Interference of Pyridine Derivatives in Arsenic Determination Insecticide Division,
C. R. GROSS Bureau of Chemistry a n d Soils, U. S. D e p a r t m e n t of Agriculture,
Washington,
D. C.
Low results f o r arsenic are obtained in analyzthis condition is certain to lead RGANIC p r o d u c t s of to low results of analysis. It is ins tobacco by the Gutzeit method after acid digesplant or animal origin for are These low results are caused by undigested aformed well-known by a given fact that amount stains of tion. arsenic by the Gutzeit method, the material first being digested residues Of nicotine, which retard the arsine under conditions of slow of hydrogen and arsine during analysis. Every with nitric and sulfuric acids in or retarded evolution of hydroother pyridine derivative tested, as well as Pyrigen are deposited over a shorter order completely to oxidize the length of strip than when the organic matter and place the dine itself, behaves similarly. Data are offered It evolution arsenic in Except for to show the quantitative nature of the interference a few minor differences, which conditions that the stains are normal. formed produced by the pyridine studied* under conditions of retarded do not affect the final results, this l a b o r a t o r y employs the A procedure is given for eliminating such interevolutionwill be under-evaluated ference so that correct results can be obtained bv when they are compared with technic of the Association of the standard stains of known official Agricultural Chemists the Gutzeit method. value during the course of a (1). This method gives very normal evolution. low yields when used for the 3. The arsenic is not completely converted to arsine during determination of arsenic in tobacco. In most cases of routine analysis, the recoveries amount to only 60 to 95 per cent the usual period of evolution (1,5 hours). Appreciable stains of the arsenic present, and in extreme cases they may be less result when the evolution is continued for an additional period after fresh acid, zinc, and mercuric bromide strips have been than 50 per cent. added. NICOTINEINTERFERENCE INTERFERENCE OF PYRIDINE COMPOUNDS IN GENERAL Abnormal conditions of hydrogen evolution caused by undigested organic residues of the tobacco alkaloid nicotine were The nicotine molecule is composed of a pyridine ring comidentified as being responsible for the low results encountered. bined with an N-methylpyrrolidine ring. The author's inThese abnormal conditions, and their effects on the results of vestigations showed that the pyridine ring of the molecule is analysis, may be described as follows: the specific cause of the low results. The following experi1. As the evolution of hydrogen proceeds, the solutions be- ment was made in demonstrating this point: come darkened by a very fine suspension of tin originating Separate acid digestions were made of pure samples of nicofrom the stannous chloride reagent. Apparently none of the tine, pyridine, and N-methylpyrrolidine, to which had been tin adheres to the zinc. Under normal conditions, the tin added known amounts of arsenic. The existence of undigested organic residues could be demonstrated in all these 101 solutions by the charring which took place when individual portions were evaporated to dryness and ignited. PI Analyses by the Gutzeit method showed that the presence of N-methylpyrrolidine residues interfered in no way with the results of analysis, but that nicotine and pyridine residues 10 caused the same abnormal conditions to appear during analysis and the same condition of low results that had been 60 encountered in analyzing tobacco. Subsequently, a number of pyridine derivatives were tested, and all acted in a similar so manner. It should therefore be noted that interference of this sort is not necessarily confined to tobacco, but can be ex40 pected during the analysis of any product containing pyridine or its derivatives. 30
0
MILLIGRAMS OF NICOTINE DlGE5TkD PER ALIQUOT
METBODSFOR ELIMINATING INTERFEREXCE Interference caused by pyridine residues can be eliminated by dry-ashing the digested solution. This method is not convenient, however, because of the difficulties connected with disposing of the large amounts of sulfuric acid evaporated. The most convenient method developed to eliminate the interfering compounds makes use of the A. 0. A. C. procedure (9) for the treatment of coal-tar food colors preliminary to analysis by the Gutzeit method. By means of this method, the arsenic is precipitated along with the phosphate by treatment of the digested solution with ammonium hydroxide,
FIGURE 1. EFFECTOF NICOTINEON DETERMINATION OF ARSENICBY ACID DIGESTIONAND GUTZEITMETHOD
coats the zinc and forms gray flocculent masses, which finally drift to the surface, leaving the solution clear. I n the present case, the failure of the tin to adhere to and properly sensitize the zinc accounts, a t least in part, for the retarded rate of evolution described next. 2. The presence of undigested nicotine residues slows down the rate of evolution of hydrogen to a marked degree. Those familiar with the Gutzeit method will a t once recognize that 58
January 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
phosphoric acid, and magnesia mixture. The precipitate containing the arsenic can then be filtered and washed free of the soluble pyridine residues, redissolved in a hydrochloric acid solution, and analyzed by the Gutzeit method. The method, as modified and simplified for use in analyzing tobacco, may be described as follows:
