IT IS NOT DIFFICULT FOR AN amateur to detect microgram quantities of metals in a variety of samples, as is done in several types of monitoring for pollution. Sam Epstein, a chemist at the Hyperion Treatment Plant in Los Angeles, has described to me a method that he calls chemical-spot testing. Although it requires no elaborate equipment, it can be employed to detect the presence of nearly any metal in samples of alloys, minerals, water and air.

As an example of the method, Epstein described how to detect copper, iron and nickel. The first step is to prepare test solutions of each metal. (Usually, of course, the experimenter does not have the benefit of knowing what is in the test sample.) The copper solution is prepared by dissolving copper sulfate (CuSO₄.5H₂O) in about 10 milliliters of distilled water containing one drop of concentrated hydrochloric acid. The acid is necessary to prevent the metal from forming insoluble hydroxides that would block its participation in the chemical reactions of the test procedure. The test solutions for iron and nickel are prepared in the same way with nickel sulfate (NiSO₄.6H₂O) and ferric chloride (FeCl₃).

A sample solution can also be prepared by dissolving .1 gram or less of a metal in 10 milliliters of a warm mixture made by adding 2.5 milliliters of hydrochloric acid to 7.5 milliliters of distilled water. (Always slowly add acid to water; never add water to acid. When you work with acids and bases, always wear safety goggles and work either in a well-ventilated area or under a chemical hood. Whenever water is called for in this article, distilled water is meant.) To the iron and copper solutions you should also add a few drops of 3 percent hydrogen peroxide (antiseptic grade). In the copper solution the peroxide reacts with the hydrochloric acid to form chlorine gas, which is necessary to dissolve copper in the acid. In the other solution it converts the iron into the ferric state that is required for the test.

To prepare a nickel solution a nickel plated object is dipped in the acid long enough to remove the thin nickel coating. A nickel-plated screw would do. One or two drops of peroxide might be necessary here too. A final step is to boil each solution gently to eliminate any remaining peroxide and chlorine.

Another set of solutions must be prepared to serve as the indicators for the metals. The indicator reagent for nickel is dimethylglyoxime (DMG), a solution of which is prepared by adding a gram of solid DMG to about 100 milliliters of warm rubbing alcohol. Shake the reagent well to ensure that the solution is saturated. The indicator for both copper and ferric iron is I percent potassium ferrocyanide (K₄Fe(CN)₆), which is made by dissolving one gram of the solid in 100 milliliters of water.

To demonstrate the detection of nickel Epstein puts one drop of the nickel solution on a piece of filter paper. Then he holds the paper over the mouth of an open bottle of concentrated ammonium hydroxide for about 15 seconds. This operation is commonly called fuming. Next he puts one drop of the DMG solution at the same place on the filter paper. After a few seconds a bright scarlet spot appears, revealing the presence of nickel on the paper.

The procedure for indicating the presence of copper or iron is similar. The paper is prepared by adding a drop of the copper or iron solution. Then a drop of ferrocyanide is added. The spot turns blue green if ferric iron is present and red brown with copper.

The testing can be done on a "spot plate" instead of paper. The plate is a flat slab of smooth porcelain with small circular depressions, in one of which a drop of the test solution is placed, followed by a drop of the indicator reagent. The colored compounds revealing the presence of the metals appear almost immediately.

If nickel is being tested for, a drop of 50 percent ammonium hydroxide is also added. The mixture is stirred with a rod of

glass or plastic. A drop of DMG is added and the mixture is stirred again. I the test solution contains nickel, nickel DMG, which is red, precipitates out, in dictating that the test solution has nickel among its contents.

Epstein demonstrates the sensitivity of the testing procedure by diluting the test solutions repeatedly. Each time he adds five milliliters of water to five milliliters of the test solution. After every third dilution he adds another drop of hydrochloric acid. The telltale colors, he says, are still detectable even after a number of dilutions.

A sample collected from the environment may contain several metals. Sometimes a single test is not conclusive because the indicator reagent might be able to indicate more than one metal. Ferrocyanide, for example, reveals both copper and iron. If both are in the sample, two colored compounds appear in the test, confusing the results.

Several techniques have been developed to overcome interference of this kind. For most metals several color forming reagents are available. By experimenting you may be able to isolate one reagent that reacts with only one of the metals in the solution. For example, copper can be detected in the presence of ferric iron if one of the indicator reagents for copper other than ferrocyanide is employed.

