper is pressed against the paper. The bulb of the dropper should be squeezed so that a small amount of suction will be supplied when the bulb is released. The tip of the tube with the filter paper is slid across into the sample drop, and the pressure on the bulb is released. If the dropper tube is not pressing too hard on the filter paper, the fluid will be drawn up into it through the filter paper, and it can be picked up and moved to another slide. As described, this procedure for separating iron is not selective, and would also leave behind other elements that precipitate at high values of pH, notably aluminum. Something else is needed to separate iron and aluminum (See "Aluminon Test" below). It takes a little practice to get this technique just right, but it opens a lot of possibilities when mixtures of ions interfere with one another. It helps to roughen the end of the tip of the dropper tube with fine sandpaper to prevent it from slipping off the filter paper when sliding it along the glass.


It is sometimes necessary to protect a glass slide or cover slip from the action of hydrogen fluoride. Plastic slides can often be used in these situations, or the glass can be coated with a hydrophobic material. Smearing grease on the glass works, but it's difficult to get a uniform thickness, and the irregularity of the coating can interfere with visibility. It works well to keep on hand a thin solution of microscope grease dissolved in xylene for this purpose. A drop is spread easily over the slide, and the xylene evaporates quickly, leaving a thin film of grease that prevents the hydrogen fluoride from attacking the glass."

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