Rapid Method for Determination of Small Amounts of Arsenic C. E. LACHELE, National Canners Association, Western Branch Laboratories, San Francisco, Calif.
T
HE method is essentially a modified G u t ~ e i prot
A method is given for the determination of arsenic by estimating the evolved arsine deposited upon a mercuric-bromide-impregnated diuphragm. It is applicable f o r estimating minute quantities of arsenic in the presence Of impurities such as iron, tin, antimony, or reducible sulfur and phosphorus compounds. Modifying the procedure makes it possible to determine the arsenic Content of many products Using an undigested sample.
free SnC12.H20 in concentrated hydrochloric acid and make up to cc. with the same strength acid.
that the the a r s i n e cedure9 is l i b e rexcept a t e d from sample in a manner to C a r e f u l l y select filter PAPER. sheets, that employed by Smith (15) similar to S & S NO. 589 black ribbon paper for weight and texand T a b e r (17) b u t u s i n g ture, and soak one hour in a saturated alcoholic s o l u t i o n of mers m a l l e r q u a n t i t i e s * Taber curic bromide. After sensitizing, modified Smith's procedure by remove sheets and dry by means adopting a larger generator flask of an air blast, and cut in disks of to accommodate a larger volume the same diameter as the diaof liquid a n d f i t t i n g t o i t a phragm tube. Avoid t o u c h i n g condenser so that heat could be the sensitized disks with the hands as far as possible. Disks should applied to the flask t o secure proper reduction of the arsenic and a vigorous ebullition of not be used when more than 5 or 6 days old. ZINC. Cut arsenic-free zinc in pieces about 1 cm. in length gas. The estimation of arsenic by Taber's distillation method (approximately grams). Activate the pieces by covering with Was based upon a n iodine titration of the reaction Products hydrochloric acid (1 3) containing about 2 cc. of special stannous formed when arsine is passed into mercuric chloride solution. chloride solution for each 100 cc. of acid. Allow the action to The method described in this paper differs from that of continue for 15 minutes and wash well with distilled water. POTASSIUM IODIDE SOLUTION. Dissolve 15 grams of potassium Taber and the official Gutzeit method in that the evolved iodide in water and dilute to cc. arsine is impinged upon a n impregnated mercuric b m k k FERROUS AMMONIUM SULFATE. Use this substance or ferrous diaphragm securely held between two ground-glass joints. sulfate crystals free from arsenic. The colored disks are then developed in a solution of iodide c h ~ ~ ~ ~ grams ~ of cuprous ~ (1, 7 ) and compared with standards. workers have CADMIUM IODIDE SOLUTION.Dissolve 20 grams of cadmium employed the principle of stain deposition upon a sensitized iodide in water and dilute to 100 cc. Dissolve 1 gram of arsenic STANDARD ARSENICSOLUTION. paper disk (2, 3, 4, 11, l a ) , but the sen- trioxide in 25 CC. of 20 per cent sodium hydroxide. Saturate solution with carbon dioxide and dilute to 1 liter with recently sitid'% used the boiled water. One cc. of this solution contains 1 mg. of arsenic Was mercuric C h b trioxide. Dilute 40 cc. of this solution to 1 liter, diluting 50 cc. ride, Various devices of the resulting solution to 1 liter. One cc. of this latter solution for securing the dia- contains 0.002 mg. of arsenic trioxide and is used to prepare standphragm in place have ard stains. Prepare fresh dilute solutions at frequent intervals. Nitrogen a 1so b e e n suggested APPARATUSAND PROCEDURE H~EOW (6, 8~ 9~ '49 16, Use an Erlenmeyer flask connected by a two-hole stopper to wply 18,lQ); some of these a Liebig or Allihn condenser. A gas such as nitrogen or hydrogen were unsatisfactory is introduced through a tube extending below the surface of the because they did not contents of the flask. The upper end of the condenser is packed with absorbent cotton, the lower half of which is saturated with the the cuprous chloride solution which acts as a scrubber to remove to obtain a staln with impurities, such as phosphine, stibine (IO),and hydrogen sulfide, a s h a r p l y d e f i n e d from the evolved arsine. A chamber having a diameter that will boundary, and others accommodate a sensitized diaphragm of a size sufficient to comwith all the arsine is fitted to the condenser. The chamber were subject to criti- bine consists of two thick-walled tubes of the same diameter with cism for reasons re- ground-glass joints held together by an outside sleeve of Gooch lated to simplicity of rubber tubing. The impregnated paper is fitted between the two ground joints as pictured in Figure 1. FIGURE 1. APPARATUS FOR ARSENIC d e s i g n 01 e a s e of An aliquot obtained from a sample digested with sulfuric and manipulation. DETERMINATION nit8ricacid in the usual manner (or the entire digestion in case of Forminuteamounts small amounts) is placed in a 1000-cc. Erlenmeyer flask with of arsenic the method is especially