Application of Enclosed Torch to Estimation of Arsenic in Foods ROEE. REMINGTON, E. JACKCOULSON,' AND HARRY VON KOLNITZ Medical College of the State of South Carolina, Charleston, S. C.
T
HE method of the Association of Official Agricultural Chemists (I) for the determination of traces of arsenic
a t high temperatures and correspondingly greater amounts of nitric acid. With acid-digested tobacco, Gross (2) experienced difficulty which he attributed to the presence of pyridine, and was able to get better recoveries if the arsenic was precipitated by magnesia mixture before marshing, thus separating it from the pyridine. Pyridine is obtained on dry distillation of many proteins, however, and Sorensen and Anderson (6) have noted that if nitrogen determinations yield lower values by the Kjeldahl than by the GunningArnold method, the sample contains either ring-like nitrogenous compounds such as pyridine or piperidine which are not decomposed, or substances which yield these compounds on closure of the ring; hence the difficulty encountered by Gross in working with tobacco might occur in many foods rich in protein. TABLEI. DISTRIBUTION OF ARSENIC IN DIFFERENT PARTS OF ABSORPTIONTRAIN
in foods provides for the destruction of organic matter by wet combustion with nitric and sulfuric acids. Wet ashing, properly conducted, gives concordant results and good recoveries on most food products. In applying it to dried shrimp and cod liver oil, however, the authors experienced certain difficulties, which, together with the long time required for the digestion, led them to try combustion in the enclosed torch devised in this laboratory (7) for the estimation of iodine in foods.
METHOD The apparatus is set up and handled exactly as for the estimation of iodine (7). It was found, however, that 1 or 2 cc. of nitric acid in each absorber could be substituted for the sodium hydroxide, with equally efficient absorption and the advantage that less acid is required in the later treatment. After combustion, the water and ash of the cu are transferred to a beaker, the cup and flask rinsed with very chute nitric acid, and the rinsings added t o the beaker together with the contents of the absorption bottles. The combined solutions are evaporated to small volume, then 5 to 20 cc. of sulfuric acid are added, the beaker is covered with a watch glass, and the solution evaporated to fumes. The concentration to small volume before addition of sulfuric acid insures oxidation and expulsion of chlorine, a precaution that is of particular importance in analysis of sea food or other products rich in salt. In a properly conducted combustion there should be no darkening with sulfuric acid, but if darkening does occur, a few drops of nitric acid will clear the solution. From this point the solution or an aliquot of suitable size is treated as provided in the official method. It was found that while a larger proportion of arsenic than of iodine stays in the combustion chamber, absorption in the washing bottles is not so efficient, so that in some cases the addition of a third washing bottle is desirable. On account of its larger orifice, the Friedrichs wash bottle is very much less efficient than the Milligan in this case, even though the two bottles are alike in principle. Table I gives data from experiments with both types of bottles.
(10 grama dried shrimp) NITRICACID SODIUM HYDROXIDB UBBD IN
ABBORBBRB Combustion chamber 1st Friedrichs bottle 2nd Milligan bottle 3rd Friedrichs bottle
1
Associate biochemist, U. S. Bureau of Fisheries.