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REAGENTS.All reagents should be arsenic-free. Ammonium hydroxide, containing not less than 26 per cent by weight of NHa. Dilute ammonium hydroxide (1 to 9). Dilute phosphoric acid, 19 cc. of HaPO4,85 per cent, diluted to one liter. Magnesia mixture, containing 55 grams of hydrated magnesium chloride, 55 grams of ammonium chloride, and 88 cc. of ammonium hydroxide per liter. Dilute hydrochloric acid (1 Lo 4). PROCEDURE. Make an acid digestion of a suitable sample in the usual manner. Transfer an ali uot of the diluted digested solution representing about 0.2 mg. arsenic trioxide to a 400-cc. beaker. (In the absence of a rough idea regarding the amount of arsenic contained by the sample, the size of this aliquot must be determined by trial.) Dilute to 150 to 200 cc. Make alkaline with ammonium hydroxide, add 20 cc. of the dilute phosphoric acid, and then slowly add 25 cc. of the magnesia mixture, stirring meanwhile. Add 5 cc. of c. P. ammonium hydroxide, stir, and then allow the mixture to stand for at least 15 minutes. Filter through an 11-cm. paper, and rinse the precipitate with several 15-cc. portions of the dilute ammonium hydroxide and finally with 10 cc. of. water. Drain for at least 15 minutes, occasionally tamping the glass funnel against the funnel support to settle the precipitate and encourage the drainage of the last few cubic
59
arsenic ranging from about 0.010 to 0.040 mg. of arsenic trioxide. Each digested solution was analyzed by the Gutzeit method both before and after phosphate precipitation. Since the A. 0. A. C. method permits the use of either granulated or stick zinc, a complete set of analyses as described above was made with each type of zinc. The results before precipitation were then calculated as percentages of the amounts found after precipitation.
07
100 90 80
70 60
50
$0 JO PO IO
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I
I
I
I
I
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DIGESTED PER ALIQUOT FIGURE3. ARSENICRECOVERED IN PRESENCE OF PYRIDINE COMPOUNDS, USINGSTICKZINC FOR HYDROGEN GENERATION
The relation between the yields of arsenic and the amounts of nicotine represented by the aliquot is shown in Figure 1. Although individual observations are somewhat scattered, the composite curves indicate that a distinct correlation exists. It is observed abo that the recoveries obtained with stick zinc are distinctly superior to those with 20-mesh zinc. The reason for this was not discovered, but the circumstance should not be interpreted as meaning that stick zinc is consequently to be preferred for general purposes of analysis. Our tests with these two forms of zinc show that under normal conditions they give results of practically equal accuracy.
/O
QUBXTITATIVE INTERFEREKCE PRODUCED BY DIFFERENT PYRIDINE COMPOUKDS In view of the fact that pyridine or its derivatives may be present in products other than tobacco which are assigned to FIGURE 2. ARSENICRECOVERED IN PRESENCE OF PYRIDINE COMPOUNDS, USING20-MESHZINCFOR HYDROGEN GENERATION the chemist for arsenic analysis, it is of interest to know to what degree different pyridine compounds interfere with the centimeters of filtrate. Dissolve the precipitate in 40 cc. of the results of analysis by the Gutzeit method. With this end in dilute hydrochloric acid, pouring the acid on the filter in small view, the following pyridine compounds were made t h e . portions and allowing it to drain into a 100-cc. volumetric flask. subjects of experiment: Rinse the filter in the same way with about 50 cc. of water and dilute the solution to volume. Use 5- to 20-cc. aliquots for the Pyridine C5H6N Gutzeit analysis. Add to the acid in the aliquot sufficient c. P. Nicotinic acid C5H4N COOH concentrated hydrochloric acid to make its total volume 5 cc. r-7-Dipyridyl (CbH4N)z Complete the determination as directed in the A. O.,,A. C. method (S), beginning with “Add 5 cc. of the KI reagent. Piperidine CBHllN Nicotine CioH& QUAKTITATIVE INTERFEREIiCE OB h-ICOTINE Quinine CzoH240zNz Analyses were made to determine the extent to which the PROCEDURE. Two grams of each compound were thoroughly amounts of nicotine normally present in tobacco had lowered digested with nitric and sulfuric acids to as nearly a colorless soluthe results of analysis. The method of analysis described tion as could be produced. Each digested solution was diluted to one liter after sufficient standard arsenic pentoxide solution above was used for comparison. Eight samples of powdered had been added t o make the diluted solution contain the equivadry tobacco leaves which carried arsenical residues ranging lent of 2 micrograms of arsenic trioxide per cubic centimeter. from the heaviest to the lightest encountered in routine analy- This solution was designated in each case as solution 1. Theresis were selected for analysis. One portion of each sample was after, solutions 2, 3, 4, etc., were prepared from a portion of each solution in such a wa that each contained the same analyzed for nicotine, while a second portion was digested in preceding concentration of arsenic and sulfuric acid, but one-half as the usual manner for arsenic analysis. The digested solu- much of the digested pyridine compound as had been contained tions were diluted until 5- to 15-cc. aliquots of each contained in the preceding solution.
