JULI- 15, 1938
.4X.4LUTIC,4L EDTTIOX
Method of Separation In the course of analyses for the pyrethrins by the Seil method it was observed that low-boiling petroleum ether does not extract the chrysanthemum dicarboxylic acid from an acidified solution. Since the chrysanthemum monocarboxylic acid is soluble in petroleum ether, it should be possible to separate the mono- from the dicarboxylic acid by such a selective extraction. A few preliminary experiments were sufficient to demonstrate that petroleum ether does not extract the dicarboxylic acid; but, in order to prove that it is possible to separate the two acids, several artificial mixtures of the two acids were prepared and subjected to the following analysis: Aliquots from standardized, freshly prepared, alkaline solutions of the acids were mixed, acidified, and then extracted with lowboiling petroleum ether as in the Seil method, with the exception that care was taken to retain quantitatively the water layer and washings. The petroleum et,her extract was titrated with standardized 0.0200 N base in the usual manner, but the water layer was again retained. The acidified water layer and washings from the petroleum ether extraction w r e then extracted with diethyl ether, and the chrysanthemum dicarboxylic acid was determined by the Seil method. The acids were then recombined and the mixture was subjected to analysis for both acids by steam distillation (the Seil method). TABLE 111. SEPAR.4TION
O F CHRYBAKTHEYUM MOXO- .4SD CARBOXYLIC ACIDS
DI-
(Both acids expressed in cc. equivalent of 0.0200 N base) Acid Found Loss by Selective Solvent Steam DisAcid Present Extraction Steam Distillation tillation, .MonoDiMonoDiMonoDiMono-
Sam*le
% 29.0 34.1 38.7 45.5 38.4 0.0 33.1 35.2
29.2 34.2 38.7 45.6 38.5 0.1 33.1
39.8 41.9 81.4 40.4 41.9 44.0 0.0 49.8
..
25.2 30.3 32.6 41.4 36.0 0.1 30.0 31.6
39.7 41.8 81.2 40.3 41.8 44.0
0.1
..
40 2 42.1 81.7 40.6 42.0 44.1 0.1 49 9
13.1 11.3 16 0 9 0 6 3 9' 4 10 3
Several such determinations were carried out, and the results are summarized for comparison in Table 111. It is apparent that tihe separation of the two acids by the selective solvent extraction method is quantitative, well
387
within the experimental errors involved. The destructive effect of steam distillation on the monocarboxylic acid is again demonstrated; for, while the results of the two methods for the dicarboxylic acid are comparable, those for the monocarboxylic acid differ by over 10 per cent. The use of this method for the estimation of the pyrethrin content of pyrethrum flowers and its extracts was suggested. But, when it was applied to samples prepared in accordance with the Seil method, the results were somewhat high for both acids, perhaps because of the presence of other acidic constituents. Further work is being pursued, and there are indications of a successful solution. Selective extraction of the monoacid by other solvents is also being investigated.
Conclusion Because of the destructive effect of steam distillation on the chrysanthemum monocarboxylic acid, the acid methods of pyrethrin I analysis are inaccurate and unreliable (8). It is possible to separate the two chrysanthemum acids by the selective extraction of the monocarboxylic acid with lowboiling petroleum ether, but this method requires further study before it can be applied to the estimation of the pyrethrin content of pyrethrum flowers and their commercial extracts.
Acknowledgments The writer is greatly indebted to F. Y. Klock and A. A. Srebren, formerly of the Murray and Nickell Manufacturing Company, who made this investigation possible by granting the use of their laboratory facilities and supplies.
Literature Cited (1) Gnadinger, C., and Corl, C., S. Am. Chem. SOC.,51, 3054 (1929). (2) Graham, J. T., IND,ENG.CHEM.,Anal. E d . , 8, 222 (1936). (3) Graham, J. T., J . &soc. Oficial A g r . Chem., 20, 388 (1937). (4) Haller, H. L., and .4rree, F., Jr.. IND.ESG. CHEX.,Anal. Ed., 7. 343 (1935). (5) R i p e r t l J . : - A n n . j a l s , 28, 27-38 (1935). (6) Seil, H. A . , Soap. 10, 89 (1934); Chem. Trade J., 95, 168 (1934). (7) Tattersfield, F., and Hobson, R., J . A g r . Sci., 19, 433-7 (1929). ( 8 ) Wilcoxon, F., Contrh. Boyce T h o m p s o n Inst., 8 , 175 (1936).
RECEIVED March 1 1 , 1938.
