356
INDUSTRIAL AND ENGINEERING
amounting to 3,716,000 pounds valued a t $2,262,000, whereas in 1932 only 1,585,000 pounds valued at $147,000 were imported.
GUMS,RESINS,AND NAVALSTORES As usual, crude chicle took first place on the list of imports of this class of goods. Shipments of 4,066,000 pounds were valued a t $1,081,000, a falling-off of 18 per cent in amount and 46 per cent in value. Shellac, with a value of $972,000, showed a 35 per cent loss and crude lac, $873,000, a 14 per cent loss. Crude natural camphor was imported to the extent of 1,989,000 pounds valued at $411,000, an increase of about 10 per cent in quantity and a decrease of 20 per cent in value; natural refined camphor, 1,630,000 pounds valued a t $490,000, gained 57 per cent in quantity and 37 per cent in value; and synthetic camphor, 1,460,000 pounds valued at $406,000 in 1932, dropped to 880,000 valued a t $216,000 last year.
CHEMISTRY
Vol. 26, No. 3
Exports of naval stores, gums, and resins more than recovered the ground they lost in 1932, the total value rising $14,714,000, whereas in 1932 it,stood a t $11,587,000 and in 1931 at $14,304,000. Trade in the various articles in the last two years is shown in Table IX. TABLEIx. EXPORTS OF NAVALSTORES,GUMS,AND RESINS (In thousands)
-1932Quantity Value
Naval stores:
Gum rosin, barrels 938 Wood rosin, barrels 160 Gum spirits of,turpentine, gallons 10,940 Wood turpentine, gallons 529 Tar and pitch of wood, barrels 6 Other guma and resins, pounds 3,682 Total
$5,345 911 4,410 229 56 636
-
11,587
-1933Quantity
Value
994 $6,540 219 1,324 13,388 5,781 851 343 .~. 7 65 4,248 661
-
14,714
RECEIYED February 10, 1934.
Tabular Method of Reading Arsenic Strips BERTRAMD. THOMAS, University of Washington, Seattle, Wash.
T
RACES of arsenic in such common foodstuffs as fruit are usually determined by some modification of the Gutzeit evolution method. The sample after appropriate treatment is placed in an evolution flask and treated with stannous chloride to reduce arsenates to arsenites and with zinc and hydrochloric acid to reduce all the arsenic to arsine. This is swept out of the flask by the excess of hydrogen which is formed a t the same time, and passed over strips of paper impregnated with mercuric bromide. The arsine colors the strip, and by using suitable standards the amount of arsenic in the sample may be estimated by measuring the length of the stain produced on the strip and comparing with the lengths of the standard stains. The comparison is usually effected by plotting the values of the standards and the corresponding stain lengths on coordinate paper, passing a smooth curve through the points, and reading the values of the samples graphically by interpolating along the curve. This is a somewhat laborious process; and since it must be repeated for every set of standards, considerable time is required for the mere mechanical details of plotting the data. The method also has the additional disadvantage, common to most graphical methods, of allowing choice as to the way the curve is drawn. More than one curve may usually be drawn through a set of points, and the differences are sometimes considerable. Where a large number of determinations are being made, the results may be obtained much more rapidly and probably more accurately by means of tables calculated for particular sets of standards. By examination of a large number of determinations it has been found that while the lengths of a given standard vary widely from set to set, the relative lengths of the various standards in the sets are quite constant. The sum of the lengths of the standards is sufficient to characterize the curve and therefore the table necessary to calculate the values of the samples. Suppose values of the standards are taken containing 0.01, 0.02, 0.03, and 0.04 mg. of arsenic trioxide. When these are evolved in the ordinary manner, the lengths of the stains of a
particular set are found to be 8.5, 13.5, 17.0, and 20.0 mm,, respectively; and the total length, 59 mm. This total length suffices to determine the table from which the results may be read. Practically it has been found that about 25 tables are necessary to cover a variation in the total length of the standards from 45 to 70 mm. These may be arranged to permit interpolation for fractions of a millimeter, depending upon the requirements and usage of the laboratory and, if desired, to read directly in grains per pound if spray residues are being determined on fruit. Average values of the lengths of the standards are given with each table to permit immediate detection of a gross error. Careful comparison of results calculated by the graphical method and those taken from the tables has shown that the differences are usually less than 5 per cent. Greater deviations can always be attributed to faulty standards, which are made immediately apparent by the average values given in the table, or to an actual choice in the curve drawn through the points. In this latter case the tables are much more reliable. Even in the case of a relatively large error in one of the lower standards, the tables still seem to give results within the experimental error of the Gutzeit method (1). The tables must be calculated to fit the particular conditions under which the determinations are made, although these conditions may vary quite widely without producing appreciable change in the relative values of the standards. The tables may be easily computed by using the data which accumulate during the determinations themselves, preferably by plotting the arsenic equivalent of particular strip lengths against the total length of the standards in the set from which the strip is taken and interpolating values for the tables. The 5-, lo-, 15-, and 20-mm. lengths are sufficient to permit the rest of the values to be interpolated. It is desirable, of course, to have as many sets of standards available as possible in plotting the curves. LITERATURE CITED (1) Barnes and Murray, IND. ENQ.CHEV.,Anal. Ed., 2, 29 (1930). RECEIVED December 8, 1933