Another way (known as masking) to eliminate interference is to add to the test solution a chemical that will combine with one of the metals, preventing it from reacting with the indicator reagent. Only the other metal is left to react. With an unknown sample from the environment you would obviously have to experiment to hit on the correct reagent.

Sometimes the pH of the drop being tested can be altered to make possible the detection of two metals in a sample. For example, both nickel and palladium react with DMG. The nickel yields a scarlet compound, but only in a basic solution, and the palladium a yellow one, but only in an acid solution. If you think both metals are in the sample, put ammonium hydroxide in the sample to ensure that it is not acidic. When the DMG is added, a scarlet compound indicates the presence of nickel. Now add hydrochloric acid to the test drop. The scarlet color disappears and the palladium combines with the DMG to form a yellow compound.

A different procedure to eliminate the interference of colors makes use of a simple device called a ring oven. The procedure allows the detection of several metals simultaneously and also increases the sensitivity of the test. A flat metal plate with a hole in the center is put on an electric hot plate. Small drops of solution are added to a filter paper placed over the hole. The liquid spreads through the paper to the edge of the hole. There the solvent and other volatile compounds evaporate, leaving the dissolved solids in a thin ring. A wash solvent is added drop by drop to the paper to wash the test solution completely out to the ring. This detection scheme is more sensitive than the others I have described because all the dissolved materials are concentrated in the thin ring. When the filter paper is dried, it can be cut into pie-shaped segments that can be tested individually for different metals by the filter-paper technique.

Epstein's ring oven consists of three parts: an electric hot plate, an aluminum plate with a hole in it and a dropper guide by which he aligns a medicine dropper to deposit a drop of solution or reagent. The aluminum plate is put on the hot plate and a filter paper is placed over the hole. The aluminum plate has two brass screws by which Epstein positions the dropper guide over the paper. Liquid is deposited on the paper through a medicine dropper inserted in the dropper guide. Since the guide is fixed in position, successive drops land at the same spot on the paper.

The filter paper is five and a half or seven centimeters in diameter and is moderately retentive; the Whatman No. 40 filter is suitable. The hot plate is at a temperature of a few degrees above the boiling point of water. To collect a liquid sample Epstein dips the tip of the dropper into the liquid, allowing capillary action to move a small amount of it into the opening of the tip. Then he puts the dropper in the guide hole, which he has designed to make a snug fit for good alignment. He lowers the dropper until the liquid touches the filter paper; then he raises the dropper.

Although the amount of liquid deposited on the filter paper is small, it is enough. The wet spot on the paper is approximately a quarter inch in diameter. If the liquid spreads beyond the edge of the hole in the aluminum plate, discard the paper and try to make a smaller spot on another paper.

To move all the test solution out to the edge of the hole in the aluminum plate Epstein deposits drops of a weakly acidic or basic solution with another dropper. About 10 drops of this wash solution are needed. Again the liquid should not spread beyond the edge of the hole, otherwise uneven or multiple rings appear. The wash solution is prepared by adding 10 drops of concentrated acid or ammonium hydroxide to 100 milliliters of water. Epstein points out that you should use several medicine droppers to avoid contaminating the various solutions and reagents with one another.

As an example of how to work with the ring oven Epstein described to me his test for iron in the ferric chloride solution. One drop of the solution is applied to the filter paper. Then 10 drops of weak hydrochloric acid are added, drop by drop, to wash the ferric chloride out to the edge of the hole in the aluminum plate. After the first five drops the ring becomes visible. When the paper is dry, it is removed from the oven assembly. When Epstein puts a drop of ferrocyanide on the ring, the blue green color signifying the presence of ferric iron appears.

Epstein also explained how to do an analysis for two metals in a solution. He mixes some of the nickel solution with the copper solution and puts a drop of the mixture on a piece of filter paper in the ring oven. Hydrochloric acid is added to facilitate the movement of the material outward to form a ring. After the paper dries he cuts it in half. On one half he puts ferrocyanide to get the red brown color indicative of copper. He fumes the other half over ammonium hydroxide and adds a drop of DMG solution. The scarlet color characteristic of nickel appears.