51.95 6.43 40.58 1.04
-
8 8 ~ 0 3
-
P. p . m.
100.00 P . p . m.
157.1
154.0
TABLE11. RECOVERY OF ARSENICFROM DRIEDSHRIMP WBIGHT A8108
RBCO~BRY OFADDED AszOa RBMARKS % ._ NO stain W e t o x i d a t i o n at low temperature No stain W e t o x i d a t i o n at low temperature No stain None Wet oxidation at _" Intemperature Trace Above solution pptd. with magnesia mixture Trace Above solution pptd. with magnesia mixture 1.03 103 Above solution pptd. with magnesia mixture 0.72 Wet oxidation with Cut304 at high temperature 0.73 Burned in torch 0.725 Burned in torch 1.69 97 Burned in torch 0.275 Burned in torch 0.289 Burned in torch 0.288 Wet oxidation with CuSO4 at high temperature 0.109 Burned in torch I. 090 98 Burned in torch 0.215 Wet oxidation with CuSOi at high temperature 0,048 Burned in torch 0.049 Burned in torch 0.045 Burned in torch 0.097 Wet oxidation with CuSOi at high temperature 0.900 Burned in torch 0.980 Burned in torch 0.470 Wet oxidation without cuso4 0.988 Aliquot of preceding redigested with CuSOr AssOs
SAMPLE TAKBNADDED FOUND Grams Mu. Ms. ' 10
20 10
1.0
10 107
'
20
10
1.0
10
IMPORTANCE OF OXIDATION Usual directions for wet digestion do not stress sufficiently the necessity for constantly maintaining oxidizing conditions. Whenever charring of the sample takes place, arsenic may be reduced from the pentavalent to the trivalent state. According to Rushton and Daniels (6) the vapor pressure of arsenious oxide is practically negligible below 250 ", and increases to 32 mm. a t 275" and 144 mm. a t 338"C., the boiling point of sulfuric acid. Appreciable losses might therefore be expected if the digest were heated to vigorous fuming while any reduced material was still present. Analysts generally avoid this danger by keeping the temperature relatively low, with frequent additions of nitric acid, until satisfied that oxidation is complete and arsenic consequently in pentavalent form . Occasionally substances are encountered which are very difficult to oxidize completely by the wet method, or which contain compounds which interfere with the evolution of arsine in the generator. Fats and oils require long treatment
%
52.83 4.20 41.69 1.28
100.00
Total
USEDI N ABSORBBRB
%
10 10
,lo
1.0
108
102
113
117
1.0
..
APPLICATION OF TORCH METHOD The authors digested a sample of dried shrimp by the official method, but failed to recover more than traces of arsenic, either with or without precipitation with magnesia mixture. When 1 mg. of arsenic trioxide was added to 10
280
July 15, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
grams of dried shrimp before digestion, none was recovered by direct marshing of an aliquot, but 1.03 mg. after precipitation with magnesia mixture. The authors concluded that the arsenic of the shrimp was not liberated by the digestion, and that some substance, possibly pyridine, was present in the digest and inhibited the evolution of added arsenic, until removed by separation. This same sample of shrimp, when burned in the torch, yielded duplicate values of 73 and 72.5 parts per million of arsenic trioxide. When digested with acid a t higher temperature and with the addition of copper 72 parts sulfate as a catalyst, as suggested by Maechling (4, per million were recovered. A sample of pipe tobacco yielded 17.4 parts per million of arsenic trioxide by wet oxidation, and 24.8 parts after precipitation with magnesia mixture, confirming the observation of Gross. Another sample of tobacco gave 28.0 when treated according to Gross, and 27.4 when burned in the torch. The results shown in Table I11 indicate that the method can be applied to various kinds of dried vegetable matter.
28 1
turns with the feed plate, the bottom of it is split and the two halves are pressed down on the pins of the plate. Twenty samples of American cod liver oil analyzed by this method gave values ranging from 1.9 to 6.7 parts per million of arsenic trioxide (3). Duplicate analyses of cotton rolls, to which had been added 1 0 7 of arsenic dissolved in alcohol, yielded identical results of 10.4 y. No detectable stain was obtained from the cotton alone.
CONCLUSION The method is applicable to products which contain sufficient combustible matter to burn freely in a current of oxygen. Consequently it can hardly have wide application in the estimation of spray residues on vegetables, which would have to be dried and ground before burning, except as a check on acid digestion. Its principal advantages, when applied to dried material or oils, are economy in time and reagents and the elimination of acid fumes from the air of the laboratory. Dry samples ranging in size from 5 to 100 grams can be burned continuously in one operation, a t a rate of 1 to 3 grams per FROM MISCELLANEOUS minute. The amount of oil that can be burned in one operaTABLE111. ARSENICRECOVERED FooDs BY ENCLOSED TORCHCOMBUBTION tion is limited to that which a cotton roll will absorb without AS203 RECOVERED leaking when subjected to the heat of the torch.
