Heavy metals are often present as impurities in essential oils. It is especially important that oils be free from such impurities if they are to be used for medicinal purposes or in foodstuffs. Furthermore, the presence of heavy metals in perfume oils will often cause discoloration in such products as soaps and cosmetic creams.
A very sensitive test for heavy metals has long been official in "The United States Pharmacopoeia"140 to insure the absence of lead and copper.
The test is based upon the fact that hydrogen sulfide will react with the chlorides of these metals to give dark colored sulfides. The sulfides of most metals are black or brownish black. The following represent the exceptions : the sulfides of cadmium, arsenic and tin (stannic form) are yellow; of antimony, orange; and of zinc, white. This test is especially satisfactory for the determination of small amounts of copper or lead.
Procedure: Shake 10 cc. of the oil with an equal volume of distilled water to which 1 drop of concentrated hydrochloric acid has been added, and pass hydrogen sulfide through the mixture until it is saturated. No darkening in color in either the oil or the water is produced in the absence of heavy metals. In order to discern any darkening if only traces of heavy metals are present, it is necessary to carry out simultaneously a blank determination to which no hydrogen sulfide is added: a comparison of the blank and the run will clearly indicate traces of heavy metals if present. Test tubes may be conveniently used for these determinations.
Often a scum will form at the surface between the oil and water layers. The formation of the scum is no indication of the presence of heavy metals, unless the scum is dark in color.
Oils manufactured in primitive stills or oils improperly stored in metal containers (especially if the oils are not thoroughly dried) or oils containing large amounts of free acids will often contain heavy metals. Anise, bay, sweet birch, cajuput, clove, geranium, and sassafras oils usually contain heavy metals when distilled commercially. Therefore, it is well to test those oils to ascertain whether or not they have been properly treated to remove such impurities.
A metallic impurity frequently encountered in essential oils is iron. Oils distilled using iron condensers and oils stored in imperfectly lined drums frequently show the presence of this impurity. Oils rich in phenols, or containing a phenol group, such as the salicylates, are often contaminated. Iron will not be precipitated by hydrogen sulfide in an acidic medium and, therefore, will not give a positive heavy metals test. Ammonium sulfide or sodium polysulfide will precipitate black ferrous sulfide.
The test as described above shows a high degree of sensitivity: ten parts per million of metallic Jead in oil of -cloves gives a positive test; the threshold yalue is Approximately fivfc parts per million.
Removal of Heavy Metals. For the removal of metallic impurities from essential oils, citric or tartaric acid is frequently employed, giving rise to complex citrates and tartrates which are insoluble and which may be filtered off.
Procedure: Add to the oil a small amount of dry tartaric acid (usually 1 to 1 per cent will prove sufficient) and shake thoroughly. Permit the acid to settle and filter the supernatant liquid. If this method should fail to remove all the metallic impurities, agitate the oil with 1/2 to 1% of a saturated aqueous solution of tartaric acid, separate the oil, shake thoroughly with salt, and filter.
The removal of heavy metals from clove, bay, pimenta, and geranium oils frequently requires several treatments.
When reporting the presence of heavy metals in an oil, it is well to indicate the relative amount found by terms such as : strongly positive; positive ; positive small amounts; positive traces.
134 The filter is best prepared by thoroughly washing a small filter paper in a glass funnel with distilled water until the water passing through fails to show any turbidity or opalescence when treated with a drop of nitric acid and 1 cc. of the silver nitrate solution.
136 Some filter papers contain sufficient amounts of chlorine to give a positive test.
136 Gildemeister and Hoffmann, "Die atherischen Ole," 3d Ed., Vol. I, 779.
137 "The United States Pharmacopoeia," Thirteenth Revision, 104.
138 Directions for the ignition with metallic sodium have been admirably described by Mulliken in his classical work, "A Method for the Identification of Pure Organic Compounds," John Wiley & Sons, Inc., New York Vol. I (1904), 10.
139 "The National Formulary," Eighth Edition, 153.
140 Thirteenth Revision, 658.

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