The ring oven also overcomes the problem of interference when an indicator reacts with more than one metal. Epstein illustrates the possibility by mixing solutions of copper and ferric iron. A drop of the solution to be tested is put on the filter paper and washed outward by a weak acid solution to form a ring. This time, however, a pie-shaped segment is cut from the dried paper and fastened to half of a fresh filter paper. A tiny bit of Duco cement is needed to hold the tip of the wedge on the fresh filter so that enough contact is made to bring about the transfer of liquid between the two.

The combined filters are inserted into the ring oven with the contact area centered over the hole in the supporting aluminum plate. Ammonium hydroxide is applied drop by drop to wash the copper out into a new ring. The iron ring remains on the filter wedge because it consists of insoluble ferric hydroxide. Once the wedge dries it is removed from the oven and fumed over concentrated hydrochloric acid. A drop of ferrocyanide solution is deposited on each ring segment. The outer ring turns red brown to reveal the presence of copper and the inner ring turns blue green to reveal ferric iron.

Still another way of overcoming interference is to make one of the metals precipitate in the area where the test drop is placed. Then the other metal can be washed outward to form a ring and be detected. This technique will work for a mixture of copper, nickel and ferric iron. A drop of the mixture is put on the filter paper and ammonium hydroxide is added to wash material out to form a ring. The hydroxide, however, also reacts with the iron to form ferric hydroxide, which remains where the drop was placed. The copper and nickel are washed out to make rings because they are soluble in ammonium hydroxide. Test part of the ring for nickel by adding DMG (and fuming with ammonia if necessary). Test another part of the ring for copper by fuming with hydrochloric acid and then adding ferrocyanide. Finally detect the presence of iron at the center of the paper by fuming that section with the acid and then adding ferrocyanide. If several metals precipitate out in the center of the paper, you can continue the testing by cutting out the center and putting it on fresh filter paper. Add appropriate solvents drop by drop to wash the material out to form a new ring on the fresh filter.

Epstein described how one might analyze the metal in a coin such as a dime o a quarter, each of which is made out o an alloy of nickel and copper. File off tiny bit of metal from the edge of the coin and dissolve the scrapings in a mixture of hydrochloric acid and hydrogen peroxide. Pick up a drop of the solution and put it on a filter paper in the ring oven. Follow the procedure I have described to test for copper and nickel in the deposited drop.

If the coin is too valuable to damage, you can employ another method that is essentially the reverse of electroplating. The coin is made to act as the anode in the circuit shown in the illustration above. The filter paper, spotted with a 10 percent solution of an electrolyte such as sodium nitrate, is put on the aluminum plate. Against the spot of electrolyte place a part of the coin that is free of dirt and grease. Epstein suggests an edge. Touch the wire from the positive terminal of the battery to the coin allowing current to flow for about 10 seconds.

Practice might be necessary to regulate the time, the current and the pressure on the contact. If the procedure is successful, a small amount of dissolved metal is left in a depression on the paper. Put the paper in the ring oven, apply the appropriate wash solutions and analyze the rings with the proper reagents. The coin is not visibly altered.

To investigate a silver coin, which can be obtained from a coin dealer, file a bit of metal from the edge, add the filings to a few milliliters of 10 percent nitric acid and gently boil the acid to eliminate oxides of nitrogen. When the solution has cooled to room temperature, put a drop of it on a filter paper. Add a drop of I percent potassium chromate. The spot will turn red brown to reflect the presence of silver.

You could also use the electric circuit to deposit a small amount of the silver alloy directly on the filter paper. Older American silver coins consist of 90 percent silver and 10 percent copper. Both can be detected if the sample spot is run through the ring oven to create a ring. Use 1 percent nitric acid as the wash solution. Add potassium chromate to art of the ring to signal the presence of silver. Ferrocyanide, however, will fail reveal the copper. For a more sensitive test Epstein suggests using a saturated solution of rubeanic acid (dithiooxnide) in alcohol and a 20 percent solution of malonic acid. Add the malonic acid and then the rubeanic acid to part the ring. That part of the ring turns back if copper is present.

Samples of water can be tested for metals, including iron, copper and zinc. Epstein cautions that if the first trials in ring oven prove negative, you should concentrate the sample by boiling it in a Pyrex flask. Add a few drops of hydrochloric acid. Then run the tests again, using ferrocyanide for the iron test and malonic and rubeanic acids for the copper test.

