V O L U M E 24, NO. 2, F E B R U A R Y 1 9 5 2 limit of detection to about 0.4microgram per liter, and means were therefore sought t o extend the sensitivity downward to t h a t attainable with pure beryllium standards. The preferential chelating ability of ethylenediaminetetraacetic acid on calcium and magnesium a t a neutral pH appeared promising. Previous work in these laboratories showed this compound to have virtually no chelating power toward beryllium under these conditions. Manganese was chosen to serve as a gathering agent, as it is only moderately complexed by ethylenediaminetetraacetic acid and a t the same time yields a gelatinous precipitate with alkali. It was found possible to precipitate a very minute quantity of manganese phosphate by this procedure, which on sparking proved to contain virtunlly all of the beryllium and yield intensities comparable to those attainable with pure beryllium solutions. Varying amounts of manganese phosphate precipitate do not depress beryllium line intensities. Of the several metals tried as internal standards, aluminum gave most satisfactory results. .4relatively week reference line of aluminum was chosen to avoid the pclssibility of error due to extreneous sources of this metal in uri le and reagents. S o interferences due to manganese or other variables were manifest in this line. What appears t o be a water bar d spectrum presents a faint line a t 3130.4A. This primarily acco i n t s for the failure of blank sample ratios to pass through zero. 'Phis background line is sufficiently constant to permit the estbnation of beryllium on the concentrations cited. For concentratj on greater than 0.005 micro-
409
gram of beryllium per ml., Be 3131.2 A. may be used instead of 3130.4 A. Other beryllium lines may be used for greater concentration than those described, the most useful of which has been found to be Be 2348 A. This line first appears a t a concentration of about 0.005 microgram of beryllium per ml. When it is desired to extend the lower limit of detection of beryllium in urine, 100-ml. samples of urine may be handled if larger centrifuge tubes are used, a6 difficulty may be experienced in keeping potassium perchlorate in solution. Limiting the final volume of solution sparked to 1 ml. further increases the sensitivity of the method. LITERATURE CITED
Barnes, E. C., Piros, W.E., Bryson, T. C., and Wiener, G. W., h . 4 ~ CHEM., . 21, 1281 (1949). Boyle, A. J., Whitehead, T., Bird, E. J., Batchelor, T. hf., Iseri, L. T., Jacobson, S.D., and Myers, G. B., J . Lab. Clin. Med., 34, 625 (1949). Cholak, J., and Hubbard, D. M., ANAL.CHEY.,20, 73 (1948). Ibid., p. 970. Klemperer, F. IT., and Martin, A. P., Ibid., 22, 828 (1950). Peterson, G. E., Welford, G. A., and Harley, J. H., Ibid., 22, 1197 (1950). Smith, R. G., Craig, P., Bird, E. J., Boyle, A. J., Iseri, L. T., Jacobson, S. D., and Myers, G. B., Ana. J . Clin. Path., 20, 263 (1950). RECEIVED July 19, 1961. Presented before the Division of Analytical Chemistry at the 119th Meeting of the AVERICANCHExrcAL SOCIETY,Boston, Mass.
Photometric: Determination of Microquantities of Arsenic CLARK E. BRICKER AND PHlLIP B. SWEETSER Princeton University, Princeton, N . 3. Comparatively few applica .ions of the photometric method for detecting the t nd points in volumetric determinations have been recorded. No previous use of the ultraviolet regicn of the spectrum or of the Beckman spectrophbto neter has been found for the determination of photometric end points. Ceric ions show a strong absoiption band at 320 mp, whereas cerous, arsenious, and arsenic ions do not absorb at this wave lengti. With an inexpensive titration cell for use on tlie Beckman DU spectrophotometer, it is possible to detect the end point photometrically in the titrr tion of microgram quantities of arsenious acid with ceric sulfate. The ac-
N
UMEROUS applications of the photometric titration
principle have been cited in 1. review by Osburn, Elliot, and Martin ( 4 ) . Later work may be found in the bibliography of a paper by Goddu and Hume ( 1 ) . This work was almost exclusively done with a special oorimercial photometer. The adaptation of the Coleman Model 1i spectrophotometer for use in photometric titrations was desci ibed by Goddu and Hume ( 1 ).
