INDUSTRIAL AND ENGINEERING CHEMISTRY
96
this relation of acidity to the coefficient of friction is much less regular. IODINE NUMBER-Judging from these and former investigations, iodine number is an unsatisfactory property upon which to base any a priori opinion as to the lubricating value of an oil, or the power of the oil to reduce friction. It would seem, however, that, other things being equal, a more unsaturated oil would be a more efficient lubricant than a more saturated one. Oil No. 3, an asphaltic base oil with an iodine number of 8.88, shows a slightly lower coefficient of static friction than oil No. 2, a paraffin-base oil with an iodine number of 13.7. In a previous report by one of the writers" an analogous case was cited. 11
THISJOURNAL, 18, 499 (1926).
VOl. 19, N o . 1
Conclusion
The results obtained in the course of this investigation do not indicate a definite relation between any of these properties and the value of an oil as a lubricant. At the present stage of this work (further investigation is in progress) the authors hesitate to express further conclusions. Treatment with ozone reduces the coefficient of static friction of paraffin-base oils, so far invesitigated, when used as lubricants between steel surfaces; but similar treatment produces a less regular and less pronounced change in the asphaltic-base oils so far investigated. In general, the readings of the Deeley machine are considerably more regular after the lubricant has been treated with ozone than before treatment.
Separation of Minute Quantities of Gold from Ferric Oxide' By Edward Wichers2 NATIONAL
BUREAUOF
STANDARDS,
u. s.
DEPARTMENT O F COMMERCB,
ECENTLY it became necessary for the writer to determine a minute quantity of gold, .possibly of the order of 0.005 mg., in a residue consisting mainly of ferric oxide and weighing slightly more than 1gram. It was questionable whether a quantity of gold less than 0.01 or 0.02 mg. could be satisfactorily identified in such a material by fire assay methods. The color reaction with benzidines seemed to offer a suitable method for an approximate determination of the gold if it could be obtained in a chloride solution separated from the iron and from any other interfering substances. There are other sensitive methods for the detection of gold, but no comparative tests of this method with others were made after it was found possible to detect 0.001 mg. of gold per cubic centimeter and an absolute quantity of 0.0001 mg. of gold (0.1 cc. of the above solution contained in a small porcelain crucible). The reaction between the solution of benzidine (1 gram in 50 cc. of water and 10 cc. of glacial acetic acid) and dilute solutions of gold chloride is marked by a clear blue color, which rapidly fades and then changes to a violet shade less intense than the first color. It is not a characteristic reaction for gold. Ferric chloride and chlorine water give the same color, but cupric chloride does not cause a visible reaction. The color is doubtless that of an oxidation product of benzidine. Chloroplatinic acid gives a dark flocculent precipitate on standing, this reaction being sensitive to at, least 0.01 mg. of platinum per cubic centimeter if the test is allowed to stand overnight. Once the small quantity of gold was separated from the large mass of residue, the elimination of any interference from other substances (except chlorine) was simplified by using chlorine water to dissolve the metallic gold. Chlorine water, of course, has no effect on most ignited substances, but dissolves finely divided gold readily. Digestion on the steam bath and evaporation of most of the water usually served to remove all of the excess of chlorine, although special precaution was necessary to make sure of this. Too long digestion permitted a possible reduction of gold by dust particles or in other ways. Usually two or three portions of
R
Received June 29, 1926. Published by permission of the Director, National Bureau of Standards. Malatesta and di Nola, Boll. chim.-farm., 62, 461 (1013); J. Cham. SOC.(London), 104, 883A (1913). 1
1
WASHINGTON,
D. c.
chlorine water were subjected to digestion and evaporation under the same conditions, so as to provide checks on the elimination of all chlorine from the solution containing the gold. Experiments were made to determine whether the gold might be separated from the ferric oxide by fusion with potassium pyrosulfate. Volumes of gold chloride solution known to contain 0.005 mg., 0.01 mg. (duplicates), and 0.05 mg. of gold were added to each of four solutions of ferric nitrate equivalent to slightly more than 1 gram of ferric oxide. The solutions were evaporated to small volume and absorbed in filter papers, which then were burned off in porcelain crucibles. Each residue was fused with about 50 grams of potassium pyrosulfate until the melt was clear. The melts were dissolved in water acidified with sulfuric acid and filtered through S. & S. No. 589 blue ribbon papers which had been treated with a suspension of paper pulp and wet with dilute sulfuric acid. Although the filters were washed thoroughly with dilute acid, on burning off the paper the ash contained some finely divided ferric oxide, which yielded a colored suspension and thereby interfered with the subsequent color reaction. The fusion was therefore repeated, using a very small quantity of potassium pyrosulfate, and the resulting solutions were filtered in the same manner. The papers were burned off and the slight residue was digested with chlorine water. At the same time four similar porcelain crucibles containing 0.001 mg., 0.005 mg., 0.01 mg., and 0.05 mg. of gold as chloride were treated with chlorine water in the same manner. Two crucibles containing chlorine water only were used as blanks to insure the elimination of chlorine in the digestion and evaporation which followed. Upon the addition of several drops of the benzidine reagent all four samples gave a positive test for gold. In each case the color was less intense than that in the corresponding known quantity of gold which had not passed through the pyrosulfate fusion. It was apparent, however, that more than half of the gold was recovered in each case. By comparing the depth of color produced by the unknown material with that of the samples which had been separated from a similar quantity of ferric oxide, and assuming a proportionate recovery of gold, it was possible to give an approximate figure for the quantity of gold in the original material.
