The Beilstein Test: Screening Organic and Polymeric Materials for the Presence of Chlorine, with Examples of Products Tested – Canadian Conservation Institute (CCI) Notes 17/1

CCI Note 17/1 is part of CCI Notes Series 17 (Spot Tests in Conservation)

Introduction

Chlorinated organic materials are generally considered unsuitable for long-term conservation and museum applications due to their potential harm to objects. These materials may degrade and produce acidic gases, or additives such as plasticizers may migrate to objects. These products can be analyzed in detail in the laboratory, but conservators may use a simple test — the Beilstein Test — to screen their own materials for the presence of chlorine without having to submit samples for laboratory analysis.

The test is based on the reaction of chlorine with copper compounds at the high temperatures found in burner flames. These conditions produce excited, green-coloured copper atoms or ions that cause the normally colourless (or very slightly blue) flame to flare brilliant green (or sometimes blue-green).

Procedure

Use a copper wire thick enough not to melt too quickly (e.g., I2- or 14-gauge copper wire, stripped of its insulation, that is used for wiring houses). Heat the copper wire to glowing red in the flame of a Bunsen burner or propane torch.

Continue heating until no colour (other than the nearly invisible, slightly blue torch flame) is visible. There should be no green colour in the flame. Wash the wire intermittently with water and dilute nitric acid (10%) to remove materials that cause unwanted colouration. If washing fails, try a new wire. Once it is clean, take care to avoid touching the wire with fingers or objects other than the test material. When the flame is colourless, the test can proceed.

The test is best carried out in subdued lighting so that the colour of the flame can be seen easily. Vapours, fumes, or smoke from the sample must envelop the hot copper wire in the flame so that reactions between the tested material and the hot copper can occur. This can be accomplished in several ways:

1. Heat the wire to red-hot, then quickly touch a fragment of the sample to it and immediately return the wire to the flame. A plume of green is a positive test for the presence of chlorine. Do not touch the entire sample with the hot wire. Some plastics (e.g., cellulose nitrate, Celluloid) may burst into flame. (This reaction is usually considered a positive test for cellulose nitrate.)
2. With the wire red-hot and still at the edge of the flame, bring a fragment of the sample near the flame in the vicinity of the wire until the fragment chars and the smoke produced envelops the wire. A green flame is a positive test for the presence of chlorine.
3. With the red-hot wire in the flame, place a fragment of the sample near or into the flame until it starts to char. Quickly move the smoking sample to the air intake at the base of the burner so that the smoke is drawn in with air and is intimately mixed with the flame gases. A green flame surrounding the copper wire is a positive test for chlorine.
4. Heat the wire to red-hot. Then immediately touch the wire with a piece of the sample held beside the air intake at the base of the burner or torch, so that some of the fumes produced are drawn into the flame. A green flame is a positive test for chlorine.

The fourth method described is the most sensitive. It is also the best method to detect the presence of a volatile chlorinated material — perhaps a solvent such as methylene chloride.

Discussion

The Beilstein Test has been used for many decades to analyze organic and polymeric materials (Shriner et al. 1964; Vogel 1966). A very similar test is now used by refrigeration technicians to detect and localize leaks of freon-type refrigerants (i.e., chlorinated and fluorinated hydrocarbons). This test has also been recommended for detecting PVC coin storage products (Sharpless 1980).

The Beilstein Test is quite sensitive and requires a very small sample. There are, however, several possible sources of error. Residues from fingerprints can give weak false positive results. Chlorinated inorganic materials (i.e., pigments, fillers) can also give a false positive result, but these are usually not present so they rarely interfere with the test. The major difficulty encountered is that the sample may volatilize too rapidly, especially if it is a solvent. These volatiles are lost so quickly that they do not have time to react with the copper wire. To avoid this problem, the fourth method described above — introducing the fumes at the air intake for the flame — is recommended.

The test is suitable for screening a wide variety of products including plastic films and sheets, adhesives, rubbers, coatings, solvents, and fabricated items. Common materials that give positive results include poly(vinyl chloride), poly(vinylidene chloride), chlorinated rubbers, chlorinated epoxies, chlorinated solvents, and any compositions containing these materials.

The Beilstein Test has been used to examine a variety of materials at the Analytical Research Services laboratory at CCI. The presence or absence of chlorinated organics was confirmed by infrared spectroscopy or by radio-isotope excited X-ray energy spectrometry. The results are presented in the tables below.

Examples of Beilstein Test Results for Various Polymeric Organic Materials

References

1. Sharpless, Thomas W. "Corrosion: the Problem of Storage." The Numismatist, October 1980, p. 2453.
2. Shriner, Ralph L., Reynold C. Fuson and David Y. Curtin. The Systematic Identification of Organic Compounds: A Laboratory Manual. 5th ed. New York: John Wiley and Sons Inc., 1964, p. 64.
3. Vogel, Arthur I. Elementary Practical Organic Chemistry. Part 2: Qualitative Organic Analysis. 2nd ed. London: Longmans, Green and Co. Ltd., 1966, p. 96.

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Texte également publié en version française.

© Government of Canada, 1993
Cat. Nº NM95-57/17-1-1989E
ISSN 0714-6221