Solutions for Colorimetric Standards VI. Ferric Chloride 41. G . MELLON and C. T. KASLINE, Purdue University, Lafayette, Ind.
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OR the preparation of certain colorimetric standards various individuals (1) have proposed aqueous solutions of ferric chloride acidified with hydrochloric acid. Apparently recognizing some correlation between the hue of the solution and the amount of acid used, Arny made the acid concentration 1 per cent I n preparing and using such a standard it is desirable both to recognize the factors affecting its color and t o specify a set of conditions, following which will yield a reproducible system. The object of the present study, therefore, was t o determine the significant facts concerning the colorimetric characteristics of the solution, including quantitative measurements, by means of a new spectrophotometer, of the effect of different amounts of acid. A number of observations related to.this problem have been previously reported as facts. The pale yellow hue of a freshly prepared, dilute aqueous solution becomes brownish on standing a t roono temperature, because of the formation of a sol of hydrous ferric oxide (6). This hydrolysis may be reversed by a small quantity of hydrochloric acid (ZG), but the resulting brilliance is not quite that of the original (18). The more dilute the unacidified solution, the less is the proportion of total iron present in true solution (10, 19). Acid stabilizes the solution (15). Adding concentrated hydrochloric acid turns a dilute, aqueous solution a deep, yellowish brown (19). This reaction, together with several of a similar nature between such a solution and variouri other reagents, indicates the formation of complex ions (6, 16,19, 21). Over considerable ranges of concentration. Beer’s law does not hold for solutions containing solme acid (11,12). The character and intensity of absorption of light by the solution depends upon the concentration, hvdrolysis, and . Lcidity (8).
the three slits to give the desired width of spectral band at the given wave length, and reading the transmission on the photometer scale. The general practice was to make readings a t intervals of 10 mp from 400 to 700 mp and then to check the readings back to the starting oint. All data were obtained by dividing the transmission for t t e solution by that for the solvent. Measured cells were used and the data calculated to a thickness of 10.0 mm. by means of a s ecial Keuffel and Esser slide rule based upon the application o?Lambert’s law. To test the accuracy of the instrument the wave-length scale was calibrated in terms of lines from a mercury arc lamp and a helium tube (7), and the photometer scale was checked before each series of measurements by means of Bureau of Standards glasses (blue, amber, and red). Two solutions recommended by the Bureau of Standards (5) served to supplement the glasses. It is believed the calibration rendered the wave-length values accurate to 1 mp, Since the instrument checked the values for the standard glasses within 0.5 per cent, the error in the transmittancies may be expected to be of this order of magnitude. The readings going up and down any curve were generally reproducible within 10.2 per cent, indicating satisfactory precision. The average was taken for the values obtained going in the two directions. For relatively flat parts of a curve the slits were set for a 10 mp band and then reduced for the steep parts to 5 or 3 mp. These narrower bands somewhat decreased the sensitivity and precision.
It seems appropriate t o mention certain advantages of t h e instrument in comparison with the visual one used for previous papers in this series. Of outstanding interest was the elimination of the personal equation in matching intensities, with its attendant optical fatigue. The time required for operation
Experimental Work MATERIALS. Using ferric chloride, prepared by chlorinating iron wire (2), and concentrated hydrochloric acid, two stock solutions were prepared, one 4.10 M in ferric chloride and 0.1 M in acid, and one 10.9 M in acid only. Mixtures of these with water rovided the different concentrations desired. 8olloidal dispersions of hydrous ferric oxide were made by adding definite amounts of the stock solution of ferric chloride to boiling water and diluting to 100 ml. when cool.
PROCEDURE. The transmission m e a s u r e ments were made with a new photoelectric s p e c t r o p h o t o m e t e r built by the General Electric Company according to the design of A. C . Hardy. As the literature thus far contains only an a b s t r a c t (9) c o n c e r n i n g the principle of the instrument and a paragraph (16) on its construction, brief reference is made here to significant items regarding its use.
FIGURE 1. SPECTRAL TRANSMISSION CURVES Brqken linea, solutions.0.1 M in ferrio chloride and varying molarities in hydrochloTio mid. Solid, tines, colloidal. dispersions prepared from 1 and 10 ml. of a solution 4.1 M in ferric chloride and 0.1 M in hydroohloria aoid.