09
ANALYTICAL EDITION
60
Solutions 1, 2, 3, etc., of each digested compound were then analyzed by the Gutzeit method until a dilution was reached which caused no visible interference with the results of analysis. In each case, 5-, lo-, and 15-cc. aliquots, corresponding to 10,20, and 30 micrograms of arsenic trioxide, were used. One set of analyses was made with stick zinc, and a second set was made with granulated zinc (20-mesh) at the rate of 2 grams per generator bottle. The results of analysis were calculated as percentage recovery of the total amount of arsenic present. The data showed that the percentage of arsenic recovered depends solely on the amount of interfering compound present, and is not influenced by the amount of arsenic contained by the aliquot, a t least not through the range of 10 to 30 micrograms. Figure 2 shows the average percentage of the total arsenic recovered in the presence of pyridine compounds ranging from 0 to 20 mg. digested per aliquot used. Granulated zinc (20-mesh) a t the rate of 2 grams per generator bottle was used in these analyses. It will be observed that y-y-dipyridyl exerts a tremendous degree of depression on the results of analysis, and that nicotine and pyridine produce a marked depression; whereas nicotinic acid, piperidine, and quinine cause much less interference. It would not be expected that quinine would have a great restraining influence, inasmuch as its one pyridine ring is only a small part of its total molecular structure. Figure 3 is based upon the results of analyses similar to those used for Figure 2, except that stick zinc was used for evolution purposes. It will be noted that with the exception of y-y-dipyridyl the results of analysis are distinctly higher when stick zinc instead of granular zinc is used. This same circumstance was noted previously in connection with the tobacco analyses shown in Figure 1. A comparison of the nicotine curves in Figures 1, 2, and 3 shows that much lower results were caused by nicotine during routine tobacco analysis than when the digestions were made with pure nicotine. This would indicate that the method of nicotine analysis does not detect all the interfering compounds present in tobacco.
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It may be mentioned here that the ability of pyridine compounds to cause these disturbed evolution conditions does not originate as a result of digestion with nitric and sulfuric acids. The pyridine compounds before digestion cause the same type of interference to an equal or a greater degree than they do after digestion. GENERALCONSIDERATIONS Aside from tobacco, not many products which are analyzed for arsenic are likely to contain pyridine or its derivatives in amounts sufficient to cause low results of analysis. Among the possibilities are materials containing animal hides, bones, or bone oil. It is possible also that apples sprayed late in the season with nicotine sulfate may contain nicotine residues sufficient to interfere with the determination of arsenical residues deposited during previous sprayings with lead arsenate. Unsuspected sources of pyridine interference may likewise be encountered at times in organic products of unknown origin or composition. When materials of this nature are being analyzed for arsenic, it would be advisable to observe closely the character of the evolution and, in case of any abnormal appearance, to verify the results by making analyses after phosphate precipitation. ACKNOWLEDGMENT The author wishes to express his appreciation for the assistance rendered by Julian Hiley in carrying on much of the analytical work connected with this investigation. LITERATURE CITED (1) Assoo. Official Agr. Chem., Official and Tentative Methods, pp, 109, 306-9 (1930). (2) Ibid., p. 203. (3) Ibid.,p. 308, section 4. RECEWED April 15, 1932. Presented before the Division of Agricultural and Food Chemistry at the 82nd Meeting of the American Chemical Society, Buffalo, N. Y., August 31 to September 4, 1931.
Analysis of White Metals and Their Smelter Products HANSNEUBERT, R. F. D. No. 1, Box 150-B,Rahway, N. J.
T
HE samples as delivered from the sample mill consist
mostly of a bar and slag which have to be weighed out in proportional parts. The metallic part, however, is generally not uniform, so that a larger sample has to be taken in order to get a good average. This difficulty obviously does not exist in the case of slags which are perfectly well miscible. The method of analysis as used in the laboratory of the U. S. Metals Refining Company begins differently from the methods generally used, which consist in a separation of the tin from the rest of the other metals by precipitating it as metastannic acid in nitric acid solution. This method of attacking the white metals with nitric acid has the undeniable disadvantage that the metastannic acid is never chemically pure, that a small part of the tin goes into solution, and that a very difficult operation ensues as soon as a greater weight of metal is taken for analysis. Therefore, experience has shown that it is preferable in any case to dissolve the metallic part to a clear solution in
hydrochloric acid, and take this solution as the starting point for further operation. The antimony and copper, which are attacked with difficulty by hydrochloric acid, are dissolved by an addition of sodium chlorate dissolved in water, or by a few drops of nitric acid. The oxidizing substance is chosen according to a preliminary test for copper. If this test shows that the copper is low, then the antimony and copper are oxidized with sodium chlorate so that a straight volumetric determination can be made later. The method used for the bar is as follows: The proportional part corresponding to 20 grams of the sample is dissolved in a 500-cc. flask with concentrated hydrochloric acid a t moderate heat, After the reaction ceases, the antimony and copper are dissolved by adding just enough oxidizing agent for the purpose. The flask is then cooled and filled up to the mark with concentrated hydrochloric acid, the contents are mixed, and the flask held in readiness for the slag part. If it is intended to run the tin by the straight volumetric