Epstein's test for zinc requires two solutions. For one mercuric thiocyanate prepared by dissolving nine grams ammonium thiocyanate (NH₄SCN) and eight grams of mercuric chloride (HgCl₂) in 100 milliliters of water. (Good ventilation is essential.) The solution must then stand for two or three days. The other necessary mixture is a .02 percent solution of a cobalt salt in 4 percent hydrochloric acid. Epstein says that cobalt chloride (CoCl₂), cobalt sulfate (CoS0₄) and cobalt nitrate (Co(NO₃)₂) are suitable.

The water sample is applied to a filter paper in a ring oven as usual. When the ring has formed, test a wedge for zinc by spotting it with a drop of cobalt solution and dipping it several times in a small amount of mercuric thiocyanate. If zinc is present, the ring turns blue almost immediately. If zinc is not present, the precipitation is delayed by two or three minutes.

To practice the test you might employ hydrochloric acid to dissolve a small amount of zinc from a piece of galvanized iron or the case of a flashlight battery. Zinc can also be obtained from certain aluminum alloys if a small amount is dissolved in hydrochloric acid. If the treatment results in a gray, insoluble residue, which is silicon, the sample should be filtered before the test is made. Copper alloys may also serve as a source of zinc. A small amount of the metal is dissolved in hydrochloric acid and hydrogen peroxide and then boiled. To remove the copper, which would interfere with the test, add iron filings. When the blue copper color disappears, filter the solution and test it for zinc.

Epstein points out that aluminum bronze, which is an extremely hard copper alloy containing no zinc, can be identified by a negative zinc test followed by a positive aluminum test. In a depression on a spot plate a drop of the solution to be tested is mixed with a drop of I percent ammonium acetate solution. Then a drop of .1 percent aluminum solution is added. (Aluminon is the ammonium salt of aurintricarboxylic acid.) The mixture turns red when aluminum is present. The test can also be run on filter paper. You can practice the test with aluminum foil dissolved in 10 percent hydrochloric acid.

The aluminum compound responsible for the buffering effect of buffered aspirin can be detected with filter paper or a spot plate. Dissolve a tablet of aspirin in warm, dilute hydrochloric acid. You may have to filter the solution to eliminate sediments. Then follow the test procedure for aluminum.

Zinc is a major constituent of manganese bronze, a copper alloy prized for its high tensile strength in such things as ship

propellers. Smaller amounts of manganese and aluminum are also present. The alloy can be distinguished from aluminum bronze and ordinary yellow brass (an alloy of copper and zinc) by testing it for zinc, manganese and aluminum. To test for the manganese add a drop of the sample to a filter paper, followed by a drop of concentrated ammonium hydroxide and then a drop of 10 percent silver nitrate. Manganese is revealed by the black color that appears.

Many of the procedures I have described can be reversed to show the presence of what normally serves as the indicator of a metal. For example, a nickel solution can be added to a test sample in order to reveal the presence of DMG. Similarly, manganese will serve to test for silver. A filter paper bearing a sample drop is fumed with hydrochloric acid and treated with one drop each of 1 percent manganese nitrate solution and .5 percent sodium hydroxide. (Household lye can serve as the source of sodium hydroxide.) If silver is present, a black spot appears. Epstein says this test for silver is more conclusive than the one with potassium chromate because metals other than silver can form colored compounds with chromates. One compound is lead chromate, a yellow insoluble substance.

One interesting source of water with which you might work is the discharge from industrial plants. Metalworking plants, electroplating shops, foundries and mining and smelting operations may discharge waste water with relatively large concentrations of toxic metals such as copper, zinc, lead and chromium. Test for chromium by using the ring oven to create a ring. Hydrochloric acid is the wash agent. On a segment of the ring place a drop of concentrated ammonium hydroxide and a drop of hydrogen peroxide. Dry the filter over the hot plate. Add a drop of freshly prepared I percent diphenylcarbazide solution in alcohol and a drop of 5 percent sulfuric acid. If chromium is present, the ring segment turns violet. Epstein cautions that a sample of distilled water should be tested as a control because the procedure can give rise to faint color even when no chromium is present.