The :wembly consists essentially of two units, a manualily adjusted double monochromator and an automatic balancing photoelectric polarizing photometer The photoelectric cell serves merely as a null device for detecting differences in intensity in the two light beams entering an integrating s here. In o eration a Rochon prism is rotated automatically until!the two 1ig& beams match in intensity. At this point the percentage transmission may be read directly on a counter connected in the automatic mechanism. The operation of this instrument ww relatively simple, measurements being made by setting the wave-length scale, adjusting
was much less. With the sample and light source nearly 3 meters apart and with most of the optical system between them, heating of the sample was practically eliminated and only monochromatic light passed through it. Illumination of the dark room did not affect the measurements. The accuracy and precision in the blue and red regions was undoubtedly much superior to that obtained in the previous 187
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Calculation of monochromatic data (13) for solutions 0.5 M in ferric chloride and 0.273 to 8.8 M in acid showed that the colorimetric purity is high in all cases (97 to 99 per cent). The relative brilliance decreases with increasing acidity a t least up to 8.8 M while the dominant wave length shows the reverse change. Solutions 0.3 M in ferric chloride and 0.26, 1.0, and 7.5 M in acid were diluted to five times their original volume, keeping the acidity constant, and measured in 5-cm. cells. The curve for the data thus obtained for the 0.25 and 1.0 M solutions came 6 to 10 m p to the left of those in Figure 2 for the same s o l u t i o n measured before dilution in 1-cm. cells. For these two concentrations this represents distinct divergence from conformity to Beer’s law, but the data obtained in the same way for the 7.5 M solution were practically identical. Little or no dichromatism was indicated for a solution 0.25 M in ferric chloride and 0.44 M in acid when the luminosities were calculated according to Clark’s method (4) and plotted against wave lengths. The curves for three solutions, 0.1666 M in ferric chloride and 0.27 M in acid, one 5 years old, one 18 months old, and one freshly prepared, each made by a different individual, checked each other within the experimental error, confirming Amy’s observations (1) on FIGURE 2. SPECTRAL TRANSMISSION CURVES stability and reproducibility. However, soluBroken lines, solutions 0.3 M in ferric ch1,oride and varying molarities, in hydroohlorio apid. Solid lines, solutions 1.0 M in hydrochloric acid and varying molaritles in ferric ohlonde. tions 0.02 in acid were n o t sufficiently acidic to prevent hydrolysis, with resultant fading and formation of a precipitate. I n addition to the determination of the transmission data qualitative experiments were made which resulted, in general, Sidgwick has discussed (17) certain striking properties of ferric chloride which he attributes to its electronic structure. in a confirmation of the observations recorded in the literature I n a variety of organic solvents carbon tetrachloride alone cited. failed to dissolve it in substantial amounts. Curves for Figures 1 and 2 with wave lengths as abscissas and transsolutions in chloroform and in glacial acetic acid resembled mittancies as ordinates, represent typical data. The soluclosely those shown here for the lower acidities. Many of tions studied ranged in concentration from 0.0155 to 1.000 M in the organic solutions fade more or less rapidly (14), presumferric chloride, and from 0.0275 to 10.66M in hydrochloric acid. ably from the reduction of the iron, rendering them worthless as colorimetric standards. Discussion
papers. Preliminary checking of a few solutions has indicated considerable uncertainty in values there reported for wave lengths below 450 and above 650 mpu. At that time, however, the data were the best that could be obtained with the facilities available.
The curves shown in Figures 1 and 2 are indicative of the absorptive characteristics of the solutions. Only at the higher acidities is there evidence of divergence from the general regularity. On one solution, 0.1 M in ferric chloride and 1.1 M in acid, readings were made each 2 m p with a 5 m p band, but there were revealed a t this slit width no maxims nor minima, such as those recently reported by Brode (3) for cobaltous chloride or by Zscheile (25’)for chlorophyll. These data, together with visual observation of the systems, indicate that the solutions most likely to be of use for colorimetric standards are those showing smooth curves with acidities under 5 M . Several individuals have reported (11,la)that the brownish hue in very acidic solutions reaches a maximum when the concentration of acid is approximately 9 M, the exact point depending upon the concentration of iron. For this phenomenon Figure 1 does not show the expected reversal of curves. Solutions of such acidities are probably of no value for colorimetric work. In Figure 1 the curves for the interesting colloidal dispersion of hydrous ferric oxide should be considered as merely illustrative of the form characteristic of such systems. While a carefully prepared dispersion seems relatively stable, its reproducibility is very questionable unless under very closely controlled conditions. Apparently it is a problem of obtaining particles of the same size.
Summary Attention has been directed to various factors affecting the color of solutions of ferric chloride. Spectral transmission curves show the type of absorption under different conditions. The best range of concentrations for colorimetric standards is probably from 0.5 to 0.02 M in ferric chloride and from 5.0 to 0.05 M in hydrochloric acid.
Literature Cited (1) Amy, H. V., 8th Intern. Congr. Appl. Chem., 26, 319 (1912); J . Am. Pharm. Assoc., 12, 839 (1923); Amy, H. V., and Piokhardt, E. G., Druggists‘ Circ., 57, 131 (1914); Amy, H.V., and Ring, C. H., J. Franklin Inst., 180, 198 (1916); J. IND. ENO.C H ~ M .8, , 309 (1916); Crookes, W.,Odling, W., and Tidy, C. M., Chem. News, 43, 174 (1881); Kolthoff, I. M., Pharm. Weekblad, 59, 104 (1922); Ritohie, K. S., IND.ENG. can^., 19, 1289 (1927); Taub, A., J. Am. Pharm. Assoc., 16, 116 (1927). (2) Biltz, H.,and Biltz, W., “Laboratory Methods of Inorganic Chemistry,” p. 69,New York, John Wiley & Sona, 1909. (3) Brode. W. R..J . Am. Chem. Soc., 53,2457 (1931). Clark,’ W. M., “Determination of Hydrogen Iona,” p. 160, Baltimore, Williams and Wilkina, 1928. (5) Davis, R.,and Gibson, K. S., Bur. Standards Miscellaneoua Pub. 114,41 (1931). (6) Donnan, F. G., and Basset, H., J. Chem. Soc., 81, 939, 956 (1902); Piokering, S. U., Ibid., 105, 483 (1914). (7) Gibson, K. S.,et al., J . Optical SOC.Am., 10, 169 (1925).