SAMPLE 753 1265 856 837 796 493 794 388 812
857
A B
DESCRIPTION
Carrot tops Carrot topa Cabbage S C ) Cabbage IS: C:) Cabbage (N. Y.) String beans ( 6 . C . ) String beans (Ga String beans (S.d.1 Spinach (Va Spinach (5. Dried kelp Dried kelp
2.)
Dry basis Fresh basis P. p . m.. P . p . m. 0.34 0.046 0.16 0.024 0.35 0.035 0.12 0.008 0.04 0.003 0.32 0.052 0.29 0.025 0.20 0.020 0.59 0.084 0.94 0.116 58.0 ... 97.0
...
The authors also found the torch applicable to the burning of cod liver oil, the oil (usually 5 cc.) being pipetted onto a 6-inch (15-cm.) piece of No. 3 dental cotton roll, the roll enclosed in a segment of Visking sausage casing to prevent loss of oil by contact with the side of the feed tube of the apparatus, and the roll burned. To insure that the roll
LITERATURE CITED (1) Assoc. Official Agr. Chem., Official and Tentative Methods, 3rd ed., pp. 306-8 (1930). (2) Gross, C. R., IND. EXG.CHEM.,Anal. Ed., 5, 58 (1933). (3) Holmes, A. D., and Remington, R. E., IND. ENG.CHEM.,26, 573
(1934). (4) Maechling, E. H., and Flinn, F. B., J . Lab. Clin. M e d . , 15, 779 (1930). (5) Rushton, E. R., and Daniels, F., J . Am. Chem. SOC.,48, 384 (1926). (6) Sorensen, S. P. L., and Anderson, A. C., 2. physiol. Chem., 44, 429 (1905). (7) von Kolnitz, H., and Remington, R. E., IND.ENG.CHEM., Anal. Ed., 5, 38 (1933). RECEIVED March 9, 1934. Presented before the Division of Agricultura and Food Chemistry at the 87th Meeting of the American Chemical Society, St. Petersburg, Fla., March 25 to 30, 1934.
Determination of Base Exchange in Soils with Copper Nitrate E. A. FIEGER, J. GRAY,AND J. F. REED,Louisiana State University, Baton Rouge, La.
T
HE determination of the total base-exchange capacity and the amounts of the various exchangeable bases in soils is important in many soil and agronomic investigations. The results of such determinations have been applied with singular success in the study of morphological and genetical problems pertaining to soils; the study of soil acidity and its related problem, soil liming; the investigation of the availability of plant food cations and their retention when applied as fertilizers; and finally the problem of adequately explaining the development of so-called alkali soils. It is evident that a simple, accurate, and rapid method for the determination of the base-exchange capacity of soils and the amounts of the various exchanged bases should be available to the soil investigator. By the term “base exchange’, is meant the exchange or replacement of the adsorbed cations of the soil by some other cation. Those cations which have been replaced or removed are known as the exchanged or replaced bases. Theoretically any salt can be used to supply the replacing cation, provided
it will form an aqueous solution of sufficiently high concentration and be highly ionized. I n laboratory practice the soil is usually leached with solutions of one of the following salts: sodium chloride, potassium chloride, ammonium chloride, or ammonium acetate, or with 0.05 N hydrochloric acid. Each of these substances as a source of cations has certain inherent objections. When sodium or potassium salts are used as the replacing agents, it is impossible to determine the amounts of these elements in the replaced bases. Another serious objection is that exceedingly small quantities of the replaceable bases, a few milligrams, must be determined in the presence of a high concentration of the replacing agent, usually several grams, or some special method must be used in order to remove these salts, such as volatilization in the case of ammonium salts with possible mechanical losses, or evaporation with nitric acid, which is tedious, disagreeable, and expensive when many determinations are to be carried out, as previously pointed out ( 2 ) . Also it is important to note that all the methods so far proposed and used require for the estimation of