applicable, as the entire enough distilled water to make about 200 cc. of solution. Then digest of a sample can be used, With a disk 20 mm. in 2 to 3 grams of solid ferrous ammonium sulfate or ferrous sulfate, 10 to 15 drops of stannous chloride solution and 50 cc. of 1 to 1 diameter 0.001 to 0.040 mg. of arsenic trioxide can be directly hydroch~oricacid are added. Just before connecting to the condetermined. The analysis can be made in the presence of im- denser, introduce two or three pieces of activated zinc. A conpurities such as iron, tin, antimony, or reducible sulfur or phos- tinual stream of nitrogen is kept flowing through the system to phorus compounds, which tend to affectthe evolution rate carr all traces of evolved arsine through the scrubber cotton to t i e paper diaphragm impregnated with mercuric bromide. of arsine or to deposit interfering stains on the mercuric The contents of the flask are boiled until all the arsine is evolved, bromide strips used in the regular Gutzeit method. which usually requires about 15 minutes. When the reaction is complete, the disk is removed and developed in cadmium iodide solution. This fixes the stain more permanently to light and REAGENTS water than does potassium iodide and changes the color from yelHYDROCHLORIC ACID. Make up 50 cc. of 35 per cent arsenic- low to brown. The disk is allowed to remain in the iodide solufree hydrochloric acid to 100 cc. with water. tion until all traces of red mercuric iodide, which immediately STANNOUS CHLORIDE SOLUTION.Dissolve 40 grams of arsenic- forms, have been dissolved. After prolonged use, the cadmium 256
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July 15, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
iodide solution turns yellow from the accumulation of complex cadmium mercuric iodides and should be replaced by fresh reagent. The excess reagent is removed from the disk by a water wash followed by a rinse with alcohol, and the disk is then dried between blotters. When dry the stains are compared with standards. The standards are prepared in a similar manner to the unknowns in steps of 0.0025 mg. of arsenic trioxide, keeping them free from excess light exposure. The stains have a tendency to bleach slightly with age and it is desirable to prepare fresh standards frequently, especially in the approximate range of the unknown. With some modifications the method is applicable to the undigested material, thus shortening the total time of an arsenic determination to 15 to 30 minutes and eliminating the costly procedure of digestion with nitric and sulfuric acids. From 5 to 100 grams of the undigested sample may be placed directly in the 1000-cc. Erlenmeyer flask together with 100 cc. of 1 to Y hydrochloric acid, 1 to 2 cc. of stannous chloride solution, 15 cc. of potassium iodide solution, and 1 to 2 grams of solid ferrous sulfate. The activated zinc (three or four pieces) is added, and the flask immediately connected to the condenser unit. The reaction is usually complete after 15 minutes of boiling except that, when using large amounts of sample, a somewhat longer time may be necessary to evolve all the arsine. The scrubber cotton should be saturated with a 15 per cent solution of cu rous chloride in 1 to 1hydrochloric acid to remove hydrogen sulfi&, phosphine, and stibine, if present. In case these gases have not been efficiently removed, the yellow stain imparted to the disk is not chan ed to brown when immersed in the cadmium iodide solution. Wien using large samples of some products that contain considerable reducible sulfur or phosphorus compounds, the scrubber cotton may not remove all impurities, and a preliminar treatment is necessary. The sample is heated a short time wit{ an alkaline solution, after which it is slightly acidified with hydrochloric acid and the excess sulfide removed by boiling. The remaining sulfur and reduced phosphorus compounds are oxidized with bromine water, and then the bromine is removed by boiling before adding the sample to the reaction flask. TABLE 1. RECOVERY OF ADDED INORGANIC ARSENIC UNDIGESTED FOODPRODUCTS
DISCUSSION OF RESULTS Known amounts of added inorganic arsenic were directly recovered from undigested material in such products as fruits, apple sauce, apple butter, cider, sauerkraut, celery, and other vegetables. However, in the case of marine products, such as shrimp, i t was not possible to determine
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Cotton
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TABLE11. TESTSON LYE-PEELED PEACHES
Blank
0.00125 mg.
0.0025 mg.
GTJTZ~IT DISK SAMPLEM ~ T H O D METHOD Mg./gram Mg./oram 52 0.00010 0.00017 89 0.00015 0.00036 54 0.00014 0 00028 136 0.00011 0.00017 137 0.00011 0.00007 183 0.00004 0.00004
0.006 mg.
0.0075 mg.
J’
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Cotton plug s a t u r a t e d w i t h s p e c i a l Cuprous Chloride s o l u t i o n
Milligan a b s o r p t i o n bulb aontnining 1ooco. or