Xo previous reference has been found to the use of the ultra-
violet region of the spectra for photometric titrations, where the molar extinction coefficients for many volumetric reagents have their maximum value. The application of the ultraviolet region t o photometric titrations should lead to a greatly increased sensitivity, better adherence to Beer's law, the use of more dilute
curacy of these determinations is 1 to 2 parts per thousand. The preparation and stability of very h') ceric sulfate solutions have been studdilute ied and a method for preparing a solution which is stable for at least 2 months is described. As the photometric determination of the end point in the determination of arsenic with a standard ceric solution eliminates the use of indicators and of indicator errors, and also obviates the necessity of titrating to the exact end point, this method is very accurate and rapid. Use of the ultraviolet region of the spectrum for photometric titrations suggests many more applications of this method.
volumetric reagents, and a greatly increased number of volumetric reagents which show no strong characteristic absorption in the visible region of the spectrum. The advantages of working in the ultraviolet region are verified in this paper, where a very dilute ceric(1V) sulfate solution is used for the determination of micro quantities of arsenic. APPARATUS
The Beckman Model DU spectrophotometer, being one of the most widely used spectrophotometers and having a useful wavelength range of 220 to 1000 mM, was chosen for this work. The adaptation of this instrument for use in photometric titrations was very simple and inexpensive. .4 titration cell was made along the lines described by Goddu and Hume ( I ) , except that a rrctangular tube (1 8 X 3.0 cm ) was
ANALYTICAL CHEMISTRY
410 attached to a 200-ml. beaker. The beaker and the upper part of the tube were painted with a flat black; a Bakelite cover was made for the beaker to prevent stray light from reachin the phototube. Two small holes were drilled in the cover, so t f a t a buret and a tube for stirring the solution could be inserted. The solution was mixed within 30 to 45 seconds by employing a stream of carbon dioxide from a very fine orifice. The rectangular tubing of the titration cell was wide enough to allow continuous bubbling of the carbon dioxide without variation of the galvanometer reading, provided the orifice was positioned EO that the gas bubbles did not actually enter the light path. The titration cell was positioned in the spectrophotometer by utilizing a n extra cell cover into uhich a slot the size of the tubing was cut. It was necessary to fit this compartment cover with a light-tight felt washer. Using the ordinary hydrogen discharge bulb as the light source, photometric end points a t wave lengths as low as 290 mp were possible with this borosilicate glass cell. A 10-ml. microburet was used for all the titrations described in this paper. This buret was fitted with a special capillary tip, so that the tip could be kept below the surface of the solution in the titration cell. No appreciable amount of diffusion was observed from this capillary tip in 15 minutes. PREPARATION OF SOLUTIONS
The preparation of the standard ceric(1V) sulfate solutions was found to be very important for the titrations involving very dilute solutions of this reagent. The difficulties described by Kirk ( 2 ) were encountered in the titration of arsenic with dilute ceric sulfate solutions prepared in the usual manner. Thus a 0.10 N solution which would give very dependable results would often lead to unsatisfactory results when diluted to 1 X 10-8 or 1 X 10-4 N . This difficulty was overcome by preparing the very dilute ceric(1V) sulfate solutions by a method similar to that given by Kirk. The standard stock solution of 0.10271 iV ceric(1V) sulfate was prepared in the usual manner ( 6 ) ,although Kirk's procedure might have been advantageous. This solution was standardized against a standard arsenious acid solution and checked against ferrous ethylenediammine sulfate tetrahydrate. The standardieations with these two reagents checked to v, ithin 3 parts per 10,000. I n the preparation of the more dilute cerium(1V) solutions it was found that ceric(1V) sulfate as dilute as 4 X 10-4 A- could be accurately prepared using redistilled sulfuric acid and water with an appropriate aliquot of the standard cerium(1V) stock solution. When 1 X S ceric(1V) sulfate solution was prepared, the resulting solution was made 2 N in sulfuric acid; for 4 X 10-4 N cerium(1V) solutions, the solution was only 1 ,V in sulfuric acid. Table I gives the results of the determination of arsenic with the cerium( IV) solutions prepared in this manner.
Table 11.
Stability of Ceric(1V) Sulfate Solutions Time of Standing
Soln. No.
Normality of Ce(SO4)r
%
0 12 28 61 0
1
( 2 N HzSOd 2
(1 N HzSOb)
10
24 48
1.007 X 10-3 1 . 0 0 8 3 X 10-2 1.0065X l o - ' 1.001 x 10-3
0.13 0.05 0.60
4.0047X 4.008oX 3.995 x 4.012 X
0.08 0.24 0.18
EXPERIMENTAL PROCEDURE
I n the determination of arsenic, a known quantity of a trivalent arsenic solution was added to the titration cell and this solution was then diluted to 100 ml. with sulfuric acid and water, so that the final solution was 1 to 1.2 N with sulfuric acid. T h e actual concentration of the acid was not critical. Three or four drops of 0.01 M osmium tetroxide were then added. The slit width of the spectrophotometer was adjusted so that the solution showed zero absorption. Known volumes of the ceric(1V) sulfate solution were added from the microburet and the solution was stirred 30 to 40 seconds before the optical density of t h e resulting solution was measured. The end point was determined by the usual procedure of plotting absorption against milliliters of ceric(1V) sulfate added. S o volume correction was required under any of the reported conditions.
