396
I
ANALYTICAL EDITION
one-kg. portions of honey for distillation, it is possible to detect as little as 10 per cent orange in a mixed honey. The distillation of quantities greater than one kg. would make it possible to detect less than 10 per cent orange in a mixed honey. A one-kg. portion is used, as it represents the largest amount that can be conveniently distilled with ordinary laboratory equipment. Since pure orange honey can usually be recognized by its characteristic flavor, it might seem superfluous to resort to chemical tests as a means for detecting it. The test described is easy to carry out, however, and might be particularly useful
VOl. 4, No. 4
for detecting the presence of orange in mixed or blended honey. In addition, it eliminates the personal equation involved in an organoleptic test (based on gustatory senses). The method might also be used t o obtain a rough estimate of the relative amount of orange present in a blended honey. LITERATURE CITED (1) Nelson, IND.ENG.CHEM.,22, 448 (1930). (2) Power, J. Am. Chem. Soc., 43, 377 (1921).
RECEIVED June 15, 1932. Contribution 125 from the Carbohydrate Division, Bureau of Chemistry and Soils, U. S. Department of Agriculture.
Determination of Arsenic Iodometric Acidimetric Method R. C. Wiley, J. P. Bewley,
AND
R. IREY,University of Maryland, College Park, Md.
RSENIC readily forms volatile compounds. A very
A
common method for determining arsenic is by distilling it as arsenious chloride. Attempts have also been made to determine arsenic by converting it to arsine and absorbing the arsine in some standard solution. Lochman (8) determined arsenic qualitatively by passing the arsine formed through mercuric chloride solution. Ericsson ( I ) conducted arsine through 0.1 N silver nitrate and precipitated the silver with hydrochloric acid. The arsenic was then titrated with 0.002 N iodine. Many other methods have been devised for measuring arsine by passing it through standard solutions. The well-known Gutzeit method is, of course, a colorimetric means of measuring arsine.
FIGURE 1. DIAGRAM OF APPARATUS
The basis for the method described here is, first, conversion of the arsenic to arsine, and next absorption of the arsine by a standard iodine solution. This is followed by the titration of the residual iodine and the titration of the acids formed in the oxidation. For this investigation the following solutions were prepared: 0.1 N I 0.01 N AsnOj 0.1 N Na&Os 0.01 N NazSIOa 0.1 N NaOH 0.1N AsrOa I n addition, arsenic-free sulfuric acid and zinc were used. All solutions were carefully standardized and the zinc and sulfuric acid were found nearly free of arsenic by the Gutzeit test. A blank was run on all reagents. All rubber tubing and stoppers used were first boiled in a solution of sodium hydroxide, washed, and boiled in dilute hydrochloric acid. They were then soaked overnight in water.
PROCEDGRE The solution of the sample, in which the arsine must be in the arsenious state, is placed in the flask and about 10 grams of 20-mesh zinc are added. The volume of the solution should not be more than 20 to 30 cc. The stopper is now placed in the flask and a measured amount of standard iodine solution distributed through the Meyer bulb tubes, C and D of Figure 1, with sufficient water to bring the volume in each tube to about 75 cc. The bulb tube, E , which is not shown, contains a 1 per cent solution of potassium iodide to dissolve any iodine vapor from D. As a further precaution, another bulb tube, F , is connected to E and is filled with water. On rare occasions F is found useful. A piece of mercuric chloride paper is inserted in F to indicate any escaping arsine. The tubs, G and H, should be filled with cold water. The separatory funnel, B, is now filled with 75 per cent sulfuric acid and about 5 cc. are allowed to run into A . The acid is added in 10-cc. portions a t about 10-minute intervals for about 40 minutes. At the end of this time most of the zinc will be disselved. The contents of the flask are then heated to gentle boiling for about 5 minutes. The solution in C,- however, should not become warm. The contents of C, D, E , and F are then washed into a beaker and titrated with standard sodium thiosulfate using starch for an indicator. After the titration with thiosulfate, one of the following procedures was adopted: 1. When a large amount (more than 0.05 to 0.07 gram of arsenious oxide) of arsenic was present, the solution was neutralized with sodium bicarbonate and titrated back with standard iodine. 2. When a small amount (less than 0.05 t o 0.07 gram of arsenious oxide) of arsenic was present, the solution was titrated with standard sodium hydroxide, using phenolphthalein as indicator.
DISCUSSION I n method 2, when the arsine is completely oxidized, the following reaction takes place:
+ +++
AsHa 41z 4Hz0+HsAsO4 HaAsOa 8HI 10NaOH+NapHAsOa
+ 8HI and + 8NaI + lOHzO
It will thus be seen that 1 cc. of 0.1 N iodine is equivalent to 0.001237 gram of arsenious oxide, and 1cc. of 0.1 N sodium hydroxide is equivalent to 0.0009896 gram of arsenious oxide. The above reaction between arsine and iodine goes to completion when the arqine generated from not more than 0.05 to
October 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
397
ticles of dark-colored material will dissolve. No doubt arsine is also dissolved in the liquid in the flask, and this is liberated upon boiling. Sufficient standard iodine solution should be present in tubes C and D so that all of it is not reduced by the arsine, else unreliable results will be obtained. TABLE 1. ARSENIOUS OXIDEFOUND AND PRESENT BY IODOMETRIC AND ACIDIMETRIC METHODS
0.07 gram of arsenious oxide is passed into 100 cc. of 0.05 N solution of iodine. The determination should be arranged so that not more than this amount of arsenic is present if it is desired to titrate both the residual iodine and acid formed.