Evaluation of Commercial Arsenious Oxide by Titration with Iodine P4UL LUNDXIAS, Bolidcns Grul .4/BB>produhtslaboratorium,Ronnskar. Skclleftehamm, Sweden
T
HE following procedure has been used in the analysis of
commercial arsenic trioxide: Weigh out samples of 5.000 grams, cover with 70 ml. of 6 N sodium carbonate solution, and boil until completely dissolved. Cool, make up to 1liter, and take a 50-ml. aliquot for the analysis. Make the solution barely acid with 6 iV sulfuric acid, add 3 grams of sodium bicarbonate, and titrate with 0.1 N iodine to a starch end point. TABLE I. ANALYSISOF ARSENICTRIOXIDE (95 to 97 per cent pure Xs203)
% Probable purity Direct titration value Artificial standard 100 - (nonvolatile Nonvolatile residue
+
Ass01
so3
SbnOs
Se
%
%
%
96.44 97.01 96.50 96.34 3.01 0.20 0.45 0.46 0 21
95.79 96.58 96.00 95.89 3.27 0.34 0,50 0.55 0.17
95.06 95.48 95.07 95,2S 3.96 0.31 0.43 0.78 0.21
If the iodine solution is standardized in exactly the same way against a sample of arsenic trioxide known to be pure, the results are very exact, provided nothing other than arsenic is present that is oxidized by iodine. It has been known for some time, however, that the values thus obtained are usually too high. The experiments described in this paper were undertaken to ascertain the reason for the error and t o see if the difficulty could not be overcome in some simple manner. I n Table I, the results obtained in the analysis of four samples containing 95 to 97 per cent of arsenic trioxide are given. The first horizontal column gives the actual purity of the sample obtained by determining all the impurities known to be present and subtracting the sum of these values from 100. The second column gives the values obtained by direct titration as outlined above; these values are about 0.5 per cent too high. The third column gives values which were
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
388
obtained by standardizing the iodine solution against pure arsenic trioxide, to which the impurities found t o be present in the sample to be analyzed had been added intentionally. The fourth column gives values obtained by adding together the nonvolatile, arsenic pentoxide, and sulfur trioxide contents and subtracting this value from 100. These results show"c1early that the values obtained by direct titration are about 0.5 per cent too high. Good results were obtained, however, by standardizing the 0.1 N iodine against pure arsenic trioxide to which impurities had been added intentionally, or by subtracting the nonvolatile residue plus arsenic pentoxide plus sulfur trioxide from 100 per cent. TABLE11. ANALYSISOP ARSENICTRIOXIDE (98 to 99 per cent pure 4siOd 1 2 3 4 5 6
%
%
98.42 98.26 1.00
97.93 97.79
!::!:i:~
Probable purity Direct titration value Artificial standard 100 - (nonvolatile .hOs) Nonvolatile AsiOs
+
7 Sb10a 8 Se
E:;
1.10
0.056
0.044
%
%
",:E
!::ti
9s. 30 0.87
9s. 60 98.59 0.069
0.027
0.25
98.46
:::::
Table 11 gives obtained in the analysis of samples of 98 to gg per centpurity, H~~~again, the values obtained by direct titration are about 0.45 per cent t'oo high on an average, but the results are good if the iodine solution is
VOL. 10, NO, 7
standardized against arsenic trioxide to which the same impurities have been added. TABLE 111. ANALYSISOF ARSENICTRIOXIDE (90 t o 9 1 per cent pure AszOs) 1 2
4
AsaOs
5 6 7 8 9
As205 803 sulfide S SbiOs
+
Se
+
%
%
%
%
%
89.89 90.49
90.35 91.61
91.15 92.42
89.65 90.25
90.57 89.94
90.29 7.22 0.40 2.09 1.00 0.55
90.31 6.41 0.15 3.13 0.97 0.41
In Table 111, the results obtained in the analysis of samples of arsenic trioxide which were 90 to 91 per cent pure are given, Here the results obtained by direct titration are erratic. In two cases the results are 1.25 per cent too high but in one case the value is 0.63 per cent too low. These samples contain appreciable quantities of selenium dioxide, antimony trioxide, and sulfur trioxide sulfides. They were hard to titrate and the end points were not easy to find. I n samples of this character, the only thing t o do is to determine the total arsenic (by distilling as arsenic trichloride and titrating the neutralized distillate with iodine) and to make a separate determination of arsenic pentoxide. The arsenic trioxide Can then be found by difference.
+
RECEIVED February 6, 1938.
Chemical Determination of Quartz (Free Silica) in Dusts AND W. THURBER F.ILES, Bureau of Vital Statistics, Baltimore City Health Department, Baltimore, Md.
EMANUEL KAPLAN, Division of Chemistry, Bureau of Laboratories,
I
N THE Knopf method ( I ) for the determination of quartz in the presence of silicates, the silicates are dissolved in hydrofluosilicic acid, leaving behind a residue containing quartz. The acid, however, decomposes quartz also with a resulting average rate of loss of 0.7 per cent per day of the original weight of quartz present in the sample. Knopf (I) notes that "by using the above factor of error it is possible to compute a t the end of an analysis the maximum possible loss in weight of quartz originally present, thus obtaining a maximum figure of quartz.'' Similarly, in a more recent method (2) which proposes the use of fluoboric acid in place of hydrofluosilicic acid, Line and Aradine state that "the free silica content of the residue must be corrected for the amount of free silica dissolved during the time required to decompose the silicate. The correction factor is 0.34 per cent per day." Neither of the above methods indicates the manner in which its respective correction factors are to be applied. Accordingly, the following mathematical treatment is presented for the interest of individuals routinely using these methods of analysis. The problem involves an application of the frequently encountered compound interest law, also called the law of organic growth, or the snowball law, which may be expressed for rates of decrease by the differential equation dy/ds = -ky. This expression is the first derivative of the exponential equation y = yOe-k', or in the logarithmic form log y
= log yo
- k x log e
I n the present problem = yo = k = z =
y
log e
mg. of quartz in residue mg. of quartz originally present rate of loss (0.7 per cent per day in the Knopf method) time of action in days = 0.43429
For example, in a determination of quartz in the presence of refractory silicates by the Knopf method, 500 mg. of dust required 10 days of treatment with hydrofluosilicic acid, leaving a residue corresponding to 50 mg. of quartz. The amount of quartz originally present was calculated in the following manner : log yo = log y $- 122 log e = log 50 (0.007 X 10 X 0.43429) = 1.72937 yo = 53.63 mg. of quartz originally present, corresponding to 10.7 per cent of quartz in the original sample of dust taken for analysis.
+
Literature Cited (1) Knopf, A, U. S. Pub. Health Service, Pub. Health Repts., 48, No. 8, 183-90 (1933). (2) Line, W. R., and Aradine, P. W., IXD.ENG.CHEM., Anal. Ed., 9,60-3 (1937). RECEIVED April 20, 1938,