I
I 2 4
28
32
36
40
4 4
; 6 6
48
52
56
60
64
CrlSO,),
Figure 1. Titration of 66.7 Micrograms of Arsenic in 75 M1. of Solution with Standard Ceric(1V) Sulfate
Table I. Analyses of Arsenic Samples Arsenic Found
10-4 10-4 10-4 10-4
The absorption spectrum of ceric(1V) sulfate shows a maximum a t approximately 320 mp, with a molar extinction coefficient of 5.58 X IO3 in 1 N sulfuric acid (3). Thus a wave length of 320 mp was employed in all titrations with ceric sulfate. Cerium(II1) solutions, arsenious acid, and arsenic acid do not absorb appreciably a t this wave length.
ML OF 4706XIO-*N
No. of Detns.
Deviation
Days
Normality of Ce(S01)z
Volume of Solution
Arsenic Taken
Average Error
A! I . 150 100 100 75
MQ.
.MQ.
%
0.10271 1.0224 X 10-8 1,0224 x 10-a 4.106 X 10-4
33.44 0.33286 0.16630 0,06678
33.42 0.33276 0.16613 0.G6G72
0.11 0.09 0.10 0.18
Very dilute ceric( IV) sulfate solutions which were prepared by merely diluting 0.1 N cerium( IV) solutions with redistilled sulfuric acid and water were stable for only about 3 days. However, stable solutions of this oxidant could be prepared in the following manner. An appropriate sized aliquot of about 0.1 Ai ceric sulfate was diluted with redistilled sulfuric acid and water to the desired volume and then heated on a steam bath for 7 to 8 hours. The resulting solutions were standardized against a standard arsenious acid solution and it was found that these ceric solutions, when properly protected from dust, etc., were stable for a t least 2 months. The results showing the stability of two very dilute ceric solutions are shown in Table 11. The standard arsenious acid was prepared by the usual procedure ( 6 ) . The more dilute arsenious solutions were prepared fresh each day from a standard stock solution (ca. 0.09 N ) to prevent possible air oxidation. The 0.01 M osmium tetroxide was prepared by the usual method (6).
The time and tediousness of plotting separate graphs for the end-point determinations were avoided by placing a sheet of linen tracing paper over a piece of graph paper with appropriate labeled ordinates. Thus, readings for an individual titration were plotted on the tracing paper and could be erased quickly after the end point had been obtained. RESULTS
Figure 1 shows a typical plot of the titration of arsenic with 4.106 X 10-4 N ceric sulfate solution. In most titrations with arsenic, the optical density prior to the end point may be taken as zero in the plots without appreciable error. Four to six points were taken after the end point, although fewer points could be used when slightly less accuracy was permissible. It is apparent from the results shown in Table I that various concentrations of standard ceric sulfate can be used with comparable accuracy in the photometric titration of arsenious acid. These results do not show the minimum amount of arsenic t h a t may be determined by this method. By the use of smaller volumes, a smaller buret, and more dilute ceric sulfate solutions,
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2
41 1
it should be possible to determine considerably smaller amounts of arsenic with a reasonable degree of accuracy. Ceric(1V) sulfate solutions as dilute as 1 X 10-3 and 4 X 10-4 N can be prepared which show only small changes, if any, in normality over a period of 2 months. Further application of the use of dilute ceric solutions for photometric titrations is in progress. Preliminary work by the authors would indicate that the range of usefulness of dilute ceric solutions in photometric titrations could be extended by increasing the oxidation potential of the cerium(1V) by the use of perchloratocerate. The application of very dilute perchloratocerate solutions using nitroferroin indicator to the determination of calcium in blood plasma and to the microdetermination of arsenic and iron has been described by Smith and coworkers (6, 7 ) . The use of hydrochloric acid in the ceric sulfate titrations is not Dossible because of the rapid oxidation of the chloride ion
when excess cerium(1V) is present. However, in the presence of excess sulfuric acid, significant amounts of chloride ion can be tolerated. LITERATURE CITED
(1) Goddu, R. F., and Hume, D. N., ANAL.CHEM.,22,1314 (1950). (2) Kirk, P. L., “Quantitative Ultramicroanalysis,” pp. 129-31. New York, John Wiley & Sons, 1950. (3) Medalia, A. I., and Byrne, B. J.,ANAL.CHEM.,23,453 (1951). (4) Osburn, R. H., Elliot, J. H., and Martin, A. F., Ibid., 15, 642 (1943). (5) Salomon, K., Gabrio, B. W., and Smith, G. F., Arch. Biochem., 11. 433 (1946). (6) Smith, G. F., “Cerate Oxidimetry,” pp. 39-42, Columbus, Ohio, G. F. Smith Chemical Co., 1942. (7) Smith, G. F., and Fritz, J. S., ANAL.CHEM.,20, 874 (1948).