Cram
I I1 I11 IV V VI VI1 VI11 IX X XI XI1
0.02475 0,02475 0.02475 0.02475 0.02475 0.02475 0.02475 0.002475 0,04950 0.04950 0,04950 0.04950
Gram
LITERATURE CITED
AstOa BY ACIDIMETRIC METHOD Present Found Diff.
AszOa B Y IODOMETRIC METHOD S A X P L I Present Found Diff Gram
Gram
-0.00014 0.02461 +0.00005 0.02480 +0.00017 0.02492 +o .00002 0.02477 +o. a0017 0.02492 0.02480 + O . 00005 0.02476 0.00000 0.002596 +o. 00012 0.04941 -0.00009 $0.00029 0.04979 $0.00004 0.04954 + O . 00014 0.04964
0.02475 0.02475 0.02475 0.02475 0.02475 0.02475 0.02475 0.002475 0.04950 0.04950 0.04950 0.04950
Gram
(1) Ericsson, A,, Svensk Farm. Tid., 18, 473-8 (1914). (2) Lochman, R., 2. fisterr. Apoth. Vet-., 45, 744-5 (1907).
Gram
0.02475 0.00000 0.02493 $0.00018 0 , 0 2 4 5 8 -0,00017 0,02459 -0.00018 0.02464 -0.00011 0.02483 $0.00008 0.02464 -0.00011 0.00282 +O. 00033 0.04928 -0.00022 0.04948 -0.00002 0,04948 -0.00002 0.04928 -0.00022
RECEIVED June 23, 1932.
A Laboratory Extractor
H. G. TANNER, E. I. du Pont de Nemours & Co., Wilmington, Del. ARSEN-
TABLE11. RESULTS OBTAINEDBY PROCEDURE 1 WITH ATES AND ARSENITESPRESENT IN RECEIVINQ IODINE SOLUTION SAMPLE AsrOa PRESENT Gram
AmOr FOUND
DIFFERENCE
Cram
Gram
0.10019 0.09809 0.09840 0.09842 0.09809 0.08032 0.11976 0.09987
$0.00116 -0.00084 -0.00053 -0.00061 0.00084 +0.00032 -0.00024 -0.00013
-
When larger than the above-mentioned amounts of arsenic are present in the sample, the receiving iodine solution will contain both arsenate and arsenite. In such cases, the volume of iodine originally added and the amount used in back titration are added, and from this is subtracted the volume of thiosulfate used in titration The arsenic present is found by multiplying the difference by the proper factor. For example, suppose 120 cc. of 0.1 N iodine were used as the receiving solution and 30 cc. of 0.1 N thiosulfate were used to titrate the residual iodine, and also that 7 cc. of 0.1 N iodine were used to titrate back after neutralization. 0.1 N I, cc.
120 30
0 . 1 N Na&Oa, cc.
-
0 . I N I used in back titration, cc. 97 X 0.001237 = 0,119989 gram As1Oa present
90
-7 97
Tables I and I1 show results obtained.
COMMENTS Since in going from arsine to orthoarsenic acid arsenic changes 8 in valence, it is possible to determine very small quantities of arsenic in a volumetric way. It has been found practical to use 0.01 N solution of iodine, one cc. of which equals 0.0001237 gram of arsenious oxide. Since one can check. readily to about 0.2 cc., it has been found possible to determine quantities of arsenic as small as 0.00002 gram. The method herein described should be four times as sensitive as the ordinary titration of arsenic with iodine. The acidimetric titration is a valuable check on the accuracy of the iodometric titration. The work done shows that it is impossible to liberate all the arsenic from a sample as arsine without boiling. The boiling has gone far enough when the last particle of metal has been dissolved. Although the reaction of the acid on the material in the flask may have ceased before boiling, it will be found that when the contents of the flask are brought to boiling par-
T
HE accompanying photograph shows an extractor of considerable capacity which was constructed almost entirely by the assembly of common laboratory apparatus. Within a tall-form liter beaker was placed a glass tripod which was readily constructed of glass rod. A 35 by 70 mm. crystallizing dish rested in an inclined position on this tripod. A 15-cm.folded filter within this dish contained the mat e r i a l to be e x t r a c t e d . Suspended in the mouth of the beaker was a 250-cc. distilling flask which served as the condenser. The extractor is particularly well adapted to the purification of organic compounds. It r e s e m b l e s in some respects an extractor described in the literature ( I ) , wherein a large funnel is used to support the filter. It h a s n u m e r o u s a d vantages, however, over the “funnel” extractor. Comparative tests showed that it has a much higher speed of extraction; the solvent is c o n d e n s e d more effectively when boiled vigoro u s l y ; the drip from the c o n d e n s e r flask does not channel through the charge in the filter; “choking” of ascending vapor with condensate in the funnel stem is avoided; and contamination by m a t e r i a l f r o m stoppers does not occur. When used with 200 cc. of carbon tetrachloride as the extraction medium, no appreciable amount of solvent escaped during a 6-hour run. Decolorizing charcoal, intimately mixed with the material to be extracted prior to filling up the filter, frequently assisted purification, LITERATURE CITED (1) Conant, J. E., “Organic Syntheses,” Vol. 11. p. 49, 7’v ley, 1922. RECEIVED June 21, 1932. Contribution 102 from the Experimental Station, Nemours & Co
a. I. du Pont de