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ASALYTICAL EDITIOS
Cimelin, L., “Handbuch der anorganischen Chemie,” No. 59, part B, p. 257, Leipzig, Verlag Chemie. G . m. b. €I., 1929. Hardy, A. C., J . Optical SOC.Am., 24, 162 (1934). Heymann, E., Z. anorg. Chem., 171, 26 (1928). Hostetter, J. C., J . Am. Chem. SOC.,41, 1535 (1919); Jones, H. C., and Anderson, J. A,, Am. Chem. J., 41, 202 (1909). Lemoine, G., Ann. chim. phys., [6], 30, 376 (1893); Moore, B. E., Phys. Rev.,12, 166 (1901). Rdellon, M. G., J. Phys. Chem., 33, 1931 (1930). Rilellon, M. G., and Foster, Violet, Ibid., 33,963 (1930). Rduller, E., J. prakt. Chem., 90, 129 (1914). Nutting, R. D., J . Optical SOC.Am., 24, 135 (1934).
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(17) Sidgwick, X. V., “The Electronic Theory of Valency,” p. 93, Oxford University Press, 1929. (18) Smith, T. B., “Analytical Processes,” p. 69, Arnold and Co., 1929. (19) Smith, T. B., Ibid., p. 329. (20) Spring, W., Rec. trav. chim.,18,238 (1899). (21) Weinland, R., “Einfuhrung in die Chemie der Komplex-Verbindung,” p. 100, F. Enke, 1924. (22) Zscheile, F. P., J. Phys. Chem., 38, 1 (1934).
RECEIVED Msroh 20, 1935. Previous papers of the seriea were publiahed in the Journal
of
Physical Chemistry.
Combination of Catalysts to Reduce Digestion Time in Determination of Nitrogen I. In Organic Compounds CHARLES F. POE and MARGARET E. NALDER, University of Colorado, Boulder, Colo.
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ONFIRMING the work of other investigators, it has been found in this laboratory (3) that 30 per cent hydrogen peroxide hastens the digestion in the determination of nitrogen in organic compounds by the Gunning process. Various combinations of catalysts, with and without strong hydrogen peroxide, have also been used in this laboratory in the determination of nitrogen in dairy products (6). I n this communication, there is reported the quantitative reduction in time when nitrogen is determined in organic compounds with the aid of strong hydrogen peroxide and a mixture of copper, mercury, and selenium as catalysts.
I n every case, the samples containing catalysts cleared much more rapidly than the controls. The use of the combination of copper, selenium, and mercury effectsthe greatest saving of time. I n some cases the use of strong hydrogen peroxide with these three catalysts somewhat reduces the digestion time over that obtained when these catalysts were used without the hydrogen peroxide, but not enough time was saved to warrant the additional expense and trouble. The use of selenium alone or in combination with other catalysts has been claimed by several investigators (%‘,C,6, 7) to cause some loss of nitrogen. The results of the present
The method employed was the Gunning modification of the original Kjeldahl method, which is official with the Association of Official Agricultural Chemists (1).
experiments, however, show no loss of nitrogen by the use of this catalyst. Use of a mixture of selenium, copper, and mercury for the determination of nitrogen in organic compounds does not seem to have been tried by other investigators.
In each of a number of digestion flasks, 0.25 gram of the organic compound was placed with 10 grams of nitrogen-free potassium !sulfate and 20 cc. of concentrated sulfuric acid. To four of the flasks were added the following catalysts: (1) HgO 0.5 gram; (2) CuSOr.5Hz0, 1 gram; (3) H e , 0.3 gram; Cu&O,.5H20,0.5gram; and Se, 0.1 gram; (4)the same ingredients as (3) with the addition of 1 cc. of strong hydrogen peroxide every 5 minutes until the solution cleared. The fifth flask contained no added catalyst. The contents of each flask were digested over electrically heated plates, all units of which were of the same construction and gave the same amount of heat. Each sample was heated until the liquid cleared. Table I gives the comparisons of the times necessary for the clearing to take place and some quantitative results.
Literature Cited (1) Assoc. Official Agr. Chem., “Official and Tentative Methods of Analysis,” 3rd ed., 1930. (2) Davis, C. F., and Wise, M., J . Cereal Chem., 10, 488 (1933). (3) Dewey, B. T., thesis, University of Colorado. (4) Osborn, R. A., and Krasnitz, A., J . Assoc. Oficial Agr. Chem., 17, 339 (1934). (5) Sandstedt, R. M., J . Cereal Chem., 9, 156 (1932). (6) Schafer, R. R., thesis, University of Colorado. (7) Snider, S. R., and Coleman, D. A., J . Cereal Chem., 11, 414 (1934).
RECEIVDD April 15, 1935.