RECEIVED June
21, 1951.
Determination of Free Carbon in Atmospheric Dust ROBERT MCCARTHY AND CARL E. MOORE’ University of Louisville, Louisville, K y .
HE soiling property of atmospheric dust is of major economic T i m p o r t a n c e in heavily populated areas. Because this property is due to a considerable extent to the proportion of free carbon present, design and test studies in the air cleaning industry have made necessary the development of a convenient analytical method for the determination of free carbon in this dust. METHOD
The sample is treated with 70% nitric acid and heated to destroy the organic matter. The residue which contains the free carbon and insoluble inorganic matter is collected in a porcelain filter crucible, dried, and weighed. Free carbon is t h m determined by the loss of weight of the crucible on ignition. Procedure. Duplicate samples of 0.2 to 0.8 gram are weighed into 250-ml. beakers and 25 ml. of 70% nitric acid are added. Small samples are desirable with material of high free carbon content. The mixture is covered with a tvatch glass and boiled for 20 minutes, diluted with 125 ml. of 6 A’ nitric acid, and allowed t o stand overnight. (It is necessary that the supernatant liquid be strongly acid, or the carbon xi11 become dispersed and be impossible to separate by filtration.) The supernatant liquid is carefully decanted through a tared porcelain filter crucible and the insoluble material collected. The crucibles and contents are dried for 2 hours a t 140” C., cooled, and weighed; then they are ignited for 2 hours a t 700” C., cooled, and weighed; the loss in weight is free carbon.
Table I.
Digestion of Carbon Black w i t h i 0 q o Nitric .4cid
Free Carbon Taken Gram 0.5897 0.3249 0.3421 0.3374 0.3405 0.3063
Free Carbon Found Gram
Difference Gram
Time Digested Min.
0.5906 0.3263 0.3422 0.3381 0.3419 0.3080
+0.0009 f0.0004 +0.0001 + O . 0007 +0.0014 f0.0017
15 30 45 60 75 90
destroyed by wet oxidation with nitric acid. Part of the inorganic matter is dissolved by the nitric acid, and the insolubIc portion is collected with the free carbon. The free carbon is the loss of weight occurring when the inorganic matter and free carbon collected are ignited. The wet-oxidation technique of determining free carbon in rubber products is a standard procedure in the rubber industry ( 1 , Z),where the treatment necessary to degrade the rubber polymer yielded high results, a t first attributed to the formation of graphitic acids ( 2 ) but now considered due to insufficient drying ( 3 ) . The analytical method outlined in this paper is not seriously affected by this error (Tables I and 11). To test the validity of the proposed analytical method, the
Table 11. Digestion of Synthetic Rlixtures w-ith i o 7 0 Nitric Acid Free Carbon Taken Gram
Atmospheric dust contains organic matter, inorganic matter, and free carbon. The organic matter, largely lint, is easily 1
Present address, 4062 Edgehill Drive, Columbus, Ohio.
Difference Gram
Composition, 70% wood flour, 207, SiOz, 10% free carbon 0.1077 0,1081 0.1012 0.1065 0.1135
0,1080 0.1067 0.1001 0.1047 0.1133
+0.0003 -0.0014 - 0 0011 -0 0008
- 010002
Composition, 25% wood flour, 257, SiO2, 50% carbon 0,5432 0.4339 0,4730 0.5358 0.4691
0.5435 0.4343 0.4715 0.5360 0.4692
+ O . 0003
f0.0004 f0.0015 0002 fO.OOO1
+o
Composition, 10% wood flour, 70% SiOl. 20% carbon 0.2282 0.2384 0.2048 0 2041
0.2099
DISCUSSION
Free Carbon Found Gram
0.2276 0.2886 0.2046 0.2036 0.2105
-0.0006 +0.0002 - 0.0002 -0.0005 0.0000
+
Composition, 10% wood flour, 10% SiOz, 80% carbon 0.9051 0.9016 0.9127 0.8400 0.8736
0.9032 0.9012 0.9127 0.8406 0.8744
-0.001a -0.0004
+o.oooo +0.0006 +0.0009