When the water sample is tested for lead, the solution should be washed into a ring with nitric acid rather than hydrochloric acid, which produces insoluble lead chloride that precipitates out in the center of the paper. Add a drop of freshly prepared .2 percent sodium rhodizonate to the ring. If lead is present, the ring segment turns blue. You can practice this test with a solution of lead nitrate (Pb(NO3)2) or metallic lead dissolved in 10 percent nitric acid.

Epstein has also described how minerals can be tested for metals. Since a mineral sample will often not dissolve easily in acid, it must be solubilized by bead formation, a standard mineralogical procedure. A platinum-wire loop is heated in a flame and dipped in sodium carbonate. (Ordinary washing soda serves here.) The loop is reheated until the fluxing material clinging to the wire fuses to form a clear bead. You may have to repeat the procedure until a solid bead forms.

While the bead is still hot it is immersed in a powdered test sample and then reheated until a homogeneous colored bead is shaped. This technique may require a rather hot flame, which can be generated with a Meker burner and compressed air or with a mineralogy blowpipe. Epstein also says that in some instances borax might work better than sodium carbonate.

When a bead has formed, it is broken out of the wire loop, pulverized and poured into a test tube. Add a few milliliters of 50 percent nitric acid to the tube, which is then heated carefully until its contents become dry. (Do not overheat the tube.) After the tube cools add three milliliters of hydrochloric acid and boil the mixture for a few seconds. Add five milliliters of water and boil the mixture again briefly. Filter it to remove insoluble silica. Now the solution can be utilized in any of the spot testing techniques.

The electrical arrangement set up to analyze coins will also work with other large samples such as a piece of metal or a length of wire. The rig can be employed not only to test pure metals and alloys but also to analyze surface coatings and detect a lack of uniformity such as pinholes in platings.

As an example Epstein described how nickel can be detected in a five-cent coin. A one-liter solution of 50 grams of sodium carbonate and five grams of sodium chloride serves as an electrolyte. Four pieces of filter paper form a pad that is dipped in the solution for a few seconds and sandwiched between two paper towels to remove the excess liquid. This pad is placed on the aluminum cathode plate.

When the circuit is closed, an imprint of one side of the coin is transferred to the paper. The top filter is removed from the pad and fumed with ammonium hydroxide. Two or three drops of DMG are added. Epstein says a good reproduction of either Thomas Jefferson or Monticello appears in bright scarlet.

With a similar method you can produce a gray black print of a silver coin. Make the electrolytic transfer. Remove the top filter and hold it near a light bulb. The silver that was transferred during the flow of current has combined with the chloride in the solution to form silver chloride, which is sensitive to light. In the light from the bulb the filter behaves like a photographic plate exposed to light.

The copper in the coin can be detected by working with the other side of the coin. The filter paper is fumed with ammonium hydroxide to generate the deep blue of the copper-ammonium ion. (After each test the aluminum plate should be washed and dried.) Although these tests for silver and copper in a coin are satisfactory for electrographic analysis, they lack the sensitivity for microspot testing.

The equipment and chemicals required for spot testing can be obtained from many chemical-supply houses. Epstein got his chemicals from Pfaltz & Bauer, Inc. (375 Fairfield Avenue, Stamford, Conn. 06902). Some of the chemicals can be bought at drug and grocery stores. Denatured alcohol is adequate for the alcoholic solutions.

Proper laboratory procedures are crucial when you handle potentially dangerous chemicals. Do not touch the chemicals with your bare hands. If a solution splashes on you or the work area, immediately wash off the splash with plenty of cool water. When you are diluting concentrated acids, particularly sulfuric acid, add the acid to the water by running it slowly down the wall of the container while you stir the mixture. Beware also of the heat that is generated by the process.

Much more can be done with chemical-spot testing. At another time I shall describe Epstein's procedure for detecting air pollution, including acid rain. If you test environmental samples of water or soil, I should like to hear about your results.

Bibliography

STANDARD METHODS OF CHEMICAL ANALYSIS. Wilfred W. Scott. D. Van Nostrand Company, 1939.

MICROANALYSIS BY THE RING OVEN TECHNIQUE. Herbert Weisz. Pergamon Press, 1961.

SPOT TESTS IN INORGANIC ANALYSIS. Fritz Feigl and Vinzenz Anger. Elsevier Publishing Company, 1971.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.